The Astronomy Book (Big Ideas Simply Explained)

300 DARK ENERGY T he Big Bang theory has It’s everywhere, really. It’s
at its heart a simple idea— between the galaxies. It is in
IN CONTEXT the universe started out
very small and then expanded. this room. We believe that
KEY ASTRONOMERS In 1998, two teams of scientists everywhere that you have
Saul Perlmutter (1959–) discovered that the expansion of space, empty space, you
Brian Schmidt (1967–) the universe is itself speeding up. cannot avoid having some
Adam Riess (1969–) This discovery revealed that what
astronomers can directly detect of this dark energy.
BEFORE makes up just 5 percent of the total Adam Riess
1917 Albert Einstein adds a mass and energy in the universe.
cosmological constant to his Invisible dark matter makes up galaxies are not only moving away
field calculations to keep the another 24 percent, while the from Earth; they are expanding
universe static. rest is a mysterious phenomenon, away from everywhere all at once.
known simply as dark energy.
1927 Georges Lemaître In 2011, three Americans, Saul Better picture
suggests that the universe Perlmutter, Brian Schmidt, and Subsequent observations helped
could be dynamic, not static. Adam Riess, won the Nobel Prize to tell the history of the expanding
for Physics for this discovery. universe. The 1964 discovery of
1928 Edwin Hubble finds the cosmic microwave background
evidence of cosmic expansion. Expanding space (CMB), a cold glow left over
The year after Georges Lemaître’s from the Big Bang, showed
1948 Fred Hoyle, Hermann paper hypothesized the Big Bang, that the universe has been
Bondi, and Thomas Gold Edwin Hubble found proof of the expanding for approximately
propose the steady-state theory expanding universe when he 13.8 billion years. Surveys of
of the expanding universe. showed that galaxies were moving
away from Earth—and the ones
AFTER that were farther away were
2013 The Dark Energy Survey moving faster. These were not
begins to map the universe. simply objects blasting away from
each other through space; this was
2016 The Hubble Space space itself growing in size and
Telescope shows that cosmic moving the matter with it. The
acceleration is 9 percent faster
than originally measured.

The expansion of the universe is Measuring this deceleration should reveal
assumed to be slowing down due the ultimate fate of the universe.

to the force of gravity.

However, when measured, cosmic expansion
is found to be accelerating.

This acceleration must be due to a previously unknown
force that works against gravity, called dark energy.

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See also: The theory of relativity 146–53 ■ Spiral galaxies 156–61 ■ The birth of the universe 168–71 ■ Beyond the Milky
Way 172–77 ■ Searching for the Big Bang 222–27 ■ Dark matter 268–71 ■ Redshift surveys 274–75

the large-scale structure of the By the mid 1990s, two programs If you’re puzzled by what
universe have since revealed that were under way to measure the dark energy is, you’re
billions of galaxies are clustered rate of expansion of the universe. in good company.
together around vast empty voids The Supernova Cosmology Project Saul Perlmutter
(p.296). This structure corresponds was headed by Saul Perlmutter
to minute ripples in the CMB that at Lawrence Berkeley National 1995. The survey found that type
show how observable matter—the Laboratory, while Brian Schmidt, 1a supernovae could be used as
stars and galaxies—emerged in based at the Australian National standard candles, or objects that
anomalous regions in otherwise University, led the High-Z Supernova can be used to measure distances
empty space. However, the future Search Team. Adam Riess, of across space. A standard candle
of the universe was uncertain. It the Space Telescope Science is an object of known brightness, ❯❯
was unknown whether it would Institute, was the lead author
expand forever or one day collapse for the latter project. The project
under its own gravity. leaders considered merging, but
had different ideas about how
Decelerating universe to proceed, and so opted instead
Throughout the 20th century, for a healthy rivalry.
cosmologists assumed that the rate
of expansion was slowing down. Both projects were using a
Following a rapid initial expansion, discovery made by the Calán/
gravity would start decelerating. Solodo Supernova Survey, carried
It seemed there were two main out in Chile between 1989 and
possibilities. If the universe was
heavy enough, its gravity would
eventually slow the expansion to
a stop and begin to pull matter
back together in a cataclysmic
Big Crunch, a kind of Big Bang
in reverse. The second possibility
was that the universe was too
light to stop the expansion, which
would therefore continue forever,
gradually slowing down. This
process would result in heat death,
where the material of the universe
had broken up, become infinitely
dispersed, and ceased to interact
in any way at all. A measurement
of the deceleration of the universe’s
expansion would tell cosmologists
which possible future the universe
was heading for.

The Chandra X-ray Observatory took
this image of the remnant of type 1a
supernova SN 1572 in Cassiopeia.
It is also known as Tycho’s nova,
as it was observed by Tycho Brahe.

302 DARK ENERGY

and so its apparent magnitude the temperature and pressure are A computer simulation shows a
(brightness as seen from Earth) such that a runaway nuclear fusion white dwarf star exploding in a type 1a
shows how far away it is. explosion ignites the star, creating supernova. A flame bubble forms inside
an object billions of times brighter the star (left), rises above the surface
A type 1a supernova is a little than the sun. (center), and envelops the star (right).
different from a standard supernova,
which forms when large stars run Distance and motion The brightness, or magnitude, of
out of fuel and explode. A type 1a Both surveys used the Cerro Tololo each star gave the distance—often
forms in a binary star system, in Inter-American Observatory in billions of light-years—while its
which a pair of stars orbit each Chile to find type 1a supernovae. redshift indicated its speed relative
other. One is a giant star, the other The plan was not simply to plot the to Earth, caused by the expansion
is a white dwarf. The white dwarf’s positions of the supernovae. They of the universe. The teams were
gravitational pull hauls stellar used the Keck Telescope in Hawaii aiming to measure the rate at
material over from the giant. The to take spectra of each explosion, which the expansion was changing.
material accretes on the surface of giving its redshift (the lengthening The rate of expansion, as indicated
the white dwarf until it has grown the spectra have undergone). by more distant objects, was
to 1.38 solar masses. At this point, expected to be tailing off. Exactly
In 2013, the Dark Energy Survey how fast it was doing this would
Dark Energy Survey began a five-year project to map show if the universe was “heavy”
the expansion of the universe or “light.” However, when the teams
in detail. The project uses the looked beyond about 5 billion light-
Dark Energy Camera (left) at years (meaning that they were
Cerro Tololo Inter-American looking 5 billion years into the
Observatory, Chile. The camera past), they found that the opposite
has one of the widest fields of was happening—the expansion
view in the world. In addition to of the universe was not slowing
searching for type 1a supernovae, down but speeding up.
the project is looking for baryon
acoustic oscillations. These are Dark energy
regular ripples in the distribution This result was first thought to
of normal matter about 490 million be an error, but successive checks
light-years apart, which can be showed it was not—and both
used as a “standard ruler” to teams found the same thing. In
show up cosmic expansion. 1998, Perlmutter and Schmidt went

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public with their findings. The This discovery has led us particles, which exist for a Planck
results shook the scientific world. to believe that there is some time (10-43 seconds, the smallest
Using Einstein’s field equations for unknown form of energy that possible amount of time) and then
general relativity, Adam Riess had is ripping the universe apart. disappear again. Dark energy may
found that the results appeared to match this idea—a form of energy
give the universe a negative mass. Brian Schmidt arising from these virtual particles,
In other words, it appeared that which creates a negative pressure
a kind of antigravity force was expanding or contracting— that pulls space apart, and
pushing matter apart. This source Einstein dropped the constant from represents a nonzero value for
of energy was named dark energy, his theories, calling it a mistake. the cosmological constant.
because it was a complete mystery.
The value of Einstein’s The expansion was not always
In 2016, new observations were cosmological constant is set to accelerating. There was a time
used to calculate a more accurate, match the energy contained in when gravity and other forces
and slightly faster, figure for the a vacuum—in empty space. This pulled matter together and was
acceleration of the universe’s was assumed to be zero. However, more powerful than dark energy.
expansion. If dark energy continues according to quantum theory, However, once the universe
to push the universe apart (it may even a vacuum contains “virtual” became big and empty enough,
not, no one really knows), it will the effects of dark energy appear
disperse the galaxies so that to have become dominant. It may
eventually they would all be too far be that a different force takes over
away to be seen from Earth (which in the future, or dark energy’s
itself will no longer exist). Eventually, effects may continue to grow. One
it may scatter the stars within the suggestion is that a Big Rip would
Milky Way until the sky goes dark. be so powerful that dark energy will
The sun and the planets in the solar tear apart spacetime itself, creating
system would be pulled apart, and a singularity—the next Big Bang. ■
finally the particles in atoms will
also be scattered, resulting in a form Four possible futures
of heat death dubbed the Big Rip.

Reviving Einstein’s mistake If the average If the density If the density Observations
Dark energy may indicate that the density of the is equal to the is below the suggest that
universe is not as homogenous as universe is above critical density, critical value, the the universe’s
cosmologists think it is, and that a certain critical the universe’s universe should expansion is
the apparent acceleration seen is value, it should geometry will be open and accelerating due
due to the fact that it is inside a be closed, and be flat, and the expand forever, to mysterious
region with less matter in it than end with a Big universe ought to to end eventually “dark energy.”
elsewhere. It may also be showing Crunch. The continue into the in a heat death. The measured
that Einstein’s theory of gravity critical value is future, neither density is very
is incorrect on the largest scales. On estimated to be expanding nor close to the
the other hand, dark energy might the equivalent of contracting. critical density,
also be explained by a mathematical five protons per but dark energy
device Einstein created in 1917 cubic meter. is accelerating
called the cosmological constant. expansion.
Einstein used this as a value that
would counteract the pull of gravity
and make the universe a static,
unchanging place. However,
when Lemaître used Einstein’s
own equations to show that the
universe could only be dynamic—

304

BPOEIVLEELRRIION1N3G.Y5BEAACRKS

STUDYING DISTANT STARS

IN CONTEXT T he James Webb Space An artist’s impression of the JWST
Telescope (JWST) is in space shows the layered stack of
KEY DEVELOPMENT designed to be the most the sunshield unfolded beneath the
James Webb Space powerful astronomical tool in space, telescope. The beryllium mirror is
Telescope (2002–) able to see farther than even the coated in gold for optimal reflection.
Hubble Space Telescope. Named
BEFORE in 2002 after the NASA director Conceived in 1995 as the successor
1935 Karl Jansky shows that who oversaw the Apollo program, to Hubble, the JWST has had a long
radiation other than light can the JWST is an infrared telescope road to completion, encountering
be used to view the universe. equipped with a 21-ft (6.5-m)-wide multiple technical hurdles. When
gold-plated mirror. This will allow launched in 2018, it will take up
1946 Lyman Spitzer Jr suggests it to see more than 13.5 billion a tight orbit around L2 (Lagrange
placing telescopes in space to light-years into the distance—to point 2), a location 1 million miles
avoid atmospheric interference. the time when the universe’s first (1.5 million kilometers) beyond
stars were forming. Earth’s orbit, away from the sun.
1998 The Sloan Digital Sky
Survey begins to make a 3D
map of the galaxies.

AFTER
2003 The Spitzer Space
Telescope, an infrared
observatory, is launched.

2014 The European Extremely
Large Telescope project is
approved, with a primary
mirror 39 m (128 ft) in diameter.

2016 LIGO announces the
discovery of gravitational
waves, suggesting a possible
means for looking even farther
than the JWST.

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See also: Radio astronomy 179 ■ Space telescopes 188–95 ■ A digital view of the skies 296 ■
Gravitational waves 328–31 ■ Lagrange (Directory) 336

The light from the first stars has been
shining through expanding space.

The expansion has Infrared is mostly To see the first stars,
stretched the light into invisible from a giant infrared
infrared wavelengths. Earth’s surface.
telescope must be
sent into space.

L2 is a place in space where the that penetrates the top layer is then are a prime target of observation
gravity of the sun and Earth work radiated sideways by successive for the JWST. At the same time,
together to pull an orbiting object inner layers so that almost nothing this ultra-sensitive eye on the
around the sun at the same rate as reaches the telescope itself. infrared sky has three other main
Earth, making one orbit every year. goals. It will investigate how
This means the JWST will be largely First light galaxies have been built over
in the shadow of Earth, blocking The light waves from the first stars billions of years, study the birth of
out any heat pollution from the to form have been stretched as stars and planets, and provide data
sun and allowing the telescope they shine through the expanding about extrasolar planets. NASA
to detect very faint infrared sources universe, changing them from hopes that the telescope will be
in deep space. NASA claims that visible light to infrared, so they in operation for at least 10 years. ■
the telescope could detect the heat
of a bumblebee on the moon. L4

Heat seeker Earth’s orbit
The JWST’s vast primary mirror
is seven times the area of Hubble’s JWST’s orbit
and, instead of polished glass, the
mirror contains 18 hexagonal units L3 L2
made from beryllium for maximum L1
reflection. The 270-sq-ft (25-m2)
mirror is too large to be launched Moon’s orbit
flat, so it is designed to unfold
once in orbit. L5

To pick up the faint heat JSWT will not be exactly at the L2 point, but will circle around it in
signatures of the most distant stars, a halo orbit. Lagrange points are positions in the orbit of two large bodies
the telescope’s detectors must where a smaller object can keep a stable position relative to those two large
always be extremely cold—never bodies. There are five L points in the orbital plane of Earth and the sun.
more than –370°F (–223°C). To
accomplish this, the JWST has
a heat shield the size of a tennis
court. Again, this is folded away
for launch. The shield is made from
five layers of shiny plastic that reflect
most of the light and heat. Any heat

OUR MISSION

COMETIS TO LAND ON A

UNDERSTANDING COMETS



308 UNDERSTANDING COMETS

IN CONTEXT B y studying comets, Giotto ignited the planetary
astronomers hope to science community in Europe.
KEY DEVELOPMENT shed new light on
ESA—Rosetta (2004) various questions about the early Gerhard Schwehm
solar system, the formation of
BEFORE Earth, and even the origins of life. Giotto Project scientist
1986 The Halley Armada of
eight spacecraft, led by ESA’s Earth is the only planet known rock. This revived the theory that
Giotto, make observations to have a surface ocean of liquid this is where Earth’s oceans came
of Halley’s comet. water. The origin of this water is from. One theory concerning the
one of the enduring mysteries of origin of life was that the complex
2005 The Deep Impact Earth science. A leading theory is chemical building blocks necessary
mission fires a probe at comet that the hot, young planet sweated for life, such as amino acids and
Tempel 1 to create a crater out the water from its rocks, nucleic acids, arrived on Earth
in the surface, and analyzes releasing water vapor into the from space. Perhaps these organic
what is underneath. atmosphere. Once the planet compounds were also delivered to
had cooled sufficiently, this vapor Earth by comets. The only way to
2006 The Stardust mission condensed and fell as a deluge of find out was to send a spacecraft
collects a capsule of cometary rain that filled the oceans. Another to meet up with a comet and land
dust from the tail of comet theory argues that at least some on its surface. In 2004, the ESA-led
Wild 2 and returns to Earth. of the water arrived from space, Rosetta mission blasted off on a
specifically in the hundreds of 10-year journey to do just that.
AFTER thousands of icy comets that rained
2015 New Horizons flies down on Earth during the first half Fresh target
by Pluto and begins an billion years of its existence and Rosetta’s intended target
exploration of the Kuiper belt. vaporized on impact. was Comet 67P/Churyumov–
Gerasimenko, or 67P for short. In
2016 NASA’s OSIRIS-REx A flyby of Halley’s comet in 1959, this comet had been captured
spacecraft is launched with a 1986 by a flotilla of spacecraft led by by the gravity of Jupiter, which had
mission to collect and return ESA’s Giotto got the first close-up pulled it into a shorter six-year orbit
a sample from the asteroid look at a comet’s core, or nucleus. of the sun. Before that, 67P had
101955 Bennu. The Halley encounter provided been circling the sun much farther
conclusive proof that comets are away. This excited the Rosetta
largely made from water ice mixed scientists because the tail of a
with organic dust and chunks of comet—its most familiar feature—
is caused by solar radiation heating
the surface of the nucleus, which

In 2005, the Deep Impact impactor
collided with comet Tempel 1,
releasing debris from the comet’s
interior. Analysis showed the comet
to be less icy than expected.

THE TRIUMPH OF TECHNOLOGY 309

See also: The Oort cloud 206 ■ The composition of comets 207 ■ Investigating craters 212 ■
Exploring beyond Neptune 286–87 ■ Studying Pluto 314–17

Comets are the
leftovers from the

formation of
the planets.

Earth’s water and the
chemicals needed for

life may have come
from comets.

creates streams of dust, gas, and An artist’s impression shows To find out,
plasma hundreds of millions of Rosetta releasing the Philae lander we need to land
miles long. The material in the tail above comet 67P. The lander bounced
is lost from the comet forever. 67P on landing, flying up from one lobe of on a comet.
had only been close to the sun a the comet to land again on the other.
handful of times in its existence. First indications
That meant it was still “fresh” with a central body about as large are that Earth’s water and
its primordial composition intact. as a small van. A folded solar organic chemicals did not
array unfurled to provide 690 sq ft
All aboard (64 m2) of photovoltaic cells, which come from comets.
Rosetta was launched by an Ariane would power the craft throughout
5 rocket from ESA’s space center the mission. the surface, Philae would pick up
in French Guiana. The spacecraft signals from CONSERT, sent out
weighed just under 3 tons, with Most of Rosetta’s instruments while Rosetta was orbiting on
were designed to study the comet the far side. Philae was equipped
We didn’t just land once— while in orbit. They included various with solar panels and rechargeable
maybe we landed twice! spectroscopes and microwave batteries and was designed to
radars for studying the composition work on the comet’s surface in
Stephan Ulamec of the comet surface and the dust order to analyze its chemistry.
and gases that would be released
Philae landing manager when 67P neared the sun and Both the names Rosetta and
began to heat up. One of the most Philae referred to ancient Egyptian
important instruments on board artifacts. The Rosetta Stone
was CONSERT (Comet Nucleus has a carved inscription in three
Sounding Experiment by Radiowave languages: Hieroglyphs, Demotic
Transmission), which would blast Egyptian, and ancient Greek. ❯❯
a beam of radio waves through the
comet to find out what lay inside.
CONSERT would operate with the
help of the lander Philae. Once on

310 UNDERSTANDING COMETS

and was soon bearing down on We are on the surface of the
67P at great speed. For the journey comet! Whatever we do has
to deep space, Rosetta had been never been done before. The
powered down to save energy, data we get there is unique.
but it powered back up and
contacted Earth on schedule as it Matt Taylor
neared the comet in August 2014.
Rosetta’s controllers then began Rosetta Project scientist
a series of thruster burns to make
Rosetta zigzag through space and
slow from 2,540 ft/s (775 m/s) to
26 ft/s (7.9 m/s). On September 10,
the spacecraft went into orbit
around 67P, offering the first look
at the target world.

Rosetta captured this image of Bumpy landing A landing zone on the “head” of
comet 67P/Churyumov−Gerasimenko Comet 67P is about 2.5 miles (4 km) the comet was selected, and at 8:35
on July 14, 2015, from a distance of long and turned out to be more GMT on November 12, 2014, Philae
96 miles (154 km), as the comet irregular in shape than expected. was released from Rosetta. It took
neared its closest point to the sun. From some viewpoints, the comet almost eight hours to confirm that
looks like a vast rubber duck, with its Philae was on the surface, much
In the early 19th century, it two lobes, one larger than the other, longer than expected. The lander
allowed scholars to decipher the connected by a narrow neck. (It is was designed to touch down at a
hieroglyphic writing system, thus assumed the comet was formed slow speed—slower than an object
unlocking the meaning of many from two smaller objects making dropped from shoulder height
ancient Egyptian writings. Philae a low-speed impact.) The surface on Earth—and attach itself to
refers to an obelisk with multiple was riddled with boulder fields the ground using harpoons fired
inscriptions that was used in a and ridges, and the Rosetta team from the tips of its legs. However,
similar way. Comets are remnants struggled to find a clear location something had gone wrong. It is
left over from the formation of the to set down the Philae lander.
solar system, so these names were
chosen because the Rosetta and Rosetta received gravity assists from both Earth and Mars en
Philae missions at comet 67P route to Comet 67P. As it swung around the planets, their gravitational
were intended as a way to unlock fields threw the spacecraft forward at greatly increased speed.
new knowledge of the primordial
material that formed the planets. Launch November 13, 2007
March 2, 2004 2nd Earth gravity assist
Comet cruise 4
Rosetta took a circuitous route
to the comet, using three flybys 21 February 25, 2007
of Earth and one of Mars (a 1st Earth Mars gravity assist
risky maneuver, skimming its gravity assist
atmosphere only 150 miles [250 km] 3
up) to boost speed through gravity
assists. This process took five June 8, 2012
years, after which Rosetta had Enters deep space hibernation
enough speed to fly through the 5
asteroid belt (getting a very close
look at some asteroids), and out
beyond the orbit of Jupiter. There,
it began to swing back around,

THE TRIUMPH OF TECHNOLOGY 311

On July 16, 2016, Rosetta was just
8 miles (12.8 km) from the center of
comet 67P. This image covers an area
about 1,500 ft (450 m) across. It shows
a dust-covered rocky surface.

thought that the lander landed of the ways in which the comet September 30 by making a controlled
awkwardly and hit a boulder, and was changing as it entered the crash-landing, returning data right
the very low gravity of the comet warmer part of the solar system. up to the moment of impact.
meant Philae bounced right off
again. It was later calculated that In mid-June 2015, Philae received Alien water
Philae bounced up about 3,000 ft enough sunlight to wake up, and The amount of deuterium (“heavy
(1 km) from the surface before falling began intermittent communication hydrogen”) found in 67P’s water
back again, tumbling to a resting with Rosetta, allowing further is much greater than in the water
place on the edge of the target CONSERT scans. In early July, found on Earth, evidence against
landing zone. Unfortunately, the however, it fell silent again. the idea that Earth’s water is of
lander ended up in the shadow of a Fortunately, it was spotted by the extraterrestrial origin. The mission
cliff and appeared to be at an angle. OSIRIS camera on September 2, has found many carbon-based
Without sunlight to recharge its 2016, as it approached within compounds, but only one amino acid
batteries, Philae had only about 1.7 miles (2.7 km) of the comet. (the building block of proteins) and
48 hours of power to perform its Knowing Philae’s precise landing no nucleic acids (the ingredient of
primary science missions, returning spot allows scientists to place the DNA) have been detected in the data.
data on the chemical composition information it sent back a year
of the dust and ice, and performing earlier into context. Rosetta’s results will allow
scans with the CONSERT astronomers to better understand
instrument on Rosetta. A last-ditch After comet 67P passed comets and whether 67P is a typical
plan to push the lander out into the perihelion in August 2015, the body. Combined with discoveries
sunlight using the harpoons (which solar power available to Rosetta fell from the Kuiper belt, this is hoped
had not fired on landing) failed, and rapidly. In September 2016, Rosetta to reveal what the solar system was
Philae shut down into safe mode. was instructed to get slowly nearer made of as the sun formed. ■
to the comet. It ended its mission on
Approaching the sun
Despite this setback, the
perilous Philae landing was
deemed a success. The hope
was that Philae’s shaded location
would become sunnier as the
comet approached the sun.
The comet would reach its
perihelion, or closest point, in
August 2015. On the approach,
comet 67P began to heat up and
its surface erupted with jets of dust
and plasma. Rosetta was sent on
a complex orbital path so that it
could fly low over the comet and
pass through the denser regions
of the coma, or cloud of material,
that was forming around 67P.
Its path also took it farther out,
providing a more complete picture

312

  STB  HOIRELTAVHRIOOSLFYESTNHTTEEM

THE NICE MODEL

IN CONTEXT B y the start of the 21st Surrounding all these bodies was
century, the solar system a distant sphere of comet material,
KEY ASTRONOMERS was known to contain many called the Oort cloud.
Rodney Gomes (1954–) kinds of object. In addition to the
Hal Levison (1959–) planets and the asteroid belt, there It was difficult to explain how
Alessandro Morbidelli (1966–) were cometlike bodies called a system like this had evolved
Kleomenis Tsiganis (1974–) centaurs located in between the from a proto-solar cloud of dust
giant planets, trojan asteroids and gas. Evidence from extra-solar
BEFORE sharing the orbits of many planets, systems showed that giant planets
1943 Kenneth Edgeworth and the outer Kuiper belt had were often much closer to their
suggests that Pluto is just also just been discovered. star than was previously thought
one of many objects in the possible. It was at least feasible,
outer solar system. The solar system is filled therefore, that the giant planets of
with many kinds of object, Earth’s solar system had formed
1950 Jan Oort suggests that closer to the sun.
long-period comets come from all orbiting the sun.
a distant cloud surrounding Planetary migration
the solar system. The arrangement of these In 2005, four astronomers in Nice,
objects formed as the France, used computer simulations
1951 Gerard Kuiper proposes to develop a theory to explain the
that a comet belt existed outermost planets Saturn, evolution of the solar system.
beyond Pluto in the early Uranus, and Neptune This is now known as the Nice
stages of the solar system. model. They suggested that
migrated out from the sun. the solar system’s three outer
1993 American planetary planets, Saturn, Uranus, and
scientist Renu Malhotra The outermost planets Neptune, were once much closer
suggests that planet migration swept away a vast disk to the sun than they are now.
took place in the solar system. of material, leaving the Jupiter was slightly farther away
than it is now at 5.5 astronomical
1998 The Kuiper belt is system seen today. units (AU), but Neptune was much
confirmed to exist. closer, at 17 AU (it now orbits at
30 AU). From Neptune’s orbit, a
AFTER vast disk of smaller objects called
2015 Spacecraft New Horizons planetesimals spread to 35 AU.
reaches the Kuiper belt. The giant planets pulled these

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See also: The discovery of Ceres 94–99 ■ The Kuiper belt 184 ■ The Oort cloud 206 ■ Investigating craters 212 ■
Exploring beyond Neptune 286–87

Rodney Gomes similar to those used in building The Nice model changed
the Nice model to understand the whole community’s
Brazilian scientist Rodney the motion of several Kuiper belt perspective on how the planets
Gomes is a member of the Nice objects (KBOs) that appear to formed and how they moved
model quartet of scientists that be following unusual orbits. In in these violent events.
came to prominence in 2005. 2012, he shook up the accepted
It also includes American Hal view of the solar system yet Hal Levison
Levison, Italian Alessandro again. Gomes proposes that a
Morbidelli, and Greek Kleomenis Neptune-sized planet (four times
Tsiganis. Gomes, who has as heavy as Earth) is orbiting
worked at Brazil’s national 140 billion miles (225 billion km)
observatory in Rio de Janeiro from Earth (at 1,500 AU) and
since the 1980s, is a leading that this mysterious planet
expert in the gravitational is distorting the orbits of the
modeling of the solar system, KBOs. The search is now on
and has applied techniques to locate this “Planet X.”

planetesmials inward and, in of thousands of meteorites were asteroid belt. Planetesimals were
return, Saturn, Uranus, and Neptune punched from the outer disk and also scattered farther out, including
started slowly edging farther away rained down on the inner planets. the dwarf planets Sedna and Eris,
from the sun. Planetesimals discovered in 2003 and 2005.
encountering Jupiter’s powerful Much of the planetesimal disk
gravity were fired out to the edge became the Kuiper belt, tied to The Nice model works well for
of the solar system to form the Neptune’s orbit at 40 AU. Some many starting scenarios for the
Oort cloud, and this had the effect planetesimals were captured by solar system. There is even one
of shifting Jupiter inward (its the planets to become moons, in which Uranus is the outermost
current orbital distance is 5.2 AU). others filled stable orbits as trojans, planet, only to swap places with
and some may have entered the Neptune 3.5 billion years ago. ■

Resonant orbit
Eventually, Saturn shifted to a
resonant 1:2 orbit with Jupiter,
which meant that Saturn orbited
once for every two orbits of Jupiter.
The gravitational effects of this
resonant orbit swung Saturn, then
Uranus and Neptune into more
eccentric orbits (on more stretched
ellipses). The ice giants swept
through the remaining planetesimal
disk, scattering most of it, to create
what is known as the Late Heavy
Bombardment, which occurred
about 4 billion years ago. Tens

During the Late Heavy Bombardment,
the moon would have glowed as it was
struck by meteorites. Most of the early
Earth’s surface was volcanic.

314 IN CONTEXT

SOVAIODCEDLWLABORAOSLEFSL-YAUOSNPFTETMHE KEY ASTRONOMER
Alan Stern (1957–)
 STUDYING PLUTO
BEFORE
1930 Clyde Tombaugh
discovers Pluto, which is
named as the ninth planet.

1992 Pluto is found to be
one of many Kuiper Belt
Objects orbiting the sun
beyond Neptune.

2005 Another Pluto-sized
object is found beyond the orbit
of Neptune. It is called Eris.

AFTER
2006 Pluto, Eris, and several
other objects are reclassified
as dwarf planets.

2016 A skewing of the orbits
of Kuiper Belt Objects suggests
that there is a Neptune-sized
planet much farther out in
space, orbiting the sun every
15,000 years. The search is
now on for this object.

I n January 2006, NASA’s
New Horizons spacecraft
lifted off from Cape Canaveral
on a voyage to the planet Pluto
and beyond. The moment was
testament to the perseverance
of the principal investigator for
New Horizons, Alan Stern.

Planetary demotion
At the time, nobody knew what Pluto
actually looked like. It was small
and far away on the inner rim of the
Kuiper belt, and even the mighty
Hubble Space Telescope could only
render it as a pixelated ball of light
and dark patches. Plans to explore
Pluto close-up were thwarted during
the 1990s as NASA budgets were

THE TRIUMPH OF TECHNOLOGY 315

See also: The Oort cloud 206 ■ The composition of comets 207 ■
Exploring the solar system 260–67 ■ Exploring beyond Neptune 286–87

Just as a Chihuahua is still was a planet at all. The IAU agreed Alan Stern
a dog, these ice dwarfs that the new body, to be named
are still planetary bodies. Eris, was not a planet. Its gravity Born Sol Alan Stern in New
Alan Stern was too weak to clear other bodies Orleans, Louisiana, in 1957,
from its orbit. The planets from Stern’s fascination with
squeezed. By 2000, the plans had Mercury to Neptune are big enough Pluto began in 1989 when
been shelved, but Stern made the to do this, but the bodies of the he worked with the Voyager
case for sending a mission to Pluto, asteroid belt manifestly are not— program. While he was there,
the smallest, most distant planet, and nor was Pluto. However, Stern witnessed the final
which had been discovered by US Pluto and Eris were not like most encounter of Voyager 2
astronomer Clyde Tombaugh in 1930. asteroids. They were massive as it flew past Neptune and its
enough to be spherical rather than moon Triton. Triton appeared
In 2003, Stern’s New Horizons irregular chunks of rock and ice. as a ball of ice, and looked
proposal was given the green So the IAU created a new class of very much like the Pluto
light, and the 2006 launch set the object: that of dwarf planet. Pluto, Stern and other scientists
spacecraft on a nine-year flight to Eris, and several large Kuiper Belt had imagined. (Triton is
Pluto. It occurred not a moment too Objects (KBOs) were given dwarf thought to be a Kuiper Belt
soon. In August 2006, prompted planet status, as was Ceres, the Object that has been captured
by the discovery of a possible largest body in the asteroid belt. by Neptune.)
tenth planet beyond the orbit of For most of these objects, this was
Pluto, the general assembly of the a promotion in the hierarchy of In the 1990s, Stern trained
International Astronomical Union the solar system, but not for Pluto. as a Space Shuttle payload
(IAU) gathered in Prague to discuss If Pluto had been declassified as specialist (technical expert),
issues raised by the new discovery. a planet prior to New Horizons’ but he never got the chance
The first question was whether it launch, it is uncertain whether to fly into space. Instead, he
the mission would have happened. returned to the study of Pluto,
the Kuiper belt, and the Oort
Long journey cloud. In addition to leading
Although Pluto’s orbit does bring the New Horizons mission
it closer to the sun than Neptune as principal investigator,
for some of its 248-year revolution Stern is active in developing
around the sun, the New Horizons new instruments for space
probe had the longest journey to the exploration and more cost-
most distant target in the history ❯❯ effective ways of putting
astronauts into orbit.
Pluto is too far away The only way to
to observe details with study Pluto is to send Key work

a telescope. a spacecraft. 2005 Pluto and Charon: Ice
Worlds on the Ragged Edge
The spacecraft has revealed that of the solar system
Pluto’s icy structure is a completely

new kind of planetary body.

316 STUDYING PLUTO

New Horizons’ scientific instruments were Range Reconnaissance Imager), a
switched off to conserve power for most of its telescopic camera, would produce
10-year journey, but they were powered up for the highest resolution pictures
one month a year so that checks could be made. of the Pluto system; SWAP (Solar
Wind Around Pluto) would, as the
REX name suggests, observe Pluto’s
PEPSSI interaction with the solar wind,
while PEPSSI (Pluto Energetic
Alice Particle Spectrometer Science
Investigation) detected the plasma
Ralph given off by Pluto. This would help
in understanding the way the
SWAP dwarf planet’s atmosphere is
formed by sublimation (the change
LORRI VBSDC of a solid directly into a gas) from
the icy surface during Pluto’s
of space exploration—30 AU, or be leaving Pluto behind. It takes “summer” as it nears the sun
2.7 billion miles (4.4 billion km) from radio signals from Pluto 4.5 hours (and then freezes again in winter).
Earth. To achieve this, the spacecraft to reach Earth, plus the same again Finally the SDC (Student Dust
was given the fastest launch ever, to send a return message. Therefore, Counter) was an instrument
blasting off with an escape velocity it would take at least nine hours to operated by university students
of 36,373 mph (58,536 km/h). A year make even tiny course corrections, throughout the mission. This
after launch, the spacecraft reached by which time the primary mission experiment was renamed VBSDC
Jupiter. In addition to making some would be almost over. for Venetia Burney, the British girl
observations of the Jovian system, who had proposed the name Pluto.
New Horizons used Jupiter’s gravity New Horizons carried seven
to gain a 20 percent speed boost. instruments. They included two Destination reached
This cut the flight time to Pluto imaging spectrometers, built to New Horizons began its approach in
from over 12 years to 9.5 years. work together and named after the January 2015. One of the first things
characters in the 1950s US sitcom it did was to make an accurate
Instruments on board The Honeymooners. Ralph was the measurement of Pluto’s size. This
The precision of the trajectory from visible and infrared spectrometer had always been a tricky problem to
Jupiter was crucial to the success of used to make maps of Pluto’s solve. When it was first discovered,
New Horizons. If it was just slightly surface, while Alice was sensitive
off, the craft would miss Pluto to ultraviolet and was tasked with It used to be that Pluto was a
altogether. The main observation studying Pluto’s thin atmosphere. misfit. Now it turns out that
window was about 12 hours long, REX (Radio science EXperiment)
after which New Horizons would would take the temperature of Earth is the misfit. Most
Pluto and its moons; LORRI (Long planets in the solar system
look like Pluto, and not like

the terrestrial planets.
Alan Stern

THE TRIUMPH OF TECHNOLOGY 317

it was estimated that Pluto was (12,472 km) above Pluto, its closest This view from New Horizons zooms
seven times the size of Earth. By approach. Its instruments were in on the southeastern portion of
1978, it was clear that Pluto was collecting huge quantities of Pluto’s great ice plains, where the
smaller than Earth’s Moon. However, data to be fed back to Earth. plains border rugged, dark highlands.
it also had a huge satellite, named The close-up view of Pluto showed
Charon (the boatman of the dead in it to be a world of pale ice plains are named after past missions:
Greek mythology), which was about and dark highlands. The ice is Voyager, Venera, and Pioneer. Two
a third of the size of Pluto, and the largely frozen nitrogen, which main mountain ranges have been
two bodies moved around each makes Pluto a very bright object imaged clearly: Norgay Montes and
other as a binary system. At launch, for its size. The highlands are also Hillary Montes, named after the first
planners also took into account two ice (although mixed with tar-like two climbers to reach the summit of
more tiny moons, Nix and Hydra, hydrocarbons). The ice is thrust Mount Everest. However, the central
but by 2012, with New Horizons into lumpy peaks that tower 2 miles feature of New Horizons’ partial
already well on the way, it was (3 km) above the plains. Quite map of Pluto is Tombaugh Regio,
found that there were two more— how such huge features arose a heart-shaped plain. Half of this
Kerberos and Styx—which could on such a cold and small body is area is made up of Sputnik Planitia,
potentially disrupt the mission. one of the mysteries of the New a vast ice floe riddled with cracks
Horizons mission. In addition, and troughs, but with no craters,
Measuring Pluto craterlike structures have been which suggests it is a young feature
In the end, these worries were identified as possible ice volcanoes. that is carving out new surface
misplaced, and LORRI was able features like glaciers on Earth.
to obtain a measurement for all Naming the landmarks
of these bodies. Pluto is 1,470 miles Pluto’s surface features have been Now past Pluto, the craft is
(2,370 kilometers) wide, meaning given unofficial names by NASA on course to meet up with other
that it is larger than Eris (although scientists. Cthulhu Regio is a large KBOs. Its nuclear power source
Eris is heavier). On July 14, 2015, whale-shaped dark patch in the should last until around 2030, and
New Horizons flew 7,750 miles southern hemisphere. Other regions the mission should make many
more discoveries. ■

MARSA LABORATORY ON

EXPLORING MARS



320 EXPLORING MARS I n August 2012, the Mars Mars has been flown by,
Science Laboratory Rover, orbited, smacked into,
IN CONTEXT better known as Curiosity, radar-examined, and rocketed
landed on Mars. This 2,000-lb onto, as well as bounced
KEY ORGANIZATION (900-kg) wheeled vehicle, which upon, rolled over, shoveled,
NASA—Mars exploration is still roaming the Martian surface, drilled into, baked, and even
is a mobile laboratory equipped to blasted. Still to come: Mars
BEFORE conduct geological experiments
1970 The Soviets’ Lunokhod 1 aimed at figuring out the natural being stepped on.
becomes the first vehicle to be history of the red planet. It is the Buzz Aldrin
used on another body when it latest robot explorer to reach
touches down on the moon. Mars, and the largest and most
advanced in a long line of rovers
1971 The Lunar Roving sent to explore other worlds.
Vehicle is driven on the moon
for the first time during Wanderers spent more than 22 hours outside.
NASA’s Apollo 15 mission. The potential of rovers in space In their rover, they covered 22 miles
was clear as far back as 1971, (36 km) in total, with one drive
1977 NASA’s Sojourner is when Apollo 15 carried a four-wheel taking the pair 4.7 miles (7.6 km)
the first rover to reach Mars. Lunar Roving Vehicle to the moon. from their spacecraft. The Lunar
This agile two-seater widened the Roving Vehicle, or moon buggy,
AFTER scope of lunar exploration for the was used to collect rocks. The six
2014 Opportunity breaks the last three Apollo missions. For Apollo missions returned to Earth
distance record for a rover on instance, during the first moon with 840 lb (381 kg) of them.
an extraterrestrial body. landing in 1969, Neil Armstrong
and Buzz Aldrin spent just two Analysis of these rocks revealed
2020 The NASA Mars 2020 and a half hours moonwalking, and much about the history of the moon.
rover is set to be launched as the farthest they moved from their The oldest were about 4.6 billion
a replacement for Curiosity. lunar module was 200 ft (60 m). years old, and their chemical
By contrast, however, in the final composition clearly showed a
2020/21 The ExoMars rover Apollo moon mission, Apollo 17, common ancestry with rocks on
is due to be deployed by the in 1972, the crew of two—Eugene Earth. Tests revealed no evidence
European Space Agency in Cernan and Harrison Schmitt— of organic compounds, indicating
Oxia Planum, a depression that the moon has always been
filled with clay-bearing rocks. a dry and lifeless world.

Lunokhod 1
The Soviet lunar program, which
began in the early 1960s, relied on
unmanned probes to explore the
moon. Three of the Soviet Luna

Geologist−astronaut Harrison
Schmitt collects samples from the lunar
surface during the 1972 Apollo 17
mission. He spent many hours exploring
the surface on the moon buggy.

THE TRIUMPH OF TECHNOLOGY 321

See also: The Space Race 242–49 ■ Exploring the solar system 260–67 ■ Understanding comets 306–11 ■
Studying Pluto 314–17

The Soviet Lunokhod 1 rover, seen
here in tests on Earth, was the first
rover ever to land on an alien world—its
predecessor, Lunokhod 0, was launched
in 1969 but never reached orbit.

probes returned with a total of moon. An X-ray spectrometer as a heater to keep the machinery
11.5 oz (326 g) of rock. Then, in was used to analyze the chemical working. The rover received
November 1970, the Soviet lander composition of rocks, and a device commands from controllers on
Luna 17 arrived at a large lunar called a penetrometer was pushed Earth about where to go and when
plain called the Sea of Rains (many into the lunar regolith (soil) to to perform experiments. A human
lunar areas are named after the measure its density. might have done a better job,
weather conditions they were once but rovers could stay in space for
thought to influence on Earth). Lunokhod was powered by months on end, and did not require
Luna 17 carried the remote- batteries that were charged by day food and water from Earth.
controlled rover Lunokhod 1 using an array of solar panels that
(Lunokhod means “moonwalker”). folded out from the top of the rover. Lunokhod 1 was designed
This was the first wheeled vehicle At night, a source of radioactive to work for three months, but
to traverse an extraterrestrial world, polonium inside the machine acted lasted almost 11. In January 1973,
arriving about eight months before Lunokhod 2 landed in the Le
the first Apollo buggy. The concept Over time you could terraform Monnier Crater on the edge of the
behind it was simple—instead of Mars to look like Earth … Sea of Serenity. By June, Lunokhod
sending moon rocks to Earth, the 2 had traveled a total of 24 miles
rover would do the analysis there. So it’s a fixer-upper of a planet. (39 km), a record that would stand
Elon Musk for more than three decades.
Remote-controlled explorer
The Lunokhod rover was 7½-ft Canadian space entrepreneur Martian walker
(2.3-m) long and resembled a As Lunokhod 1 was exploring the
motorized bathtub. The wheels moon, the Soviet space program
were independently powered so was eyeing an even greater prize:
that they could retain traction on a rover on Mars. In December 1971,
the rough lunar terrain. Lunokhod two Soviet spacecraft, code-named
was equipped with video cameras Mars 2 and Mars 3, sent modules
that sent back TV footage of the to land on the red planet. Mars 2
crashed, but Mars 3 made a
successful touchdown—the first-
ever landing on Mars. However,
it lost all communications just
14.5 seconds later, probably due
to damage from an intense dust
storm. Scientists never found out
what happened to Mars 3’s cargo:
a Prop-M rover, a tiny 10-lb (4.5-kg)
vehicle designed to walk on two
ski-shaped feet. It was powered
through a 50-ft (15-m) umbilical
cord, and once on the surface was
designed to take readings of the
Martian soil. It is unlikely that ❯❯

322 EXPLORING MARS

the Prop-M ever carried out its We landed in a nice flat spot. traveled just 300 ft (100 m) during
mission, but it was programmed Beautiful, really beautiful. its 83-day mission, and never
to operate without input from Adam Steltzner ventured farther than 40 ft (12 m)
Earth. A radio signal between from the lander. Now named the
the moon and Earth travels in less Lead landing engineer, Curiosity Carl Sagan Memorial Station, the
than 2 seconds, but a signal to lander was used to relay data from
or from Mars takes between 3 huge protective airbags inflated the rover back to Earth. Most of the
and 21 minutes to arrive, varying around the lander, and retrorockets rover’s power came from small solar
with the planet’s distance from on the spacecraft holding the tether panels on the top. One of the goals
Earth. For a Martian rover to be fired to slow the speed of descent. of the mission was to see how
a successful explorer, it needed The tether was then cut, and the these panels stood up to extreme
to work autonomously. lander bounced across the Martian temperatures and what power
surface until it rolled to a stop. could be generated in the faint
Bounce down Fortunately, once the airbags had Martian sunlight.
In 1976, NASA’s two Viking landers deflated, the lander was the right
sent back the first pictures of Mars. side up. The three upper sides or The rover’s activities were
Following this success, many more “petals” of the tetrahedral lander run from NASA’s Jet Propulsion
rovers were planned, but most of folded outward, revealing the Laboratory (JPL) in California, and
these projects never reached their 24-lb (11-kg) rover. JPL has remained the lead agency
destination, succumbing to what the in developing Martian rovers.
press dubbed the “Martian Curse.” During development, the With the time delays inherent in
rover was called MFEX, short communicating with Mars, it is
NASA eventually had a success for Microrover Flight Experiment. not possible to drive a rover in real
with its 1997 Mars Pathfinder However, it was known to the time, so every leg of a journey must
mission. In July of that year, the public as Sojourner, meaning be preprogrammed. To achieve
Pathfinder spacecraft entered the “traveler” and chosen for its link this, cameras on the lander were
Martian atmosphere. Slowed first to Sojourner Truth, a 19th-century used to create a virtual model
by the friction of a heat shield and US abolitionist and rights activist. of the surface around Sojourner.
then by a large parachute, the Human controllers could view the
spacecraft jettisoned its outer area in 3-D from any angle before
shielding, and the lander inside mapping a route for the rover.
was lowered on a 65-ft (20-m)
tether. As it neared the surface, Spirit and Opportunity
Despite its limitations in terms of
size and power, Sojourner’s mission
was a great success, and NASA

Rolling on Mars Whatever the reason you’re on
Sojourner was the first rover to Mars is, I’m glad you’re there.
take a tour of the Martian surface. And I wish I was with you.
However, the Pathfinder mission
was really a test for the innovative Carl Sagan
landing system and the technology
that would power larger rovers in in a message for future explorers
the future. The minuscule vehicle

During its 83 days of operation,
the tiny Sojourner rover explored
around 2,691 sq ft (250 sq m)
of the planet’s surface and
recorded 550 images.

THE TRIUMPH OF TECHNOLOGY 323

An artist’s impression portrays a
NASA Mars Exploration Rover. Rovers
Opportunity and Spirit were launched
a few weeks apart in 2003 and landed
in January 2004 at two sites on Mars.

pressed ahead with two Mars team did not know whether the the rising sun in order to maximize
Exploration Rovers (MERs). In June solar-powered rovers would retain electricity generation and to top
2003, MER A, named Spirit, and adequate power to keep working. off the batteries. All nonessential
MER B, Opportunity, were ready for Of all the solar system’s rocky equipment was shut down so
launch. They were about the same planets, the seasons of Mars are the that power could be diverted
size as a Lunokhod rover, but were most Earth-like, due to the similar to heaters that kept the rovers’
much lighter, at around 400 lb tilts of the planets’ rotational axes. internal temperature above
(180 kg). By the end of January the Martin winters are dark and bitterly –40°F (–40°C).
following year, both were traveling cold, with surface temperatures
across the Martian deserts, hills, falling to as low as –225°F (–143°C) Continuing mission
and plains, photographing surface near the polar ice caps. The hibernation worked, and
features and chemically analyzing incredibly, JPL has managed to
rock samples and atmosphere. They As predicted, Martian winds extend the rover missions from
sent back the most glorious vistas blew fine dust onto the solar arrays, a few days to several years. More
of the Martian landscape ever seen, cutting their generating power; but than five years into its mission,
enabling geologists to examine the the wind also blew the panels clean however, Spirit became bogged
large-scale structures of the planet. from time to time. As winter drew down in soft soil; all attempts
nearer, the JPL team searched for to free it by remote control from
Spirit and Opportunity had suitable locations in which the Earth failed, and unable to move
landed using the same airbag- rovers could safely hibernate. To to a winter refuge, Spirit finally
and-tether system as Sojourner. do this, they used a 3-D viewer lost power 10 months later. It
Like Sojourner, both relied on built from the images taken from had traveled 4.8 miles (7.73 km).
solar panels, but the new rovers the rover’s stereoscopic cameras. Opportunity, meanwhile, has ❯❯
were built as self-contained units, They chose steep slopes that faced
able to wander far from their
landers. Each vehicle’s six wheels
were attached to a rocking
mechanism, which made it
possible for the rovers to keep at
least two wheels on the ground
as they crossed rugged terrain.
The software offered a degree of
autonomy so that the rovers could
respond to unpredictable events,
such as a sudden dust storm,
without needing to wait for
instructions from Earth.

Low expectations
Nevertheless, expectations
for these rovers were low. JPL
expected that they would cover
about 2,000 ft (600 m) and last
for 90 Martian sols (equivalent
to about 90 Earth days). During
the Martian winter, however, the

324 EXPLORING MARS

avoided mishap and continues to vaporized samples of ground rock to In the “Kimberley” formation
operate. In 2014, it beat Lunokhod reveal their chemicals. In addition, on Mars, photographed by Curiosity,
2’s distance record, and by August the rover monitored radiation levels strata indicate a flow of water. In the
2015 it had completed the marathon to see whether the planet would be distance is Mount Sharp, named after
distance of 26.4 miles (42.45 km). safe for future human colonization. US geologist Robert P. Sharp in 2012.
This was no mean feat on a planet
located some hundreds of millions Considerably larger than by the sheer distance from Earth)
of miles from Earth. previous rovers, Curiosity was was 14 minutes, and the journey
delivered to Mars in an unusual through the atmosphere to the
Curiosity needed way. During the landing phase of surface would take just seven—all
Spirit and Opportunity were the mission, the radio delay (caused on autopilot (not remotely controlled
equipped with the latest detectors; from Earth). This created “seven
including a microscope for imaging The Seven Minutes of Terror minutes of terror”: the engineers
mineral structures and a grinding has turned into the Seven on Earth knew that by the time a
tool for accessing samples from Minutes of Triumph. signal arrived informing them that
the interiors of rocks. John Grunsfeld Curiosity had entered the Martian
atmosphere, the rover would
However, Curiosity, the next NASA associate administrator already have been on the ground
rover to arrive on the planet in for seven minutes—and would be
August 2012, carried instruments operational or smashed to pieces.
that not only studied the geology
of Mars but also looked for Safe landing
biosignatures—the organic As Curiosity’s landing craft moved
substances that would indicate through the upper atmosphere,
whether Mars once harbored life. its heat shield glowed with heat,
These included the SAM or Sample while rockets adjusted the descent
Analysis at Mars device, which

THE TRIUMPH OF TECHNOLOGY 325

speed to reach the Gale Crater, an be detached and blasted clear of the ExoMars
ancient crater caused by a massive area so that its eventual impact did
meteorite impact. A parachute not upset any future exploration. In 2020, the European Space
slowed the craft to about 200 mph Agency, in collaboration with
(320 km/h), but this was still too Having survived the landing, the Russian space agency,
fast for a landing. It continued to Curiosity signaled to Earth that Roscosmos, will launch its
slow its descent over a flat region it had arrived safely. Curiosity’s first Mars rover, ExoMars
of the crater, avoiding the 20,000-ft power supply is expected to last (Exobiology on Mars), with
(6,000-m) mountain at its center. at least 14 years, and the initial the goal of landing on Mars
The craft reached about 60 ft two-year mission has now been the following year. In addition
(20 m) above the surface and then extended indefinitely. So far, it has to looking for signs of alien life,
had to hover, since going too low measured radiation levels, revealing the solar-powered rover will
would create a dust cloud that that it may be possible for humans carry a ground-penetrating
might wreck its instruments. The to survive on Mars; discovered an radar that will look deep
rover was finally delivered to the ancient stream bed, suggesting a into Martian rocks to find
surface via a rocket-powered past presence of water and perhaps groundwater. The ExoMars
hovering platform called a sky even life; and found many of the key rover will communicate with
crane. The sky crane then had to elements for life, including nitrogen, Earth via the ExoMars Trace
oxygen, hydrogen, and carbon. ■ Gas Orbiter, which was
launched in 2016. This system
Distances traveled by extraterrestrial rovers will limit data transfer to
twice a day. The rover is
Lunokhod 1 designed to drive by itself;
1970–71 its control software will build
moon: 6.5 miles a virtual model of the terrain
(10.5 km) and navigate through that.
Apollo 17 Rover The rover software was taught
Dec. 1972 how to drive in Stevenage,
moon: 22.2 miles England, at a mockup of the
(35.74 km) Martian surface called the
Lunokhod 2 Mars Yard (above).
Jan.–Jun. 1973
moon: 24.2 miles The ExoMars rover is
(39 km) expected to operate for at
Sojourner least seven months and to
Jul.–Sep. 1997 travel 2.5 miles (4 km) across
Mars: 0.06 miles the Martian surface. It will be
(0.1 km) delivered to the surface by a
Curiosity robotic platform that will then
2011–present remain in place to study the
Mars: 8.1 miles area around the landing site.
(13.1 km)
Spirit
2004–2010
Mars: 4.8 miles
(7.7 km)
Opportunity
2004–present
Mars: 26.6 miles
(42.8 km)

DISTANCE IN MILES 0 6 12 18 24

326

ETYHEE OBNIGTGHEESTSKY

LOOKING FARTHER INTO SPACE

IN CONTEXT D espite its name, the ESO, Telescope, the new technology
or European Southern being adaptive optics that reduce
KEY DEVELOPMENT Observatory, is located the blurring effect on images
European Extremely in northern Chile, a region of dry caused by the turbulence of the
Large Telescope (2014–) desert and high mountains ideal atmosphere. In 1999, it opened
for ground-based astronomy. its Very Large Telescope, which
BEFORE This collaborative organization comprises four 27-ft (8.2-m)
1610 Galileo Galilei makes of 15 European countries, along reflecting telescopes that can be
the first recorded astronomical with Brazil and Chile, has been used together. The Atacama Large
observations using a telescope. pushing the limits of astronomy Millimeter Array, a vast radio
for more than 50 years. telescope with 66 antennae, then
1668 Isaac Newton makes the became operational in 2013. This
first usable reflecting telescope. Big telescopes is the largest ESO program to date,
The ESO uses literal names for and the largest ground-based
1946 Lyman Spitzer Jr. its telescopes. In 1989, it began astronomical project of all time.
suggests putting telescopes operating the New Technology In 2014, however, the ESO received
in space to avoid Earth’s
atmospheric interference. European Southern Observatory

1990 The Hubble Space Formed in 1962, the ESO today Observatory, an ultra-modern
Telescope is launched. has 17 member countries: science center in the remote
Austria, Belgium, the Czech desert. The observatory’s
AFTER Republic, Denmark, Finland, subterranean living quarters
2015 Construction begins in France, Germany, Italy, the were used as a Bond villain’s lair
Chile on the US-led 72-ft (22-m) Netherlands, Poland, Portugal, in the 2008 movie Quantum of
Giant Magellan Telescope. Spain, Sweden, Switzerland, and Solace. The site’s new Extremely
the UK, with Chile and Brazil. Large Telescope is costing
2016 LIGO detects the It is located in the Atacama $1.1 billion (€1 billion) to build.
gravitational waves of objects Desert of Chile, chosen for its The ESO opted for this project
in space. clear, moisture-free skies and after rejecting the far costlier
the absence of light pollution. OWL (Overwhelmingly Large
2018 The James Webb The ESO’s headquarters is Telescope), the proposed design
Space Telescope will become near Munich, Germany, but of which had a 330-ft (100-m)
the largest telescope ever its working base is the Paranal wide primary mirror.
launched into space.

THE TRIUMPH OF TECHNOLOGY 327

See also: Galileo’s telescope 56–63 ■ Gravitational theory 66–73 ■ Space telescopes 188–95 ■
Studying distant stars 304–05

The E-ELT’s dome is shown opening
as the sun sets over the desert in this
artist’s impression. The completed
structure will be 256 ft (78 m) high.

funding for the European Extremely artificial “star,” which is created can ripple and warp in real time
Large Telescope (E-ELT). When by firing a laser into the sky. M4 to counteract any atmospheric
completed in 2024, this will be can alter its shape 1,000 times a distortions. Finally, M5 directs
the largest optical telescope ever second using 8,000 pistons housed the image into the camera.
built, with a resolution 15 times underneath. In other words, the 798
sharper than the Hubble Space segments of this astonishing mirror The E-ELT will pick up a
Telescope (pp.172–77). narrower band of the spectrum
than space telescopes, but it can
Giant mirror do so on a much larger scale. As
The E-ELT has an unusual five- a result, the E-ELT will be able
mirror design housed inside a to see exoplanets, protoplanetary
dome half the size of a football discs (including their chemistry),
stadium. The primary mirror (M1), black holes, and the first galaxies
which collects the visible light in greater detail than ever before. ■
(and near infrared) is built from
798 hexagonal segments that are Secondary Fourth
4 ft 10 in (1.45 m) wide. Together mirror (M2) mirror (M4)
they will make a mirror that is 129 ft
(39.3 m) across. In contrast, the Fifth Third
Hubble’s primary mirror is just mirror mirror
7 ft 11 in (2.4 m) wide; even the (M3)
E-ELT’s secondary mirror (M2) is (M5)
larger than that, at 13 ft 10 in (4.2 m).
Primary
The shape of M1 can be fine- mirror
tuned to account for distortions (M1)
caused by temperature changes,
and by the gravitational effect as At the heart of the E-ELT’s complex arrangement of mirrors is the huge
the telescope swings into different dish of the primary mirror. It will gather 13 times more light than the largest
positions. M2 directs the light from existing optical telescopes, and will be aided by six laser guide star units.
M1 through a hole in the fourth
mirror (M4) onto the third mirror
(M3). From there light is reflected
back onto M4, the adaptive optics
mirror, which greatly reduces
atmospheric blurring of the image.
M4 follows the twinkling of an

328 IN CONTEXT

RSTHIPPARPCOLEUETGSIHME KEY ORGANISATION
LIGO (2016)
 GRAVITATIONAL WAVES
BEFORE
1687 Isaac Newton formulates
the universal law of gravitation,
which sees gravity as a force
between masses.

1915 Albert Einstein presents
the general theory of relativity,
which explains gravity as the
distortion of spacetime by
mass, predicting the existence
of gravitational waves.

1960 American physicist
Joseph Weber attempts to
measure gravitational waves.

1984 Rai Weiss and Kip
Thorne found LIGO.

AFTER
2034 eLISA is scheduled to
search for gravitational waves
using three spacecraft in
heliocentric orbits, between
which lasers will be fired.

I n 1916, as he worked on his
theory of relativity, Albert
Einstein predicted that, as
a mass moved, its gravity would
cause ripples in the fabric of
spacetime. Every mass would
do this, although larger masses
would make bigger waves, in
the same way that a pebble
dropped in a pond makes an ever-
increasing circle of ripples, while
a meteor impacting the ocean
creates tsunami-sized waves.

In 2016, 100 years after Einstein’s
predictions, a collaboration
between scientists working under
the name LIGO announced that
they had discovered these ripples,
or gravitational waves. Their

THE TRIUMPH OF TECHNOLOGY 329

See also: Gravitational theory 66–73 ■ The theory of relativity 146–53

Within a period of 20 milliseconds,
the two black holes LIGO had detected
increased their orbital speed from
30 times a second to 250 times a
second before colliding.

decades-long search had revealed space is doing: the speed of light. by the mass of objects curving
the gravitational equivalent of Light behaves like a wave, but it the space around them. What is
tsunamis created by two black does not require a medium through understood as the “pull of gravity”
holes spiraling around each which to travel. Instead, light is a small mass appearing to alter
other and then colliding. (and any kind of electromagnetic its motion and “fall” toward a larger
radiation) is an oscillation of an mass as it encounters a region of
It is hoped that the discovery electromagnetic field: in other warped space.
of gravitational waves will provide words, light is a disturbance in
a new way of observing the a field permeating all of space. All masses are in motion—
universe. Instead of using light planets, stars, even galaxies—and
or other electromagnetic radiation, Gravitational waves can be as they move, they leave a trail of
astronomers are hoping to map understood as disturbances in the gravitational disturbances in their
the universe by the gravitational gravitational field that permeates wake. Gravity waves propagate in
effects of its contents. While the universe. Einstein described a comparable way to sound waves,
radiation is obscured in many ways, how these disturbances are caused by distorting the medium through
including by the opaque plasma which they travel. In the case
of the early universe up to 380,000 of sound waves, that medium is
years after the Big Bang, gravitational made of molecules, which are made
waves pass through everything. to oscillate. In the case of gravity,
This means that gravitational the medium is spacetime, the very
astronomy could see back to the fabric of the universe. Einstein
very beginning of time, a trillionth predicted that the speed of gravity ❯❯
of a second after the Big Bang.
Relativity reveals that gravity is
the warping of spacetime by mass.

Wave behaviors Moving objects create ripples through
LIGO stands for Laser Interferometer spacetime, or gravitational waves.
Gravitational-Wave Observatory.
It is a remarkable set of instruments Gravitational waves can Gravitational waves
for measuring expansions and be detected by measuring let astronomers see
contractions in space itself. This farther into space
is no easy task. A ruler cannot do it the expansion and
because, as space changes in size, compression of spacetime. than ever before.
so does the ruler, so the observer
measures no change at all. LIGO
succeeded using the benchmark
that remains constant whatever

330 GRAVITATIONAL WAVES

would be the same as the speed With no gravitational waves, LIGO’s light waves cancel one
of light, and that the ripples in another out when they are recombined. Gravitational waves stretch
spacetime would move outward one tube while compressing the other, so that the waves are no
in all directions. The intensity of longer perfectly aligned, and a signal is produced.
these ripples diminishes rapidly
with distance (by a square of the Normal Gravitational
distance), so detecting a distinct situation wave detection
gravity wave from a known object
far out in space would require
a very powerful source of waves
and a very sensitive instrument.

Laser-guided No signal Signal
As its name suggests, LIGO
employs a technique called laser out, disappearing completely. half a wavelength farther than
interferometry. This makes use LIGO’s source of waves is a laser, the other (a difference of a few
of a property of waves called which is a light beam that contains hundred billionths of a meter).
interference. When two waves a single color, or wavelength, When the beams meet each other
meet, they interfere with one of light. In addition, the light in again, they are exactly out of phase
another to create a single wave. a laser beam is coherent, which as they interfere, and promptly
How they do this depends on means that its oscillations are all disappear—unless a gravitational
their phase—the relative timing perfectly in time. Such beams wave has passed through space
of their oscillations. If the waves can be made to interfere with while the beams were traveling.
are exactly in phase—rising and one another in very precise ways. If present, a gravitational wave
falling perfectly in sync—they would stretch one of the laser
will interfere constructively, The laser beam is split in two tracks and compress the other, so
merging to create a wave with and the resulting beams are sent the beams would end up traveling
double the intensity. By contrast, off perpendicular to one another. slightly altered distances.
if the waves are exactly out of They both hit a mirror and bounce
phase—one rising as the other straight back to the starting point. Noise filter
falls—the interference will be The distance traveled by each The laser beams are split and sent
destructive. The two waves will beam is very precisely controlled on a 695-mile (1,120-km) journey up
merge and cancel one another so that one has to travel exactly and down LIGO’s 2.5-mile (4-km)
long arms before being recombined.
Rai Weiss and Kip Thorne This gives LIGO the sensitivity to
detect minute perturbations in space
LIGO is a collaboration between by LIGO, working from the that add up to a few thousandths
Caltech and MIT, and also initial ideas of Joseph Weber, of the width of a proton. With the
shares its data with a similar one of the inventors of the laser. distances put very slightly out of
experiment called Virgo, which In 1984, Weiss cofounded LIGO sync, the interfering beams would
is running in France and Poland. with Thorne, a counterpart at no longer cancel each other out.
Hundreds of researchers have Caltech, who is a leading expert Instead, they would create a
contributed to the discovery of on the theory of relativity. LIGO flickering pattern of light, perhaps
gravitational waves. However, is the most expensive science indicating a gravity wave passing
there are two people, both project ever funded by the US through LIGO’s corner of space.
Americans, who stand out government, with a current cost
among them all: Rainer “Rai” of $1.1 billion. After 32 years of The difficulty was that such
Weiss (1932–) and Kip Thorne trying, in 2016 Weiss and Thorne a sensitive detector was prone to
(1940–). In 1967, while at MIT, announced their discovery of distortions from the frequent seismic
Weiss developed the laser gravitational waves at a news waves that run through Earth’s
interferometry technique used conference in Washington, D.C. surface. To be sure that a laser flicker

THE TRIUMPH OF TECHNOLOGY 331

LIGO’s precision instruments must Colliding black holes the sun. Lasers will be fired
be kept completely clean. Maintaining On September 14, 2015, at 9:50:45 between the spacecraft, making
the purity of the laser beams is one GMT, two black holes a billion light- a laser track 2 million miles
of the project’s biggest challenges. years away collided and unleashed (3 million km) long that is many
huge warps in the fabric of space. times more sensitive to
was not an earth tremor, two In fact, this event occurred a billion gravitational waves than LIGO.
identical detectors were built at years ago but it had taken that long
opposite ends of the United States: for the ripples they had released The discovery of gravitational
one in Louisiana, the other in to reach Earth—where they were waves has the potential to transform
Washington state. Only signals detected by both LIGO detectors. astronomers’ view of the universe.
registering on both detectors were The researchers took another few The patterns in the fluctuations
gravitational waves (the signals months to check their result and in the light signals from LIGO and
are in fact 10 milliseconds apart— went public in February 2016. future projects will produce new
the time it takes for light, and information, providing a detailed
gravitational waves, to travel from The search is now on for more map of mass across the universe. ■
Louisiana to Washington). Ligo gravitational waves, and the best
operated from 2002–2010 with no place to do it is from space. In Gravitational waves will
success, then started up again in December 2015, the spacecraft bring us exquisitely accurate
2015 with enhanced sensitivity. LISA Pathfinder was launched. maps of black holes—maps
It is headed to an orbit at L1, which
is a gravitationally stable position of their spacetime.
between the sun and Earth. Kip Thorne
There, the spacecraft will test
laser interferometry instruments
in space, in the hope that they
can be used in an ambitious
experiment called eLISA (evolved
Laser Interferometer Space
Antenna). Provisionally scheduled
for 2034, eLISA will use three
spacecraft triangulated around

Mirror LIGO splits one beam of laser light and sends beams Mirror
down two tubes at 90° to each other. To prevent unwanted
interference, the tubes are vacuums at one trillionth of the
pressure of Earth’s atmosphere. LIGO also has to make
adjustments to allow for the tidal pull of the sun and the moon.

Beam splitter

2.5-mile (4-km) tube 2.5-mile (4-km) tube

Laser
source

Light detector

DIRECTO

RY

334

DIRECTORY

F or a field of enquiry as broad as astronomy, there has not been
room to include every significant scientist as a main entry in
this book. The following pages list astronomers who have also
made important contributions across time, from the 7th century BCE to
the present day. In its early stages, astronomy usually involved individuals
or small groups making observations and calculations. Modern high-tech
“big astronomy,” meanwhile, often requires large-scale collaborations
of hundreds or thousands of scientists. Whether they are booking time
for an experiment at a particle accelerator or requesting that a space
telescope be pointed in a particular direction, today’s astronomers form
part of a huge community developing the big ideas of tomorrow.

ANAXIMANDER OF MILETUS the circumference of Earth vernal equinoxes), and the lunar
by comparing the angle of the month. He used his calculations to
610–546 BCE noon shadow at midsummer in predict four solar eclipses correctly.
Alexandria with that in Syene Zu measured the length of Jupiter’s
Greek philosopher Anaximander (present-day Aswan). He knew the year as 11.858 Earth years, which is
provided one of the earliest distance between the two locations, less than 0.1 percent away from the
attempts at a rational explanation and his measurement allowed him current accepted figure.
of the universe. He speculated that to figure out the proportion of the See also: The solar year 28–29
the celestial bodies made full circles entire circumference that this
around Earth, which led him to the represented. He also produced AL-BATTANI
conclusion that Earth must float an accurate measurement of Earth’s
freely and unsupported in space. He axial tilt, measured the distances c.858–929
also stated that celestial bodies lie to the sun and the moon, introduced
behind one another, meaning that the leap day to correct the length Arab astronomer and
there was depth to the universe— of the year, and produced one of mathematician Al-Battani made
the first recorded conception of the the first ever maps of the world. accurate observations to refine
idea of “space.” Anaximander placed See also: Consolidating the figures for the length of the
the celestial bodies in the wrong knowledge 24–25 year, the inclination of the ecliptic,
order, however, believing that the and the precession of the equinoxes.
stars were nearest to Earth, followed ZU CHONGZHI He developed trigonometric
by the moon, and then the sun. methods to improve on Ptolemy’s
See also: The geocentric model 20 429–500 CE calculations, and showed that
the distance of the sun from Earth
ERATOSTHENES Tasked with producing a new varies over time. Al-Battani’s most
calendar by the Emperor Xiaowu, influential work was a compilation
c.276–c.194 BCE Chinese mathematician Zu of astronomical tables, which
Chongzhi made highly accurate was translated into Latin in the
The third chief librarian at the measurements of the lengths of 12th century and was a major
famous Library of Alexandria, the sidereal year (Earth’s rotation influence on Copernicus.
Greek scholar Eratosthenes period measured relative to the See also: Consolidating knowledge
made major contributions to the background stars), the tropical year 24–25 ■ The Copernican model
field of geography. He measured (the period between successive 32–39

DIRECTORY 335

IBN AL-HAYTHAM Although he made and used project to calculate the time of day
telescopes, he preferred to map using the eclipses of the moons
c.965–1040 star positions with just a sextant of Jupiter, a method first proposed
and the naked eye, making him by Galileo to solve the problem of
Also known by his Latinized name the last major astronomer to do so. measuring longitude at sea. Over
Alhazen, Ibn al-Haytham worked at Hevelius’s second wife, Elisabetha, a number of years, Rømer carefully
the court of the Fatimid Caliphate whom he married in 1663, helped timed the eclipses of the moon Io
in Cairo. A pioneer of the scientific him to compile a catalog of and found that their duration varied
method, whereby hypotheses are more than 1,500 stars, which she depending on whether Earth was
tested by experiment, al-Haytham completed and published following moving toward Jupiter or away from
wrote a work popularizing his death. A tireless and skilled it. He reasoned that this variation
Ptolemy’s Almagest and, later, a observer in her own right, was due to a difference in the time it
book casting doubts on aspects Elisabetha was one of the first took the light from Io to reach Earth,
of Ptolemy’s system. notable female astronomers. and estimated that light takes
See also: Consolidating See also: The Tychonic model 22 minutes to travel a distance equal
knowledge 24–25 44–47 to the diameter of Earth’s orbit of
the sun. This gave the speed of light
ROBERT GROSSETESTE CHRISTIAAN HUYGENS as 140,000 miles/s (220,000 km/s),
about 75 percent of its true value.
c.1175–1253 1629–1695 Rømer’s finding that light has a
finite speed was confirmed in 1726,
English bishop Robert Grosseteste Dutch mathematician and when James Bradley explained the
wrote treatises concerning optics, astronomer Christiaan Huygens phenomenon of stellar aberration
mathematics, and astronomy. He was fascinated by Saturn and the in terms of light speed.
translated Greek and Arabic texts strange “handles” that telescopes See also: Stellar aberration 78
into Latin, introducing the ideas of revealed to protrude from either side
Aristotle and Ptolemy into medieval of it. With his brother Constantijn, JOHN MICHELL
European thought. In his work De he constructed a powerful telescope
luce (On light), Grosseteste made with improved lenses through which 1724–1793
an early attempt to describe the to study the planet. Huygens was
entire universe using a single set of the first to describe the true shape English clergyman John Michell
mathematical laws. He called light of Saturn’s rings, explaining that studied a wide range of scientific
the first form of existence, which, he they were thin and flat, and tilted fields, including seismology,
said, allowed the universe to spread at an angle of 20 degrees to the magnetism, and gravity. He designed
out in all directions, in a description plane of the planet’s orbit. He the torsion balance, which his
reminiscent of the Big Bang theory. published his findings in 1659 in friend Henry Cavendish later used
See also: The geocentric model the book Systema Saturnium. Four to measure the strength of gravity.
20 ■ Consolidating knowledge 24–25 years earlier, he had discovered Michell was also the first person to
Titan, Saturn’s largest moon. propose that an object might be so
JOHANNES HEVELIUS See also: Observing Saturn’s massive that light would be unable
rings 65 to escape its gravitational pull. He
1611–1687 calculated that a star 500 times the
OLE RØMER size of the sun would be such an
ELISABETHA HEVELIUS object, which he called a “dark star.”
1644–1710 Michell’s idea was largely forgotten
1647–1693 until the 20th century, when
Working at the Paris Observatory, astronomers started to take the
From an observatory he built on Danish astronomer Ole Rømer concept of black holes seriously.
top of his house, Polish astronomer demonstrated that light has a finite See also: Curves in spacetime
Johannes Hevelius made detailed speed. Rømer was working on a 154–55 ■ Hawking radiation 255
maps of the surface of the moon.

336 DIRECTORY

JOSEPH-LOUIS LAGRANGE BENJAMIN APTHORP GOULD parts of the sun were rotating
at different speeds.
1736–1813 1824–1896 See also: Galileo’s telescope
56–63 ■ The surface of the sun 103
French–Italian mathematician and A child prodigy, American Benjamin
astronomer Joseph-Louis Lagrange Apthorp Gould graduated early from ISAAC ROBERTS
studied celestial mechanics and Harvard University before moving
the effects of gravity. He explored to Germany to study under the 1829–1904
mathematically the ways in which renowned mathematician Friedrich
the gravitational pulls within a Gauss in 1845. In Europe, he earneda In the 1880s, British amateur
system of three bodies, such as the Ph.D. in astronomy—the first astronomer Isaac Roberts made
sun, Earth, and the moon, combine American to receive a doctorate in important advances in the field
with one another. His work led to the the subject. He returned to the US in of astrophotography, enabling
discovery of positions with stable 1849 determined to raise the profile photographs of the night sky to
orbits for a small body orbiting two of American astronomy. To this reveal structures invisible to the
larger ones, now called Lagrange end, he founded The Astronomical naked eye for the first time. Roberts
points. Space telescopes are often Journal to publish research from the developed an instrument that
placed near Lagrange points for their United States; the journal continues allowed very long exposure times,
orbits around Earth and the sun. to this day. Between 1868 and 1885, and thus the collection of more light.
See also: Gravitational theory Gould worked in Argentina, where He kept the telescope pointing at
66–73 ■ Studying distant stars he founded the National Observatory exactly the same point in the sky by
304–05 in Córdoba. He also helped to adjusting it to compensate for the
set up the Argentine National rotation of Earth. Roberts’ most
JEAN BAPTISTE JOSEPH Weather Service. Gould produced famous image is an 1888 photograph
DELAMBRE a comprehensive catalog of of the Andromeda nebula, now
the bright stars visible from the known to be a galaxy, which
1736–1813 southern hemisphere, which revealed its spiral structure in
he published in 1879 as the unprecedented detail.
A leading figure in scientific circles Uranometria Argentina. See also: Astrophotography
during the French Revolution, in 118–19
1792 Delambre was tasked with RICHARD CARRINGTON
measuring the length of the arc of the HENRY DRAPER
meridian from Dunkirk to Barcelona. 1826–1875
This was to refine the new metric 1837–1882
system, which defined the meter as British amateur astronomer
1/10,000,000 of the distance from Richard Carrington carried out A pioneer of astrophotography,
the North Pole to the equator. He careful observations of the sun medical doctor Henry Draper
completed the task in 1798. From over the course of many years. resigned as dean of medicine at
1804, Delambre served as the In 1859, he was the first person to New York University in 1873 to
director of the prestigious Paris observe a solar flare—a magnetic devote himself to astronomy. With
Observatory. His astronomical explosion on the surface of the the assistance of his wife, Anna
work included the production sun that causes a surge of visible Mary, Draper photographed the
of accurate tables showing the light. The flare was followed by transit of Venus in 1874, was
positions of Jupiter’s moons. In disruption to worldwide telegraph the first to capture the Orion nebula
1809, he estimated that light from systems, and Carrington suggested on camera in 1880, and was also
the sun takes 8 minutes 12 seconds that such solar activity might have the first to take a wide-angled
to reach Earth (the figure is now an electrical effect on Earth. photograph of a comet’s tail in
measured at 8 minutes 20 seconds). In 1863, through his records of 1881. He developed new techniques
See also: Gravitational the movements of sunspots, he for astrophotography, but died of
disturbances 92–93 demonstrated that different pleurisy in 1882, a few years before

DIRECTORY 337

photography began to be taken sunspots uncovered a correlation HEBER D. CURTIS
seriously by astronomers as a means between their number and Earth’s
of discovery. After his death, his wife climate. This led them to discover 1872–1942
created a foundation in his name, a period of reduced solar activity
which funded the Henry Draper between 1645 and 1715, now called American classics professor Heber
Catalogue, a huge photographic the Maunder Minimum, which Doust Curtis switched to astronomy
survey of the stars carried out by coincided with lower-than-average in 1900 when he became a volunteer
Edward C. Pickering and his team temperatures in Europe. When the observer for the Lick Observatory in
of female astronomers. ban on women at the society was California. After receiving his Ph.D.
See also: The star catalog lifted in 1916, Annie Maunder in astronomy in 1902, Curtis enjoyed
120–21 ■ The characteristics of was elected a Fellow of the Royal a long association with the Lick
the stars 122–27 Astronomical Society, after Observatory, carrying out a detailed
which her observations were survey of the known nebulae, which
JACOBUS KAPTEYN published under her own name. he completed in 1918. In 1920, he
Prior to that, much of her work took part in the “Great Debate” with
1851–1922 had appeared in papers under fellow astronomer Harlow Shapley
her husband’s name. at the Smithsonian museum. Curtis
Using photographic plates supplied See also: The surface of the sun argued that distant nebulae were
to him from South Africa by David 103 ■ The properties of sunspots 129 separate galaxies far from the Milky
Gill, Dutch astronomer Jacobus Way, while Shapley asserted that
Kapteyn cataloged more than E. E. BARNARD they lay within it.
450,000 southern stars. After See also: Spiral galaxies 156–61 ■
grouping stars in different parts 1857–1923 Beyond the Milky Way 172–77
of the galaxy and measuring their
magnitudes, radial velocities, and US astronomer Edward Emerson JAMES JEANS
proper motions, Kapteyn carried out Barnard was a renowned observer,
vast statistical analyzes that revealed who discovered about 30 new 1877–1946
the phenomenon of star streaming— comets and numerous nebulae.
which shows how the motions of In 1892, Barnard discovered British mathematician James Jeans
stars are not random, but grouped a fifth moon around Jupiter, called worked on a variety of theoretical
together in two opposite directions. Amalthea, which was to be the problems relating to astrophysics. In
This was the first definitive evidence last moon to be discovered through 1902, he calculated the conditions
that the Milky Way galaxy is rotating. visual observation rather than under which a cloud of interstellar
See also: Astrophotography through the study of photographic gas becomes unstable and collapses
118–19 plates. Himself a pioneer of astro- to form a new star. In developing his
photography, Barnard produced theory of gases in 1916, he explained
EDWARD WALTER MAUNDER a series of stunning long-exposure how gas atoms can gradually escape
photographs of the Milky Way, from a planet’s atmosphere over time.
1851–1928 which was published posthumously In later life, Jeans devoted his time to
in 1927 as the Atlas of Selected writing and became well-known for
ANNIE SCOTT DILL Regions of the Milky Way. Barnard’s his nine popular books, including
MAUNDER star is named after him; in 1916, he Through Space and Time and The
discovered that this faint red dwarf Stars in Their Courses. He promoted
1868–1947 has the largest known proper motion an idealist philosophy that saw
(rate at which a star changes its both mind and matter as central to
British husband and wife team position on the celestial sphere) understanding the universe, which
Edward Walter Maunder and of all known stars. he described as “nearer to a great
Annie Maunder (née Scott Dill) See also: Galileo’s telescope thought than to a great machine.”
collaborated at the Greenwich 56–63 ■ Astrophotography See also: Inside giant molecular
Royal Observatory in the study 118–19 clouds 276–79
of the sun. Their investigations of

338 DIRECTORY

ERNST ÖPIK status of dwarf planet. Following after the end of World War II. To
his discovery, Tombaugh earned a conduct further radio investigations
1893–1985 degree and pursued a career as in clear atmospheric conditions,
a professional astronomer. in 1954 Reber moved to Tasmania,
Estonian astrophysicist Ernst See also: Spiral galaxies 156–61 ■ where he remained for the rest
Öpik obtained his doctorate at the Studying Pluto 314–17 of his life.
University of Tartu, Estonia, where See also: Radio astronomy 179
he worked from 1921 to 1944, VICTOR AMBARTSUMIAN
specializing in the study of minor IOSIF SHKLOVSKY
objects such as asteroids, comets, 1908–1996
and meteors. In 1922, he estimated 1916–1985
the distance of the Andromeda Soviet–Armenian astronomer Victor
galaxy using a new method based Ambartsumian was a founding In 1962, Soviet astrophysicist
on the galaxy’s speed of rotation. figure in the field of theoretical Iosif Shklovsky wrote a popular
This method is still used today. astrophysics, contributing to book examining the possibility
Öpik also suggested that comets theories of star formation and of extraterrestrial life, which was
originated from a cloud beyond galactic evolution. He was one of republished four years later in an
Pluto, now known commonly as the first people to suggest that young expanded edition, co-authored by
the Oort cloud, but sometimes stars formed from protostars. In Carl Sagan, as Intelligent Life in
referred to as the Öpik–Oort cloud. 1946, he organized the construction the Universe. In this later edition,
As the Red Army approached of the Byurakan Observatory in paragraphs by the two authors
Estonia in 1944, Öpik fled into Armenia, where he was the director are alternated with one another,
exile, eventually settling in until 1988. A popular lecturer with as Sagan provides a commentary
Northern Ireland, where he took a colorful and engaging style, and expansion on Shklovsky’s
a position at Armagh Observatory. Ambartsumian served as the original points. Many of the latter’s
See also: The Oort cloud 206 president of the International ideas were highly speculative,
Astronomical Union from 1961–64, including a suggestion that an
CLYDE TOMBAUGH and hosted several conferences on observed acceleration of Mars’s
the search for extraterrestrial life. moon Phobos was due to the fact
1906–1997 See also: Dense molecular clouds that it was a hollow artificial
200–01 ■ Inside giant molecular structure, a monument to a long-
In the late 1920s, the Lowell clouds 276–79 gone Martian civilization.
Observatory in Arizona embarked See also: Life on other planets
upon a systematic search for a GROTE REBER 228–35
planet believed to be causing
perturbations to the orbit of 1911–2002 MARTIN RYLE
Uranus. To carry out the work, the
director Vesto Slipher hired the In 1937, American radio engineer 1918–1984
young amateur astronomer Clyde Grote Reber built his own radio
Tombaugh, who had impressed him telescope in his backyard after Like many pioneer radio
with drawings of Jupiter and Mars hearing of Karl Jansky’s discovery astronomers, Briton Martin Ryle
made using a homemade telescope. of galactic radio waves. Over started his career developing radar
After 10 months examining the next few years, Reber was technology during World War II.
photographs, on February 18, 1930, effectively the only radio astronomer Subsequently, he joined the
Tombaugh discovered an object in the world, conducting the Cavendish Radio Astronomy Group
orbiting the sun beyond Neptune. first radio survey of the sky and in Cambridge, where he worked
Named Pluto after the Roman god publishing his results in astronomy alongside Antony Hewish and
of the underworld, it was initially and engineering journals. Reber’s Jocelyn Bell Burnell, developing
classified as the ninth planet, but work was to form the basis for the new techniques in radio astronomy
has since been demoted to the development of radio astronomy and producing a number of catalogs

DIRECTORY 339

of radio sources. Deeply affected Penrose has proposed a theory of a life to develop. Since 1986, Carter
by his experiences of war, Ryle cyclic cosmology, in which the heat has been the director of research
devoted his final years to the death (end state) of one universe at the Paris–Meudon Observatory.
promotion of the peaceful use produces the conditions for the He has also made contributions
of science, warning against the Big Bang of another universe. to understanding the properties
dangers of nuclear weapons and Penrose has also produced a of black holes.
power, and advocating research series of popular science books See also: Life on other planets
into alternative energy. in which he explains the physics 228–35 ■ Hawking radiation 255
See also: Radio astronomy 179 ■ of the universe and suggests
Quasars and pulsars 236–39 novel explanations for the origins JILL TARTER
of consciousness.
HALTON ARP See also: Curves in spacetime 1944–
154–55 ■ Hawking radiation 255
1927–2013 As director of the Center for SETI
SHIV S. KUMAR Research in California, Jill Tarter
A staff astronomer at the Mount was a leading figure in the search
Wilson Observatory in California 1939– for extra-terrestrial life for more
for nearly 30 years, Halton Arp than 30 years, lecturing widely on
gained a reputation as a skilled Indian-born astronomer Shiv S. the subject before her retirement in
observer. In 1966, he produced Kumar earned a doctorate in 2012. In 1975, she coined the term
his Atlas of Peculiar Galaxies, astronomy at the University of “brown dwarf” for the type of star,
which cataloged, for the first Michigan and has made his career discovered by Shiv S. Kumar, that
time, hundreds of odd structures in the United States, working on is not massive enough to sustain
that had been seen in nearby theoretical problems concerning nuclear fusion. Carl Sagan based
galaxies. Today it is known that matters including the origin of the the protagonist in his novel and
many of these features are the solar system, the development of film Contact on Tarter.
result of galaxies colliding. Later life in the universe, and exoplanets. See also: Life on other planets
in his career, Arp found himself In 1962, Kumar predicted the 228–35
professionally marginalized when existence of low-mass stars that
he cast doubt on the Big Bang would be too small to sustain MAX TEGMARK
theory. He contended that objects nuclear fusion. Later named
with very different degrees of brown dwarfs by Jill Tarter, their 1967–
redshift were close to one another existence was confirmed in 1995.
and not at vastly different distances. See also: Exoplanets 288–95 Swedish cosmologist Max
See also: Beyond the Milky Way Tegmark’s research at MIT has
172–77 BRANDON CARTER focused on developing methods
to analyze the vast amounts of
ROGER PENROSE 1942– data produced by surveys of the
cosmic microwave background.
1931– In 1974, Australian physicist Tegmark is a leading proponent
Brandon Carter formulated the of the idea that the results of
In the 1960s, British mathematician anthropic principle, which states quantum mechanics are best
and physicist Roger Penrose that the universe must necessarily explained by the existence of a
figured out much of the complex have certain characteristics for multiverse. He has developed the
mathematics relating to the humankind to exist. That is to say mathematical universe hypothesis,
curvature of spacetime around a that the physical properties of the which proposes that the universe
black hole. In collaboration with universe, such as the strength of is best understood as a purely
Stephen Hawking, he showed how the fundamental forces, must fall mathematical structure.
matter within a black hole collapses within very narrow limits for See also: Observing the CMB
into a singularity. More recently, sunlike stars capable of sustaining 280–85

340

GLOSSARY

Absolute magnitude A measure Big Bang The event with which Comet A small, icy body in orbit
of the intrinsic brightness of a the universe is thought to have around the sun. When a comet
star. It is defined as the apparent begun, at a particular time in the approaches the sun, gas and dust
magnitude of the star from a distance past, from a hot, dense initial state. evaporate from its nucleus (solid
of 10 parsecs (32.6 light-years). core) to produce a cloud called
Black body A theoretical, idealized a coma and one or more tails.
Accretion The process by body that absorbs all the radiation
which smaller particles or that falls on it, reflecting nothing. Constellation One of 88 named
bodies collide and join together A black body would emit a regions on the celestial sphere,
to form larger bodies. spectrum of radiation with containing an identifiable pattern
a peak at a particular wavelength, of naked-eye stars.
Aphelion The point on its elliptical depending on its temperature.
orbit around the sun at which a Cosmic Microwave Background
planet, asteroid, or comet is Black hole A region of spacetime (CMB) Faint microwave radiation
farthest from the sun. surrounding a mass that is so that is detectable from all directions.
dense that its gravitational pull The CMB is the oldest radiation
Apparent magnitude A measure allows no mass or radiation to in the universe, emitted when the
of the brightness of a star as seen escape from it. universe was 380,000 years old.
from Earth. The fainter the object, Its existence was predicted by the
the higher the value of its apparent Blueshift A shift in a spectrum Big Bang theory, and it was first
magnitude. The faintest stars of light or other radiation toward detected in 1964.
visible to the naked eye are of shorter wavelengths that occurs
a magnitude 6. when the source of the light is Cosmic rays Highly energetic
moving toward the observer. particles, such as electrons and
Armillary sphere An instrument protons, that travel through space
that models the celestial sphere. Bok globule Small, dark clouds of at close to the speed of light.
At its center is Earth or the sun, cold gas and dust, within which it is
around which is a framework of thought that new stars are forming. Cosmological constant A term
rings representing lines of celestial that Albert Einstein added to
longitude and latitude. Brown dwarf A starlike ball his general relativity equations,
of gas that is not massive enough which may correspond to the dark
Asteroid A small body that to sustain nuclear fusion in its core. energy that is accelerating the
orbits the sun independently. expansion of the universe.
Asteroids are found throughout Celestial sphere An imaginary
the solar system, with the greatest sphere surrounding Earth. The Dark energy A little-understood
concentration in the asteroid belt positions of stars and other celestial form of energy that exerts a repulsive
between the orbits of Mars and bodies can be defined by their force, causing the expansion of the
Jupiter. Their diameters range from places on this sphere if they were universe to accelerate.
a few yards to 600 miles (1,000 km). imagined to be attached to it.
Dark matter A form of matter that
Astronomical unit (AU) Cepheid variable A pulsating does not emit radiation or interact
A distance equal to the average star whose brightness increases with other matter in any way other
distance between Earth and the and decreases over a regular than through the effect of its gravity.
sun. 1 AU = 92,956,000 miles period. The more luminous it is, the It comprises 85 percent of all mass
(149,598,000 km). longer the period of its variation. in the universe.

GLOSSARY 341

Degeneracy pressure An Equinox A twice-yearly occasion Gravitational wave A
outward pressure within a when the sun is directly overhead distortion of space that travels
concentrated ball of gas, such at a planet’s equator, meaning that at the speed of light, generated
as a collapsed star, that is exerted day and night are of roughly equal by the acceleration of mass.
due to the principle that no two duration across the entire planet.
particles with mass can exist in Harvard Spectral Classification
the same quantum state. Escape velocity The minimum A system first devised by the
velocity an object needs to be Harvard Observatory in the late
Doppler effect The change in traveling at to escape the 19th century to classify stars by
frequency of radiation experienced gravitational pull of a larger the appearance of their spectra.
by an observer in relative motion body such as a planet.
to the source of the radiation. Heliocentric Of a system or an
Event horizon A boundary orbit, treated as having the sun
Dwarf planet An object in orbit around a black hole beyond at the center.
around a star that is large enough to which no mass or light can
have formed a spherical shape but escape its gravity. At this point, Hertzsprung–Russell diagram
that has not cleared its orbital path of the escape velocity of the black A scatter diagram on which stars
other material. Examples in the solar hole equals the speed of light. are plotted according to their
system include Pluto and Ceres. luminosity and surface temperature.
Exoplanet A planet that orbits
Dwarf star Also called a main a star other than the sun. Hubble’s law The observed
sequence star, a star that shines by relationship between the redshifts
converting hydrogen to helium. About Fraunhofer lines Dark absorption and distances of galaxies, which
90 percent of stars are dwarf stars. lines found in the spectrum of the shows galaxies receding with
sun, first identified by German a velocity proportional to their
Eclipse The blocking of light Joseph von Fraunhofer in the distance. The number that
from one celestial body, caused 19th century. quantifies the relationship is
by another body passing between called Hubble’s constant (H0).
it and an observer, or it and a light Galaxy A large collection of stars
source that it reflects. and clouds of gas and dust that is Inflation A short period of rapid
held together by gravity. expansion that the universe
Ecliptic The apparent path along is thought to have undergone
which the sun travels across the Galilean moon One of the four moments after the Big Bang.
celestial sphere. It is equivalent biggest moons of Jupiter, first
to the plane of Earth’s orbit. discovered in 1610 by Galileo. Ionization The process by which
an atom or molecule gains or loses
Electromagnetic radiation General theory of relativity electrons to gain a positive or
Waves that carry energy through A theory that describes gravity negative charge. The resultant
space in the form of oscillating as a warping of spacetime by charged particles are called ions.
electric and magnetic disturbances. the presence of mass. Formulated
The electromagnetic spectrum by Albert Einstein in 1916, Kepler’s laws of planetary
ranges from short, high-energy many of its predictions, such as motion Three laws devised by
gamma rays to long, low-energy gravitational waves, have now Johannes Kepler to describe the
radio waves, and includes the been confirmed experimentally. shapes and speeds of the orbits
visible spectrum. of the planets around the sun.
Geocentric Of a system or an orbit,
Electron A subatomic particle treated as having Earth at the center. Kuiper belt A region of space
with negative charge. In an atom, beyond Neptune in which a large
a cloud of electrons orbits a central, Gnomon The part of a sundial number of comets orbit the sun. It
positively charged nucleus. that casts a shadow. is the source of short-period comets.

342 GLOSSARY

Light-year (ly) A unit of distance Oort cloud Also known as the Proton A subatomic particle with
that is the distance traveled by Oort–Öpik cloud. A spherical region a positive charge, made of three
light in one year, equal to 5,878 at the edge of the solar system quarks. The nucleus of the element
million miles (9,460 billion km). containing planetesimals and hydrogen contains a single proton.
comets. It is the origin of long-
Main sequence See dwarf star. period comets. Protostar A star in the early
stages of its formation, comprising
Messier object One of the nebulae Orbit The path of a body around a collapsing cloud that is accreting
first cataloged by Charles Messier another, more massive, body. matter but in which nuclear fusion
in 1781. has not yet begun.
Parallax The apparent shift in
Meteorite A lump of rock or position of an object due to the Pulsar A rapidly rotating neutron
metal that falls from space and movement of an observer to a star. Pulsars are detected on
reaches the surface of Earth in different place. Earth by their rapid, regular
one piece or in many fragments. pulses of radio waves.
Perihelion The point on its
Nebula A cloud of gas and dust in elliptical orbit around the sun Quadrant An instrument for
interstellar space. Before the 20th at which a planet, asteroid, measuring angles of up to 90°.
century, any diffuse object in the or comet is closest to the sun. Ancient astronomers used
sky was known as a nebula; many of quadrants to measure a star’s
these are now known to be galaxies. Perturbation A change in the position on the celestial sphere.
orbit of a body, caused by the
Neutrino A subatomic particle gravitational influence of other Quark A fundamental subatomic
with very low mass and zero orbiting bodies. Observed particle. Neutrons and protons are
electric charge, which travels perturbations in the orbit of made of three quarks.
at close to the speed of light. the planet Uranus led to the
discovery of Neptune. Quasar Short for “quasi-stellar
Neutron A subatomic particle radio source,” a compact but
made of three quarks with zero Planet A non-luminous body powerful source of radiation
electric charge. that orbits a star such as the sun, that is believed to be an active
is large enough to be spherical galactic nucleus.
Neutron star A very dense, in shape, and has cleared its
compact star composed almost neighborhood of smaller objects. Radial velocity The part of the
entirely of densely packed neutrons. velocity of a star or other body
Neutron stars form when the core Planetesimal A small body of that is along the line of sight
of a high-mass star collapses in rock or ice. The planets formed directly toward or directly
a supernova explosion. from planetesimals that joined away from an observer.
together by the process of accretion.
Nova A star that suddenly Radio astronomy The branch of
becomes thousands of times Precession A change in the astronomy that studies radiation
brighter before returning to its orientation of a rotating body’s in the long radio wavelength, first
original brightness over a period axis of rotation, caused by discovered to be coming from
of weeks or months. the gravitational influence space in the 1930s.
of neighboring bodies.
Nuclear fusion A process whereby Red dwarf A cool, red,
atomic nuclei join together to Proper motion The rate at which low-luminosity star.
form heavier nuclei, releasing a star changes its position on the
energy. Inside stars like the sun, celestial sphere. This change is Red giant A large, highly luminous
this process involves the fusion caused by the star’s motion relative star. A main sequence star becomes
of hydrogen atoms to make helium. to the motion of other stars. a red giant near the end of its life.

GLOSSARY 343

Redshift A shift in a spectrum Spacetime The four-dimensional Stellar parallax See parallax.
of light or other radiation toward combination of the three
longer wavelengths that occurs dimensions of space and one Subatomic particle One of the
when the source of the light is of time. According to the theory many kinds of particle that are
moving away from an observer. of relativity, space and time do smaller than atoms. These include
not exist as separate entities. electrons, neutrinos, and quarks.
Reflecting telescope A telescope Rather, they are intimately
in which an image is formed by linked as one continuum. sunspot An area on the surface of
reflecting light on a curved mirror. the sun that appears dark because
Spectrum The range of the it is cooler than its surroundings.
Refracting telescope A telescope wavelengths of electromagnetic sunspots are found in areas of
that creates an image by bending radiation. The full spectrum concentrated magnetic field.
light through a converging lens. ranges from gamma rays, with
wavelengths shorter than an atom, Supernova The result of the
Relativity Theories developed to radio waves, whose wavelength collapse of a star, which causes an
by Albert Einstein to describe the may be many feet long. explosion that may be many billions
nature of space and time. See also of times brighter than the sun.
general theory of relativity. Spectroscopy The study of the
spectra of objects. The spectrum Time dilation The phenomenon
Satellite A small body that of a star contains information about whereby two objects moving
orbits a larger one. many of its physical properties. relative to each other, or in different
gravitational fields, experience
Schwarzschild radius The Spiral galaxy A galaxy that a different rate of flow of time.
distance from the center of a takes the shape of a central
black hole to its event horizon. bulge or bar surrounded by TNO Short for Trans-Neptunian
a flattened disk of stars in a Object. Any minor planet (dwarf
SETI Short for Search for pattern of spiral arms. planet, asteroid, or comet) that
Extra-Terrestrial Intelligence, orbits the sun at a greater average
the scientific search for alien life. Standard candle A celestial body distance than Neptune (30 AU).
that has a known luminosity, such
Seyfert galaxy A spiral galaxy as a Cepheid variable star. These Transit The passage of a celestial
with a bright, compact nucleus. allow astronomers to measure body across the face of a larger body.
distances that are too large to
Sidereal Relating to the stars. measure using stellar parallax. Wavelength The distance
A sidereal day corresponds to between two successive peaks
Earth’s rotation period measured Star A luminous body of hot gas or troughs in a wave.
relative to the background stars. that generates energy through
nuclear fusion. White dwarf A star with low
Singularity A point of infinite luminosity but high surface
density at which the known laws of Steady State theory A theory temperature, compressed by
physics appear to break down. It is proposing that matter is constantly gravity to a diameter close to
theorized that there is a singularity created. The theory was an attempt that of Earth.
at the center of a black hole. to explain the universe’s expansion
without the need for a “Big Bang.” Zodiac A band around the
Solar wind A stream of fast- celestial sphere, extending 9° on
moving, charged particles Stellar aberration The apparent either side of the ecliptic, through
emanating from the sun that motion of a star caused by movement which the sun, moon, and planets
flows out through the solar system. of an observer in a direction appear to travel. The zodiac crosses
It consists mostly of electrons perpendicular to the direction the constellations that correspond
and protons. to the star. to the “signs of the zodiac.”

344

INDEX

Numbers in bold refer apparent magnitude 135, 136, 138 Becquerel, Henri 111, 140, 166
to a main entry Aquinas, Thomas 20
archaeology 12 Bell Burnell, Jocelyn 179, 180, 205,
40 Eridani 141, 178 Archimedes 21 218, 236–9
51 Pegasi-b (Bellerophon) 290, 291, 293 Arecibo message 233, 234
67P/Churyumov–Gerasimenko 206, 207, Aristarchus of Samos 18, 21, 34, 36, 38, 102 Bessel, Friedrich 21, 78, 83, 102, 132
Aristotle 18, 20, 21, 24, 26, 34, 35, 44–5, Bethe, Hans 166, 182–3, 196, 198, 252
308–11
1054 supernova 19 46, 48, 74, 77 BICEP2 272, 273
armillary spheres 45
A Armstrong, Neil 205, 248, 320 Big Bang 116, 148, 163, 168, 171, 177,
Arp, Halton 339
absolute magnitude 135, 139, 141 Arrhenius 230 179, 182, 196, 197, 198, 199, 220,
absorption lines 125–7, 128, 163 Aryabhata 19, 26 222–7, 272, 273, 277, 282–3, 284,
accretion disk 221 Asphaug, Erik 186
active galactic nuclei (AGNs) 185 asteroid belt 82, 90, 91, 97, 312 300, 329
active galaxies 185, 221 asteroids 65, 72, 82, 83, 90–91, 96, 99, 308
Adams, Fred 277 Aston, Francis 182 Big Crunch 301, 303
Adams, John Couch 107 astrobiology 15
Adams, Walter 124, 138, 141, 178, 180 astrochemistry 15 Big Rip 303
adaptive optics (AO) 192 astrology 13, 25, 52
al-Battani 334 astronomy Big Splash 187
al-Sijzi 26 observations 14–15
al-Sufi, Abd al-Rahman 24, 27, 30, 87 origins of 12–13 binary stars 49, 86, 110, 214, 216, 217,
Albertus Magnus 230 purpose of 14
Aldrin, Buzz 248, 320 scope of 15 294, 302
Alfonsine Tables 24 astrophotography 118–19, 120
Alfonso, Giovanni 69 astrophysics 15 Biot, Jean-Baptiste 91
Almagest (Ptolemy) 18, 19, 21, 24, 25, 27, rise of 108–41
Atacama Large Millimeter Array 259, black bodies 283–4
30, 34, 86
Alpha Centauri 102, 180 326–7 black dwarfs 127
Alpher, Ralph 116, 182, 196–7, 198, Atkinson, Robert 166, 167, 182, 183, 198 black holes 14, 82, 145, 148, 153, 154–5,
atomic clocks 13
224–5, 226, 272 atomic theory 112, 114 178, 179, 181, 205, 214, 216, 217,
Alvarez, Luis 212 atoms 144, 145 218–21, 239, 269
Amalthea 63 at center of Milky Way 154, 297
Ambartsumian, Victor 338 B
Anaxagoras 231 colliding 329, 331
Anaximander of Miletus 18, 334 Baade, Walter 137, 140, 141, 145, discovering 254
ancient world 12–13, 18–25 180–81, 236 radiation emissions 255
Anders, Bill 247 supermassive 154, 179, 217, 221, 297
Anderson, Carl 140 Babcock, Horace 270
Andromeda galaxy/nebula 27, 87, 110, Babylonians 13, 18, 24, 25 blue dwarfs 279
Backer, Donald 236
132, 136, 137, 159–60, 161, 174, 216, Baghdad 19 blue supergiants 126
221, 270 Bahcall, John 253
antineutrinos 252 Bailey, Solon I 136 blueshift 159, 160, 270
Apianus, Petrus 76, 77 Barnard, Edward 63, 200, 337
Apollo missions 14, 186, 205, 244–9, Barringer, Daniel 212 Bode, Johann Elert 79, 85, 96, 97, 98, 99
320, 325
Bohr, Niels 112, 114
Bok, Bart 200–201, 276
Bok globules 200–201, 276, 278
Bolton, Tom 254

Bondi, Hermann 290, 300

Borman, Frank 247

Bournon, Jacques-Louis de 90

Bouvard, Alexis 106

Bowen, Ira 114

Boyle, Robert 167

Boyle’s Law 167
Bradley, James 39, 43, 78

Brahe, Tycho 20, 30, 31, 36, 39, 42, 43,
44–7, 48, 52–4, 74–5, 102, 180

Braun, Wernher von 208, 245

brown dwarfs 127, 258, 293, 294

Bruno, Giordano 42, 230

Bryson, Bill 271

Bunsen, Robert 110, 112, 113, 114

Burney, Venetia 316

INDEX 345

C CMB see cosmic microwave background Davis, Ray 252–3
COBE (Cosmic Background Explorer) Deep Impact mission 308
Caesar, Julius 28 Delambre, Jean Baptiste Joseph 83,
calendars 28–9 224, 227, 282, 284–5
Callisto 62, 63 Cocconi, Giuseppe 204, 210–11, 231 93, 336
camera obscuras 49 Cohen, I Bernard 60 Delta Cephei 48, 86, 132
Cannon, Annie Jump 111, 113, 120, Coma cluster 270 Democritus 27
comets 46, 69–70, 72, 73, 110, 184, 206, density wave theory 276, 277, 278
124–7, 133, 138, 162 Dicke, Robert H 224–7
Cape Observatory (South Africa) 79, 119 287, 312 Digges, Thomas 34
carbon 199 composition of 207, 286, 308 disk galaxies 240
carbon-nitrogen-oxygen (CNO) cycle Halley’s 74–7 Dolland, John 43
landing on 306–11 Doppler, Christian 158, 159, 274
166, 183 Compton Gamma Ray Observatory 195 Doppler effect 158, 159–61, 176, 238,
Carrington, Richard 336 computer technology 259
Carte du Ciel project 100, 119 Comte, Auguste 15 274
Carter, Brandon 230, 339 constellations 24, 25, 79 Doppler spectroscopy 291
Cassini, Giovanni Domenico 43, 65 Copernican principle 62, 230, 235, double-star systems see binary stars
Cassini Division 65 Drake, Frank 210, 231–2, 233
Cassiopeia 45 290–91, 292 Draper, Henry 100, 110, 118, 120, 121,
Cassiopeia B 45 Copernicus, Nicolaus 19, 21, 22, 23,
Caterpillar Bok globule 200 124, 336–7
Cat’s Eye nebula 115 24, 26, 30, 32–9, 44, 46–7, 52, 58, Draper, John 118
Cavendish, Henry 68, 70–71 62–3, 291 Draper, Mary 121
celestial equator 22 Corot-3b 294 Draper Catalogue of Stellar Spectra 111,
celestial mechanics 15, 92–3 cosmic inflation 272–3, 274, 282
Celestial Police 97–9 cosmic microwave background (CMB) 121, 124
celestial sphere 22, 25 179, 195, 196, 197, 204, 224–7, 272, dwarf galaxies 100–101
Cellarius, Andreas 36 280–85, 300–301 dwarf planets 84, 90, 96, 99, 184, 287,
centaurs 312 cosmic radiation 214–17
Cepheid variables 86, 111, 120, 132–7, cosmic radio waves 58, 219 313, 314, 315
cosmic rays 111, 140, 198, 254 dwarf stars 126, 138, 139, 180
161, 174, 175, 177 cosmic wind 267
Ceres 82, 83, 90, 94–9, 315 cosmological constant 176, 177, 300, 303 E
Cernan, Eugene 249, 320 cosmology 15
Cerulli, Vincenzo 117 Cowan, Clyde 252 Earth
Chadwick, James 236 Crab nebula 19, 140, 237, 239 age of 186
Chaffee, Roger 247 Crabtree, William 64 atmosphere 20, 140, 190–91
Chamberlin, Thomas Chrowder 250 craters 212 composition of 187
Chandra X-ray Observatory 195, 214, cubewanos 287 distance from sun 64
Curiosity rover 259, 320, 324–5 geocentric model 18, 20, 24, 26, 34–6,
216–17, 237, 297, 301 Curtis, Heber D. 161, 174, 175, 337
Chandrasekhar, Subrahmanyan 141, Cygnus X-1 214, 218, 254 47, 62
gravity 72–3, 187
145, 154, 178, 180, 181 D life on 73, 231, 235, 294
chaos theory 92 risks from space 14
charge-coupled devices (CCDs) 258–9 Dalton Minimum 103 rotation of 13, 26, 35, 36, 37, 39
Charles II, King 13 Daly, Reginald 186–7 spin axis 22, 35, 78
Charon 262, 317 Dampier, William 55 Tychonic model 47
Chiron 184 dark energy 12, 148, 177, 180, 259, 271, eccentricity 54
Chladni, Ernst 82, 83, 90–91, 96 eclipses 23
chondrules 91 272, 296, 298–303 lunar 20
Christian Church Dark Energy Survey 300, 302 solar 14, 116, 144
dark matter 12, 15, 164, 165, 196, 240, eclipsing binary systems 86
and geocentric model 18, 34 ecliptic 22, 52
and heliocentric model 39, 63 258, 268–71 Eddington, Arthur 14, 116, 132, 141, 144,
chromatic aberration 58 D’Arrest, Heinrich 107
chromosphere 116 Darwin, Charles 231 145, 148, 152–3, 166–7, 170, 182–3
Clairaut, Alexis 77 Darwin, George 186, 187 Edgeworth, Kenneth 184, 206, 286, 312
Ehman, Jerry 210, 234

346 INDEX

Einstein, Albert 150, 170, 171, 329–30 Fernie, J. Donald 76 giant molecular clouds (GMCs)
cosmological constant 176, 177, Fisher, Richard 240 276–9
Fizeau, Hippolyte 103
300, 303 Flamsteed, John 13, 69, 84, 88 giant stars 138, 139, 241, 279, 302
general theory of relativity 14, 68, 73, Flandro, Gary 262, 265 Gilbert, Grove 212
Fleming, Williamina 113, 120, 124, 125, Gilbert, William 129
106, 107, 144, 148–53, 154, 167, 168, Gill, Sir David 79, 118–19, 120
169, 181, 182, 220, 259, 268, 303, 328 126, 128, 133, 141 Giotto spacecraft 207, 308
Einstein Observatory 214, 216 focal length 61 Glenn, John 245
electromagnetic radiation 190–91, 239 Foote, Albert E. 212 globular clusters 136, 137, 164–5
electromagnetic spectrum 204, 205 Ford, Kent 270 Goethe, Johann von 34
electromagnetism 111, 148 Foucault, Léon 26, 39, 103 Gold, Thomas 238–9, 300
electron degeneracy pressure 178 Fowler, Ralph 141 Goldilocks zone 294–5
electrons 282 Frankland, Edward 116 Gomes, Rodney 312–13
elements 144, 145, 162, 163, 166, 198–9 Fraunhofer, Joseph von 78, 112, 113 Goodricke, John 48, 86, 132
eLISA 328, 331 Fraunhofer lines 112, 162 Gould, Benjamin Apthorp 83
elliptical galaxies 105, 161, 241 free orbits 39 Grand Unified Theory (GUT) 272, 273
elliptical orbits 39, 50–55, 68–9, 75, 76, 92 Friedman, Herbert 215, 216 gravitational lensing 14, 153
Encke, Johann 74 Friedmann, Alexander 168, 169–70 gravitational theory 14, 43, 55, 66–73,
epicycles 35, 39
equinoxes 25 G 75, 92–3, 106, 118, 148, 151, 152,
precession of the 22 268, 269, 328
equivalence principle 151 Gagarin, Yuri 204, 208, 244 gravitational waves 12, 14, 73, 148, 259,
Eratosthenes 18, 334 galactic “walls” 274, 275, 282, 296 268, 272, 297, 326, 328–31
Eris 184, 286, 287, 313, 314, 315, 317 galaxies Great Comet 45, 46, 69–70, 119
ESA 177, 195, 217, 227, 285, 308–11 colliding 161, 217, 221, 271 “Great Debate” 144, 161, 174–5
escape velocity 73, 181 distance of 137, 164, 240–41, 274 Greeks, ancient 18–19, 20–22, 24–5
Eudoxus 20 evolution of 240–41, 283, 285 Greenstein, Jesse 220
Euler, Leonhard 78 mapping 27 Gregorian calendar 28, 29
Europa 62, 71, 234, 264 nebulae as 89, 115, 136, 145, 158–61, 170 Grisson, Virgil “Gus” 247
European Extremely Large Telescope rotation of 15, 269–71 Grosseteste, Robert 335
(E-ELT) 290, 293, 296, 304, 326–7 see also by name Grunsfeld, John 324
European Southern Observatory (ESO) galaxy clusters 214, 274–5, 282, 296, 301 Guo Shoujing 19, 28–9
258, 259, 326–7 Galilean moons 60–63, 265 Gush, Herb 226–7
event horizon 153, 154, 155, 255 Galilei, Galileo 12, 27, 34, 39, 49, 56–63, Guth, Alan 258, 272–3, 274, 282
Ewen, Harold 210
ExoMars rover 320, 325 58, 65, 107, 129 H
exoplanets 230, 259, 288–95, 305, 327 telescope 42, 44, 55, 326
extraterrestrial intelligence 12, 204, Galle, Johann 106–7 Hadley, John 43
210–11, 228–35, 238, 267, 294, Gamow, George 171, 182, 196–7, 198, Hagecius, Thaddaeus 46
295, 325 Hale, George Ellery 103, 129
extraterrestrial rovers 320–25 224, 272 Hale Telescope 129, 218, 220
Ganymede 62, 63, 71, 265 Hall, Asaph 62
F gas dwarfs 294 Halley, Edmond 22, 43, 47, 52, 64, 69–70,
gas laws 167
Fabri de Peiresc, Nicolas-Claude 61 Gassendi, Pierre 64 74–7, 87, 206, 207
Fabricius, David 48–9, 86, 132 Gaultier de la Vatelle, Joseph 61 Halley’s comet 43, 70, 74–7, 207, 308
Fakhri sextant 31 Gauss, Carl Friedrich 83, 98 Harding, Karl 99
false vacuum 273 Geller, Margaret 274–5 Harriot, Thomas 61
FAST (Five-hundred-meter Aperture geocentric model 18, 20, 24, 26, 34–6, 62 Harrison, John 62
geodesic 152–3 Hartmann, William 186
Spherical Telescope) 234 geometry 18 Harvard College Observatory 100, 111,
Fath, Edward 185 Gerard of Cremona 19
Fermi, Enrico 231 Ghez, Andrea 154, 218, 297 120–21, 124, 125, 128, 132–3, 162
Fermi Space Telescope 140 Giacconi, Riccardo 214–17 Harvard Spectral Classification System
Fernández, Julio 184 Giant Magellan Telescope 326
113, 120, 125–6
Haumea 184, 287

INDEX 347

Hauser, Mike 282, 284 Huchra, John 274–5 K
Huggins, Margaret 110, 114, 115, 116
Hawking, Stephen 148, 154, 174, 254, Huggins, William 87, 88, 104, 105, 110, Kant, Immanuel 158, 161, 250
255, 284 Kappa Andromedae b 293
114–15, 116, 158 Kapteyn, Jacobus 119, 164–5, 337
Hawking radiation 254, 255 Hulse, Russell 236 Keck Observatory (Hawaii) 292, 297, 302
Humason, Milton 175 Keenan, Philip 126
Heath, Sir Thomas 21 Huygens, Christiaan 14, 43, 58, 65, 335 Kellman, Edith 126
heliocentric model 18, 19, 21, 32–9, 55, hydrogen 113, 115, 116, 124, 125, 126, Kelvin, Lord 166
Kennedy, John F. 204, 244
62–3, 291 127, 129, 144, 162–3, 166, 167, Kepler, Johannes 48, 49, 53, 71, 96,
182–3, 196, 197, 198, 201, 226,
helioseismology 213 252, 272, 282 168–9
comets 75, 76–7
helium 110, 116, 124, 125, 126, 162–3, I elliptical orbits 23, 34, 39, 42, 47,

166, 167, 182–3, 196, 197, 198, 226, Iapetus 65 52–5, 92
Ibn al-Haytham 19, 335 laws of planetary motion 44, 64,
252, 272, 282 inflation 258, 272–3
Infrared Astronomical Satellite 250 68–9
Helix planetary nebula 127 infrared telescopes 304–5 telescope 61
inside-out model 277, 278 Kepler 10b 294
Helmholz, Herman von 166 International Ultraviolet Explorer 190 Kepler 442-b 290
interplanetary scintillation (IPS) 237 Kepler Telescope/Observatory 190, 195,
Henderson, Thomas 102 interplanetary space 43, 90, 267
interstellar communications 210–11 230, 292, 293
Heraclides Ponticus 20, 26, 35 Io 43, 62, 71, 265, 266 Kerr, Roy 255
ionization 140, 162, 163 Kirch, Gottfried 207
Herbig-Harp Objects 200 iron 199 Kirchhoff, Gustav 110, 112, 113, 114, 116,
Islamic scholars 19, 27, 30–31
Herman, Robert 197, 224–5, 226 124, 162
J Kohlhase, Charles 263, 264, 266
Herschel, Caroline 82, 85 Korolev, Sergei 208–9
Herschel, John 82, 83, 88, 100–101, 104, James Webb Space Telescope (JWST) Koshiba, Masatoshi 252, 253
190, 195, 276, 279, 290, 293, 296, Kowal, Charles 184
105, 107 304–5, 326 Kranz, Gene 244
Herschel, William 82, 83, 84–5, 88–9, Kublai Khan 28, 29
Jansky, Karl 145, 179, 190, 218, 219, Kuiper, Gerard 184, 287, 312
96, 98, 99, 100, 103, 104, 106, 114, 297, 304 Kuiper belt 84, 184, 206, 259, 286–7,

141, 158 Janssen, Jules 116, 124 308, 311, 312, 313
Janssen, Sacharias 59 Kuiper Belt Objects 286, 287, 314, 315
Hertz, Heinrich 179 Jeans, James 337 Kumar, Shiv S 258, 339
Hertzsprung, Ejnar 86, 111, 128, 128, Jewitt, David 184, 206, 286–7
Jing Fang 23 L
135–6, 138, 139, 158 Jodrell Bank (UK) 210, 211
John of Worcester 103 Lacaille, Nicolas-Louis de 77, 79, 87
Hertzsprung–Russell diagram 124, 125, Jupiter Lagrange, Joseph-Louis 336
128, 138, 139 exploration of 262, 263, 264, 265–6, 316 Laika 208, 209
gravity 93, 291, 313, 316 Lalande, Joseph 77
Hess, Victor 111, 140 moons 34, 42, 59, 60–63, 65, 71, 148, Laniakea Supercluster 275
Hevelius, Elisabetha 335 Laplace, Pierre-Simon 82, 83, 92–3, 106,
Hevelius, Johannes 75, 79, 335 234
Hewish, Antony 179, 180, 236–9 X-rays from 214, 216 107, 154, 155, 250, 251
Jutzi, Martin 186 Large Hadron Collider (LHC) 151
Hidalgo 96 Large Magellanic Cloud 27, 100–101,

High-Z Supernova Search 174 133, 134, 181
Hipparchus 22, 23, 24, 30, 47, 86

Hipparcos satellite 100

Holmdel Horn 225–6

Holmes, Arthur 186

Holwarda, Johannes 132
Homestake experiment 252–3

Hooke, Robert 69

horoscopes 13, 52
Horrocks, Jeremiah 42, 64

hot Jupiters 293

Houtermans, Fritz 182, 183, 198

Howard, Edward 90

Hoyle, Fred 52, 145, 168, 171, 196,
198–9, 226, 300

Hubble, Edwin 27, 86, 102, 120, 132,

136–7, 144, 158, 161, 164, 168, 170,
174–7, 185, 193, 240, 274, 296, 300
Hubble Constant 174, 177

Hubble Space Project 174

Hubble Space Telescope (HST) 137, 177,
190, 193–5, 217, 219, 221, 258, 259,

277, 279, 300, 304, 314, 326, 327

348 INDEX

Large Space Telescope (LST) 192–3 magnetic monopoles 272 Moon (cont)
laser interferometry 330 phases of 13
Lassell, William 106 main sequence stars see dwarf stars seen with naked eye 58
Late Heavy Bombardment 313 moons, planetary 65, 313
Le Verrier, Urbain 68, 83, 84, 85, 106–7 Makemake 184, 287 Moore-Hall, Chester 43
Leavitt, Henrietta Swan 48, 86, 102, 111, Morbidelli, Alessandro 312–13
Malhotra, Renu 312 Morgan, William Wilson 120, 126
120, 132–7, 174 Morrison, Philip 204, 210–11, 231
Leighton, Robert 129, 213 Marconi, Guglielmo 179 Mouchez, Amédée 100
Lemaître, Georges 145, 148, 168–71, Mount Wilson Observatory (California)
Mariner spacecraft 117, 204
174, 176, 196, 224, 272, 300 129, 137, 141, 174, 175
Lemonier, Pierre 84 Marius, Simon 61–2 Mueller, George 246, 247
lenses 43, 58, 59–60 Muirhead, Phil 294
Leonard, Frederick C 184 Markarian, Benjamin 185 multiverse 271
Leonov, Alexei 208 Murdin, Paul 254
Lepaute, Nicole-Reine 77 Mars Musk, Elon 321
Leuschner, Armin O 184 exploration of 262, 318–25 mythology 18
Leviathan of Parsonstown 104–5
Levison, Hal 312–13 gravity 72 N
Lexell, Anders Johan 85, 106
Lick Observatory (California) 61, 63 life on 230, 234, 325 NASA
light James Webb Space Telescope 279,
curved 144, 152–3 moons 62, 65
pollution 12 304–5
spectrum analysis of 15, 110 retrograde motion 35, 37, 38, 53 Mars exploration 117, 234, 259,
speed of 83, 148, 149–51, 272–3, 329 surface of 117, 259
wavelengths 110, 112, 329–30 320–25
LIGO (Laser Interferometer Gravitational- Maskelyne, Nevil 85 New Horizons 259, 314–17
Mather, John 282–5 observations 64, 97, 177, 190, 192,
Wave Observatory) 148, 153, 259,
268, 272, 297, 304, 326, 328–31 matter 14, 148, 196, 271 194, 195, 205, 216, 227, 230, 282,
Lin, Chia-Chiao 276, 277 285, 308
Lindblad, Bertil 164–5, 268, 269 Matthews, Thomas 220 Project Cyclops Report 232, 237
Lipperhey, Hans 42, 59 Maunder, Edward and Annie 129, 337 Space Race 244–9
Lizano, Susana 277 Voyager Mission 258, 262–7
Local Group 133, 275 Maury, Antonia 111, 120, 124–5, 128, navigation 13–14
Lockyer, Joseph Norman 110, 113, 116, 124 Near Earth Asteroids (NEAs) 99
longitude 62 133 nebulae 82, 83, 87, 88, 89, 101, 104–5,
Lovell, James 247 145, 158, 159–60
Lowell, Percival 15, 117, 158, 159, 230 Maxwell, James Clerk 65, 110, 111, 148 spectra of 114–15, 116
Lowell Observatory 158, 160 Mayor, Michel 259, 290–95 nebular hypothesis 205, 250–51
lunar eclipses 20 Neptune
Lunar Roving Vehicle 249, 320, 325 Mercury discovery of 82, 83, 84, 85, 106–7, 184
Lunokhod 1 and 2 320–21, 323, 325 exploration beyond 286–7
Luu, Jane 184, 206, 286–7 orbit 106, 107, 152, 269 exploration of 263, 264, 266–7
Luyten, Willem 178 moons 106
Lynden-Bell, Donald 297 transit of 64 orbit of 269
planetary migration 312–13
M Mesopotamia 12–13, 18 Neugebauer, Gerry 213
Messier, Charles 82, 84, 87, 88, 100, neutrinos 252–3
McClean, Frank 119 neutron stars 141, 145, 154, 178, 180–81,
MACHOs (Massive Compact Halo 101, 104 205, 217, 236, 237, 238–9
Messier objects 87 neutrons 180, 181, 196–7, 236
Objects) 271 meteorites 83, 90–91, 96, 212, 313 New Horizons spacecraft 259, 262, 308,
Magellanic Clouds 27, 100–101, 133, 134, 312, 314–17
meteors 190 New Technology Telescope (NTT) 258
135, 181
Metius, Jacob 59
Michell, John 70, 82, 254, 335

middle ages 26–31
Milky Way 12, 27, 58, 82, 88–9, 101, 104,

137, 221, 275, 276
shape of 164–5, 268, 269–70

size of 136

spiral nebulae 158–61
supermassive black hole 154, 297

Millikan, Robert 140

Milner, Yuri 235

mini-Neptunes 294
Mira Ceti 48–9, 86, 132

MKK system 126

Montanari, Geminiano 48

Moon

composition of 244, 246, 248, 320–21

early theories about 23

eclipses 20
landings 204, 205, 209, 244–9, 320,

325

movement of 38
origin of 186–7