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New Horizons flies past Ultima Thule p.28

JUNE 2019

The world’s best-selling astronomy magazine Bob Berman on
finding difficult
How fast
is the sky objects
universe
EXPANDING? p. 12

Answering cosmology’s www.Astronomy.com
big question p. 20
BONUS
EXPLORING ONLINE
The Outer Limits CONTENT
universe p.44 CODE p. 4

PLUS Astronomy’s Mars rover Vol. 47 • Issue 6
paper mission
Behind the collectibles p.50 update p.56
scenes at
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Online Content Code: ASY1906 JUNE 2019
Enter this code at: www.astronomy.com/code
VOL. 47, NO. 6
to gain access to web-exclusive content

ESA/HUBBLE & NASA

ON THE COVER

Expansion of the universe governs
how galaxies like the Antennae
(NGC 4038–9) interact.

CONTENTS 28

FEATURES 38 56 COLUMNS
StarDome and Mission complete for
20 COVER STORY Path of the Planets Opportunity rover Strange Universe 12
Tension at the heart
of cosmology RICHARD TALCOTT; After 15 years exploring the Red BOB BERMAN
ILLUSTRATIONS BY ROEN KELLY Planet, the Mars Exploration
Astronomers have found two Rover mission has ended. Binocular Universe 14
different — and mutually exclusive 44
— values for the expansion rate of The Outer Limits universe ALISON KLESMAN AND JOHN WENZ PHIL HARRINGTON
the universe. ROBERT NAEYE
Objects briefly seen on television 60 Observing Basics 16
28 over 50 years ago make terrific The story behind
New Horizons swings targets for today’s amateur Stellarvue GLENN CHAPLE
past Ultima Thule astronomers. MICHAEL E. BAKICH
From the start, this company’s Secret Sky 64
The historic encounter gave us 50 goal was the perfect telescope.
our first close-up look at a Kuiper The paper trail STEPHEN JAMES O’MEARA
Belt object, the most distant body of astronomy TONY HALLAS
we have ever explored. QUANTUM GRAVITY
Transient treasures of word and 68
DAVID J. EICHER art show that our science has a Ask Astro Snapshot 9
rich and colorful past. Astro News 10
36 Solar storms.
Sky This Month RAYMOND SHUBINSKI IN EVERY ISSUE

Jupiter dazzles all night. From the Editor 6
Astro Letters 8
MARTIN RATCLIFFE AND Advertiser Index 65
New Products 66
ALISTER LING Reader Gallery 70
Breakthrough 74

ONLINE Astronomy (ISSN 0091-6358, USPS 531-350)
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4 ASTRONOMY • JUNE 2019

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W W W.ASTRONOMY.COM 5

FROM THE EDITOR Editor David J. Eicher
Art Director LuAnn Williams Belter
BY DAVID J. EICHER
EDITORIAL
The Senior Editors Michael E. Bakich, Richard Talcott
expanding Production Editor Elisa R. Neckar
cosmos Associate Editors Alison Klesman, Jake Parks
Copy Editor Dave Lee
The famous Hubble parsec, one of the fundamen- radiation left from the Big Editorial Assistant Amber Jorgenson
constant has been a tal units of distance in the Bang, suggest a somewhat
source of heated con- cosmos, is 3.26 light-years.) smaller number. They are ART
troversy since its earli- not direct measurements of Graphic Designer Kelly Katlaps
est days in the 1920s. This month’s cover story the Hubble constant, but Illustrator Roen Kelly
by science writer Robert Production Specialist Jodi Jeranek

The number that defines the Naeye examines the history imply a number more like 67. CONTRIBUTING EDITORS
Bob Berman, Adam Block, Glenn F. Chaple, Jr., Martin George,
expansion rate of the uni- of decoding the expansion Data from the Planck satel- Tony Hallas, Phil Harrington, Korey Haynes, Jeff Hester,
Liz Kruesi, Ray Jayawardhana, Alister Ling, Steve Nadis,
verse, the Hubble constant rate of the universe and all lite, with the known compo- Stephen James O’Meara, Tom Polakis, Martin Ratcliffe, Mike D.
Reynolds, Sheldon Reynolds, Erika Rix, Raymond Shubinski
(or H0) is estimated primarily of its consequences. It trans- sition of the universe — dark
SCIENCE GROUP
by measuring redshifts: lates into big implications energy, dark matter, and Executive Editor Becky Lang
Design Director Dan Bishop
changes in a galaxy’s spec- for the cosmos, including its normal matter — yield this
EDITORIAL ADVISORY BOARD
trum attributed to the fast age, fate, and size, among smaller result. Buzz Aldrin, Marcia Bartusiak, Timothy Ferris, Alex Filippenko,
Adam Frank, John S. Gallagher lll, Daniel W. E. Green, William K.
motion of the galaxy away other things. The implications are big. Hartmann, Paul Hodge, Edward Kolb, Stephen P. Maran,
Brian May, S. Alan Stern, James Trefil
from an observer. Spectral Naeye describes how Right now, astronomers
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For decades, astronomers based on a Hubble Vice President, Content Stephen C. George
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amount, giving away have argued about the constant of 67 from Vice President, Human Resources Sarah A. Horner
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This may all sound constant and faced off 73, it would mean a Advertising Sales Director Scott Redmond
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6 ASTRONOMY • JUNE 2019

P35557 CELEBRATING

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ASTROLETTERS We welcome your comments at Astronomy Letters, P. O. Box 1612, Waukesha, WI 53187;
or email to [email protected]. Please include your name, city, state, and country.
Letters may be edited for space and clarity.

No Bull writing about 100 years ago, once specu- patterns that later became identified as
lated about the manner in which life might mathematics. — Donald Craig, Indianapolis, IN
“Specters of past constellations” in your exist on other planets. Writing philosophi-
February issue is a fine article on failed cally, he said: “If the Lord God Almighty, ‘Oumuamua annihilation
constellations, but it missed Poniatowski’s by combining carbon and the three gases,
Bull. It was created in honor of Stanisław can make an Ambassador to the Court of I find it incredible that, in our lifetime, an
Poniatowski, king of Poland from 1764 St. James, I see absolutely no reason why extrasolar object like ‘Oumuamua sailed
to 1795. This starry tribute goes unrec- he cannot create a monad of helium and right through our solar system. From this
ognized today, but it grips you none- fluorine.” — Paul Campion, New York, NY discovery, a scientist extrapolated such an
theless. When I showed the Bull to my object can be found in the solar system
survey class in astronomy last semester, it Historic equations at least once a year. So what does this
looked back at us through its eye, double mean for interstellar space travel, where a
star 70 Ophiuchi. At a mere 17 light- Jeff Hester’s article in the February issue, fast spaceship could be annihilated after
years away, Poniatowski lives on! And a “Wigner’s anachronism,” is an enlight- colliding with a speck of dust? Hitting
wide-field scope makes this bull’s-eye a ened review of Eugene Wigner’s accom- an “ ‘Oumuamua” would light up the
showpiece. — Michael Farney, Mitchell, SD plishments and their importance to all sky, like a miniature supernova! Perhaps
of science. His definition of mathematics that’s why we have not seen any aliens yet.
Alien commentary was written in 1960 and in the framework
provided at that time. More advanced — Guenter Hoernig, Penticton, British Columbia, Canada
Kiona N. Smith wrote compellingly in concepts have altered our thinking and
February’s “How to build aliens in the the utility of mathematics. While it may Correction
lab” about the attempts to create non- be “trendy” to talk about how the uni-
carbon-based life, like what might exist verse is fine-tuned for our existence, we The photo of Abell 39 on p. 59 of the
on Titan. The beauty of this comes from must remember that eons before there February 2019 issue was credited incor-
the scientists’ creativity in devising were any creatures living on this planet, rectly. The photo credit should have read:
non-carbon-based cell parts. The news- the universe was evolving with the same Adam Block/Mount Lemmon SkyCenter/
paper writer and critic H.L. Mencken, University of Arizona.

P35798Take Part in Astronomy’s Readers Choice Awards

You know what you like! Your chance to elect your favorite astro gear is coming soon!
We’ll take your votes from May 15-30, 2019 and share the results in our September 2019 issue.
Watch for announcements on our website and social media for when the ballot box is open!
PLUS all voters will be eligible to win an Astronomy prize package*.

*Rules will be included in the Astronomy Readers Choice Awards online voting survey.

8 ASTRONOMY • JUNE 2019

QUANTUM

QG GRAVITY
EVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH . . .

HOT BYTES>> PUZZLE PIECE TALK IT OUT DUSTY DISCOVERY
Neptune’s newly A Northwestern University While searching data for
TRENDING discovered smallest model will use data from brown dwarfs, citizen
TO THE TOP moon, Hippocamp, isolation experiments to scientist Melina Thévenot
may be a piece broken predict and avoid crew discovered the first white
off from the larger communications problems dwarf surrounded by
moon Proteus. on missions to Mars. multiple dusty rings.

SNAPSHOT

Glowing
galaxies

A tiny neutron star shines brightly.

This vivid image shows the slow NASA/JPL-CALTECH, IPAC. TOP FROM LEFT: ESA/HUBBLE, NASA, L. CALÇADA; NASA; NASA’S GODDARD SPACE FLIGHT CENTER/SCOTT WIESSINGER
merger of the two galaxies that make
up the Whirlpool Galaxy (M51).
They both sit 23 million light-years
from Earth. The photo combines
optical imagery (appearing in blue)
from the Sloan Digital Sky Survey
with X-ray data (shown in green)
from NASA’s NuSTAR mission.

Much to astronomers’ surprise,
the supermassive black holes in the
centers of these two galaxies are not
shining as intensely in X-rays as
expected. Researchers think this may
be due to a brief dip in brightness as
the black holes “flicker” over time.

Odder still, the X-ray emission
from these behemoths is matched by
an object millions of times less mas-
sive: a single, incredibly dense neutron
star about 12 miles (20 kilometers)
across and two times the mass of the
Sun. This neutron star is situated
on the left side of the main galaxy,
where its green glow can easily be
seen. Astronomers think the neutron
star’s strong magnetic field could be
causing it to blast out high-energy
X-rays, and that the other green spots
of X-ray emission in the colliding
galaxies could also be neutron stars.

— Amber Jorgenson

W W W.ASTRONOMY.COM 9

ASTRONEWS SYSTEMS GO. NASA plans to launch the Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices
Explorer, or SPHEREx, in 2023 to observe the entire sky in almost 100 different wavelengths and illuminate the origins of the universe.

MARS LANDER DIGS IN, THEN GRINDS TO A HALT

One of InSight’s instruments is stuck
in the ground; engineering models
on Earth could solve the problem.

NASA’s Mars InSight lander touched
down in November and immediately
got to work studying Mars’ deep
interior, including what it’s made of
and how the planet’s layers move. InSight
spent months studying the area around its
landing site, practicing its movements and
scouting the best locations to place instru-
ments. Then, in late February, it started
digging — and promptly got stuck.

ABRUPT STOP NASA/JPL-CALTECH
InSight’s Heat Flow and Physical Properties
Package (HP3) instrument includes a “mole” SITTING PRETTY. The HP3 instrument sits on the martian surface in this image taken March 19 by InSight’s
— a self-hammering spike — designed to Instrument Deployment Camera. NASA is trying to find out why a hammering spike on HP3 has stopped digging.
burrow up to 16 feet (5 meters) underground.
But the mole made it only about a foot Spohn says. “The hope is that what we’re dig, as the lander runs on solar power and NASA, ESA, A. SIMON (NASA GODDARD SPACE FLIGHT CENTER), AND M.H. WONG AND A. HSU (UNIVERSITY OF CALIFORNIA, BERKELEY)
(0.3 m) deep before stopping. In March, HP3 hammering against is a small rock, say half was designed for two Earth years of duty.
principal investigator Tilman Spohn told the size of the mole’s length. We could push
Astronomy the team’s best guess was that the that aside by continuing to hammer.” Spohn STILL GOOD SCIENCE
mole hit a rock or a gravel layer shortly after calls this the “brute force” approach. The mole needs to descend at least 10 feet
beginning to dig February 28. (3 m) to measure heat flow from Mars’
One possible tactic would be to press interior. If it can’t continue, “we would lose
To find out more, the NASA team down on the mole or its support structure, a significant amount of science,” Spohn
turned to InSight’s camera and other sen- probably with InSight’s arm, lending the admits. But InSight’s other instruments are
sors. They also set to work on re-creating mole more force and limiting recoil. But the working as planned, and NASA could still
the problem with engineering models on arm wasn’t designed for such a move. “If get information from the dirt InSight has
Earth: InSight has a twin in Berlin, and you make a mistake, it’s gone,” Spohn says. already dug through. “It would still be stuff
copies of its various instruments, including But he also points out that if the mole is that hasn’t been done before,” Spohn says.
the mole, also exist. Any potential solutions able to dig freely again, it could reach its “Not as bold as originally planned, but still
will be tested extensively using the models target depth within about four hours. good science.” — Korey Haynes
on Earth before trying them on the real InSight has plenty of power to finish the
InSight on Mars.

But some scenarios might stop the
$800 million mission where it stands. “If it’s
a 1-meter (3 feet) block of rock at the spot,
there’s no way we can handle that situation,”

Summer storms on Uranus and Neptune CLOUDY SKIES. Astronomers use the Hubble
Space Telescope to check on our outer solar system
neighbors annually. These images, taken in September
and November 2018, show weather on the ice giants.
At right, a dark storm spanning about 6,800 miles
(11,000 kilometers) rages on Neptune. It is the fourth
such tempest Hubble has imaged on the planet
since 1993. To the right of the dark vortex are white
companion clouds, which form as methane gas is
thrust upward by the storm’s vortex and freezes into
clouds. On the left is Uranus, with a large, white cloud
over its north polar cap. Researchers believe the
cloud is a result of the planet’s unique rotation and
tilt, which expose the north pole to uninterrupted
sunlight during the long summer season. Astronomers
aren’t sure how the narrow band of clouds around
the equator formed. Charting the weather on these
two ice giants helps scientists better understand the
differences — and similarities — between the planets
that share our Sun. — Alison Klesman

10 ASTRONOMY • JUNE 2019

ASTRONEWS KEEP COOL. Research suggests the insulating crust of dwarf planet Ceres could keep salty water (called cryomagma)
in liquid form beneath the surface for up to 10 million years, explaining the various ages of its surface deposits.

In Andromeda, QUICK TAKES
a star explodes
like clockwork GROWING DARKER

This nova has erupted once Dark energy’s repulsive force
a year for millions of years, may be increasing over time,
leaving behind the largest
known cloud of stellar debris suggests a new study that
around a white dwarf. used nearly 1,600 quasars to
ASTRONOMY: ROEN KELLY AFTER SOHO/EIT CONSORTIUM/MDI TEAM
X-RAY: NASA/CXC/RIKEN/D. TAKEI ET AL.; OPTICAL: NASA/STSCI; RADIO: NRAO/VLAAstronomers have discovered that aNOT DEAD YET. Like M31N 2008–12a, GK Persei (pictured) is a prime example of•track the universe’s expansion.
star in the Andromeda Galaxy (M31) a nova remnant. GAUGING GRAVITY
has been regularly erupting for the
past million years, leaving behind one When this material reaches the sur- IT KEEPS GOING AND GOING Scientists have repurposed a
of the biggest shells of ejected mate- face, it’s heated and compressed movement-tracking device on
rial ever seen. The new research, pub- thanks to the white dwarf’s intense The massive size of the remnant is the Curiosity rover to measure
lished January 9 in the journal Nature, gravitational pull. Eventually, the not its only claim to fame. Indeed,
not only marks the first discovery of hydrogen reaches a breaking point M31N 2008–12a also now holds the tiny variations in Mars’
such a super-remnant in another gal- and spontaneously fuses to create title of most frequently recurring gravitational field, revealing
axy; it also paves the way for detect- helium, resulting in a powerful sur- nova, as it erupts at least once a year. new clues about the formation
ing a potentially huge population face explosion called a nova. of Mount Sharp, which the
of repeatedly exploding stars called “When we first discovered that
recurrent novae. This burst of fusion causes the M31N 2008–12a erupted every year, •rover is currently climbing.
white dwarf to temporarily brighten we were very surprised,” said FEELING SPACE
SWING YOUR PARTNER up to a millionfold as it ejects material co-author Allen Shafter of San Diego
outward at up to 3 percent the speed State University in a press release. Astronomers teamed up with
The star responsible for this expan- of light. In the case of M31N 2008–12a, Most recurrent novae explode only teachers to design a workshop
sive remnant, which stretches more over time these repeated explosions about once a decade. where those with hearing loss
than 400 light-years across, is actu- have created an extensive and ever-
ally a diminutive white dwarf, the expanding cocoon of gas and dust Even though the white dwarf has can feel vibrations from
Earth-sized remnant of a Sun-like around the white dwarf. According spent the past million years or so cosmic sources like stars,
star. But in the case of this rem- to the paper, “Larger than almost all exploding, researchers don’t think it
nant, which bears the catchy name known remnants of even supernova will last forever. The study concludes •galaxies, and supernovae.
M31N 2008–12a, the culprit is not an explosions, the existence of this shell that in less than 40,000 years, the
ordinary white dwarf: This star has a demonstrates that the nova M31N white dwarf will either explode one SHORTER TIMELINE
dance partner. 2008–12a has erupted with high last time as a type Ia supernova or
frequency for millions of years.” collapse under the weight of the According to data from the
As the tiny white dwarf and its accreted material into a neutron Gaia satellite, the Milky Way
nearby companion star orbit each star. — Jake Parks
other, the white dwarf rapidly will collide with the
siphons hydrogen from its buddy. Andromeda Galaxy in some
3.9 billion years, not 4.5 billion
HOW BIG
IS THE SUN? •years as previously thought.
SALTY STAR
Jupiter New ALMA observations

Earth revealed the first detection
The Moon of sodium chloride (table salt)

400,000 300,000 200,000 100,000 0 in the disk surrounding a
Miles young, massive star in the
2,158 miles
(3,475 km) 86,882 miles •Orion Nebula (M42).
(139,823 km) ROCK ON

7,918 miles Asteroid Day, which aims to
(12,742 km) raise awareness about

The Sun’s diameter asteroids and the search to
864,340 miles (1,391,020 km) find them, will celebrate its
fifth anniversary June 30 with
SUN WORSHIP. Our solar system is so FAST thousands of events hosted
named because it’s dominated by Sol, the FACT
Sun. This image, taken July 19, 2000, includes a •around the world.
vast array of sunspots. But it’s hard to imagine how
just how big those sunspots — and the Sun itself — IN PLAIN SIGHT
are without a visual comparison. We’ve depicted
Jupiter, Earth, and the Moon to scale here, so you can An international team of
see how they stack up against our local star. researchers uncovered
—Michael E. Bakich hundreds of thousands of
previously unseen galaxies
Some 1.3 million Earths in radio data from the Low
could fit inside the Sun.
•Frequency Array telescope.
SYNTHETIC LIFE

Research funded by NASA has
created a DNA-like molecule
that can store and transmit
information, which may force

scientists to rethink life

•beyond Earth.

SHOT IN THE DARK

Japan’s Hayabusa2 spacecraft
fired a tantalum bullet into the

surface of asteroid Ryugu to
dislodge and collect samples
that will be brought to Earth

in December 2020. — J.P.

W W W.ASTR ONOMY.COM 11

STRANGEUNIVERSE

BY BOB BERMAN

Flirting with

Some phenomena can be

futilitychallenging to find, but
well worth the effort.

With sky observ- chance of success was minus-
ing, as in most
things, success cule. And so it happened; M33
is more satisfy-
ing than fail- wasn’t there. The point, how-
ure. If you want to show people
Saturn’s rings, there is no ever, is that it was fun to try.
upside to turbulent air blurring
those rings into an incoherent Then I noticed a star where The Triangulum Galaxy (M33) is a difficult target for the naked eye, requiring utterly
mess. So why attempt an obser- there normally is only a blank perfect conditions. Even though it is rarely visible, many observers find the challenge to
vation whose chance of success piece of celestial real estate. spot it exciting. STEPHEN RAHN/FLICKR
is negligible?
“There’s Mira!” I practically
Yet I know I’m not alone in
finding futility appealing. shouted. For decades I’ve delib- out to walk to another observa- something you should not look

I saw this last November erately avoided reading when tory building. Glancing casually for if you value your eyesight.
when I invited a couple of peo-
ple to join me in the meadow Mira (Omicron [ο] Ceti) would into the clear Chilean skies at For a few seconds just as a total
behind my home for some
naked-eye exploring. I had two reach naked-eye visibility. the Large Magellanic Cloud, as eclipse is ending, immediately
targets in mind: the planet
Uranus and the Triangulum There’s no trace of it nearly the he had done a thousand times before the literally blinding
Galaxy (M33). I expected the
first quest to be successful, entire year. And then, for about before, he now saw a star in it. photosphere starts to appear,
but not the second.
a month, an obvious star sud- Oscar, one of the nicest guys in the awesome chromosphere is
Sure enough, after they’d
followed my green laser to east- denly shines to the right of the world, knows the sky far briefly exposed. Using binocu-
ern Pisces and viewed Uranus
through image-stabilized bin- Cetus’ tail. The thrill is seeing more intimately than most pro- lars longer than she should
oculars, we tried for a naked-
eye glimpse of the planet. And that blank bit of sky undergo fessional astronomers, and he have, near the end of the 2017
we all saw it. There is a strange,
wonderful satisfaction to this that metamorphosis. The under- knew then and there that he’d totality, my wife suddenly
that cannot be shared with
most of our friends. Only lying science of a pulsating giant discovered the first naked-eye shouted, “Oh my God, what is
another astronomer can under-
stand the allure of seeing the star in its final death gasps supernova in four centuries. that?” Despite being a veteran
seventh planet with no optical
assistance. If there’s even a before becoming a planetary Actually, Oscar’s story of five totalities, she had never
word for this singular pleasure,
I don’t know what it is. nebula is pretty riveting, too. doesn’t count — futility-wise even heard of the chromo-

That autumn night was dry, Comet hunters routinely do — because he wasn’t trying to sphere, but excitedly told us
clear, and moonless, with the
nearest population center of how the Sun’s western edge had
25,000 some 20 miles (30 kilo-
meters) away, so seeing M33 I know I’m not alone in finding just turned a vivid purple-red.
was a possibility. But from my Far safer is the circum-
backyard meadow, I’ve seen it
only once all these years. The futility appealing. zenithal arc (CZA), which I

used to consider a near-futile

quest. I look for the CZA

the near-futility thing. They find a supernova. His tale is almost every day, whenever the

search the sky hour after hour, more similar to that of the two Sun is lowish and a thin cirrus

year after year, and typically amateur observers (I knew one layer is overhead. This upside-

see nothing. And then in one of them) who observed the down rainbow, whose vivid

glorious pre-dawn moment, spokes on Saturn’s rings color is unsurpassed by any

they may become the first per- through their backyard tele- other phenomenon, can hover

son to witness the approach of scopes years before the Voyager directly overhead — but

a new celestial object. spacecraft first showed that nowhere else. Does successfully

Something even more aston- “impossible” feature to be real. seeing something only once out

ishing happened to Oscar A more relevant quest involving of every 50 attempts qualify as

Duhalde in 1987. He runs the the sixth planet might be you or a nearly futile project?

equipment at Las Campanas me trying to see Saturn’s Great What does? Please send your

Observatory in Chile, where the White Spot. The storm typically favorite near-futile targets.

great 6.5-meter Magellan tele- appears once every 29 years.

scopes are perched. On Normally there’s no trace of it. Join me and Pulse of the Planet’s
February 24, 1987, while per- Another impossible observa- Jim Metzner in my new podcast,
forming his technical and
instrument tasks, he stepped tion? What about the Sun’s bril- Astounding Universe, at
liant red chromosphere? This is http://astoundinguniverse.com.

BROWSE THE “STRANGE UNIVERSE” ARCHIVE AT www.Astronomy.com/Berman.

12 ASTRONOMY • JUNE 2019

ASTRONEWS DOUBLE DOWN. A NASA researcher found a 23-mile-wide (37 kilometers) impact crater beneath the Greenland ice.
The new crater is about 100 miles (160 km) from a 19-mile-wide (30 km) crater discovered just three months before.
Ultima Thule reveals
its flatter shape

INCOMING! A planetary collision is exactly as bad as you would imagine. Unlike an asteroid impact, there’s not
just a crater left behind. Instead, such a massive crash strips the surviving world of much of its lighter elements,
leaving behind the dense core. NASA/JPL-CALTECH

First evidence of exoplanet collision

For the first time ever, astronomers think elements, it should be denser than its twin, STRANGE SHAPE. The top image of Ultima
they’ve discovered an exoplanet that not less dense. According to the paper, this Thule averages 10 separate frames; the image
survived a catastrophic collision with another would “make the more-irradiated and less- below it has been processed to eliminate
planet. The evidence for the impact comes massive planet Kepler-107b denser than blurring due to motion. NASA/JHUAPL/SWRI/NOAO
from two “twin” exoplanets that seem a Kepler-107c,” which is not the case.
bit more fraternal than identical, accord- Though it originally appeared as a snowman
ing to research published February 4 in the However, there is another way that a planet made up of two globes, the Kuiper Belt object
journal Nature Astronomy. can lose a lot of mass: by getting whacked by Ultima Thule is actually more of a pancake.
another planet. This is exactly what the The new view comes from parting shots taken
The pair of planets in question (along with researchers think happened to Kepler-107c. with the Long-Range Reconnaissance Imager
at least two other planets) orbit a Sun-like star on NASA’s New Horizons on January 1, as the
in the Kepler-107 system, roughly 1,700 light- The team argues that the denser planet, spacecraft was already 5,494 miles (8,862 kilo-
years away in the constellation Cygnus the Kepler-107c, likely experienced a massive colli- meters) past the contact binary. Researchers
Swan. Known as Kepler-107b and Kepler-107c, sion with a third, unknown planet at some reconstructed the object’s shape by watch-
these planets have nearly identical sizes (both point in its past. Such a gigantic impact, the ing background stars disappear behind it in
have a radius of roughly 1.5 times that of study says, would have stripped the lighter sil- separate images, revealing that the larger
Earth), but the inner planet, Kepler-107b, is just icate mantle from Kepler-107c, leaving behind lobe, Ultima, is flattened like a pancake while
one-third the density of Kepler-107c, which is an extremely dense, iron-rich core. According the smaller lobe, Thule, is shaped more like a
more than twice as dense as Earth. to the study, Kepler-107c could be as much as dented walnut, according to a press release.
70 percent iron.
Previous studies have shown that intense Going back over previous data, researchers
stellar radiation can strip the atmosphere from Though further research is needed to con- confirmed that their new model of Ultima
a planet that sits too near its host star. But if firm the hypothesis, if proven correct, this find Thule’s shape is consistent with the character-
the inner planet lost its lighter atmospheric would become the first evidence for a plane- istics gleaned from all prior observations. “We
tary collision outside our solar system. — J.P. had an impression of Ultima Thule based on
the limited number of images returned in the
ABOVE JUPITER’S THICK ATMOSPHERE days around the flyby, but seeing more data
has significantly changed our view,” said New
A FISHY SCENE. If you stood on the Horizons principal investigator Alan Stern.

cloud tops at the south pole of Jupiter and The object’s true shape has left researchers
wondering about its formation, but will cer-
PUPPIS looked straight overhead, these are the tainly shed further light on the processes that
stars you would see. The gas giant’s axis shaped our outer solar system. — A.K.

points toward a nondescript region W W W.ASTR ONOMY.COM 13

Canopus in the constellation Dorado the

Dolphinfish, some 12° away from

DORADO the night sky’s second-brightest
star, magnitude –0.7 Canopus.

South Celestial Pole Even more impressive, the
center of the Milky Way’s
CARINA
RETICULUM galactic neighbor, the Large

Magellanic Cloud (LMC),

sits just 5° away.

LMC — Richard Talcott

ASTRONOMY: ROEN KELLY VOLANS Because Jupiter’s Galilean FAST
5° moons orbit in the planet’s FACT
equatorial plane and their axes
are perpendicular to their orbits,
observers at their south poles
would have similar views.

BINOCULARUNIVERSE May 12 q N e
22 June 1
BY PHIL HARRINGTON Path of Ceres LIBRA
11
21 July 1

Let’s go r ` 11
asteroiding! E i 21
SCORPIUS t h
Minor planets make for challenging binocular targets. OPHIUCHUS 31
t
ASTRONOMY: ROEN KELLY
t
b 1°

Ceres crosses into Ophiuchus, Scorpius, and Libra from late May through early July.

On January 1, 1801, Ceres for the Roman and Ophiuchi. As it continues to to Vesta, the fourth asteroid
Italian astrono- Sicilian goddess of grain. march westward, it passes 1.5° discovered. German physician
mer Giuseppe north of 4th-magnitude Nu (ν) Heinrich Olbers found Vesta on
Piazzi spotted a Asteroids can change dra- Scorpii on June 9, and on the March 29, 1807. Olbers had also
“star” through his matically in brightness depend- 17th, it appears 1.6° north of discovered the asteroid Pallas
telescope that was not on his ing on their distance from Graffias (Beta [β] Scorpii). five years earlier.
detailed charts of the region. He Earth. The best time to look for
returned to the same location them is when their distance During July, its retrograde Vesta always puts on a fine
over the next few nights, only to away from us is minimal. This motion will slow, allowing show around opposition. That’s
find that this new star couldn’t occurs at opposition, when the Ceres to resume its normal, next set to occur November 12.
be a star at all. It had moved asteroid rises in the east as the easterly track. It will hover At that time, Vesta will be
against the stationary backdrop. Sun sets in the west. between Graffias and Lambda within the constellation Cetus
Whatever he had discovered, it (λ) Librae, slowly arcing south, the Whale. At opposition, Vesta
had to be orbiting the Sun. June opens with Ceres just then southeast by the end of the will be 1.56 AU from Earth and
past its May 28 opposition. month. In the process, Ceres shine at magnitude 6.5. It will
At first, he thought he had From late May through early will fade by more than a magni- be 1.9° west-southwest of 4th-
discovered a comet, but later July, it appears to move in ret- tude since June 1. magnitude Omicron (ο) Tauri,
observations (by him and oth- rograde as quicker Earth passes which marks one of the Bull’s
ers) allowed an orbit to be cal- by. During this time, it slides Once you spot Ceres, make front hoofs. That night’s Full
culated. Piazzi’s new object was from Ophiuchus, through visiting it a habit over the next Moon will be only about a bin-
revolving around the Sun in a northernmost Scorpius, and few months. Note its position ocular field away, confounding
nearly circular orbit at a dis- into Libra. Throughout, it will on the chart each time you efforts to see Vesta. But take
tance of 2.77 astronomical remain around 7th magnitude. glimpse it. In the end, you’ll heart. Vesta stays bright several
units. (An AU is the average Of course, that region of the have a personal record of Ceres’ weeks before and after opposi-
Earth-Sun distance.) That sky has lots of 7th-magnitude 2019 apparition. tion: In November, it remains
placed it in the gulf between stars, so spotting which one is brighter than 7th magnitude
Mars and Jupiter. He had dis- Ceres is a bit like looking for a Ceres is not the only member and within binocular range.
covered the first asteroid, now celestial needle in a haystack. of the asteroid belt visible
reclassified as a dwarf planet by through your binoculars. More I mentioned earlier that
the International Astronomical Ceres moves past several than 60 become brighter than Eunomia would be challenging
Union. As is customary, the notable stars during the month, 10th magnitude around opposi- to search for, and the same is
discoverer of a new member of which should help in our quest. tion, and should therefore be true of most asteroids — accu-
the solar system has the honor I hope the sky is clear for you visible through binoculars. rate finder charts are a must.
of naming it. Piazzi chose May 24 and 25. On those One of the best online resources
nights, Ceres is just north On August 13, the 15th aster- for this is in-the-sky.org/
of 4th-magnitude Chi (χ) oid discovered, Eunomia, will newsindex.php?feed=asteroids.
reach opposition. That night, it Good luck with this introduc-
N ARIES will shine at magnitude 8.2, tion to asteroiding.
faint but doable through
j Path of Vesta h steadily supported 50mm bin- I’d enjoy hearing your
30 Dec 10 oculars. It will lie among the results. Contact me through my
Oct 21 31 k faint stars of Aquarius, 3.7° west website, philharrington.net.
E Nov 10 of Sadalsuud [Beta Aquarii]. Until next month, remember
You’ll need a good finder chart that two eyes are better than
TAU R U S 20 to locate this challenging target. one.
(More on that later.)
CETUS ASTRONOMY: ROEN KELLY Phil Harrington is a longtime
Surprisingly, even though contributor to Astronomy and
1° Ceres is the largest object in the
_ asteroid belt at about 600 miles the author of many books.
Vesta remains bright weeks before and after the asteroid’s opposition November 12. (960 kilometers) across, it is not
the brightest. That honor goes

14 ASTRONOMY • JUNE 2019

ASTRONEWS HIDDEN FIGURE. NASA has renamed its Independent Verification and Validation Facility, which ensures the
agency’s missions succeed by confirming their software performs correctly, in honor of Katherine Johnson.

Radioactivity may dry out water worlds

Although we imagine that Earth’s oceans make HEAT IT UP. This artist’s concept illustrates how a planet’s birthplace affects the amount of radioactive aluminum
it a watery planet, our home is actually only a it inherits. At left, a planet formed in a massive, dense star cluster is subject to significant bombardment, resulting
tiny fraction of water by mass. And in other plan- in a hotter, drier planet. The world at right, formed in a different environment, receives less aluminum-26 and stays
etary systems, water is more common than our cooler and wetter. THIBAUT ROGER
own planet implies: Some exoplanets are up to
50 percent water by mass. So, what causes some which appears unexpectedly dry. However, this new idea remains just one possibility out of
planetary systems to stay wet, while others dry Lichtenberg points out there’s no solid proof potentially many. “This is not the only method”
out? The answer might be aluminum, says a that aluminum heating caused either our solar that can dry out systems like TRAPPIST-1, he
Nature Astronomy study published February 11. system’s or TRAPPIST-1’s relative dryness, so said. “But it is a powerful one.” — K.H.

Lead author Tim Lichtenberg of the University
of Oxford told Astronomy that when large
amounts of aluminum-26 — a radioactive form
of aluminum — decay, the process can heat up
and dry out the large boulders that collide to
form planets. As a result, the amount of alumi-
num in a young star system could predict which
types of planets will evolve there.

Aluminum heating matters only for bodies
of a certain size. Small pebbles don’t have
enough aluminum-26 to cause any heating.
Full-size planets may be able to retain their
water through other methods, such as in an
atmosphere. But aluminum heating would
affect all objects in the unlucky size range
between 5 and 50 miles (8 and 80 kilometers).

A good example of this effect in action
might be the TRAPPIST-1 exoplanet system,

1.6 The amount of data in the Pan-STARRS
PETABYTES digital sky survey’s second release.
It is the largest amount of
astronomical data ever published,
equivalent to 2 billion selfies.

MEET SOME CONTACT BINARIES

ASTRONOMY: ROEN KELLYAsteroid 624Comet 67P/ ODD COUPLE. In Earth’s extended atmosphere
NASAHektorChuryumov-GerasimenkoJanuary, New Horizons
flew by the Kuiper Belt Think that Earth’s atmosphere only encompasses,
Asteroid 216 Comet 1P/Halley object Ultima Thule (MU69) well, Earth? Think again. New findings published
Kleopatra and identified it as a February 15 in the Journal of Geophysical Research:
contact binary: a double- Space Physics show that our atmosphere stretches
lobed object made up of much farther into space than previously thought.
two bodies that have
come together until they Researchers made this discovery while “dusting
are gently touching. This off” data from ESA/NASA’s Solar and Heliospheric
odd arrangement is not Observatory (SOHO), which has been studying the
unique in our solar system; Sun since 1995. Between 1996 and 1998, the space-
there are numerous other craft used its sensors to trace Earth’s geocorona —
bodies that astronomers the cloud of hydrogen atoms appearing where
believe are contact Earth’s atmosphere meets outer space. When these
binaries, many of which atoms interact with the Sun, they emit ultraviolet
are closer to home in the light, visible in this image taken by Apollo 16
main asteroid belt or astronauts on the Moon in 1972.
circling the Sun as short-
period comets. Here are a SOHO used this light to measure how far the
few of the solar system’s geocorona extends. To their surprise, the researchers
more famous contact found that the Moon, which sits about 239,000 miles
binaries. — A.K. (385,000 kilometers) from Earth, lies well within the
geocorona’s boundary, which stretches 390,000 miles
FAST (630,000 km) into space. Now, the team hopes to
FACT use this technique to detect hydrogen in the
atmospheres of more distant planets. — A.J.
As of July 2018, astronomers had
identified 83 suspected contact W W W.ASTR ONOMY.COM 15
binaries in our solar system: 69
asteroids, eight trans-Neptunian

objects, and six comets.

OBSERVINGBASICS

BY GLENN CHAPLE

Make a mask
for double stars

Some stellar pairs are too bright to separate with a large Limiting the brightness
scope. An off-axis mask can help you easily split them. of double stars with an off-
axis mask can significantly

help you separate otherwise

Over the years, I’ve pointed the scope at Castor, indistinguishable stars. The two alter ego. Still, it allowed me
always been an the fuzzy blob was indeed large, bright stars of Castor (which to shift my attention from
advocate for the transformed into two is really six stars that appear as one) faint fuzzies to close double
small-aperture beautifully separated stars. are seen in this sketch. JEREMY PEREZ stars without having to put
telescope, tout- Encouraged by my success, I the 10-inch away and bring
thickness of the telescope tube, out my 3.5-inch f/11 refractor.

ing its remarkable capabilities fashioned a more permanent place the opening a half-inch I made my off-axis mask
last January when Epsilon
in my writings and through model out of ⅛-inch plywood. from the edge of the mask. Arietis was conveniently
located in the evening sky. If
talks at astronomy conven- A few years ago, I sold the 4. With the hobby knife, you make one now, you might
want to test it on Mu (μ)
tions and club meetings. 13.1-inch Dobsonian (reluc- cut out the aperture mask and Draconis (mags 5.7 and 5.7,
sep. 2.3"), Xi (ξ) Ursae Majoris
During my salad days as an tantly!) for an easier-to-handle the opening. (4.3 and 4.8, 1.9"), or the clos-
est components to the neat
amateur astronomer, back in 10-inch f/5 instrument. 5. Attach the mask to the triple star Xi (ξ) Scorpii (4.9
and 5.2, 1.1"), whose third
the 1970s, my telescopic arse- Although there were no direc- telescope with a few strips of member (mag 7.3) is 7.6" away.

nal was made up of a 3-inch tions for making an off-axis duct tape, and you’re ready to Once you’ve made a work-
able prototype, you can move
f/10 reflector and a 2.4-inch mask, I used my previous head outside and use it. on to a more durable model. As
mentioned, I made mine out of
f/11 refractor. Modest in size experience to cobble together a On an evening of fair seeing ⅛-inch plywood, then spray-
painted it flat black on the
though they were, these high crude prototype in less than an conditions, I tested the mask inside, and secured it to the
main scope with small metal
focal ratio instruments were hour. To do the same, all you on the striking near-twin dou- L-brackets. I’ll leave it to you to
come up with a final design of
remarkably capable — espe- need is a sheet of corrugated ble star Epsilon (ε) Arietis, your own once you’ve created a
workable prototype.
cially on double stars, my cardboard greater in diameter whose diamondlike magnitude
Questions, comments, or
favorite night-sky targets. than the telescope tube, a pen- 5.2 and 5.6 component stars suggestions? Email me at
[email protected]. Next
In the early 1980s, I took cil, a hobby knife, a tape mea- were 1.3" apart. To my scope’s month: We honor an event
that happened 50 years ago.
a giant leap and purchased a sure, and a roll of duct tape. credit, the pair was reasonably Clear skies!

13.1-inch Dobsonian-mounted Glenn Chaple has been an
avid observer since a friend
reflector. This “fast” (f/4.5) A mask allowed me to shift my attention showed him Saturn through a
light-gobbling beast revealed small backyard scope in 1963.

deep-sky wonders well beyond from faint fuzzies to close double stars.

the grasp of my little scopes.

But it didn’t do well with dou-

ble stars. The bright binary Here’s the process: split at 212x without the mask.

star Castor, whose component 1. To make sure the mask is But with the mask taped in

stars were separated by 2" at the correct size, hold the card- place, I noticed a slight

the time, was resolvable in board against the top of the improvement. Although the

the 3-inch, but not so in the tube and trace out a circle that seeing conditions didn’t war-

13.1-inch. Instead of two sepa- matches the telescope’s out- rant it, I decided to go for

rate stellar images, my eye was side diameter. broke with a 3.2mm eyepiece

greeted by an elongated, scin- 2. Use the tape measure that yielded a magnification

tillating blob of light. to determine the diameter of of 397x. This time, the view

Fortunately, the big scope the largest circle that will fit using the mask was a definite

came with directions to make between the secondary mirror improvement over what I saw

an off-axis mask that would and support vanes. I found without it.

convert it into an unob- that to be a 4-inch diameter There are limitations to a

structed 5-inch f/12 long-focus hole for my 10-inch scope. “new” masked scope, however.

scope that promised to yield 3. Use a compass (or an For me, the theoretical resolu-

sharper stellar images. When I appropriately sized can) to tion limit was reduced from

placed a cardboard prototype trace out the opening for the 0.5" for my 10-inch scope to

over the front of the tube and mask. To compensate for the a little over 1" for its 4-inch

BROWSE THE “OBSERVING BASICS” ARCHIVE AT www.Astronomy.com/Chaple.

16 ASTRONOMY • JUNE 2019

ASTRONEWS NEWLY NAMED. The International Astronomical Union approved the name Statio Tianhe, which is derived from the
ancient Chinese term for Milky Way, for the landing site of the Chang’e-4 lander currently studying the Moon’s farside.

Hello there, Chang’e-4
NASA/GSFC/ARIZONA STATE UNIVERSITY
CHAO LIU, NATIONAL ASTRONOMICAL OBSERVATORIES, CHINESE ACADEMY OF SCIENCES
WARPED DISK. The Milky Way’s disk has a distinctive twist — exaggerated here for illustrative purposes.

PASSING BY. This image on February 1 from the Lunar The Milky Way has a galactic twist
Reconnaissance Orbiter shows the Chang’e-4 lander
(vertical arrow) and its rover, Yutu-2 (horizontal arrow). The shape of the Milky Way, usually pictured Chinese Academy of Sciences determined
as a flat spiral, may actually be more akin to the distance to each Cepheid in their data set
Not often do two spacecraft get to say hello to a warped and twisted disk, according to a to an accuracy of 5 percent or better. They
each other. But NASA’s Lunar Reconnaissance new study published February 4 in Nature then plotted these distances in 3D, creating
Orbiter (LRO) and the China National Space Astronomy. The study analyzed 1,339 stars a new map of the Milky Way that shows the
Administration’s Chang’e-4 have had the chance whose distances could be determined with galaxy’s disk has a distinct kink.
several times. great accuracy.
Astronomers had known about a kink in
In early February, LRO spied the Chang’e-4 Each star in the study is a Cepheid vari- our galaxy’s gas disk, but they didn’t know
lander and its Yutu-2 rover on the farside of the able, a type of pulsating star whose intrinsic that the stars would follow the same shape.
Moon from an altitude of 51 miles (82 kilome- brightness is tied to how long it takes to oscil- The new data also suggest that the warp in
ters). At the time, the rover had already traveled late between bright and dim. Normally, it’s the Milky Way’s disk precesses, or spins, like
95 feet (29 m) northwest of the lander, though it difficult to tell if a star is bright and far away a top. “The twisting of the warp is new,”
has gone even farther since then. or dim and nearby. Because a Cepheid’s astronomer and study co-author Richard
period tells astronomers how bright the star deGrijs of Macquarie University told
Sitting in Von Kármán Crater, the Chinese mis- truly is, by comparing that number with how Astronomy. “It’s been seen in a dozen other
sion will study the Moon’s topography, explore bright it appears, researchers can accurately galaxies before, but not ours.”
what lies beneath its surface, and record the pin down its distance.
composition of the crater’s rocks. Researchers The authors hypothesize that as the Milky
hope this information will reveal more about Using infrared data from the Wide-field Way’s inner disk of stars rotates, it produces
how the Moon’s farside differs from the nearside Infrared Survey Explorer, astronomers from drag on the outer disk as well, distorting the
we see from Earth. — A.J., A.K. Macquarie University in Australia and the flat spiral. — K.H.

$20,400,000 The funding awarded by the National Science Foundation to
Caltech and MIT to upgrade LIGO by 2024, allowing scientists
to search a greater volume of space for gravitational waves.

FROM HOT TO COLD PLANETARY TEMPS. Although many planets (and dwarf planets) have temperatures that

Saturn vary drastically, as a general rule of thumb, the average temperature of a planet drops as you
Neptune
go farther from the Sun. Venus, however, is the exception. Even though Mercury is the closest

planet to the Sun, Venus, at more than 850 degrees Fahrenheit (450 degrees Celsius), has the

highest average temperature of any planet in the solar system. Unlike Mercury, which has a

very thin atmosphere and slow rotation rate, Venus has an incredibly dense, heat-trapping

atmosphere that perpetuates the global greenhouse effect. — J.P. Venus

Mercury

°F –400° –300° –200° –100° 0° 100° 200° 300° 400° 800° 900°

°C –200° –100° 0° 100° 200° 400°

ASTRONOMY: ROEN KELLY Pluto Mars Earth The dayside temperature of Mercury FAST
Uranus reaches about 800 degrees Fahrenheit, FACT
while the nightside temperature drops
Jupiter as low as –290 F (–180 C), about as cold as

the ice giants Uranus and Neptune.

W W W.ASTR ONOMY.COM 17

ASTRONEWS MELTED MOLECULES. As the young, active star V883 Orionis flares up, its snow line — the distance from the star at which
water turns to ice — is pushed outward, thawing previously frozen organic molecules in the star’s protoplanetary disk.

Huge, rare jet bursts from a young star

ESO, A MCLEOD ET AL.

TEMPER TANTRUM. At just over 150,000 light-years from Earth, the Large Magellanic Cloud (LMC) is bursting with newly formed stars. In a paper published January 24
in Nature, astronomers homed in on a particularly fertile — and visually stunning — star-forming region of the LMC named LHA 120-N 180B, informally known as N180 B,
shown at left. Probing the nebula with the Multi Unit Spectroscopic Explorer (MUSE) on the European Southern Observatory’s Very Large Telescope, the researchers spotted a
fledgling star 12 times the mass of our Sun firing a huge, narrow jet into space, shown at right with the jet colorized to show its orientation pointing toward (blue) and away
(red) from Earth. At nearly 33 light-years long, it is one of the longest such jets observed to date. It is also the first time astronomers have spotted such a jet in a galaxy other
than the Milky Way using visible light. Although such narrow jets are often observed around low-mass baby stars, few have been found around stars weighing more than
8 solar masses. This rare example provides yet another piece of evidence suggesting small stars are not the only ones that throw tantrums in their early years. — J.P.

Virgin Galactic’s SpaceShipTwo reaches space for second time

On February 22, Virgin Galactic’s back to the ground, where it ENGINES ON. SpaceShipTwo, also named the VSS Unity, fires its engine during
SpaceShipTwo, also named VSS touched down like an airplane. its first successful test flight. Unity, which seats six passengers, is carried underneath
Unity, flew in space for the second a flying plane before taking off into space under its own power. VIRGIN GALACTIC
time, taking off from Mojave, SpaceShipTwo made its maiden
California, after days of weather space voyage December 13, 2018. to perform another test: determin- the flight was successful, boding
delays. The craft carried two pilots, February’s test was its fifth pow- ing how the vehicle flies with a well for the craft’s future in ferrying
one crew member, and a nearly ered flight in total. The third crew greater weight distribution. Details paying passengers in addition
full payload of science projects. member on board was Virgin will likely come later, but ultimately to cargo. — K.H.
Galactic’s chief astronaut instruc-
Unlike most spaceflights, tor and cabin evaluation lead. Her
which fire rockets from the job was to experience flying on
ground, SpaceShipTwo is carried the ship firsthand to better pre-
on the belly of a plane named pare future passengers.
WhiteKnightTwo. Once released,
SpaceShipTwo propels itself into Virgin Galactic’s goal is to ferry
the upper atmosphere. paying tourists into space for a few
minutes of weightlessness — and
During the test, SpaceShipTwo a priceless view.
was released at a height of 45,000
feet (13,700 meters) before suc- The spacecraft also carried four
cessfully firing its rocket engine to research projects from NASA’s
reach suborbital space less than an Flight Opportunities program,
hour after taking off. It coasted which pairs research institutions
there for only a few minutes of with private companies that can fly
weightlessness before heading their projects into space. Carrying
the projects allowed the company

18 ASTRONOMY • JUNE 2019

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W W W.ASTR ONOMY.COM 19

TENSION

AT THE HEART
OF COSMOLOGY

Astronomers have
found two different
— and mutually
exclusive — values
for the expansion
rate of the universe.

by Robert Naeye

Spiral galaxy M106 harbors a Something appears
water megamaser — amplified amiss in cosmology. A
microwave emission from water tension has arisen from
molecules — near its massive attempts to measure
central black hole. The maser the universe’s current
provides an independent way expansion rate, known
to measure M106’s distance and as the Hubble constant.
thus helps calibrate the cosmic
distance ladder, leading to more Large international teams have used two
accurate values for the Hubble general methods to determine it. All the
constant. NASA/ESA/THE HUBBLE HERITAGE groups have been extremely diligent in their
research and have cross-checked their
TEAM (STSCI/AURA)/R. GENDLER (FOR THE HUBBLE results, and their measurements seem rock
solid. But the practitioners of one approach
HERITAGE TEAM) can’t quite come to agreement with practi-
tioners of the other.

The stakes are high. As Nobel laureate
Adam Riess of the Space Telescope Science
Institute and Johns Hopkins University
explains, “The choices now are either a con-
spiracy of errors, not just in one measure-
ment but in multiple measurements . . . or
there’s some kind of interesting new physics
in the universe.”

The Hubble Wars

Controversies surrounding the Hubble
constant are hardly new to cosmology.
This parameter, often called H-naught
(and abbreviated H0), is fundamental to
determining the age of the universe and its
ultimate fate, giving astronomers a powerful
incentive to get it right.

To measure H0 directly, astronomers need
to observe many galaxies and glean two key
pieces of information from each one: its dis-
tance and the speed at which it moves away
from us. The latter comes directly from
measuring how much the galaxy’s light has
shifted toward the red. But determining dis-
tances proves to be much trickier.

From the 1960s to the 1980s, a team
led by Allan Sandage of the Carnegie
Observatories consistently measured values
of H0 around 50 to 55 kilometers per second
per megaparsec. (A megaparsec equals
3.26 million light-years.) A competing
team led by Gérard de Vaucouleurs of the
University of Texas obtained figures around
100. This discrepancy by a factor of two was
so extreme that the scientific dispute degen-
erated into personal animosity.

Both teams used a traditional “distance
ladder” approach to measure distances.

W W W.ASTR ONOMY.COM 21

CLOCKWISE FROM ABOVE: They monitored Cepheid variable stars around 67. And all of these measure-
The Antennae Galaxies (NGC 4038 and in far-flung galaxies. The luminosities of ments have become so precise that their
NGC 4039) lie 65 million light-years these supergiant stars correlate with their ranges of uncertainty no longer overlap.
away in Corvus. Hubble astronomers periods of variation, making them excel-
targeted this interacting pair because lent “standard candles” — objects that The Hubble constant is 73
it is one of 19 galaxies to host a type Ia radiate a well-known amount of light.
supernova (2007sr) and many Cepheid Once astronomers measure the distances There’s still a lingering suspicion among
variables, helping to forge a link to Cepheids in our Milky Way using many cosmologists that the Hubble ten-
between these two standard candles. trigonometric parallax, they can calcu- sion will eventually disappear, a result
late the distances to other galaxies by of measurement or systematic errors.
ESA/HUBBLE & NASA watching their individual Cepheids Although that position remains tenable,
brighten and fade. recent advances in measurement tools
Face-on spiral galaxy M101 in Ursa and techniques are pointing in the oppo-
Major lies a mere 21 million light-years The Hubble Wars appeared to wane in site direction. “There’s less than a 0.01
from Earth. Its proximity means that 2001, when the Hubble Space Telescope’s percent chance of this kind of difference
astronomers got particularly sharp Key Project published an H0 of 72 with occurring just by chance,” Riess says.
views when type Ia supernova 2011fe an uncertainty range of plus or minus 8.
erupted in a spiral arm in August By using Hubble, Wendy Freedman (now A case in point is the latest result from
2011. Hubble astronomers also have at the University of Chicago) and her col- SH0ES — the Supernova H0 for the
measured the periods and luminosities leagues monitored Cepheids in galaxies Equation of State — a large international
of 272 Cepheids in M101. NASA/ESA/STSCI out to about 80 million light-years. They consortium Riess leads. In 2018, the team
then used these results to calibrate other published an H0 of 73.5 with an uncer-
Supernova 2009ig exploded in the distance indicators in galaxies out to tainty of only 2.2 percent.
barred spiral galaxy NGC 1015. (The about 1.3 billion light-years. At that dis-
arrow points to the location of the tance, cosmic expansion dominates the SH0ES uses the same distance ladder
supernova.) The galaxy also contains speed of galaxies away from us, with lit- method employed by the Key Project, but
more than 40 Cepheid variable stars, tle “contamination” from the motions of it adds powerful new measurement tools.
helping to strengthen the link between galaxies within their host clusters. The most important are type Ia superno-
these two vital standard candles. vae — white dwarfs that explode with a
NGC 1015 lies 118 million light-years More recently, teams that employ the relatively uniform luminosity. Scientists
from Earth in the constellation Cetus. traditional distance ladder method have have carefully calibrated their variations
measured H0 values of about 73, consis- in intensity by studying how fast they
NASA/ESA/A. RIESS (STSCI/JHU) tent with the Key Project, but with brighten and fade, making them ideal
greater precision. However, teams that standard candles. And these supernovae
Spiral galaxy NGC 3972 hosted study the cosmic microwave background are incredibly bright, so they can be seen
Supernova 2011by. (The arrow shows (CMB), the leftover radiation from the at far greater distances than Cepheids.
the position of the star that exploded.) Big Bang, are calculating H0 values of
Astronomers have tracked several Riess and his colleagues are particu-
dozen Cepheid variables in this galaxy, larly interested in galaxies that are close
making it a key object connecting two
rungs on the cosmic distance ladder.
NGC 3972 lies 65 million light-years
away in Ursa Major. NASA/ESA/A. RIESS (STSCI/JHU)

22 ASTRONOMY • JUNE 2019

enough for Cepheid monitoring but also Supernova 2009ig
have hosted type Ia supernovae in recent
years. His team has analyzed 19 such gal- Supernova 2011by
axies to date, with another 19 to come,
giving them independent distance mea- flickering brightnesses of distant quasars derive H0 from the time delays. The team
surements. But to cross-check their that are gravitationally lensed by fore- recently measured an H0 of 72.5, with
Cepheid and supernova results, SH0ES ground galaxies. Because the light from 3 percent uncertainty.
also uses geometric distance indicators, each lensed quasar takes multiple paths
such as eclipsing binary stars in the and different amounts of time to reach “Throughout our analysis, we kept the
Large Magellanic Cloud and water Earth, the H0LiCOW astronomers can
masers in the spiral galaxy M106. Hubble constant blinded, meaning we

And even the Cepheid distances in never know what value we were getting
our own galaxy have become more accu-
rate, thanks to extremely precise parallax
measurements from the European Space
Agency’s Gaia satellite and improved
cameras on Hubble. SH0ES is getting
virtually the same H0 as the Key Project,
but its range of uncertainty has narrowed
significantly. “What’s changed from pre-
vious generations is that the data quality
is much better,” says Riess.

Yet another group, the Carnegie
Supernova Project, is reaching an identi-
cal result. Its most recent paper lists two
H0 values, 73.2 and 72.7, taken through
different wavelength filters, with uncer-
tainties of just 2.3 and 2.1, respectively.

Carnegie team leader Christopher
Burns says his group uses the same
Cepheid, eclipsing binary, and maser
data that SH0ES does, but Carnegie
employs a different method for analyzing
supernova data and making corrections
for variations in luminosity and the red-
dening effects of dust.

“We’ve done these corrections in
slightly different ways with different
assumptions and different data sets, but
we’re coming up with the same answer,”
says Burns. “So as far as supernovae are
concerned, I’m pretty confident we’re
doing the right thing.”

But Burns is quick to add that SH0ES
and Carnegie work with the same
Cepheid data and use similar methods
to study them. That part gives him a bit
of unease. “I would love to have another
method of figuring out distances to these
supernovae and making sure that agrees
as well,” he says.

Adding confidence to the SH0ES
and Carnegie results, the H0 Lenses
in COSMOGRAIL’s Wellspring
(H0LiCOW) group recently announced a
new H0 measurement. Using a completely
independent method, this international
team has spent years watching the

W W W.ASTR ONOMY.COM 23

CLOCKWISE FROM ABOVE: throughout the entire analysis,” says direct measurements of H0. Instead, they
The gravitationally lensed quasar in H0LiCOW team leader Sherry Suyu of are predictions of what H0 should be,
B1608+656 offers an independent way the Max Planck Institute for Astrophysics given known conditions in the early cos-
to determine cosmic distances. The in Garching, Germany. “That’s impor- mos and how the universe’s main ingre-
close-up view reveals two foreground tant because that avoids confirmation dients influence cosmic expansion.
galaxies that smeared the light of a bias. So it’s not that we subconsciously
more distant quasar into four arcs. The favor one H0 over another.” The material that gave rise to the
galaxies lie 5 billion light-years from CMB was forged in the Big Bang. For
Earth; the quasar is 4 billion light-years The H0LiCOW result is beautifully 380,000 years, the universe was a dense,
farther away. NASA/ESA/HUBBLE/S. SUYU (MPIA) ET AL. consistent with SH0ES and Carnegie. In opaque sea of electrically charged gas
other words, all the teams that measure known as plasma. Sound waves coursing
Spiral galaxy NGC 3370 in Leo shines H0 in the local universe are getting the through this plasma caused matter to
across 98 million light-years of space. same result: about 73. compress and rarify non-randomly into
It hosted type Ia supernova 1994ae high- and low-density regions. These are
in November 1994. It also boasts 65 No, the Hubble constant is 67 now imprinted on the CMB as slight
Cepheids astronomers have tracked to temperature irregularities. About
get an independent distance measure. Were it not for the CMB measurements, 380,000 years after the Big Bang, the uni-
the Hubble constant would probably verse had expanded and cooled enough
NASA/ESA/THE HUBBLE HERITAGE TEAM AND A. RIESS (STSCI) be considered a solved problem, and for electrons to combine with atomic
researchers would move on to other nuclei to form atoms. This enabled the
Astronomers studied 85 Cepheids in projects. But the CMB results are highly Big Bang’s remnant gas to radiate freely
spiral galaxy NGC 5584 in Virgo to learn compelling despite the fact they are not as light in all directions. Over the next
it lies 70 million light-years away. They 13.8 billion years, cosmic expansion has
then applied this value to the galaxy’s redshifted this ancient light into the
Supernova 2007af to help calibrate microwave portion of the spectrum.
distances to these far more luminous
objects. NASA/ESA/A. RIESS (STSCI/JHU)/L. MACRI (TEXAS The precise mixture of dark matter
and normal matter affected how those
A&M UNIVERSITY)/THE HUBBLE HERITAGE TEAM (STSCI/AURA) early sound waves imprinted the CMB
with temperature variations. NASA’s
TENSION IN CMB LOCAL UNIVERSE WMAP satellite and Europe’s Planck sat-
THE COSMOS ellite have measured these irregularities
with increasing precision across the entire
Values of the Hubble Planck sky, with Planck providing the most sen-
(2018) sitive map of all. A detailed analysis of the
constant measured by SH0ES (2018) Planck data, combined with other data,
enabled cosmologists to measure the uni-
direct observations verse’s contents as 68.3 percent dark
energy, 26.8 percent dark matter, and
of relatively nearby 4.9 percent “normal” matter. When

galaxies differ from Carnegie (2018, H band)

those garnered

through data on the

cosmic microwave Carnegie (2018, B band)

background. The error

bars between the two

different methods no H0LiCOW (2018)

longer overlap. 65 70 75

ASTRONOMY: ROEN KELLY Hubble constant (km/s/Mpc)

60 80

24 ASTRONOMY • JUNE 2019

cosmologists plug these numbers into this,” says CMB researcher Gary Hinshaw thus tacitly assumes that the cosmologi-
of the University of British Columbia. cal constant is the dark energy that is
Einstein’s equations from the general the- causing cosmic expansion to acceler-
The Planck result is consistent with all ate. CDM stands for “cold dark matter,”
ory of relativity, they predict an H0 of 67.4, other CMB studies. But it’s conspicuously meaning that most of the universe’s mass
with an uncertainty range of only 0.5. lower than the H0 values measured by consists of heavy particles that move
SH0ES, Carnegie, and H0LiCOW, and relatively slowly.
“The CMB predictions for H0 assume their error bars do not overlap.
that the contents of the universe are well Lambda CDM beautifully explains the
“I have to confess that as someone cosmos and is consistent with virtually
described by atomic matter, cold dark whose professional training dates to the every parcel of astronomical data. The
era of a factor-of-two uncertainty in H0, model assumes that the universe is spa-
matter, and dark energy. If this descrip- I have a difficult time becoming terribly tially flat on large scales, meaning two
agitated by disagreements of a few per- parallel light beams traveling unhindered
tion is incomplete, the predictions could cent!” says Penn State University astron- through intergalactic space will remain
omer Donald Schneider. parallel over billions of light-years. It also
be in error, but there is no evidence for assumes that Einstein’s general relativity
But what happens if this tension over explains the universe on large scales.
H0 persists?
This model has been so successful
A lot of unknown physics that cosmologists would be loath to give
it up, or even to make substantial modifi-
If future observations fail to show that cations. But as Riess explains: “There’s a
this tension results from measurement lot of unknown physics in that model.”
errors, it will throw a monkey wrench
into the prevailing cosmological model, For example, we don’t know what kind
known as Lambda CDM. Lambda is a of particle constitutes dark matter, or
Greek letter that symbolizes Einstein’s even if it is a particle. After all, numerous
cosmological constant, an unchanging experiments to detect dark matter par-
property of space that exerts a tiny but ticles have come up empty. And we don’t
inexorable repulsive force. The model

W W W.ASTR ONOMY.COM 25

CLOCKWISE FROM ABOVE: know what is causing cosmic expansion involves something about the physics of
Europe’s Planck satellite has taken the to accelerate. It might be Einstein’s cos- the early universe,” adds Riess.
best data on the cosmic microwave mological constant; it might be some
background. Combining these results kind of dynamical field that changes over Vivian Poulin of Johns Hopkins
with the standard model of the universe time; or it might be something else. University recently published a promis-
yields a Hubble constant slightly but ing idea. He and three colleagues posit
meaningfully smaller than that from “I don’t think we should be totally that a form of dark energy that modestly
nearby galaxies. ESA/PLANCK COLLABORATION stunned if we can’t explain the dynamics affected cosmic expansion infused the
The Meathook Galaxy (NGC 2442) lies of the universe across all of cosmic time universe from about 20,000 to 100,000
55 million light-years from Earth in to 1 percent when we don’t really under- years after the Big Bang. Poulin says this
Volans. This ground-based image nicely stand the physics of 95 percent of the dark energy “could explain this mis-
shows its two asymmetric spiral arms. universe,” says Riess. match in the measurements,” adding,
Close-up Hubble observations studied “The beauty of the idea is that it’s not so
Supernova 2015F and 143 Cepheids to Hinshaw adds, “It’s a great surprise that exotic. We have already observed similar
forge a link between two key rungs on Lambda CDM works as well as it does.” effects at different times.”
the universe’s distance ladder. ESO
Astronomers used light variations in A daunting challenge Another plausible idea is the existence
16 Cepheids to deduce that NGC 3982 of a fourth type of neutrino currently
in Ursa Major lies 68 million light-years The H0 tension presents a challenge to unknown to science. This ethereal par-
away. Supernova 1998aq, a far brighter theorists. Although theorists are a cre- ticle, known as a sterile neutrino, would
object, lit up NGC 3982 in April 1998. ative lot, they can’t just concoct any idea have increased the amount of radiation
Scientists use galaxies with both types to resolve this cosmic conundrum. “It’s in the early universe. When plugged into
of objects to extend the distance ladder really hard to change Lambda CDM in Lambda CDM, the extra radiation
deep into the cosmos. NASA/ESA/THE HUBBLE a way that actually fits this enormous increases the Hubble constant predicted
suite of data from the early universe and from the CMB.
HERITAGE TEAM (STSCI/AURA) the late universe in a way that works,”
explains Princeton University physicist Both of these ideas could relieve the H0
A nearby star overshadows NGC 7250, an Joanna Dunkley. tension without making radical changes
irregular galaxy 45 million light-years to Lambda CDM. But other ideas would
away in Lacerta. Host to Supernova “The consensus tends to be that if deliver more of a hammer blow.
2013dy and 22 Cepheids, NGC 7250 has you’re looking for a source, it most likely
played a significant role in measuring For example, perhaps the overall spa-
the Hubble constant. ESA/HUBBLE & NASA tial geometry of the universe is not flat
after all. A non-flat universe would be
26 ASTRONOMY • JUNE 2019 dynamically unstable, however, and it
would contradict CMB observations
showing that the universe must be
extremely close to flat. “It would be very
unusual for the universe to be almost
flat, but not quite, today. That’s hard to
engineer,” says Hinshaw.

Or perhaps dark energy is not the cos-
mological constant, but is caused by some
kind of dynamical field that changes over
time. Poulin notes that such a field would
have “exotic” properties because instead
of diluting as the universe expands, it
does the opposite. Although Poulin says
such a field is “not absolutely impossible
from a theoretical standpoint, people are
not at ease with it. It’s a bit weird.”

An even more radical proposal is that
we live in a region of the universe with
an anomalously low density. Dunkley
states the objection of many cosmologists
to this idea: “It doesn’t make sense that
our local region should be that strange
compared to the rest of the universe.”

Relieving the tension

All the observing teams express high
confidence in their methodologies and

results. Fortunately, unlike the Hubble “a completely independent, ground-up star mergers over the next decade, scien-
Wars of yesteryear, the modern discrep- recalibration of type Ia supernovae. It’s tists should be able to calibrate them as
ancy has not devolved into personal pretty exciting,” she says. “standard sirens” and nail down H0 to
animosity or professional disrespect. within 1 percent.
Instead, it has motivated an insatiable Large survey telescopes will help
desire to get to the bottom of a profound astronomers precisely measure how the Right now, cosmologists calculate that
mystery. For observers, it means reduc- density variations in the early universe the universe is 13.8 billion years old,
ing their errors even further, down to imprinted themselves on the large-scale based on Planck’s H0 of 67.4. But if H0 is
1 percent if possible. It also means new distribution of galaxies. These signatures, actually closer to 73, it could shave hun-
types of measurements. known as baryon acoustic oscillations, dreds of millions of years off the uni-
will enable scientists to measure how cos- verse’s age, depending on what changes
For example, Freedman leads a large mic expansion evolved during the uni- would be required in Lambda CDM.
international group that will soon pub- verse’s middle ages, which in turn will And more importantly, a resolution of the
lish a new H0 using the distance ladder help connect CMB observations of the Hubble tension could also shed light on
method. But instead of basing the result early universe and distance ladder mea- dark energy, which controls the universe’s
on Cepheids, her team is using Hubble to surements of the modern-day universe. ultimate fate. If dark energy is indeed
observe the most luminous red giants in Einstein’s cosmological constant, the cos-
the halos of distant galaxies, which cut Dunkley is now working with the mos will expand forever and lead to a Big
off at a specific maximum luminosity. By Atacama Cosmology Telescope in Chile, Chill. But a dynamical dark energy could
observing in galactic halos, her team can which is making detailed measurements become so powerful that it would tear
make brightness measurements that are of the CMB’s polarization. This result will all matter to shreds in a Big Rip. And
less contaminated by the light of back- provide an independent H0 measurement. according to Hinshaw, “If the dark energy
ground stars. Cepheids, in contrast, are “We’ll be able to add our data to the is unstable, it could decay into a new sub-
young stars found in crowded galactic Planck data and actually further shrink stance and change the laws of physics
disks, where other stars contribute noise the uncertainty on the Hubble constant entirely, with unpredictable results.”
to the data. Red giants are also simpler from the CMB, and then see if it’s even
objects than Cepheids, which have com- more different from the local one or If the tension still exists after observ-
plex, dynamic atmospheres. whether the tension is reduced,” she says. ers get down to 1 percent uncertainty,
we’ll have “extraordinary evidence” that
For these and other reasons, Freedman Further down the road, the LIGO and the tension is for real, says Hinshaw. This
claims the red giants are more precise Virgo gravitational wave detectors will would necessitate changes in Lambda
distance indicators than Cepheids, and make their own H0 measurements. From CDM, which would be incredibly excit-
they produce less scatter in the data. So just one event — the neutron star merger ing. He concludes, “The best scenario
far, her team has measured red giants in observed August 17, 2017 — LIGO scien- would be that all of this holds up and it
17 galaxies that also have hosted type Ia tists measured an H0 of 70, but with an points us in a direction that ultimately
supernovae. This new method provides uncertainty of about 15 percent. Once gives us more insight about the dark
LIGO registers many dozens of neutron universe — the dark matter and dark
energy — which would be fantastic.”

Former Sky & Telescope editor in chief Robert

Naeye served as an editor at Astronomy from

1995 to 2000. He is one of just two people

who have worked on the editorial staffs of

America’s two largest astronomy magazines.

W W W.ASTR ONOMY.COM 27

New Horizons
swings past

Ultima Thule

The historic encounter gave us our first
close-up look at a Kuiper Belt object, the
most distant body we have ever explored.

by David J. Eicher

An artist’s impression shows
NASA’s New Horizons spacecraft
approaching Ultima Thule,
a Kuiper Belt object 4.1 billion
miles from the Sun, on January 1,
2019. NASA/JOHNS HOPKINS UNIVERSITY APPLIED

PHYSICS LABORATORY/SOUTHWEST RESEARCH
INSTITUTE/STEVE GRIBBEN

28 ASTRONOMY • JUNE 2019





W W W.ASTR ONOMY.COM 29

It could not have been set up any better. And its master architect,
planetary scientist Alan Stern, had that in mind all along. On
New Year’s Eve and New Year’s Day 2019, the famed New Horizons
spacecraft had a date. In 2015, this intrepid explorer swung past the
dwarf planet Pluto, giving us our first close-up view of that world
and its system of moons. Now, New Horizons would take on a second target
much farther out than Pluto, a distant Kuiper Belt object (KBO).

The New Horizons science team named this extravaganza was marked by several hundred
strange object Ultima Thule (pronounced TOO- scientists and journalists converging on the
lee), Latin for “beyond the known world.” campus of the Johns Hopkins Applied Physics
Laboratory in Baltimore, the mission’s
The flyby of Ultima Thule would mark a headquarters.
hugely significant event: the most distant human
exploration of a body in world history. And the I was invited to this unique New Year’s party
timing, coinciding with New Year’s Eve parties by the lead architect and party master, Stern
from Times Square and elsewhere, would capital- himself. He serves as the mission’s principal
ize on amazing publicity that would catapult investigator, and he will contribute a story in
planetary exploration into hundreds of millions an upcoming issue of Astronomy that will sum-
of living rooms across the globe. The whole marize the depth of the scientific findings about

30 ASTRONOMY • JUNE 2019

Ultima Thule. For now, my mission was
to soak in the historic moment and to
highlight the first results, the first images.
Accompanying me was Senior Editor
Rich Talcott, who was also busy with
coverage. And the party would have a
unique contribution from a friend: Brian
May, a New Horizons team member and
guitarist of the legendary band Queen,
was on hand to debut his new song, dedi-
cated to New Horizons.

The planning for this first-ever
encounter with a KBO extended back to
days after the Pluto flyby in 2015. With
New Horizons showing us that dynamic
little world, with its light-colored, heart-
shaped Tombaugh Regio, we were all
stunned by the close-up imagery of what
had been the last unexplored major body
of the solar system. (Yes, Pluto was

CLOCKWISE FROM TOP RIGHT: New Horizons The media auditorium at Johns Hopkins
team members (from left) Alan Stern, begins to fill with several hundred curious
Helene Winters, Frederic Pelletier, and journalists.
John Spencer brief the media on initial findings
as the spacecraft approaches its target. The auditorium at Johns Hopkins Applied
Physics Laboratory fires up for a busy few
ALL PHOTOS BY DAVID J. EICHER UNLESS NOTED days to come, on December 30, 2018. The
flyby of New Horizons past Ultima Thule
Near the moment of closest encounter, Stern, would occur on New Year’s Eve and New
the mission’s principal investigator, marks the Year‘s Day.
occasion with a group of science-interested kids.
NASA Science Missions Associate
As the spacecraft passes close by Ultima Thule, Administrator Thomas Zurbuchen (far
Stern and his family celebrate success. left) leads a science panel discussion
about exploring other small bodies with
On New Year’s Eve, New Horizons team member (from left) Olivier Barnouin, Hal Levison,
and Queen guitarist Brian May unveils his Lindy Elkins-Tanton, and Stern.
first solo song in 20 years, honoring the New
Horizons mission.

W W W.ASTR ONOMY.COM 31

demoted to dwarf planet status in 2006, Two become one
but the plans had long been underway to

visit this world, viewed as being on the Ultima Thule formed as numerous planetesimals in the infant solar system gently stuck
virtual edge of our solar system.) together (left). The smaller materials were eventually cast off (center), and the two principal
spheres gently merged by gravity (right), leaving the contact binary we see today. ASTRONOMY: ROEN KELLY

DRAWING A BEAD
ON THE KUIPER BELT

Between Pluto’s discovery in 1930 and the

spacecraft’s arrival in 2015, our knowl-

edge of the framework of the solar system

changed, regardless of whether you think

of Pluto as a planet or something less. In

1992, astronomers David Jewitt and Jane

Luu discovered the first KBO — 1992

QB1, nicknamed Albion — and we now A rotating cloud of small, icy Eventually, two larger bodies remain: Ultima and Thule slowly
know that a vast population of icy bod- bodies starts to coalesce. Ultima and Thule. spiral closer until they touch,
forming the bi-lobed object
ies exists beyond the orbit of Neptune, of we see today.

which Pluto appears to be the largest.

We’d also since learned about three

distinct populations of bodies in the solar inner solar system — be they planets, 30 AU to its outer edge at some 55 AU

system: the terrestrial planets, the gas and moons, asteroids, or infalling comets — from the Sun. Sometimes it is called the

ice giants, and the Kuiper Belt. The latter are all subject to major chemical changes Edgeworth-Kuiper Belt, as the great

probably holds tens or hundreds of thou- as they evolve because the Sun warms Dutch-American astronomer Gerard

sands of small, icy bodies beyond them and makes those reactions possible. Kuiper hypothesized it, and the

Neptune. That’s roughly 40 astronomical The Kuiper Belt, however, is in a deep independent Irish astronomer Kenneth

units (AU) away from the Sun, or 40 freeze. The temperatures of the icy bodies Edgeworth helped to confirm it. The

times the Sun-Earth distance — a long there — many of which have been there name is a bit of a misnomer: The Kuiper

way out. And yet the outer edge of the since they formed, planetary scientists Belt is more of a doughnut, or torus, than

Oort Cloud, comprising some 2 trillion believe — make ordinary chemical a belt. It is somewhat thick, with many

cometary nuclei, stretches some 50,000 reactions impossible. So these objects, objects extending 10° outside its plane,

AU away from the Sun. Exploration by including Ultima Thule, offer a way for and it is inclined relative to the ecliptic,

spacecraft of the outermost reaches of the scientists to look back in time to the pris- the plane made by Earth’s orbit around

solar system still has a long way to go. tine conditions of the early solar system. the Sun, by about 2°.

What makes exploring the Kuiper Belt Maybe we can even better understand its KBOs, perhaps vast in number, are

particularly exciting for planetary scien- very formation. leftover debris and icy chunks from the

tists is plain old chemistry. Bodies in the The Kuiper Belt stretches from about solar system’s early days. Their heritage is

made more complicated by the existence

Homing in on a distant rock of Neptune, whose presence throws the
orbits of many of the objects out of

whack, making them unstable. Within a

Raw Processed zone of about 42 to 48 AU from the Sun,

most of the objects can remain stable for a

long time. The planet does establish reso-

nances, at 2:3, or 1:2. That is, an object in

2:3 resonance with Neptune completes

two orbits around the Sun for every three

Neptunian orbits. An object in 1:2 reso-

nance completes an orbit once every two

Neptunian orbits. Objects in these

resonances can remain stable, without

Neptune kicking them outward into

unstable orbits.

Finding Ultima Thule in the New Horizons camera’s field of view was not easy, particularly with PLUTO: THE DOMINANT KBO
a spacecraft whizzing by at 32,000 mph (51,000 km/h). These images depict a raw and processed
field of view, showing Ultima Thule in the center. NASA/JHUAPL/SWRI The first known KBO was not actu-
ally 1992 QB1, but Pluto. Discovered in

32 ASTRONOMY • JUNE 2019

A closer look at Ultima Thule

2018 2019

Thule

21 miles 21 miles
(33 km) (33 km)

Ultima

WHAT MAKES A DIFFERENCE: On New Year’s TWO SPHERES: Ultima Thule was named after 3 miles
Day 2019, scientists received the first close-up a Latin phrase meaning “beyond the known
images of Ultima Thule — larger than a small world.” On January 1, New Horizons scientists OCCULATION PROFILE: When Ultima Thule
number of pixels — and believed, for a time, that attached the name Ultima to the larger sphere passed in front of various stars, momentarily
the Kuiper Belt object was shaped like a bowling and Thule to the smaller sphere. NASA/JHUAPL/SWRI blocking their light, astronomers believed the
pin. NASA/JHUAPL/SWRI object may be binary in nature. The profile based
on the occultation observations turned out to be
amazingly accurate. NASA/JHUAPL/SWRI

Think of New Horizons as a time machine that has brought us back
to the very beginning of the solar system, to a place where we can
observe the most primordial building blocks of the planets.

CLOSE APPROACH IMAGE (ORIGINAL): COLOR VARIATION: Overall, Ultima Thule appears dark and reddish because its
To appreciate the targeting challenge to image icy surface, which contains organic molecules, has been exposed to radiation for
Ultima Thule, this image shows the original billions of years. The surface does show variations in color, however. NASA/JHUAPL/SWRI
snapshot of the Kuiper Belt object as seen from
New Horizons at closest approach. NASA/JHUAPL/SWRI

0.12 Brighter SURFACE VARIATIONS:
regions The geology of Ultima
0.1 Thule is not yet well
known. The object’s
0.08 unusual features include
an overall mottled
0.06 appearance, brighter
and darker regions,
Remarkable the strange neck, no
0.04 “neck” obvious impact craters,
and possible hills and
0.02 ridges. NASA/JHUAPL/SWRI

0 Darker
regions

REFLECTIVITY VARIATIONS: Variations in reflectivity on the surfaces
of Ultima Thule suggest a range of ices, particle sizes, and brightnesses
on the Kuiper Belt object‘s surface. The “neck” connecting the two spheres
is particularly bright, and scientists believe it holds highly reflective, fine-
grained particles that have slumped downward by gravity. NASA/JHUAPL/SWRI

W W W.ASTR ONOMY.COM 33

Ultima Thule’s color 50
40
Eris

30

Ultima Thule

Makemake Pluto Redness 20

Haumea

Kuiper Belt Ultima Thule (2014 MU69) 10
Flyby January 1, 2019 0

Cold classical

Other KBOs

Ultima Thule, which isn’t the only notable Kuiper Belt object, has a -10 5° 10° 15° 20° 25° 30° 35°
moderately high degree of redness that fits in well with the other so-called 0°
cold classical KBOs in relatively normal, low-tilt orbits. NASA/JHUAPL/SWRI
Orbital tilt

Sharp curves Rotation period = 15 hours large. But it’s not the only significant
KBO known. Dwarf planet Haumea
The surfaces of Ultima Thule show some 5° 10° 15° 20° 25° 30° spans 1,000 miles (1,600 km), dwarf
Local slope planet Makemake measures 890 miles
fairly extraordinary slopes, especially in areas (1,430 km) across, and 50000 Quaoar is
sparser population of icy asteroid-like more than 750 miles (1,200 km) across.
near the neck, the joint between the two bodies, in the so-called Scattered Disk.
These objects have been disrupted by The Kuiper Belt also has several popu-
bodies. Here, scientists believe, fine-grained Neptune or other bodies, and they have lations. The main one, the so-called clas-
high inclinations and eccentricities — sical population, holds about two-thirds
material has slumped down to fill the neck, 0° weird orbits that have them careening up of the objects. It contains both cold and
and it is more reflective. NASA/JHUAPL/SWRI and down at odd angles relative to the hot objects, referring not to temperatures
plane of the major solar system planets. — they’re all cold — but to orbits. The
1930 and labeled a planet, Pluto was cold classicals have nearly circular orbits
the result of the long search at Lowell Within the last generation, whether or with small eccentricities. The hot popula-
Observatory for an outer planet. Believed not you consider it a planet, Pluto has tion is characterized by much more
to be both more massive and more distant come to be recognized as the largest KBO. inclined orbits.
than Pluto turned out to be, the hypothet- With a diameter of 1,477 miles
ical planet ultimately didn’t exist — but (2,370 kilometers), Pluto is the largest Resonances also play a role in defining
the search did uncover Pluto. body in the Kuiper Belt, and it has five the objects. The 2:3 resonant group con-
satellites, one of which, Charon, is quite sists of at least 200 objects, including
By the 1990s, it was becoming clear Pluto. This gives the group the nickname
that a large population of icy objects “Plutinos.” The 1:2 resonant group is
existed in the realm beyond Neptune. more sparsely populated and is nick-
Scientists call these bodies trans- named “twotinos.”
Neptunian objects (TNOs), and many of
them, they came to find out, exist in the PARTYING WITH ULTIMA THULE
Kuiper Belt.
We arrived in Baltimore to celebrate a
Beyond the Kuiper Belt lies another, suddenly famous member of the cold
classical Kuiper Belt family, Ultima
Thule. Originally designated 2014
MU69, the object being approached by
New Horizons remained mysterious until

34 ASTRONOMY • JUNE 2019

the final days and hours of the encoun- too many to mention — but they ABOVE: When New Horizons swung past
ter. Near year’s end, the New Horizons included major mission participants like Ultima Thule on New Year’s Eve, it explored
team believed Ultima Thule to be about Cathy Olkin, John Spencer, Hal Weaver, the most distant object ever visited by
20 miles (30 km) across and quite elon- Joel Parker, Alice Bowman, Frederic human technology. ASTRONOMY: ROEN KELLY
gated. Months earlier, Earth-based Pelletier, Marc Buie, Will Grundy, Jeff
observations of Ultima Thule occulting Moore, Leslie Young, Kelsi Singer, Kirby LEFT: Graduate student and New Horizons
(passing in front of) stars led the team Runyon, Bill McKinnon, Alex Parker, and team member Mallory Kinczyk holds a
to suspect a binary shape. Andy Cheng. model of Ultima Thule she created with
Styrofoam, a wooden dowel, and clay.
The prediction turned out to be right: When it came time for the New Year’s She displayed it at a press conference just
On the first data release, we got an image Eve celebration itself, we moved to a a few hours after the release of detailed
of an object that resembled a bowling large, open area with a beautiful stage. data on the object’s shape.
pin, but with few pixels to support the Champagne in hand, we counted down to
certainty. The next day, when the the closest approach of the spacecraft to Ultima Thule, knowing that history was
highest-resolution images were released about to be made, although we wouldn’t
in early January, the object clearly Ultima Thule have the imagery and other data in hand
appeared to be a binary, giving rise to appears as a until the next morning. (The telemetry
the name “Cosmic Snowman.” In fact, floating cosmic time from the Kuiper Belt back to Earth
the team determined Ultima Thule to snowman in this was about six hours.)
be a contact binary, two objects that stereo image
gravitationally migrated inward and created by Paul To celebrate, May took to the stage and
stuck together. Thus, we had the first Schenck and the introduced the video he created, his first
close-up observations of a contact binary New Horizons solo release song in two decades, “New
in history. The team, in a tribute to sim- Imaging Team. Horizons (Ultima Thule Mix).” The
plicity, named the larger sphere Ultima Relaxing one’s crowd, stunned and amazed, loved it,
and the smaller one Thule. eyes allows and we had one tremendous party going.
the images to Then Stern mounted the stage and,
The atmosphere in the press audito- merge, revealing accompanied by a large group of
rium at Johns Hopkins was electric, as it a 3D picture. schoolkids, celebrated the spirit of
has been for many other key moments in discovery at this critical time.
space exploration. Perhaps 200 people or NASA/JHUAPL/SWRI/NOAO
so — media, scientists, family, and friends And then we crashed, getting up early
— packed the room, and at important for exciting and important press confer-
announcements of success, applause and ences — and those magical pictures of the
sometimes screams of joy erupted. Many Cosmic Snowman. The early data releases
press conferences and talks entertained tell the tale of an alien world — the most
the attendees, and Stern took center stage distant object ever visited by human tech-
much of the time, communicating the nology. We will see this story repeated,
latest news and explaining a vast amount even likely by this spacecraft, as it ven-
of knowledge for the media. Others tures deeper into the abyss.
featured prominently, too — and far
David J. Eicher is the editor of Astronomy
In stereo and a longtime fan of Pluto and other KBOs,
having enjoyed knowing Clyde Tombaugh
in his later life.

W W W.ASTR ONOMY.COM 35

SKYTHIS MARTIN RATCLIFFE and ALISTER LING describe the Visible to the naked eye
MONTH solar system’s changing landscape as it appears in Earth’s sky. Visible with binoculars
Visible with a telescope

June 2019: Jupiter dazzles all night

Small telescopes reveal details in Jupiter’s cloud bands when the gas giant more prominent. This is earth- merely a line-of-sight effect.
looms large, as it does throughout June. In this Hubble Space Telescope shine — sunlight that bounces Mars currently lies on the far
image, the Great Red Spot looks like a bloodshot eye with the shadow of off Earth, hits the Moon, and side of the Sun from Earth
Ganymede representing its pupil. NASA/ESA/A. SIMON (GSFC) reflects back to our eyes. The while Mercury is on the near
following evening, a slightly side. The view through a tele-
Although June nights may in the west-northwest a half- fatter crescent Moon appears scope confirms this: Even
be short, you can pack hour after sunset. It stands 6° to Mars’ upper left. though Mars is physically
a lot of planet view- out quite nicely in the twilight 40 percent larger than
ing into those limited glow. Four other objects join Over the next two weeks, Mercury, it appears only 3.7"
hours. Most observ- the inner planet. Capella, at Mercury climbs higher and across — half as big as the
ers will spend the bulk of magnitude 0.1 the brightest Mars drops lower. The solar inner planet’s 7.4" diameter. A
their time with Jupiter, which star in Auriga, stands 21° to system’s smallest planets are telescope also shows the con-
reaches peak visibility and is Mercury’s upper right. Mars destined for a dramatic meet- trast between ruddy Mars’
up all night. But you’ll also lies 16° to Mercury’s upper ing just after midmonth. On nearly full disk and whiter
want to watch Mercury and left and at the same altitude June 17, the two appear side by Mercury’s half-lit phase.
Mars as they have their closest as Capella. Glowing at magni- side with 28' — just less than
evening conjunction in more tude 1.8, the Red Planet may the Full Moon’s diameter — Following this close con-
than a decade. Be sure to turn be hard to see in twilight between them. (This is the junction, Mercury continues
your attention toward Saturn without binoculars. Finally, separation observers in central to edge away from the Sun. It
and its magnificent rings as the twin stars Castor and North America will see; the reaches greatest elongation
midnight approaches, then Pollux in Gemini appear 13° planets appear slightly farther June 23, when it lies 25° east
wrap up a memorable night directly above the slightly apart from the eastern part of of our star and appears 11°
with a view of Venus in morn- fainter Mars. the continent and a bit closer high a half-hour after sunset.
ing twilight. from farther west.) It then starts to sink closer
A two-day-old crescent to the horizon, ending the
But your first order of Moon joins the party June 4. On the evening of the 18th, month 3.8° to Mars’ left.
business these June nights Luna lies 6° to Mercury’s left, the two stand just 18' apart
should be to track down and both stand some 9° high — the closest they have been to As Mercury and Mars set
Mercury and Mars. On the 30 minutes after sundown. As each other in the evening sky in the northwest, Jupiter
1st, Mercury shines at magni- the sky darkens, watch the in 13 years. Mercury now lies rises in the southeast. The
tude –1.0 and appears 6° high unlit side of the Moon grow above Mars and shines at mag- solar system’s largest planet
nitude 0.1, some five times comes to opposition June 10,
36 ASTRONOMY • JUNE 2019 brighter than its companion. when it lies opposite the Sun
This close conjunction is in our sky and remains visible

Jupiter at its finest

OPHIUCHUS

LIBRA

Jupiter

Antares

SAGIT TARIUS LUPUS
Saturn
SCORPIUS

10°

June 10, 11 P.M.
Looking south-southeast

The giant planet peaks at opposition June 10, but it reigns supreme against
the backdrop of Ophiuchus all month. ALL ILLUSTRATIONS: ASTRONOMY: RICK JOHNSON

RISINGMOON

A terrific trio of conspicuous craters Ptolemaeus, Alphonsus, and Arzachel

Three magnificent large craters ancient craters hidden under a Ptolemaeus
beckon observers the evening blanket of debris.
of June 10, when the Moon is Alphonsus
less than a day past First Quarter The smaller impact that cre-
phase and appears slightly more ated Alphonsus immediately to Arzachel
than half-lit. The northernmost the south of Ptolemaeus pro-
and largest of the three craters, duces more distinct features and N
Ptolemaeus, lies just south of a higher central peak. Return to
the lunar equator. It sports a this feature every hour or so and E
rugged rim that casts long shad- note how quickly the spire’s
ows onto the relatively smooth shadow retreats under the rising The First Quarter Moon boasts three large and spectacular craters just
floor. A small crater northeast of Sun. The unusual ridge that south of the lunar equator. CONSOLIDATED LUNAR ATLAS/UA/LPL; INSET: NASA/GSFC/ASU
Ptolemaeus’ center affords a bisects Alphonsus lines up with
good test of your telescope’s other linear features that point gray spots on Alphonsus’ floor. studies show the composition
optics and seeing conditions. back toward Imbrium. These are deposits of ash of the ash is similar to that of
gently sprayed out during vol- the lava that welled up to flood
Like most large impact fea- The youngest of the three canic eruptions. Spectroscopic the large lunar maria.
tures, Ptolemaeus probably pos- craters is Arzachel to the south.
sesses a complex central peak Its rim and inner walls appear
— but you won’t see it no mat- sharper than those of its north-
ter how hard you look. Lunar sci- ern neighbors, which suffered
entists suspect that it lies buried many more millennia of impacts.
under deposits sprayed out dur-
ing the excavation of giant Mare Don’t hesitate to return
Imbrium to the northwest. You to this region on subsequent
might also glimpse a few subtle nights. Although the higher
depressions, telltale signs of Sun masks topographic relief,
it reveals surface composition.
Notice the handful of darker

all night. Opposition brings METEORWATCH
Jupiter closest to Earth, so it
shines at its brightest for the Early summer’s Pearly noctilucent clouds
year: magnitude –2.6. It brings twilight clouds
a touch of brilliance to the
otherwise faint constellation Despite the lack of major meteor June’s extended twilight offers ideal conditions for northern skygazers
Ophiuchus the Serpent-bearer. showers in June, sporadic meteors to see these highly reflective, high-altitude clouds. NASA
continue to light up the sky. These
Jupiter’s proximity also random flashes arise when tiny condense on dust particles. Search for them in twilight an
makes it appear larger than grains of dust slam into Earth’s They typically appear in early hour or two after sunset, when
at any other time in 2019. Its atmosphere and friction with air summer from latitudes our star still illuminates these
46"-diameter disk should dis- molecules incinerates them. Under between 50° and 60° north. high-altitude clouds.
play a wealth of detail through a dark sky, observers typically see
telescopes of all sizes. A paral- a half-dozen or so of these random
lel pair of relatively dark belts, meteors per hour.
one on either side of a brighter
zone that coincides with the Similarly fine meteoritic dust
planet’s equator, dominates plays a role in producing summer’s
the view. In moments of gorgeous noctilucent clouds.
steady seeing, look for a whole These silver-blue clouds form
series of alternating belts and about 50 miles up (10 times higher
zones as well as the dusky than cirrus), where Earth’s atmo-
north and south polar regions. sphere is coldest, when ice crystals
You also might notice several
dark and white spots lurking OBSERVING Jupiter reaches its 2019 peak June 10, when the gas giant planet
near the boundaries between HIGHLIGHT shines at magnitude –2.6 and spans 46" through a telescope.
the belts and zones. The Great

— Continued on page 42

W W W.ASTR ONOMY.COM 37

STAR N
DOME
b ¡ C A M E L OPA R DA L I S
How to use this map: This map portrays the
sky as seen near 35° north latitude. Located ` A PE S S A C _
inside the border are the cardinal directions _ `
and their intermediate points. To find d a O I
stars, hold the map overhead and
orient it so one of the labels matches I

NEthe direction you’re facing. The k af

stars above the map’s horizon L A C E RTA ` CEPHEUS c NCP
now match what’s in the sky. b _
_ M81
The all-sky map shows Polaris M82
how the sky looks at:
midnight June 1 `+
11 P.M. June 15
10 P.M. June 30 MINOR _
Planets are shown
at midmonth URSA d

Deneb DRACO _
_
PEGASUS b a
`

c c

¡ M27 a b CYGNUS ` Mizar ¡
a dr _ ab
i
a d cf

M15 Vega M51 d
¡ _ LYRA
` DE SAGIT TA ` a M57 ` d

EQUULEUS _ V b COMA
a`
_ L d BERENICES _

P UL /b

H _ HERCULES M13 c CBOORROENAAL I S
`
I P BOÖTES

E N E

U C

S U

L +

a A
_
¡
_
` Altair c

e CAPRICORNUS b SE _ _g ` Arcturus _
C `
_` d A Q U I L A M11
R
A Q UA R I U S A SERPENS
CAPUT
PUEDNAS

STAR h OPHIUCHUS _ c
MAGNITUDES M5
d 20 /
c ` e CE
Sirius i b
M16 ¡ i
0.0 _ +
c
1.0 3.0 SCUTUM
4.0
d a LIBRA _
2.0 5.0 M17

Saturn / mc + M20 Jupiter `
o
STAR COLORS M22 M8 e Antares m b
 h b /
A star’s color depends o _ M4 m
on its surface temperature.
SE S A a M6 ¡ SCORPIUS
• The hottest stars shine blue M7
• Slightly cooler stars appear white G I T
• Intermediate stars (like the Sun) glow yellow
• Lower-temperature stars appear orange TA R ¡ h
• The coolest stars glow red p
• Fainter stars can’t excite our eyes’ color IU S d + LUPUS
g c NGC 6231
receptors, so they appear white unless you f e d a g
use optical aid to gather more light `
d ¡ d

NORMA _

a

38 ASTRONOMY • JUNE 2019 c

S

JUNE 2019 Note: Moon phases in the calendar vary
in size due to the distance from Earth
and are shown at 0h Universal Time.

MAP SYMBOLS SUN. MON. TUES. WED. THURS. FRI. SAT.

k Open cluster 1
Globular cluster
LYNX Diffuse nebula 234 5678
Planetary nebula 9 10 11
NW Galaxy

f 12 13 14 15

MAJOR e 16 17 18 19 20 21 22 ILLUSTRATIONS BY ASTRONOMY: ROEN KELLY

_

URSA ` +h L EM ION O R 23 24 25 26 27 28 29
`

s ¡ 30
d
CANES VENATIC i c _ Regulus Calendar of events

LEOa 1 The Moon passes 3° south of SPECIAL OBSERVING DATE
Venus, 2 P.M. EDT
18 Mercury passes 0.2° north
M66 e b of Mars in evening twilight.
M65
` Denebola 2 Asteroid Pallas is stationary,

_ W 10 P.M. EDT 18 The Moon passes 0.4° south of
M64 NGP 3 New Moon occurs at Saturn, midnight EDT

SEXTANS _ 6:02 A.M. EDT 19 The Moon passes 0.07° south

4 The Moon passes 4° south of of Pluto, 7 A.M. EDT

Mercury, noon EDT 21 Mercury passes 6° south of

VIRGO b ¡ 5 The Moon passes 1.6° south of Pollux, 1 A.M. EDT

`

c Path of the Sun (ecliptic) Mars, 11 A.M. EDT Summer solstice occurs at
_ Spica M104
7 The Moon is at perigee 11:54 A.M. EDT

a

(228,978 miles from Earth), Neptune is stationary,

7:15 P.M. EDT midnight EDT

CRATER b i 10 First Quarter Moon 23 Mars passes 6° south of Pollux,

occurs at 1:59 A.M. EDT 3 A.M. EDT

S

U

RV a Jupiter is at opposition, The Moon is at apogee (251,375
¡ 11 A.M. EDT miles from Earth), 3:50 A.M. EDT
b

O

C

` 15 The Moon passes 0.9° north of Mercury is at greatest eastern
dwarf planet Ceres, 11 A.M. EDT elongation (25°), 7 P.M. EDT

a _

HYDRA 16 The Moon passes 2° north of The Moon passes 4° south of
Jupiter, 3 P.M. EDT Neptune, 9 P.M. EDT

M83 SW 17 Full Moon occurs at 25 Last Quarter Moon

US 4:31 A.M. EDT occurs at 5:46 A.M. EDT

TAU R f

E N Venus passes 5° north of 27 The Moon passes 5° south of
Aldebaran, 5 P.M. EDT Uranus, 6 P.M. EDT
NGC 5128
i
+

NGC 5139

BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart.

W W W.ASTR ONOMY.COM 39

PATHOF THE
PLANETS The planets in June 2019

LYN CAS DRA
Objects visible before dawn
AUR

AND LAC

PER CYG HER

Venus TRI LYR CrB B
TAU ARI
Sun
ORI PEG VUL
Neptune
Uranus PSC SCL SGE

SER

EQU AQL SER
Celestial equator

Eunomia SCT OPH
AQR
Parthenope
CET Melpomene Ceres
PHE
ERI Saturn of the Moon LIB
FOR Path
CAP
LEP Pluto

PsA MIC SGR Jupiter appears at its
best for the year in June

COL CAE GRU CrA SCO LU

Moon phases Dawn Midnight

5 4 3 21
30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15

The planets These illustrations show the size, phase, and orientation of each planet and the two brightest dwarf planets at 0h UT
for the dates in the data table at bottom. South is at the top to match the view through a telescope.
in the sky

Mercury Uranus

S Mars Saturn Pluto
WE Ceres
Venus Neptune
N
Jupiter
10"

Planets MERCURY VENUS MARS CERES JUPITER SATURN URANUS NEPTUNE PLUTO
Date June 15 June 15 June 15 June 15 June 15 June 15 June 15
Magnitude –0.1 –3.8 1.8 June 15 June 15 0.2 5.9 7.9 14.2
Angular size 6.8" 10.2" 3.8" 18.2" 3.4" 2.3" 0.1"
Illumination 56% 96% 98% 7.4 –2.6 100% 100% 100% 100%
Distance (AU) from Earth 0.983 1.638 2.497 9.124 20.503 29.832 32.927
Distance (AU) from Sun 0.401 0.723 1.641 0.7" 46.0" 10.051 19.843 29.936 33.819
Right ascension (2000.0) 7h12.1m 4h21.1m 7h22.7m 19h20.7m 2h11.7m 23h19.1m 19h36.6m
Declination (2000.0) 24°13' 20°40' 23°16' 100% 100% –21°47' 12°44' –5°30' –21°58'

1.801 4.285

2.777 5.298

16h08.0m 17h10.9m

–18°07' –22°23'

40 ASTRONOMY • JUNE 2019

This map unfolds the entire night sky from sunset (at right) until sunrise (at left).
Arrows and colored dots show motions and locations of solar system objects during the month.

UMa Objects visible in the evening Jupiter’s moons

LYN AUR Dots display positions

C Vn of Galilean satellites at Io
Europa
COM LMi Mercury appears bright in 11 P.M. EDT on the date
Pallas GEM evening twilight during June Ganymede
BOÖ shown. South is at the Callisto
Mars
top to match
CNC the view S

Sun through a W E

LEO (ecliptic) telescope. N
the Sun
ORI

Path of CMi

VIR SEX MON 1
H YA 2
CRV CRT 3
4
UP CMa 5
CEN 6
ANT LEP ERI 7 Io
COL 8 Ganymede
PYX PUP 9 Callisto
VEL CAE 10
11
Early evening 12
13
To locate the Moon in the sky, draw a line from the phase shown for the day straight up to the curved blue line. 14
Note: Moons vary in size due to the distance from Earth and are shown at 0h Universal Time. 15 Europa
16 Jupiter
14 13 12 11 10 9 8 7 6 5 43 17
18
Mars 19
20
Venus Mercury 21
Greatest eastern 22
elongation is June 23 23
24
Earth 25
Summer solstice 26
in June 21 27
28
Ceres 29
30
Jupiter
Opposition

is June 10

The planets Uranus ILLUSTRATIONS BY ASTRONOMY: ROEN KELLY
in their orbits
Jupiter Neptune
Arrows show the inner planets’ Pluto
monthly motions and dots depict Saturn
the outer planets’ positions at mid-
month from high above their orbits.

W W W.ASTR ONOMY.COM 41

— Continued from page 37

WHEN TO VIEW THE PLANETS Hunt down Jupiter’s four bright satellites

S

Callisto Jupiter

EVENING SKY MIDNIGHT MORNING SKY Io Ganymede
Mercury (northwest) Jupiter (south) Venus (northeast) W Europa
Saturn (southeast) Jupiter (southwest)
Mars (northwest) Saturn (southwest)
Jupiter (southeast) June 11, 4:00 A.M. EDT
Uranus (east)
Neptune (southeast) 1'

The giant world’s moons show up nicely through small scopes, but they can
be tricky to identify. Their arrangement at opposition makes the task easy.

Red Spot appears about half the planet’s face or lies near minutes, you should notice its Saturn lies in northern
the time — whenever the plan- the limb. shadow immediately to the Sagittarius, just south of that
et’s 10-hour rotation carries it moon’s east, with the two over- constellation’s Teaspoon
onto Jupiter’s Earth-facing The night of opposition lapping. Ganymede lies north asterism. It shines at magni-
hemisphere. offers a good opportunity to of Io and begins to transit at tude 0.2 in mid-June and
acquaint yourself with these 11:28 p.m. Its shadow falls on appears four times brighter
The sharpest views of the so-called Galilean moons. If the cloud tops just southeast of than any of the Archer’s stars.
giant world come when it lies you look during the morning the moon. Because Ganymede
highest in the south. Jupiter hours of June 11, you’ll see Io, lies farther from Jupiter than The best time to view
peaks at an altitude of about Europa, and Ganymede lined Io, its shadow appears slightly Saturn through a telescope
30° at 1 a.m. local daylight up to Jupiter’s east while more separated from the occurs during the early morn-
time the night of opposition. It Callisto stands alone to the moon. Io completes its transit ing hours when it climbs
reaches the same benchmark planet’s west. at 12:33 a.m., when Ganymede highest in the south. Even the
about a half-hour earlier with is about halfway across the smallest telescope delivers
each passing week. Still, the The month’s most dra- giant planet’s disk. The outer stunning views. The planet’s
planet lies nearly as high and matic scene occurs the follow- moon completes its trek disk measures 18" across
the views are almost as good ing night, however. Both Io around 1:40 a.m. while the rings span 41" and
for a couple of hours on either and Ganymede start to cross tilt 24° to our line of sight.
side of these times. Jupiter’s disk the evening of Saturn lies 30° east of Saturn’s disk shows little
June 11. But with the gas giant Jupiter, which means it trails detail, though you might spot
Jupiter’s four brightest so close to opposition, the about two hours behind its an equatorial belt and a dark
moons create a scene that shadow that each satellite bigger brother. The ringed polar hood.
changes dramatically from casts falls onto the jovian planet rises around 11 p.m.
night to night. Their motions cloud tops almost directly local daylight time June 1 and Although Saturn’s moons
sometimes appear noticeable beneath the moon. some two hours earlier by don’t glow as brightly as
even within minutes, particu- month’s end. Jupiter’s Galilean satellites,
larly when a satellite transits Io’s transit begins at small scopes reveal at least
10:22 p.m. EDT. Within five

COMETSEARCH

A killer takes aim at the Whale Comet ASASSN (C/2018 N2)

Comet observers often hope a hour before morning twilight N j
new discovery will herald a commences. Look for the faint 30
bright target to feed their pas- smudge of light as it glides
sion. This does happen, though northward between magnitude 27
not often. Two arrivals bright- 4.9 Nu (ν) Ceti and magnitude Path of Comet ASASSN
ened to 9th magnitude in late 4.3 Xi2 (ξ2) Cet. Astronomers dis-
2018, but unless we get a similar covered this comet in July 2018, 24 CETUS
surprise this month, we’ll have to and it appears destined to peak
be satisfied with a comet that at 11th magnitude this autumn. E
reaches only 12th magnitude. 21
A harder target lies on the
To catch Comet ASASSN border of Centaurus and Lupus i 18
(C/2018 N2), wait for the Moon- in early June. Comet ATLAS
free period at the end of June. (C/2017 M4) passes between June 15
ASASSN — short for the All- magnitude 2.7 Beta (β) Lupi and
Sky Automated Survey for magnitude 3.1 Kappa (κ) Centauri 0.5°
Supernovae program — then during June’s first week. You’ll
resides in northeastern Cetus need a 12-inch or larger scope to Reserve some time on June mornings to track down this faint comet as it
the Whale and rises about an spot this 13th-magnitude object. slides to the northeast against the background stars of Cetus.

42 ASTRONOMY • JUNE 2019

Mercury tangos with Mars LOCATINGASTEROIDS
Tangled up in Berenice’s tresses
Pollux Castor

Mars Mercury Asteroid 2 Pallas glows at 9th and star-hop to Pallas’ position
magnitude in June, making it a with the help of the chart below.

nice target for asteroid seekers. If you want to see the aster-

GEMINI Visible with some effort through oid move in a single evening,

binoculars, it’s a cinch to find June 12 and 26 offer the best

5° with the extra light-gathering chances. Both nights, Pallas

power of a small telescope. skims near a similarly bright star

June 17, 45 minutes after sunset Pallas lies high in the south that helps you gauge its motion.
Looking west-northwest after darkness falls. To find it, When Heinrich Olbers dis-
first locate Arcturus, the magni-
covered Pallas in 1802, astrono-

These two planets slide within 0.3° of each other June 18. They have not tude –0.04 orange giant in mers thought it might be the
passed this close in the evening sky since 2006. Boötes that ranks as the night second “missing planet” in what
sky’s fourth-brightest star. Next, seemed like an abnormally large

four of them. Titan is the easi- the time twilight starts to move 6.4° west-northwest to gap between Mars and Jupiter.
est. It shines at 8th magnitude paint the sky. The planet 5th-magnitude 6 Boötis, then Scientists later figured out that
and shows up through any shines at magnitude 5.8 another 2.4° northwest to 6th- the gravity of massive Jupiter
instrument. This large moon and shows up quite easily magnitude 2 Boo. From there, prevented any major planet
orbits Saturn in 16 days, pass- through binoculars. cross the border into eastern from forming at that distance
ing south of the ringed world Coma Berenices, Berenice’s Hair, from the Sun.
The hardest part of finding

the mornings of June 5 and 21 Uranus is zeroing in on the Pallas continues its nice run
and north of the planet on the right star field. The world lies

13th and 29th. Look for three in southern Aries, about 10° N
10th-magnitude moons — south of the Ram’s brightest

Tethys, Dione, and Rhea — star, magnitude 2.0 Hamal

closer to Saturn. (Alpha [α] Arietis). This is a

Scan 60° east-northeast of sparse area of sky, however. June 1
Saturn and you’ll arrive at First locate 6th-magnitude 6
Neptune. The solar system’s 19 Ari, which lies 8° south of BOÖTES Path of Pallas
most distant major planet rises Hamal and shows up to the E
shortly after 1 a.m. local day- naked eye from under a dark 11
light time June 15 and climbs sky. Center 19 Ari in your
16

2 21

25° above the southeastern binoculars and you’ll see 26 C OMA BERENICES
horizon by the time twilight Uranus 2.4° to its south.

begins. Neptune glows at mag- To confirm your planet sight-

nitude 7.9, so you’ll need bin- ing, swing a telescope in its 6 July 1

oculars or a telescope to see it. direction. Uranus shows a 1°
The outer world resides in 3.5"-diameter disk with a dis-

northeastern Aquarius, in the tinctive blue-green color. This 9th-magnitude object rides high in the south on June evenings as
same binocular field as magni- Morning twilight is well it traverses the sparse star fields of eastern Coma Berenices.
tude 4.2 Phi (ϕ) Aquarii. It
underway before our final

begins June 1.2° east-northeast planet appears. Venus rises

of this star and crawls 0.1° far- an hour before the Sun on

ther away by the latter half of June 1 in the company of a stands just 3° high a half-hour when the Sun lies farthest

the month. This places the slender crescent Moon. The before sunup June 30, when north in our sky, and is the

planet within 0.4° — slightly two stand about 6° apart and you’ll need a haze-free sky and reason why the days are so

less than the Full Moon’s a similar distance above the an unobstructed horizon to see long and the nights so short in

diameter — south of the mag- eastern horizon a half-hour it against the twilight glow. the Northern Hemisphere.

nitude 5.6 star 96 Aqr. When before sunrise. Although The inner planet is heading

viewed through a telescope, Venus shines brilliantly at toward its mid-August supe- Martin Ratcliffe provides plane-

Neptune displays a blue-gray magnitude –3.8, the Sun’s rior conjunction, and will dis- tarium development for Sky-Skan,

disk that appears 2.3" across. glare drowns it out within appear from view in early July. Inc., from his home in Wichita,

Uranus slowly emerges into the next 15 minutes. Earth reaches its summer Kansas. Alister Ling, who lives in

a dark sky by the end of June. Venus slowly sinks lower solstice at 11:54 a.m. EDT on Edmonton, Alberta, has watched

On the 30th, it rises around as the month progresses. It June 21. This marks the instant the skies since 1975.

2 a.m. local daylight time and GET DAILY UPDATES ON YOUR NIGHT SKY AT www.Astronomy.com/skythisweek.
climbs 15° high in the east by

W W W.ASTR ONOMY.COM 43

Objects briefly seen on television over 50 years ago mak

by Michael E. Bakich M67

or myself and other “more sea- Our first target is the “other” open cluster in the
soned” readers of Astronomy constellation Cancer — in other words, not the more
magazine, 1963 doesn’t seem famous Beehive Cluster (M44). You’ll easily find M67
all that long ago. The space race through binoculars or a small telescope 1.7° due west
with the USSR (not Russia yet) of magnitude 4.3 Alpha (α) Cancri. At magnitude 6.9,
was in full swing, and NASA M67 glows just past the limit of the human eye.
had to accomplish a Moon land-
ing to fulfill a national goal set Through a 4-inch telescope, you’ll resolve roughly
by President John F. Kennedy. It two dozen stars in M67 across an area two-thirds the
seemed everyone was talking about width of the Full Moon. Increase the aperture to
space, rockets, and astronauts. And 6 inches, and 50 stars will shine
not all the talk was strictly about science; interest in science
fiction was also on the rise. M67
In this environment, a new television program debuted
Monday, September 16. Each episode began with a brief scene RICHARD MCCOY
followed by a “Control Voice” speaking the words, “There is
nothing wrong with your television set. Do not attempt to forth. The official listed
adjust the picture. … You are about to experience the awe and diameter of M67 is 29', which is almost identical
mystery, which reaches from the inner mind to [dramatic to the Full Moon.
pause] the Outer Limits.”
A total of 49 black-and-white episodes of The Outer Limits A dozen of M67’s stars shine brighter than 11th
aired over two seasons. But although the show ran only for a magnitude. When you view the cluster through a
few years, it forever changed the face of televised science fic- telescope, you’ll undoubtedly notice a yellow star on
tion. The first episode, “The Galaxy Being,” addressed the pos- its northeastern edge. Identified as SAO 98178, this
sible danger involved in contacting an extraterrestrial. By the star shines at magnitude 7.8 but is not a member of
end of the hour, I was hooked! The last thing that caught my the cluster.
eye, however, even at the tender age of 10, was the sequence
that concluded each episode: As the credits rolled, eight
deep-sky objects appeared in the background.
The members of this group — one open cluster and
seven galaxies — are all spectacular targets, especially
for beginning observers with small- to medium-sized
telescopes who have access to a dark observing site.
In order of their appearance during the credits,
they are M67, NGC 891, M104, M101, NGC 5128,
NGC 1300, M81, and M31. Let’s take a detailed
look at each of these celestial jewels so that,
on the next clear moonless night, you can
begin your journey to [dramatic pause]
the outer limits.

e terrific targets for today’s amateur astronomers.

TV SET: CHEPKO ELELNA/DREAMSTIME

W W W.ASTR ONOMY.COM 45

KEN CRAWFORD

NGC 891

NGC 891 M104 R. JAY GABANY

You know, second place is fine if feature to spot, so it’s definitely worth It’s easy to see why the creators of The
people remember you. NGC 891 is the waiting for the seeing (atmospheric steadi- Outer Limits selected this object for the
second-best galaxy in the constellation ness) to improve. Additionally, dozens credits sequence. The Sombrero Galaxy is
Andromeda. Not bad, except the constel- of foreground stars populate the field, undoubtedly one of the finest objects you
lation’s top dog happens to be one of the which adds the third dimension of depth can see through a small telescope.
sky’s supreme wonders: the Andromeda to the view.
Galaxy (M31, the last entry on this list). But permit me a bit of science. M104
At magnifications above 200x, note the was the first galaxy astronomers discovered
Despite often being overshadowed, sections of NGC 891’s nucleus on each side with a large redshift, which refers to the
NGC 891 ranks as one of the sky’s best of the dust lane. The western section glows magnitude of the galaxy’s motion away
edge-on spiral galaxies. It inclines only
1.4° to our line of sight. Its more than slightly brighter. Likewise, the galaxy’s disk from us caused by the uni-
4-to-1 length-to-width ratio (13.0' by 2.8') to the southwest outshines its lesser half, verse’s expansion. In 1912,
and magnitude 9.9 brightness easily which lies to the northeast. American astronomer
earned it the nickname the “Silver Sliver.” Vesto M. Slipher discovered
But it has another common name as well, M104 the Sombrero Galaxy was
given to it sometime within the past half- moving away from us at a
century: the Outer Limits Galaxy. Deep within Virgo sits a showpiece spi- speed of 2.2 million mph
ral galaxy guaranteed to delight amateur (3.6 million km/h).
To find NGC 891, locate magnitude 2.2 astronomers and the general public alike. At magnitude 8.0, M104 is a bright
Almach (Gamma [γ] Andromedae) and spiral galaxy. And that brightness is
move 3½° due east. I’ve really enjoyed the confined to a relatively small area measur-
views I’ve had of this object through small ing only 7.1' by 4.4'. Because it is both bright
scopes. True, not much detail is visible, but and small, the galaxy’s surface brightness is
relatively high, which makes it easy to see.
you’ll immediately see why this object To find it, point your telescope 5½° north-
earns the “sliver” descriptor. northeast of magnitude 2.9 Algorab

You’ll do better if you can move up to
a 10-inch telescope, which reveals a sym-
metrical object about 10' long with a
noticeable but narrow central bulge. A
dark dust lane bisects the galaxy and runs
nearly its entire length. This is a really cool

46 ASTRONOMY • JUNE 2019

M101

ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA

(Delta [δ] Corvi), or 11° west of Spica 10 percent of all spiral
(Alpha Virginis).
galaxies fall into the
The Sombrero Galaxy’s lens shape and
the dark dust lane that splits it are easy to grand design
spot. What’s more, the galaxy’s two sec-
tions have unequal brightnesses — the category.
north outshines the south because M104
inclines 6° to our line of sight. The dust This wonderful
lane, therefore, appears to cross a bit south
of center from our perspective. object lies in the

Through a 4-inch telescope, you may constellation Ursa
detect the dust lane, but only near the
Sombrero’s center. The core is bright, and Major. To get a
a large halo surrounds it. It even extends
above and below the sections of the spiral rough idea of its
arms nearest the nucleus.
position, make the MICHAEL SIDONIO
M101
base of an equilateral
Only one thing prevents spiral galaxy
M101 from making every observer’s top triangle with the end NGC 5128
10 list: its surface brightness. Covering two stars of the Big
slightly more area than the Full Moon, Dipper’s handle; M101
M101’s light spreads out so much that only
large amateur telescopes (those 12 inches marks the triangle’s third
and larger in aperture) do it justice. That’s
not to say you can’t see it through smaller point above. Alternatively, it lies 1.5° east-
scopes; I just want you to reduce your
expectations a bit. northeast of the magnitude 5.7 star

M101 represents one of the sky’s 86 Ursae Majoris. M101 glows at magnitude
“grand design” spiral galaxies — one
with prominent and clearly defined spiral 7.9 and has a diameter of 40'.
arms. Usually, the arms mostly or com-
pletely envelop such galaxies. Only about From a dark site through a large telescope,

look for M101’s multiple spiral arms. The

core is concentrated but broad, not starlike. a nebula filter to tell the difference

Many star-forming regions and stellar asso- between star-forming regions and

ciations (loose open clusters) lie along M101’s associations. The filter will dim the

spiral arms. In fact, at least five — NGC 5447, stars within the associations, but not the

NGC 5455, NGC 5461, NGC 5462, and nebular gas of the star-forming regions.

NGC 5471 — are bright enough to have their This technique will help you see the

own NGC numbers. Of these, NGC 5447 is glowing hydrogen clouds better.

the most prominent. Find it 6' southwest of NGC 5128
M101’s core. Several other objects within

M101 once carried catalog designations, but One note about the objects as they

astronomers no longer recognize them. Use appeared in The Outer Limits: Starting

W W W.ASTR ONOMY.COM 47