Solar system a beginners guide1

Solar system

A Diagram showing the rotation of the Earth.

Earth seen as a tiny dot by the Voyager 1 spacecraft, four billion miles from Earth
It takes the Earth, on average, 23 hours, 56 minutes and 4.091 seconds (one sidereal day)
to rotate around the axis that connects the north and the south poles. From Earth, the main
apparent motion of celestial bodies in the sky (except that of meteors within the
atmosphere and low-orbiting satellites) is to the west at a rate of 15 °/h = 15’/min, i.e., an
apparent Sun or Moon diameter every two minutes.
Earth orbits the Sun every 365.2564 mean solar days (1 sidereal year). From Earth, this
gives an apparent movement of the Sun with respect to the stars at a rate of about 1 °/day,
i.e., a Sun or Moon diameter every 12 hours, eastward. The orbital speed of the Earth
averages about 30 km/s (108,000 km/h), which is enough to cover the planet’s diameter
(~12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.
The Moon revolves with the Earth around a common barycenter, from fixed star to fixed
star, every 27.32 days. When combined with the Earth–Moon system’s common
revolution around the Sun, the period of the synodic month, from new moon to new
moon, is 29.53 days. The Hill sphere (gravitational sphere of influence) of the Earth is
about 1.5 Gm (930,000 miles) in radius. Viewed from Earth’s north pole, the motion of
Earth, its moon and their axial rotations are all counterclockwise. The orbital and axial

planes are not precisely aligned: Earth’s axis is tilted some 23.5 degrees against the
Earth–Sun plane (which causes the seasons); and the Earth–Moon plane is tilted about
5 degrees against the Earth-Sun plane (without a tilt, there would be an eclipse every two
weeks, alternating between lunar eclipses and solar eclipses).
In an inertial reference frame, the Earth’s axis undergoes a slow precessional motion with
a period of some 25,800 years, as well as a nutation with a main period of 18.6 years.
These motions are caused by the differential attraction of Sun and Moon on the Earth’s
equatorial bulge because of its oblateness. In a reference frame attached to the solid body
of the Earth, its rotation is also slightly irregular from polar motion. The polar motion is
quasi-periodic, containing an annual component and a component with a 14-month period
called the Chandler wobble. In addition, the rotational velocity varies, in a phenomenon
known as length of day variation.
In modern times, Earth’s perihelion occurs around January 3, and the aphelion around
July 4 (near the solstices, which are on about December 21 and June 21). For other eras,
see precession and Milankovitch cycles.

Phases

Earth and Moon from Mars, imaged by Mars Global Surveyor.
From space, the Earth can be seen to go through phases similar to the phases of the Moon
and Venus. This appearance is caused by light that reflects off the Earth as it moves
around the Sun. The phases seen depend upon the observer’s location in space. The
phases of the Earth can be simulated by shining light on a globe of the Earth.
From orbit around the Earth, one can see all of the phases of the Earth in progression
from New Earth to New Earth. The speed at which one sees these phases is related to the
orbit of the observer and the speed of the observer around the Earth.

A Martian observer can see the Earth go through phases similar to those that an Earth-
bound observer sees the phases of Venus (as discovered be Galileo), going for the
Martian’s perspective from New Earth to Fat Crescent to wane to New Earth. It is can be
shown that an imaginary observer on the Sun would not see the Earth going through
phases. The sun observer would only be able to see the lit side of the earth.

Magnetic field

Earth’s magnetic field

The Earth’s magnetic field is shaped roughly as a magnetic dipole, with the poles
currently located proximate to the planet’s geographic poles. The field forms the
magnetosphere, which deflects particles in the solar wind. The bow shock is located
about at 13.5 RE. The collision between the magnetic field and the solar wind forms the
Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged
particles. When the plasma enters the Earth’s atmosphere at the magnetic poles, it forms
the aurora.

Moon

Earthrise as seen from lunar orbit on Apollo 8, 24 December 1968.

Name Diameter Mass (kg) Semi-major axis Orbital period
(km) (km)

Moon 3,474.8 7.349×1022 384,400 27 days, 7 hours,
43.7 minutes

The Moon, sometimes called ‘Luna’, is a relatively large, terrestrial, planet-like satellite,
with a diameter about one-quarter of the Earth’s. It is the largest moon in the solar system
relative to the size of its planet. (Charon is larger relative to dwarf planet Pluto.) The
natural satellites orbiting other planets are called “moons”, after Earth’s Moon.

The gravitational attraction between the Earth and Moon cause tides on Earth. The same
effect on the Moon has led to its tidal locking: its rotation period is the same as the time it
takes to orbit the Earth. As a result, it always presents the same face to the planet. As the
Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the
lunar phases: The dark part of the face is separated from the light part by the solar
terminator.

Because of their tidal interaction, the Moon recedes from Earth at the rate of
approximately 38 mm a year. Over millions of years, these tiny modifications—and the
lengthening of Earth’s day by about 17 µs a year—add up to significant changes. During
the Devonian period, there were 400 days in a year, with each day lasting 21.8 hours.

The Moon may dramatically affect the development of life by taming the weather.
Paleontological evidence and computer simulations show that Earth’s axial tilt is
stabilized by tidal interactions with the Moon. Some theorists believe that without this
stabilization against the torques applied by the Sun and planets to the Earth’s equatorial
bulge, the rotational axis might be chaotically unstable, as it appears to be for Mars. If
Earth’s axis of rotation were to approach the plane of the ecliptic, extremely severe
weather could result from the resulting extreme seasonal differences. One pole would be
pointed directly toward the Sun during summer and directly away during winter.
Planetary scientists who have studied the effect claim that this might kill all large animal
and higher plant life. However, this is a controversial subject, and further studies of
Mars—which shares Earth’s rotation period and axial tilt, but not its large moon or liquid
core—may settle the matter.

Viewed from Earth, the Moon is just far enough away to have very nearly the same
apparent angular size as the Sun (the Sun is 400 times larger, and the Moon is 400 times
closer). This allows total eclipses and annular eclipses to occur on Earth.

The relative sizes of and distance between Earth and Moon, to scale

The most widely accepted theory of the Moon’s origin, the giant impact theory, states that
it was formed from the collision of a Mars-size protoplanet with the early Earth. This
hypothesis explains (among other things) the Moon’s relative lack of iron and volatile
elements, and the fact that its composition is nearly identical to that of the Earth’s crust.

Earth has at least two co-orbital satellites, the asteroids 3753 Cruithne and 2002 AA29.

Descriptions

The first time an “Earth-rise” was seen from the moon.

Earth has often been personified as a deity, in particular a goddess (see Gaia and Mother
Earth). The Chinese Earth goddess Hou-Tu is similar to Gaia, the deification of the
Earth. As the patroness of fertility, her element is Earth. In Norse mythology, the Earth
goddess Jord was the mother of Thor and the daughter of Annar. Ancient Egyptian
mythology is different from that of other cultures because Earth is male, Geb, and sky is
female, Nut (goddess).

Although commonly thought to be a sphere, the Earth is actually an oblate spheroid. It
bulges slightly at the equator and is slightly flattened at the poles. In the past there were
varying levels of belief in a flat Earth, but ancient Greek philosophers and, in the Middle
Ages, thinkers such as Thomas Aquinas believed that it was spherical. A 19th-century
organization called the Flat Earth Society advocated the even-then discredited idea that the
Earth was actually disc-shaped, with the North Pole at its center and a 150 foot (50 m)
high wall of ice at the outer edge. It and similar organizations continued to promote this
idea, based on religious beliefs and conspiracy theories, through the 1970s. Today, the
subject is more frequently treated tongue-in-cheek or with mockery.

Prior to the introduction of space flight, these inaccurate beliefs were countered with
deductions based on observations of the secondary effects of the Earth’s shape and
parallels drawn with the shape of other planets. Cartography, the study and practice of
map making, and vicariously geography, have historically been the disciplines devoted to
depicting the Earth. Surveying, the determination of locations and distances, to an lesser
extent navigation, the determination of position and direction, have developed alongside
cartography and geography, providing and suitably quantifying the requisite information.

The technological developments of the latter half of the 20th century are widely considered
to have altered the public’s perception of the Earth. Before space flight, the popular
image of Earth was of a green world. Science fiction artist Frank R. Paul
provided perhaps the first image of a cloudless blue planet (with sharply defined land
masses) on the back cover of the July 1940 issue of Amazing Stories, a common depiction
for several decades thereafter. Apollo 17’s 1972 “Blue Marble” photograph of Earth from
cislunar space became the current iconic image of the planet as a marble of cloud-swirled
blue ocean broken by green-brown continents. A photo taken of a distant Earth by
Voyager 1 in 1990 inspired Carl Sagan to describe the planet as a “Pale Blue Dot.” Earth
has also been described as a massive spaceship, with a life support system that requires
maintenance, or as having a biosphere that forms one large organism. See Spaceship Earth
and Gaia theory.

Future

Artist’s conception of the remains of artificial structures on the Earth after the Sun enters
its red giant phase and swells to roughly 100 times its current size.

Comparison between the red supergiant Antares and the Sun. The black circle is the size
of the orbit of Mars. Arcturus is also included in the picture for comparison
The future of the planet is closely tied to that of the Sun. The luminosity of the Sun will
continue to steadily increase, growing from the current luminosity by 10% in 1.1 billion
years (1.1 Gyr) and up to 40% in 3.5 Gyr. Climate models indicate that the increase in
radiation reaching the Earth is likely to have dire consequences, including possible loss of
the oceans.

The Sun, as part of its solar lifespan, will expand to a red giant in 5 Gyr. Models predict
that the Sun will expand out to about 99% of the distance to the Earth’s present orbit (1
astronomical unit, or AU). However, by that time, the orbit of the Earth may have
expanded to about 1.7 AUs because of the diminished mass of the Sun. The planet might
thus escape envelopment.
The increased heat will accelerate the inorganic CO2 cycle,reducing it’s concentration to
lethal dose for plants (10 ppm for C4 photosynthesis) in 900 million years.But even if our
sun was eternal and stable,the continued internal cooling of earth would have resulted in a
loss of much of our atmosphere and oceans (due to lower volcanism). More
specifically, for our oceans, the lower temperatures in the crust will permit their water to

leak more deeply than today(at certain debt the water is evaporating) resulting in their
total disappearance in 1 billion years.

Jupiter

Jupiter

Orbital characteristics (Epoch J2000)

Semi-major axis 778,412,027 km
5.203 363 01 AU

Orbital circumference 4.888 Tm
32.675 AU

Eccentricity 0.048 392 66

Perihelion 740,742,598 km
4.951 558 43 AU

Aphelion 816,081,455 km
5.455 167 59 AU

Orbital period 4333.2867 d
(11.86 a)

Synodic period 398.88 d

Avg. Orbital Speed 13.056 km/s

Max. Orbital Speed 13.712 km/s

Min. Orbital Speed 12.446 km/s

Inclination 1.305 30°
(6.09° to Sun’s equator)

Longitude of the 100.556 15°
ascending node

Argument of the 274.197 70°
perihelion

Number of satellites 63

Physical characteristics

Equatorial diameter 142,984 km
(11.209 Earths)

Polar diameter 133,709 km
(10.517 Earths)

Oblateness 0.064 87
Surface area
6.14×1010 km2
Volume (120.5 Earths)

Mass 1.431×1015 km3
(1321.3 Earths)
Mean density
Equatorial gravity 1.899×1027 kg
(317.8 Earths)

1.326 g/cm3

23.12 m/s2
(2.358 gee)

Escape velocity 59.54 km/s

Rotation period 0.413 538 021 d
(9 h 55 min 29.685 s)

Rotation velocity 12.6 km/s = 45,300 km/h
(at the equator)

Axial tilt 3.13°

Right ascension 268.05° (17 h 52 min 12 s)
of North pole

Declination 64.49°

Albedo 0.52

Surface temp. min mean max
110 K 152 K N/A K

Adjective Jovian

Atmospheric characteristics

Atmospheric pressure 70 kPa

Hydrogen ~86%

Helium ~14%

Methane 0.1%

Water vapor 0.1%

Ammonia 0.02%

Ethane 0.0002%

Phosphine 0.0001%

Hydrogen sulfide <0.00010%

Jupiter (IPA: /< dʒu< pɪtə/) is the fifth planet from the Sun and the largest within the
solar system. Jupiter and the other gas giants—Saturn, Uranus, and Neptune—are
sometimes referred to as “Jovian planets”.

Overview

Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and
Venus); however at times Mars appears brighter than Jupiter.

Approximate size comparison of Earth and Jupiter, including the Great Red Spot

Jupiter is 2.5 times more massive than all the other planets combined, so massive that its
barycenter with the Sun actually lies above the Sun’s surface (1.068 solar radii from the
Sun’s center). It is 318 times more massive than Earth, with a diameter 11 times that of
Earth, and its volume is 1300 times as great as that of Earth. Quite naturally, Jupiter’s

gravitational influence has dominated the evolution of the solar system: some have
described the solar system as consisting of the Sun, Jupiter, and assorted debris. Most
planets’ orbits lie closer to Jupiter’s orbital plane than the Sun’s equatorial plane
(Mercury is the only planet which is closer to the Sun’s equator in orbital tilt), the
majority of short-period comets belong to Jupiter’s family (a result due to both Jupiter’s
mass and its relative speed), the Kirkwood gaps in the asteroid belt are mostly due to
Jupiter, and the planet may have been responsible for the Late Heavy Bombardment of
the inner solar system’s history. Jupiter has been called the solar system’s vacuum
cleaner, due to its immense gravity well.

As impressive as Jupiter’s mass is, extrasolar planets have been discovered with much
greater masses. There is no clear-cut definition of what distinguishes a large planet such
as Jupiter from a brown dwarf star, although the latter possesses rather specific spectral
lines. Currently, if an object of solar metallicity is 13 Jupiter masses or above, large
enough to burn deuterium, it is considered a brown dwarf; below that mass (and orbiting a
star or stellar remnant), it is a planet. Jupiter is thought to have about as large a diameter
as a planet of its composition can; adding extra mass would result in further gravitational
compression, in theory leading to stellar ignition. This has led some astronomers to term it
a “failed star”—although Jupiter would need to be about seventy-five times as massive to
become a star, the smallest red dwarf is only about 30% larger than Jupiter. In light of
this, it is also interesting to note that it radiates more heat than it receives from the Sun.
This additional heat radiation is produced by the Kelvin-Helmholtz mechanism. As
another symptom of this process, the planet shrinks at the rate of a few millimeters each
year. When it was younger and hotter, Jupiter was much larger than it is today, though
previously Saturn would have been even bigger than Jupiter due to its lower mass: Saturn
has a much weaker gravitational pull and with more heat, both planets would have been
more bloated (and because of Saturn’s lower core mass, this effect would have been
greater). In general, the more massive the core, the smaller the planet in size.

Aurora borealis on Jupiter.

Jupiter also has the fastest rotation rate of any planet within the solar system, making a
complete rotation on its axis in slightly less than ten hours, which results in an equatorial
bulge easily seen through an Earth-based amateur telescope. Jupiter is perpetually
covered with a layer of clouds, composed of ammonia crystals and possibly ammonium
hydrosulphide, and it may not have any solid surface in that the density may simply

increase gradually as you move towards the core. Its best known feature is the Great Red
Spot, a storm larger than Earth which was likely first observed by Giovanni Domenico
Cassini and Robert Hooke four centuries ago. Indeed, mathematical models suggest that
the storm is stable and may be a permanent feature of the planet. In 2000, three small
spots merged to form a larger spot named Oval BA, which later acquired a red hue very
similar to that of the Great Red Spot.

Historical observations

The planet Jupiter has been known since ancient times and is visible to the naked eye in
the night sky. The Romans named the planet after the Roman god Jupiter (also called
Jove). The astronomical symbol for the planet is a stylized representation of the god’s
lightning bolt.

The Chinese, Korean, Japanese, and Vietnamese refer to the planet as the wood star, < < ,
based on the Chinese Five Elements. In Vedic Astrology, Hindu astrologers refer to
Jupiter as Brihaspati, or “Guru” which means the “Big One”. In Hindi, Thursday is
referred to as Guruvaar (day of Jupiter). In the English language Thursday is rendered as
Thor’s day, with Thor being identified with the Roman god Jupiter.

In 1610, Galileo Galilei discovered the four largest moons of Jupiter, Io, Europa,
Ganymede and Callisto (now known as the Galilean moons) using a telescope, the first
observation of moons other than Earth’s. This was also the first discovery of a celestial
motion not apparently centered on the Earth. It was a major point in favor of Copernicus’
heliocentric theory of the motions of the planets; Galileo’s outspoken support of the
Copernican theory placed him under the threat of the Inquisition.

In 1892, E. E. Barnard observed a fifth satellite of Jupiter with the 36-inch refractor at
Lick Observatory in California. The discovery, a testament to his extraordinary eyesight,
made him quickly famous. The moon was later named Amalthea.

Physical characteristics

Planetary composition

Jupiter is composed of a relatively small rocky core, surrounded by metallic hydrogen,
with further layers of liquid hydrogen and gaseous hydrogen. There is no clear boundary
or surface between these different phases of hydrogen; the conditions blend smoothly
from gas to liquid as one descends.

Atmosphere

False-color detail of Jupiter’s atmosphere, imaged by Voyager 1, showing the Great Red
Spot and a passing white oval

Jupiter’s atmosphere is composed of ~90% hydrogen and ~10% helium by number of
atoms. The atmosphere is ~75%/24% by mass; with ~1% of the mass accounted for by
other substances - the interior contains denser materials such that the distribution is
~71%/24%/5%. The atmosphere contains trace amounts of methane, water vapor,
ammonia, and “rock”. There are also traces of carbon, ethane, hydrogen sulphide, neon,
oxygen, phosphine, and sulphur. The outermost layer of the atmosphere contains crystals
of frozen ammonia. Through IR and UV measurements benzene (at a relative mixing
ratio of 2x10-9 to hydrogen) and other hydrocarbons have also been found.

This atmospheric composition is very close to the composition of the solar nebula. Saturn
has a similar composition, but Uranus and Neptune have much less hydrogen and helium.

Jupiter’s upper atmosphere undergoes differential rotation, an effect first noticed by
Giovanni Cassini (1690). The rotation of Jupiter’s polar atmosphere is ~5 minutes longer
than that of the equatorial atmosphere. In addition, bands of clouds of different latitudes,
known as tropical regions flow in opposing directions on the prevailing winds. The
interactions of these conflicting circulation patterns cause storms and turbulence. Wind
speeds of 600 km/h are not uncommon.

The only spacecraft to have descended into Jupiter’s atmosphere to take scientific
measurements is the Galileo probe (see Galileo mission). It sent an atmospheric probe
into Jupiter upon arrival in 1995, then itself entered Jupiter’s atmosphere and burned
up in 2003.

The Great Red Spot

The Great Red Spot as seen from Voyager 1 in 1979.
Color animation of Jupiter’s cloud motion.

An animation of the Great Red Spot

Image of Jupiter by Pioneer 10 in 1974. The Great Red Spot appears more prominent
here than in the Voyager images because of its location in a lighter colored band of
clouds.
The Great Red Spot is a persistent anticyclonic storm on the planet Jupiter, 22° south of
the equator, which has lasted at least 340 years. The storm is large enough to be visible
through Earth-based telescopes. It was probably first observed by Giovanni Domenico
Cassini, who described it around 1665.

This dramatic view of Jupiter’s Great Red Spot and its surroundings was obtained by
Voyager 1 on February 25, 1979, when the spacecraft was 9.2 million km (5.7 million
miles) from Jupiter. Cloud details as small as 160 km (100 miles) across can be seen here.
The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily
complex and variable wave motion. To give a sense of Jupiter’s scale, the white oval
storm directly below the Great Red Spot is approximately the same diameter as Earth.

The oval object rotates counterclockwise, with a period of about 6 days. The Great Red
Spot’s dimensions are 24–40,000 km × 12–14,000 km. It is large enough to contain two
or three planets of Earth size. The cloudtops of this storm are about 8 km above the
surrounding cloudtops.

Storms such as this are not uncommon within the turbulent atmospheres of gas giants.
Jupiter also has white ovals and brown ovals, which are lesser unnamed storms. White
ovals tend to consist of relatively cool clouds within the upper atmosphere. Brown ovals
are warmer and located within the “normal cloud layer”. Such storms can last hours or
centuries.

Before the Voyager missions, astronomers were highly uncertain of its nature. Many
believed it to be a solid or liquid feature on Jupiter’s surface.

Planetary rings

Jupiter has a faint planetary ring system composed of smoke-like dust particles knocked
from its moons by energetic meteor impacts. The innermost doughnut-shaped ring, called
the halo, is almost as thick (20,000km) as it is wide (22,800km). This is followed by the
thinnest and brightest main ring, which is made of dust from the satellites Adrastea and
Metis. Metis orbits within its fluid Roche limit with Jupiter, and objects not rigidly
attached to it may freely fall away from it and into Jupiter’s gravitational field. Two wide
gossamer rings encircle the main ring, originating from Thebe and Amalthea. Finally,
there is a distant and very faint outer ring circling Jupiter backwards—retrograde of its
spin. It is not known for certain where the material for this outer ring comes from, but it
may be captured interplanetary dust.

Magnetosphere

Jupiter has a very large and powerful magnetosphere. In fact, if one could see Jupiter’s
magnetic field from Earth, it would appear five times as large as the full moon in the sky
despite being so much farther away. The magnetic field is generated by eddy currents in
Jupiter’s metallic hydrogen core. This magnetic field collects a large flux of particle
radiation in Jupiter’s radiation belts, as well as producing a dramatic gas torus and flux
tube associated with Io. Jupiter’s magnetosphere is the largest planetary structure in the
solar system.

The Pioneer probes confirmed that Jupiter’s enormous magnetic field is 10 times stronger
than Earth’s and contains 20,000 times as much energy. The sensitive instruments aboard
found that the Jovian magnetic field’s “north” magnetic pole is at the planet’s geographic
south pole, with the axis of the magnetic field tilted 11 degrees from the Jovian rotation
axis and offset from the center of Jupiter in a manner similar to the axis of the Earth’s
field. The Pioneers measured the bow shock of the Jovian magnetosphere to the width of
26 million kilometers (16 million miles), with the magnetic tail extending beyond
Saturn’s orbit.

The data showed that the magnetic field fluctuates rapidly in size on the sunward side of
Jupiter because of pressure variations in the solar wind, an effect studied in further detail
by the two Voyager spacecraft. It was also discovered that streams of high-energy atomic
particles are ejected from the Jovian magnetosphere and travel as far as the orbit of the

Earth. Energetic protons were found and measured in the Jovian radiation belt and
electric currents were detected flowing between Jupiter and some of its moons,
particularly Io.

Exploration of Jupiter

A number of probes have visited Jupiter.

Pioneer flyby missions

Pioneer 10 flew past Jupiter in December of 1973, followed by Pioneer 11 exactly one
year later. Pioneer 10 obtained the first ever close up images of Jupiter and the Galilean
moons, studied its atmosphere, detected its magnetic field, observed its radiation belts
and found that Jupiter is mainly liquid.

Voyager flyby missions

Voyager 1 took this photo of the planet Jupiter on January 24, 1979 while still more than
25 million miles (40 million kilometres) away. Click image for full caption.
Voyager 1 flew by in March 1979 followed by Voyager 2 in July of the same year. The
Voyagers vastly improved the understanding of the Galilean moons and discovered
Jupiter’s rings. They also took the first close up images of the planet’s atmosphere.

Ulysses flyby mission

In February 1992, Ulysses solar probe performed a flyby of Jupiter at a distance of
450,000 km (6.3 Jovian radii). The flyby was required to attain a polar orbit around the
Sun. The probe conducted studies on Jupiter’s magnetosphere. Since there are no cameras
onboard the probe, no images were taken. In February 2004, the probe came again in the
vicinity of Jupiter. This time the distance was much greater, about 240 million km

Galileo mission

So far the only spacecraft to orbit Jupiter is the Galileo orbiter, which went into orbit
around Jupiter on December 7, 1995. It orbited the planet for over seven years and
conducted multiple flybys of all of the Galilean moons and Amalthea. The spacecraft also
witnessed the impact of Comet Shoemaker-Levy 9 into Jupiter as it approached the planet
in 1994, giving a unique vantage point for this spectacular event. However, while the
information gained about the Jovian system from the Galileo mission was extensive in its
own right, its originally-designed capacity was limited by the failed deployment of its
high-gain radio transmitting antenna.

Jupiter as seen by the space probe Cassini. This is the most detailed global color portrait
of Jupiter ever assembled.
An atmospheric probe was released from the spacecraft in July 1995. The probe entered
the planet’s atmosphere on December 7, 1995. It parachuted through 150 km of the
atmosphere, collecting data for 57.6 minutes, before being crushed by the extreme
pressure to which it was subjected. It would have melted and vaporized shortly thereafter.
The Galileo orbiter itself experienced a more rapid version of the same fate when it was
deliberately steered into the planet on September 21, 2003 at a speed of over 50 km/s, in
order to avoid any possibility of it crashing into and possibly contaminating Europa, one
of the Jovian moons.

Cassini flyby mission

In 2000, the Cassini probe, en route to Saturn, flew by Jupiter and provided some of the
highest-resolution images ever made of the planet. On December 19, 2000, the Cassini
spacecraft, captured a very low resolution image of the moon Himalia, but it was too
distant to show any surface details.

New Horizons flyby mission

The New Horizons probe and its Atlas V launcher lifted off from Pad 41 at Cape
Canaveral Air Force Station, Florida, directly south of Space Shuttle Launch Complex
39, at 2:00 p.m. EST (1900 UTC) on January 19, 2006. New Horizons passed Lunar orbit
before midnight EST on the same day, and is scheduled to reach Jupiter in February
2007. It will pass through the Jupiter system at 21 km/s (47,000 mph), with closest
approach to Jupiter occurring at approximately 06:00 UTC February 28, 2007.

The flyby will come within about 32 Jovian radii (3 Gm) of Jupiter and will be the center
of a 4-month intensive observation campaign. New Horizons also has instruments built
twenty years after Galileo’s - particularly Galileo’s cameras, which were evolved
versions of Voyager cameras which, in turn, were evolved Mariner cameras. Because of
the much shorter distance from Jupiter to Earth, the communications link can transmit
multiple loadings of the memory buffer. The mission will actually return more data from
Jupiter than Pluto. Imaging of Jupiter began on September 4, 2006.

Future probes

NASA is planning a mission to study Jupiter in detail from a polar orbit. Named Juno, the
spacecraft is planned to launch by 2010.

Because of the possibility of a liquid ocean on Jupiter’s moon Europa, there has been
great interest to study the icy moons in detail. A mission proposed by NASA was
dedicated to study them. The JIMO (Jupiter Icy Moons Orbiter) was expected to be
launched sometime after 2012. However, the mission was deemed too ambitious and its
funding was cancelled.

Natural satellites

Jupiter’s 4 Galilean moons, in a composite image comparing their sizes and the size of
Jupiter (Great Red Spot visible). From the top they are: Callisto, Ganymede, Europa and
Io.

Jupiter has at least 63 moons. For a complete listing of these moons, please see Jupiter’s
natural satellites. For a timeline of their discovery dates, see Timeline of discovery of
Solar System planets and their natural satellites.

The four large moons, known as the “Galilean moons”, are Io, Europa, Ganymede and
Callisto.

Galilean moons

The orbits of Io, Europa, and Ganymede, the largest moon in the solar system, form a
pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter,
Europa makes exactly two orbits and Ganymede makes exactly one. This resonance
causes the gravitational effects of the three moons to distort their orbits into elliptical
shapes, since each moon receives an extra tug from its neighbors at the same point in
every orbit it makes.

A picture of Jupiter and its moon Io taken by Hubble. The black spot is Io’s shadow.

The tidal force from Jupiter, on the other hand, works to circularize their orbits. This
constant tug of war causes regular flexing of the three moons’ shapes, Jupiter’s gravity
stretches the moons more strongly during the portion of their orbits that are closest to it
and allowing them to spring back to more spherical shapes when they’re farther away.
This flexing causes tidal heating of the three moons’ cores. This is seen most
dramatically in Io’s extraordinary volcanic activity, and to a somewhat less dramatic
extent in the geologically young surface of Europa indicating recent resurfacing.

The Galilean moons, compared to Earth’s moon Luna

Name Diameter Mass Orbital Orbital
(km) (kg)
(Pronunciation radius (km) period (days)
key) eye’-oe
Io

< aɪəʊ

Io eye’-oe 3643 8.9×1022 421 700 1.77
Europa (6.5% Luna)
Ganymede < aɪəʊ (105% Luna) (120% Luna) (110% Luna)
Callisto 3.55
ew-roe’-pə 3122 4.8×1022 671 034 (13% Luna)

jʊˈrəʊpə (90% Luna) (65% Luna) (175% Luna) 7.15
(26% Luna)
gan’-ə- 5262 14.8×1022 1 070 412
meed (150% Luna) (200% Luna) (280% Luna) 16.69
< gænəmid (61% Luna)

kə-lis’-toe 4821 10.8×1022 1 882 709

kə< lɪstəʊ (140% Luna) (150% Luna) (490% Luna)

Classification of Jupiter’s moons

Before the discoveries of the Voyager missions, Jupiter’s moons were arranged neatly into
four groups of four. Since then, the large number of new small outer moons has
complicated this picture. There are now thought to be six main groups, although some are
more distinct than others. A basic division is between the eight inner regular moons with
nearly circular orbits near the plane of Jupiter’s equator, which are believed to have
formed with Jupiter, and an unknown number of small irregular moons, with elliptical and
inclined orbits, which are believed to be captured asteroids or fragments of captured
asteroids.

Europa, one of Jupiter’s many moons.

1. Regular moons
1. The inner group of four small moons all have diameters of less than 200
km, orbit at radii less than 200,000 km, and have orbital inclinations of
less than half a degree..
2. The four Galilean moons were all discovered by Galileo Galilei, orbit
between 400,000 and 2,000,000 km, and include some of the largest
moons in the solar system.

2. Irregular moons
1. Themisto is in a group of its own, orbiting halfway between the Galilean
moons and the next group.

2. The Himalia group is a tightly clustered group of moons with orbits
around 11,000,000-12,000,000 km from Jupiter.

3. Carpo is another isolated case; at the inner edge of the Ananke group, it
revolves in the direct sense.

4. The Ananke group is a group with rather indistinct borders, averaging
21,276,000 km from Jupiter with an average inclination of 149 degrees.

5. The Carme group is a fairly distinct group that averages 23,404,000 km
from Jupiter with an average inclination of 165 degrees.

6. The Pasiphaë group is a dispersed and only vaguely distinct group that
covers all the outermost moons.

It is thought that the groups of outer moons may each have a common origin, perhaps as a
larger moon or captured body that broke up.

Life on Jupiter

It is considered highly unlikely that there is any Earth-like life on Jupiter, as there is little
water in the atmosphere and any possible solid surface deep within Jupiter would be under
extraordinary pressures. However, in 1976, before the Voyager missions, Carl
Sagan hypothesized (with Edwin Ernest Salpeter) that ammonia-based life could evolve in
Jupiter’s upper atmosphere. Sagan and Salpeter based this hypothesis on the ecology of
terrestrial seas which have simple photosynthetic plankton at the top level, fish at lower
levels feeding on these creatures, and marine predators which hunt the fish. The Jovian
equivalents Sagan and Salpeter hypothesized were “sinkers”, “floaters”, and “hunters”.
The “sinkers” would be plankton-like organisms which fall through the atmosphere,
existing just long enough that they can reproduce in the time they are kept afloat by
convection. The “floaters” would be giant bags of gas functioning along the lines of hot
air balloons, using their own metabolism (feeding off sunlight and free molecules) to
keep their gas warm. The “hunters” would be almost squid-like creatures, using jets of
gas to propel themselves into “floaters” and consume them.

This diagram shows the Trojan Asteroids in Jupiter’s orbit, as well as the main asteroid
belt

Trojan asteroids

In addition to its moons, Jupiter’s gravitational field controls numerous asteroids which
have settled into the regions of the Lagrangian points preceding and following Jupiter in
its orbit around the sun. These are known as the Trojan asteroids, and are divided into
Greek and Trojan “camps” to commemorate the Iliad. The first of these, 588 Achilles,
was discovered by Max Wolf in 1906; since then hundreds more have been discovered.
The largest is 624 Hektor.

Cometary impact

Aftermath of impact of a cometary fragment. The dark scars visible on the cloudtops
were larger than Earth itself.
During the period July 16 to July 22, 1994, over twenty fragments from the comet
Shoemaker-Levy 9 hit Jupiter’s southern hemisphere, providing the first direct
observation of a collision between two solar system objects. It is thought that due to
Jupiter’s large mass and location near the inner solar system it receives the most frequent
comet impacts of the solar system’s planets.

Saturn

Saturn

Orbital characteristics (Epoch J2000)

Semi-major axis 1,426,725,413 km
9.537 070 32 AU

Orbital circumference 8.958 Tm
59.879 AU

Eccentricity 0.054 150 60

Perihelion 1,349,467,375 km
9.020 632 24 AU

Aphelion 1,503,983,449 km
10.053 508 40 AU

Orbital period 10,756.1995 d
(29.45 a)

Synodic period 378.10 d

Avg. orbital speed 9.639 km/s

Max. orbital speed 10.183 km/s

Min. orbital speed 9.137 km/s

Inclination 2.484 46°
(5.51° to Sun’s equator)

Longitude of the 113.715 04°
ascending node

Argument of the 338.716 90°
perihelion

Number of satellites 56 confirmed

Physical characteristics

Equatorial diameter 120,536 km
(9.449 Earths)

Polar diameter 108,728 km
(8.552 Earths)

Oblateness 0.097 96
Surface area
Volume 4.27×1010 km2
Mass (83.703 Earths)
Mean density
Equatorial gravity 8.27×1014 km3
(763.59 Earths)

5.6846×1026 kg
(95.162 Earths)

0.6873 g/cm3
(less than water)

8.96 m/s2
(0.914 gee)

Escape velocity 35.49 km/s

Rotation period 0.449 375 d
(10 h 47 min 6 s) 1

Rotation velocity 9.87 km/s = 35,500 km/h
(at the equator)
Axial tilt 26.73°
Right ascension 40.59° (2 h 42 min 21 s)
of North pole
Declination 83.54°
Albedo 0.47
Avg. cloudtop temp. 93 K
Surface temp. min mean max
82 K 143 K N/A K

Adjective Saturnian

Atmospheric characteristics

Atmospheric pressure 140 kPa

Hydrogen >93%

Helium >5%

Methane 0.2%

Water vapor 0.1%

Ammonia 0.01%

Ethane 0.0005%

Phosphine 0.0001%

Saturn (IPA: / satən, -ə ®n/) is the sixth planet from the Sun. It is a gas giant (also
known as a Jovian planet, after the planet Jupiter), the second-largest planet in the solar
system after Jupiter. Saturn has a prominent system of rings, consisting mostly of ice
particles with a smaller amount of rocky debris and dust. It was named after the Roman
god Saturn (the Greek equivalent is Kronos, father of Zeus). Its symbol is a stylized

representation of the god’s sickle (Unicode: ).

Physical characteristics

Saturn is an oblate spheroid, i.e. it is flattened at the poles and bulging at the equator; its
equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is
the result of its rapid rotation and fluid state . The other gas planets are also oblate, but to
a lesser degree. Saturn is the only one of the Solar System’s planets that is less dense than
water, with an average specific density of 0.69. This is a mean value; Saturn’s upper
atmosphere is less dense and its core is considerably more dense than water.

Saturn’s temperature emissions. The prominent hot spot at the bottom of the image is at
Saturn’s south pole.

Saturn’s interior is similar to Jupiter’s, having a rocky core at the center, a liquid metallic
hydrogen layer above that, and a molecular hydrogen layer above that. Traces of various
ices are also present. Saturn has a very hot interior, reaching 12,000 Kelvin (11,700°C) at
the core, and it radiates more energy into space than it receives from the Sun. Most of the
extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational
compression), but this alone may not be sufficient to explain Saturn’s heat production.
An additional proposed mechanism by which Saturn may generate some of its heat is the
“raining out” of droplets of helium deep in Saturn’s interior, the droplets of helium
releasing heat by friction as they fall down through the lighter hydrogen.

Saturn’s atmosphere exhibits a banded pattern similar to Jupiter’s (in fact, the
nomenclature is the same), but Saturn’s bands are much fainter and are also much wider
near the equator. Saturn’s winds are among the Solar System’s fastest; Voyager data
indicates peak easterly winds of 500 m/s (1116 mph). Saturn’s finer cloud patterns were
not observed until the Voyager flybys. Since then, however, Earth-based telescopy has
improved to the point where regular observations can be made.

Saturn’s usually bland atmosphere occasionally exhibits long-lived ovals and other
features common on Jupiter; in 1990 the Hubble Space Telescope observed an enormous
white cloud near Saturn’s equator which was not present during the Voyager encounters
and in 1994 another, smaller storm was observed. The 1990 storm was an example of a
Great White Spot, a unique but short-lived Saturnian phenomenon with a roughly 30-year
periodicity. Previous Great White Spots were observed in 1876, 1903, 1933, and 1960,
with the 1933 storm being the most famous. The careful study of these episodes reveals
interesting patterns; if it holds another storm will occur in about 2020.(Kidger 1992)

Recent images from the Cassini spacecraft show that Saturn’s northern hemisphere is
changing colors. It now appears a bright blue, similar to Uranus, as can be seen in the
image below. This blue color cannot currently be observed from earth, because Saturn’s
rings are currently blocking its northern hemisphere. One theory is that this shocking
color change is a result of colder temperatures, as the shadows cast by Saturn’s rings are
blocking out sunlight. This would result in the yellow clouds sinking and Saturn’s deeper
blue atmosphere being revealed.

Visual comparison of Saturn and Earth

Astronomers using infrared imaging have shown that Saturn has a warm polar vortex, and
is the only planet in the solar system known to do so.

An apparently permanent hexagonal wave pattern around the polar vortex in the
atmosphere at about 78°N was first noted in the Voyager images . HST imaging of the
south polar region indicates the presence of a jet stream, but no strong polar vortex nor
any hexagonal standing wave. However, NASA reported in November of 2006 that the
Cassini spacecraft observed a ‘hurricane-like’ storm locked to the south pole that had a
clearly defined eyewall. This observation is particularly notable because eyewall clouds
have not been seen on any other planet other than Earth (including a failure to observe an
eyewall in the Great Red Spot of Jupiter by the Galileo spacecraft).

Rotational behavior

Since Saturn does not rotate on its axis at a uniform rate, two rotation periods have been
assigned to it (as in Jupiter’s case): System I has a period of 10 h 14 min 00 s (844.3°/d)
and encompasses the Equatorial Zone, which extends from the northern edge of the South
Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian
latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is
System II. System III, based on radio emissions from the planet, has a period of 10 h 39
min 22.4 s (810.8°/d); because it is very close to System II, it has largely superseded it.

While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation
period of Saturn had increased slightly, to approximately 10 h 45 m 45 s (± 36 s). The
cause of the change is unknown — however, it is thought that this is due to a movement
of the radio source to a different latitude inside Saturn, with a different rotational period,
rather than an actual change in Saturn’s rotation.

Planetary rings

Saturn is probably best known for its planetary rings, which make it one of the most
visually remarkable objects in the solar system.

History

The rings were first observed by Galileo Galilei in 1610 with his telescope, but he was
unable to identify them as such. He wrote to the Duke of Tuscany that “The planet Saturn
is not alone, but is composed of three, which almost touch one another and never move
nor change with respect to one another. They are arranged in a line parallel to the zodiac,
and the middle one (Saturn itself) is about three times the size of the lateral ones [the
edges of the rings].” He also described Saturn as having “ears.” In 1612 the plane of the
rings was oriented directly at the Earth and the rings appeared to vanish, and then in 1613
they reappeared again, further confusing Galileo.

In 1655, Christiaan Huygens became the first person to suggest that Saturn was
surrounded by a ring. Using a telescope that was far superior to those available to
Galileo, Huygens observed Saturn and wrote that “It [Saturn] is surrounded by a thin,
flat, ring, nowhere touching, inclined to the ecliptic.”

In 1675, Giovanni Domenico Cassini determined that Saturn’s ring was actually
composed of multiple smaller rings with gaps between them; the largest of these gaps
was later named the Cassini Division.

In 1859, James Clerk Maxwell demonstrated that the rings could not be solid or they
would become unstable and break apart. He proposed that the rings must be composed of
numerous small particles, all independently orbiting Saturn. Maxwell’s theory was
proved correct in 1895 through spectroscopic studies of the rings carried out by James
Keeler of Lick Observatory.

Physical characteristics

The rings can be viewed using a quite modest modern telescope or with a good pair of
binoculars. They extend from 6,630 km to 120,700 km above Saturn’s equator, average
close to one kilometre in thickness and are composed of silica rock, iron oxide, and ice
particles ranging in size from specks of dust to the size of a small automobile. There are
two main theories regarding the origin of Saturn’s rings. One theory, originally proposed
by Édouard Roche in the 19th century, is that the rings were once a moon of Saturn whose
orbit decayed until it came close enough to be ripped apart by tidal forces (see Roche
limit). A variation of this theory is that the moon disintegrated after being struck by a
large comet or asteroid. The second theory is that the rings were never part of a moon,
but are instead left over from the original nebular material that Saturn formed out of. This
theory is not widely accepted today, since Saturn’s rings are thought to be unstable over
periods of millions of years and therefore of relatively recent origin.

While the largest gaps in the rings, such as the Cassini division and Encke division, can be
seen from Earth, the Voyager spacecrafts discovered the rings to have an intricate
structure of thousands of thin gaps and ringlets. This structure is thought to arise from the
gravitational pull of Saturn’s many moons in several different ways. Some gaps are
cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be
discovered, and some ringlets seem to be maintained by the gravitational effects of small
shepherd satellites such as Prometheus and Pandora. Other gaps arise from resonances
between the orbital period of particles in the gap and that of a more massive moon further
out; Mimas maintains the Cassini division in this manner. Still more structure in the rings
actually consists of spiral waves raised by the moons’ periodic gravitational
perturbations.

Data from the Cassini space probe indicates that the rings of Saturn possess their own
atmosphere, independent of that of the planet itself. The atmosphere is composed of
molecular oxygen gas (O2) produced when ultraviolet light from the Sun disintegrates
water ice in the rings. Chemical reactions between water molecule fragments and further
ultraviolet stimulation create and eject, among other things O2. According to models of
this atmosphere, H2 is also present. The O2 and H2 atmospheres are so sparse that if the
entire atmosphere were somehow condensed onto the rings, it would be on the order of 1
atom thick. The rings also have a similarly sparse OH (hydroxide) atmosphere. Like the
O2, this atmosphere is produced by the disintegration of water molecules, though in this
case the disintegration is done by energetic ions that bombard water molecules ejected by
Saturn’s moon Enceladus. This atmosphere, despite being extremely sparse, was detected
from Earth by the Hubble Space Telescope.

Saturn shows complex patterns in its brightness. Most of the variability is due to the
changing aspect of the rings, and this goes through two cycles every orbit. However,
superimposed on this is variability due to the eccentricity of the planet’s orbit that causes
the planet to display brighter oppositions in the northern hemisphere than it does in the
southern. (Henshaw, C., 2003).

Spokes of the rings

Spokes in the B ring, imaged by Voyager 2 in 1981.

Until 1980, the structure of the rings of Saturn was explained exclusively as the action of
gravitational forces. The Voyager spacecraft found radial features in the B ring, called
spokes, which could not be explained in this manner, as their persistence and rotation
around the rings were not consistent with orbital mechanics. The spokes appear dark
against the lit side of the rings, and light when seen against the unlit side. It is assumed
that they are connected to electromagnetic interactions, as they rotate almost
synchronously with the magnetosphere of Saturn. However, the precise mechanism
behind the spokes is still unknown.

Spokes imaged by Cassini in 2005.

Twenty-five years later, Cassini observed the spokes again. They appear to be a seasonal
phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as
Saturn comes closer to equinox. The spokes were not visible when Cassini arrived at
Saturn in early 2004. Some scientists speculated that the spokes would not be visible
again until 2007, based on models attempting to describe spoke formation. Nevertheless,
the Cassini imaging team kept looking for spokes in images of the rings, and the spokes
reappeared in images taken September 5, 2005.

Natural satellites

Four of Saturn’s moons, Dione, Titan, Prometheus (edge of rings), Telesto (top center).

Saturn has a large number of moons. The precise figure is uncertain as the orbiting
chunks of ice in Saturn’s rings are all technically moons, and it is difficult to draw a
distinction between a large ring particle and a tiny moon. As of 2006, a total of 56
individual moons have been identified, many of them quite small[citation needed]. Seven of
the moons are massive enough to have collapsed into a spheroid under their own
gravitation. These are compared to Earth’s moon in the table below. Saturn’s most
noteworthy moon is Titan, the only moon in the solar system to have a dense atmosphere.

Saturn’s rings cut across an eerie scene that is ruled by Titan’s luminous crescent and
globe-encircling haze, broken by the small moon Enceladus, whose icy jets are dimly
visible at its south pole. North is up

Traditionally, most of Saturn’s other moons are named after actual Titans of Greek
mythology. This started because John Herschel — son of William Herschel, discoverer of
Mimas and Enceladus — suggested doing so in his 1847 publication Results of
Astronomical Observations made at the Cape of Good Hope, because they were the
sisters and brothers of Cronos (the Greek Saturn).

Saturn’s major satellites, compared to Earth’s Moon.

Name Diameter Mass Orbital radius Orbital period
(km) (kg) (km) (days)

(Pronunciation key) 0.9
(3% Luna)
Mimas mye’-məs 400 0.4×1020 185,000
maɪməs (10% Luna) (0.05% Luna) (50% Luna)

en-sel’-ə- 500 1.1×1020 238,000 1.4
Enceladus dəs (15% Luna) (0.2% Luna) (60% Luna) (5% Luna)

ɛn sɛlədəs 1060 6.2×1020 295,000 1.9
(30% Luna) (0.8% Luna) (80% Luna) (7% Luna)
Tethys tee’-thəs
tiθəs

Dione dye-oe’-nee 1120 11×1020 377,000 2.7
Rhea (30% Luna) (1.5% Luna) (100% Luna) (10% Luna)
daɪˈɔʊni
1530 23×1020 527,000 4.5
ree’-ə (45% Luna) (3% Luna) (140% Luna) (20% Luna)
riə

Titan tye’-tən 5150 1350×1020 1,222,000 16
(60% Luna)
taɪtən (150% Luna) (180% Luna) (320% Luna)

Iapetus eye-ap’-ə- 1440 20×1020 3,560,000 79
təs (40% Luna) (3% Luna) (930% Luna) (290% Luna)

aɪˈæpətəs

For a timeline of discovery dates, see Timeline of discovery of Solar System planets

and their natural satellites.

Exploration of Saturn

A Hubble Space Telescope image, captured in October 1996 shows Saturn’s rings from
just past edge-on

Pioneer 11 flyby

Saturn was first visited by Pioneer 11 in September 1979. It flew within 20,000 km of the
planet’s cloud tops. Low-resolution images were acquired of the planet and few of its
moons. Resolution was not good enough to discern surface features, however. The
spacecraft also studied the rings; among the discoveries were the thin F-ring and the fact
that dark gaps in the rings are bright when viewed towards the Sun, or in other words,
they are not empty of material. It also measured the temperature of Titan.

Voyager flybys

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first
high-resolution images of the planet, rings, and the satellites. Surface features of various

moons were seen for the first time. Voyager 1 performed a close flyby of Titan greatly
increasing our knowledge of the atmosphere of the moon. However, it also proved that
Titan’s atmosphere is impenetrable in visible wavelengths, so no surface details were
seen. The flyby also changed the spacecraft’s trajectory out from the plane of the solar
system.

Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system.
More close-up images of Saturn’s moons were acquired, as well as evidence of changes
in the atmosphere and the rings. Unfortunately, during the flyby, the probe’s turnable
cameraplatform stuck for a couple of days and some planned imaging was lost. Saturn’s
gravity was used to direct the spacecraft’s trajectory towards Uranus.

The probes discovered and confirmed several new satellites orbiting near or within the
planet’s rings. They also discovered the small Maxwell and Keeler gaps.

Cassini orbiter

Saturn eclipses the sun as seen from Cassini

On July 1, 2004, the Cassini-Huygens spacecraft performed the SOI (Saturn Orbit
Insertion) maneuver and entered into orbit around Saturn. Before the SOI, Cassini had
already studied the system extensively. In June 2004, it had conducted a close flyby of
Phoebe sending back high-resolution images and data. The orbiter completed two Titan
flybys before releasing the Huygens probe on December 25, 2004. Huygens descended
onto the surface of Titan on January 14, 2005, sending a flood of data during the
atmospheric descent and after the landing. As of 2005, Cassini is conducting multiple
flybys of Titan and icy satellites. On March 10, 2006, NASA reported that the Cassini
probe found evidence of liquid water reservoirs that erupt in geysers on Saturn’s moon
Enceladus . On September 20, 2006, a Cassini probe photograph revealed a previously
undiscovered planetary ring, outside the brighter main rings of Saturn and inside the G
and E rings. The primary mission ends in 2008 when the spacecraft has completed 74
orbits around the planet.

Best viewing of Saturn

Saturn Oppositions: 2001-2029

Saturn has been known since prehistoric times. It is the most distant of the five planets
visible to the naked eye (the other four are Mercury, Venus, Mars, and Jupiter) and was
the last planet known to early astronomers until Uranus was discovered in 1781. Saturn
appears to the naked eye in the night sky as a bright, yellowish star varying usually
between magnitude +1 and 0 and takes approximately 29 and a half years to make a
complete circuit of the ecliptic against the background constellations of the zodiac.
Optical aid (a large pair of binoculars or a telescope) magnifying at least 20X is required
to clearly resolve Saturn’s rings for most people.

While it is a rewarding target for observation for most of the time it is visible in the sky,
Saturn and its rings are best seen when the planet is at or near opposition (the
configuration of a planet when it is at an elongation of 180° and thus appears opposite the
Sun in the sky.) In the opposition on January 13, 2005, Saturn appeared at its brightest
until 2031, mostly due to a favorable orientation of the rings relative to the Earth.

Saturn in various cultures

Saturn is known as “Sani” or “Shani” in Hindu Astrology. Hindus believe in the existence
of Nine Planets - known as Navagraha(s). These Navagrahas were propitiated
as planetary influences govern the life of individuals. Sani is identified as an inauspicious
planet, and is worshipped by individuals going through a “bad” phase in their life. Sani’s
father is the Sun God “Surya”.

Chinese and Japanese culture designate the planet Saturn as the earth star ( ). This is
based on Five Elements which was traditionally used to classify natural elements.

In Hebrew, Saturn is called ‘Shabbathai’. Its Angel is Cassiel. Its Intelligence, or
beneficial spirit, is Agiel (layga), and its spirit (darker aspect) is Zazel (lzaz). See:
Kabbalah.

In Bahasa Malaysia (the Malay language), its name is ‘Zuhal’.



Uranus

Discovery

Discovered by William Herschel

Discovered on March 13, 1781

Orbital characteristics (Epoch J2000)

Semi-major axis 2,870,972,220 km
19.191 263 93 AU

Orbital 18.029 Tm
circumference 120.515 AU

Eccentricity 0.047 167 71

Perihelion 2,735,555,035 km
18.286 055 96 AU

Aphelion 3,006,389,405 km
20.096 471 90 AU

Orbital period 30,707.4896 d
(84.07 a)

Synodic period 369.65 d

Orbital speed 6.795 km/s

Max. orbital speed 7.128 km/s

Min. orbital speed 6.486 km/s

Inclination 0.769 86°
(6.48° to Sun’s equator)

Longitude of the 74.229 88°
ascending node

Argument of the 96.734 36°
perihelion

Number of satellites 27

Physical characteristics

Equatorial diameter 51,118 km
(4.007 Earths)

Polar diameter 49,946 km
(3.929 Earths)

Oblateness 0.0229
Surface area
8.084×109 km2
Volume (15.849 Earths)

Mass 6.834×1013 km3
(63.086 Earths)
Mean density
Equatorial gravity 8.6832×1025 kg
(14.536 Earths)

1.318 g/cm3

8.69 m/s2
(0.886 g)

Escape velocity 21.29 km/s

Rotation period −0.718 33 d (17 h 14 min 24 s
by convention) 1

Rotation velocity 2.59 km/s = 9320 km/h (at the
equator)

Axial tilt 97.77°

Right ascension 77.31° (5 h 9 min 15 s)
of North pole

Declination +15.175°

Albedo 0.51

Cloudtop avg. temp. 55 K

Surface temp. min mean max
59 K 68 K N/A K

Adjective Uranian

Atmospheric characteristics

Atmospheric 120 kPa (at the cloud level)
pressure

Hydrogen 83%

Helium 15%

Methane 1.99%

Ammonia 0.01%

Ethane 0.00025%

Acetylene 0.00001%

Carbon monoxide trace
Hydrogen sulfide

Uranus (IPA: /jə reɪnəs/ or / jurənəs/) is the seventh planet from the Sun. It is a gas
giant, the third largest by diameter and fourth largest by mass. It is named after Uranus,
the Greek god of the sky and progenitor of the other gods. Its symbol is either

(astrological) or (astronomical). The first symbol derives from the name of its
discoverer, William Herschel. The second symbol is a combination of the devices for the
Sun and Mars, as Uranus was the personification of heaven in Greek mythology,
dominated by the light of the Sun and the power of Mars. It is also the alchemical symbol
of platinum.

NASA’s Voyager 2 is the only spacecraft to have visited the planet and no other visits are
currently planned. Launched in 1977, Voyager 2 made its closest approach to Uranus on
January 24, 1986, before continuing its journey to Neptune.

Uranus is the first planet discovered in modern times. Sir William Herschel formally
discovered the planet on March 13, 1781; the other planets (from Mercury out to Saturn)
have been known since ancient times, and Uranus’ discovery expanded the boundaries of
the solar system for the first time in modern human history. It was also the first planet
discovered using technology (a telescope) rather than the naked eye.

Discovery and naming

Uranus was the first planet to be discovered that was not known in ancient times;
although it had been observed on many previous occasions, it was always mistakenly
identified as a star. The earliest recorded sighting was in 1690 when John Flamsteed
catalogued Uranus as 34 Tauri. Flamsteed observed Uranus at least six more times. The

record belongs to a French astronomer, Pierre Lemonnier, who observed Uranus at least
twelve times between 1750 and 1771, including on four consecutive nights. (Lemonnier
is often[citation needed] called careless or even “sloppy” for this, but it is important to know
that he realized 9 of these within a short time of Herschel’s discovery and most of his
observations occurred at the stationary point in Uranus’ orbit.)

Sir William Herschel discovered the planet on March 13, 1781, but reported it on April
26, 1781, as a “comet.”

« On the 13th of March, 1781, between ten and eleven o’clock at night, while
Herschel was examining the small stars near H Geminorum with a seven-foot
telescope, bearing a magnifying power of two hundred and twenty-seven times, one
of these stars seemed to have an unusual diameter; and it was, therefore, thought to be
a comet. It was under this denomination that it was discussed at the Royal Society of
London. But the researches of Herschel and of Laplace showed later that the orbit of
the new body was nearly circular, and Uranus was consequently elevated to the rank
of a planet. »

Herschel originally named it Georgium Sidus (George’s Star) in honour of King George
III of Great Britain (c.f. American poet Elizabeth Graeme Fergusson’s “Upon the
Discovery of the Planet...” about the event). When it was pointed out that sidus means star
and not planet, Herschel rebaptised it the Georgian Planet. This name was not acceptable
outside of Britain. Lalande proposed in 1784 to name it Herschel, at the same time
that he created the planet’s (astrological) symbol (“a globe surmounted by your
initial”); his proposal was readily adopted by French astronomers. Prosperin, of Uppsala,
proposed the names Astraea, Cybele, and Neptune (now borne by two asteroids and
another planet). Lexell, of St. Petersburg, compromised with George III’s Neptune and
Great-Britain’s Neptune. Bernoulli, from Berlin, suggested Hypercronius and
Transaturnis. Lichtenberg, from Göttingen, chimed in with Austräa, a goddess mentioned
by Ovid (but who is traditionally associated with Virgo). The name Minerva was also
proposed. Finally, Bode, as editor of the Berliner Astronomisches Jahrbuch, opted for
Uranus, after Latinized version of the Greek god of the sky, Ouranos; Maximilian Hell
followed suit by using it in the first ephemeris, published in Vienna and computed by the
Benedictine priest Placido Fixlmillner. The earliest publication to include Uranus in its
title, according to NASA’s ADS, was in 1823 (Schwerd, Opposition des Uranus 1821,
Astronomische Nachrichten, Vol. 1, pp. 18-21). The name was in use in Germany at least

as far back as 1791, however (Fixlmillner, Acta Astronomica Cremifanensia, Steyr, AT:
Franz Josef Medter, 1791). Examination of earliest issues of Monthly Notices of the Royal
Astronomical Society from 1827 shows that the name Uranus was already the most
common name used even by British astronomers by then, and probably earlier. The name
Georgium Sidus or “the Georgian” were still used infrequently (by the British alone)
thereafter. The final holdout was HM Nautical Almanac Office, which did not switch to
Uranus until 1850.

The stressed syllable in the name is properly the first, antepenultimate syllable, since in
Latin the penultimate vowel a is short (ūrănŭs) and in an open syllable, and such
syllables are never stressed in Latin. The historically correct pronunciation of the name

by English-speakers is therefore [ jurənəs] or [ jurənʌs]. The historically incorrect
pronunciations [ju reɪnəs] or [jə reɪnəs], with stress on the second syllable and a “long

a” (ūrānŭs) have become very common, however, perhaps through the influence of

the related adjective “Uranian” (always pronounced [ju reɪniən] or [jə reɪniən]) or the

similarly-pronounced name of the element uranium.

In the Chinese, Japanese, Korean, and Vietnamese languages, the planet’s name is
literally translated as the sky king star ( ).

Physical characteristics

Composition

Uranus is composed primarily of gas and various ices. The atmosphere is about 83
percent hydrogen, 15 percent helium, 2 percent methane and traces of acetylene. The
interior is richer in heavier elements, most likely compounds of oxygen, carbon, and
nitrogen, as well as rocky materials. This is in contrast to Jupiter and Saturn which are
mostly hydrogen and helium. Uranus (like Neptune) is very much similar to the cores of
Jupiter and Saturn without the massive fluid metallic hydrogen envelope. Uranus’ cyan
color is due to the absorption of red light by atmospheric methane. Surface temperature
on Uranus’s cloud cover is approximately 55 K (−218 °C or −360 °F).

Axial tilt

One of the most distinctive features of Uranus is its axial tilt of ninety-eight degrees.
Consequently, for part of its orbit one pole faces the Sun continually while the other pole
faces away. At the other side of Uranus’ orbit the orientation of the poles towards the Sun
is reversed. Between these two extremes of its orbit the Sun rises and sets around the
equator normally.

At the time of Voyager 2’s passage in 1986, Uranus’ south pole was pointed almost
directly at the Sun. The labelling of this pole as “south” uses the coordinate definitions
currently endorsed by the International Astronomical Union, namely that the north pole
of a planet of satelite shall be the pole which points above the invariable plane of the

solar system (regardless of the direction the planet is spinning) . A different system is
sometimes used, defining a body’s north and south poles according to the right-hand rule
in relation to the direction of rotation . In terms of this latter coordinate system it was
Uranus’s north pole which was in sunlight in 1986. On page 47 of the September 2006
issue of the Sky at Night magazine, Patrick Moore, commenting on the issue, sums it up
with “take your pick!”

One result of this orientation is that the polar regions of Uranus receive a greater energy
input from the Sun than its equatorial regions. Uranus is nevertheless hotter at its equator
than at its poles, although the underlying mechanism which causes this is unknown. The
reason for Uranus’ extreme axial tilt is also not known. It is speculated[citation needed] that
during the formation of the Solar System, an Earth sized protoplanet collided with
Uranus, causing the skewed orientation.

It appears that Uranus’ extreme axial tilt also results in extreme seasonal variations in its
weather. During the Voyager 2 flyby, Uranus’ banded cloud patterns were extremely
bland and faint. Recent Hubble Space Telescope observations, however, show a more
strongly banded appearance now that the Sun is approaching Uranus’ equator. By 2007
the Sun will be directly over Uranus’s equator.

Magnetic field

Uranus’ magnetic field is peculiar since it is not originating from the geometric center of
the planet and is tilted almost 60° from the axis of rotation. It is probably generated by
motion at relatively shallow depths within Uranus. Neptune has a similarly displaced
magnetic field, which suggests the magnetic field is not necessarily a consequence of
Uranus’ axial tilt. The magnetotail is twisted by the planet’s rotation into a long
corkscrew shape behind the planet. The magnetic field’s source is unknown.

Explanation for bland atmosphere

The internal heat of Uranus is lower than that of Jupiter and Saturn. Both Jupiter and
Saturn radiate more energy than they receive from the Sun. This causes many powerful
convection currents to form in the atmosphere. On Uranus that heat source is much lower
due to its lower mass, with the temperature of its core roughly 7000K compared to
30000K at Jupiter’s core and 18000K at Saturn. The convection currents formed in the
Uranian atmosphere are not as strong and hence it lacks the atmosphere banding of the
larger gas giants. However, as stated, above, the weather patterns of Uranus do vary with
season, being more pronounced at the equinoxes than at the solstices.

Cloud Features

For a short period in Autumn 2004, a number of large clouds appeared in the Uranian
atmosphere, giving it a Neptune-like appearance

Planetary rings

Uranus with its rings in false color

Uranus has a faint planetary ring system, composed of dark particulate matter up to ten
meters in diameter. This ring system was discovered in March 1977 by James L. Elliot,
Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. The
discovery was serendipitous; they planned to use the occultation of a star by Uranus to
study the planet’s atmosphere. However, when their observations were analyzed, they
found that the star had disappeared briefly from view five times both before and after it
disappeared behind the planet. They concluded that there must be a ring system around
the planet; it was directly detected when Voyager 2 passed Uranus in 1986. As of 2005,
13 rings had been identified. In December 2005, the Hubble Space Telescope
photographed a pair of previously unknown rings. The largest is twice the diameter of the
planet’s previously known rings. The new rings are so far from the planet that they are
being called Uranus’s “second ring system.” Hubble also spotted two small satellites.
One shares its orbit with one of the newly discovered rings. The new data reveals that the
orbits of Uranus’s family of inner moons have changed significantly in the last decade.

In April 2006, information about the color of the outer rings was published, one of them
appearing spectrally blue and the other red. The rest of the planet’s rings appear grey.
The blue ring is thought to get its color from being swept by a moon, which may draw
away all large debris, leaving only fine dust which refracts light in much the same way
the Earth’s atmosphere does.

Natural satellites

Uranian moon montage

Uranus has 27 known natural satellites. The names for these satellites are chosen from
characters from the works of Shakespeare and Alexander Pope. The five main satellites
are Miranda, Ariel, Umbriel, Titania and Oberon.

The main Uranian moons
(compared to Earth’s Moon)

Name Diameter Mass Orbital radius Orbital period
(Pronunciation key)
(km) (kg) (km) (d)

Miranda mə-ran’-də 470 7.0×1019 129,000 1.4
Ariel (14%) (0.1%) (35%) (5%)
/mɪˈrændə/
arr’-ee-əl 1160 14×1020 191,000 2.5
(33%) (1.8%) (50%) (10%)
/ ɛəriəl/

Umbriel um’-bree-əl 1170 12×1020 266,000 4.1
/ ʌmbriəl/ (34%) (1.6%) (70%) (15%)

tə-taan’-yə 1580 35×1020 436,000 8.7
Titania (45%) (4.8%) (115%) (30%)

/tɪˈtɑ:njə/ or /tɪˈteɪnjə/

Oberon oe’-bər-on 1520 30×1020 584,000 13.5
/ oʊbərɒn/ (44%) (4.1%) (150%) (50%)

For a timeline of discovery dates, see Timeline of discovery of Solar System planets
and their natural satellites

Visibility

Size comparison of Earth and Uranus

The brightness of Uranus is between magnitude +5.5 and +6.0, so it can be seen with the
naked eye as a faint star under dark sky conditions. It can be easily found with binoculars.
From Earth, it has a diameter of four arc-seconds. In larger amateur telescopes with an
objective diameter greater than 12” (30cm) the planet appears as a pale blue disc with
distinct limb shading, and two of the larger satellites, Titania and Oberon, may be visible.
Even in large professional instruments no details can be seen on its disc. However,

infrared studies of its atmosphere using adaptive optics have yielded interesting data in
the years since the Voyager flyby.

Neptune

Neptune

Discovery

Discovered by Urbain Le Verrier
John Couch Adams
Johann Galle

Discovered on September 23, 1846

Orbital characteristics (Epoch J2000)

Semi-major axis 4,498,252,900 km
30.068 963 48 AU

Orbital circumference 28.263 Tm
188.925 AU

Eccentricity 0.008 585 87

Perihelion 4,459,631,496 km
29.810 795 27 AU

Aphelion 4,536,874,325 km
30.327 131 69 AU

Orbital period 60,223.3528 d
(164.88 a)

Synodic period 367.49 d

Avg. Orbital Speed 5.432 km/s

Max. Orbital Speed 5.479 km/s

Min. Orbital Speed 5.385 km/s

Inclination 1.769 17°
(6.43° to Sun’s equator)

Longitude of the 131.721 69°
ascending node

Argument of the 273.249 66°
perihelion

Number of satellites 13

Physical characteristics

Equatorial diameter 49,528 km
(3.883 Earths)

Polar diameter 48,681 km
(3.829 Earths)

Oblateness 0.0171
Surface area
7.619×109 km2
Volume (14.94 Earths)

Mass 6.254×1013 km3
(57.74 Earths)
Mean density
Equatorial gravity 1.0243×1026 kg
(At 1 bar) (17.147 Earths)

1.638 g/cm3

m/s2 (1.14
g)

Escape velocity 23.5 km/s

Rotation period 16.11 h (16 h 6 min 36 s) 1

Rotation velocity 2.68 km/s = 9660 km/h (at the
equator)
Axial tilt 28.32°
Right ascension 299.33° (19 h 57 min 20 s)
of North pole
Declination 42.95°
Albedo 0.41
Surface temp. min mean max
50 K 53 K N/A K

Adjective Neptunian

Atmospheric characteristics

Atmospheric pressure < 100 kPa

Hydrogen - H2 80% ±3.2%

Helium - He 19% ±3.2%

Methane - CH4 1.5% ±0.5%

Hydrogen Deuteride - 192 ppm
HD

Ethane - C2H6 1.5 ppm

Neptune (IPA: /< nɛpt(j)u< n/) is the eighth and outermost planet in our solar system. It is
the fourth-largest planet by diameter and the third-largest by mass; Neptune is 17 times
the mass of Earth and is slightly more massive than its near twin Uranus which is 14
Earth Masses, but slightly smaller due to its higher density. The planet is named after the
Roman god of the sea. Its astronomical symbol is a stylized version of the god’s trident.

Neptune’s atmosphere is primarily composed of hydrogen and helium, with traces of
methane that account for the planet’s blue appearance. Neptune’s blue colour is much
more vivid than that of Uranus, which has a similar amount of methane, so an unknown
component is presumed to cause Neptune’s intense color. Neptune also has the strongest
winds of any planet in the solar system, with estimates as high as 2,500 km/h or 1,500
mph. At the time of the 1989 Voyager 2 flyby, it had in its southern hemisphere a Great
Dark Spot comparable to the Great Red Spot on Jupiter. Neptune’s temperature at its
cloud tops is usually close to −210< (−346°F), one of the coldest in the solar system, due
to its long distance from the sun. Neptune’s center is about 7,000< (13,000°F), however,
hotter than the sun’s surface. This is due to extremely hot gases and rock in the center.
However, the outermost layers of the planet are extremely cold.

Faint dark colored rings have been detected around the blue planet, but are much less
substantial than those of Saturn. When these rings were discovered by a team led by
Edward Guinan, it was thought that they might not be complete but this was disproved by
Voyager 2. Neptune possesses thirteen confirmed moons. Neptune’s largest moon, Triton,
is notable for its retrograde orbit, extreme cold (38K), and extremely tenuous (14
microbar) nitrogen/methane atmosphere.

Discovered on September 23, 1846, Neptune is notable for being the first planet
discovered based on mathematical prediction rather than regular observations.
Perturbations in the orbit of Uranus led astronomers to deduce Neptune’s existence. It has
been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25,
1989. In 2003, there was a proposal to NASA’s “Vision Missions Studies” to implement a
“Neptune Orbiter with Probes” mission that does Cassini-level science without fission-
based electric power or propulsion. The work is being done in conjunction with JPL and
the California Institute of Technology.

Discovery

Galileo’s astronomical drawings show that he had first observed Neptune on December
27, 1612, and again on January 27, 1613; on both occasions Galileo had mistaken
Neptune for a fixed star when it appeared very close (in conjunction) to Jupiter in the
night sky. Believing it to be a fixed star, he cannot be credited with its discovery. At the
time Galileo first observed Neptune on December 28, 1612, it was stationary in the sky
because it had just turned retrograde that very day;[citation needed] because it was stationary
in the sky and only beginning the planet’s yearly retrograde cycle, its motion was far too
slight to be detected with Galileo’s small telescope.

Size comparison of Neptune and Earth

In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus.
Subsequent observations revealed substantial deviations from the tables, leading Bouvard
to hypothesize some perturbing body. In 1843, John Couch Adams calculated the orbit of
an eighth planet that would account for Uranus’ motion. He sent his calculations to Sir
George Airy, the Astronomer Royal, who asked Adams for a clarification. Adams began
to draft a reply but never sent it.

In 1846, Urbain Le Verrier, independently of Adams, produced his own calculations but
also experienced difficulties in encouraging any enthusiasm in his compatriots. However,
in the same year, John Herschel started to champion the mathematical approach and
persuaded James Challis to search for the planet.

After much procrastination, Challis began his reluctant search in July 1846. However, in
the meantime, Le Verrier had convinced Johann Gottfried Galle to search for the planet.
Though still a student at the Berlin Observatory, Heinrich d’Arrest suggested that a
recently drawn chart of the sky, in the region of Le Verrier’s predicted location, could be
compared with the current sky to seek the displacement characteristic of a planet, as
opposed to a fixed star. Neptune was discovered that very night, September 23, 1846,
within 1° of where Le Verrier had predicted it to be, and about 10° from Adams’
prediction. Challis later realized that he had observed the planet twice in August, failing
to identify it owing to his casual approach to the work.

In the aftermath of the discovery, there was much nationalistic rivalry between the French
and the British over who had priority and deserved credit for the discovery. Eventually an
international consensus emerged that both Le Verrier and Adams jointly deserved credit.
However, the issue is now being re-evaluated by historians with the rediscovery in 1998
of the “Neptune papers” (historical documents from the Royal Greenwich Observatory),
which had apparently been misappropriated by astronomer Olin Eggen for nearly three
decades and were only rediscovered (in his possession) immediately after his death. After

reviewing the documents, some historians now suggest that Adams does not deserve
equal credit with Le Verrier.

Naming

Shortly after its discovery, Neptune was referred to simply as “the planet exterior to
Uranus” or as “Le Verrier’s planet.” The first suggestion for a name came from Galle. He
proposed the name Janus. In England, Challis put forth the name Oceanus, particularly
appropriate for a seafaring people. In France, Arago suggested that the new planet be
called Leverrier, a suggestion which was met with stiff resistance outside France. French
almanacs promptly reintroduced the name Herschel for Uranus and Leverrier for the new
planet.

Meanwhile, on separate and independent occasions, Adams suggested altering the name
Georgian to Uranus, while Leverrier (through the Board of Longitude) suggested
Neptune for the new planet. Struve came out in favor of that name on December 29, 1846,
to the Saint Petersburg Academy of Sciences. Soon Neptune became the internationally
accepted nomenclature. In Roman mythology, Neptune was the god of the sea,
identified with the Greek Poseidon. The demand for a mythological name seemed to be
in keeping with the nomenclature of the other planets, all of which, except for Uranus,
were named in antiquity.

The planet’s name is translated literally as the sea king star in the Chinese, Korean,
Japanese, and Vietnamese languages (< < < in Chinese characters, < < < in Korean).

Physical characteristics

The Great Dark Spot, as seen from Voyager 2.

Relative size

At 1.0243×1026 kg Neptune is an intermediate body between Earth and the largest gas
giants: it is seventeen Earth masses but just 1/18th the mass of Jupiter. It and Uranus are
often considered a sub-class of gas giant termed “ice giants”, given their smaller size and
important differences in composition relative to Jupiter and Saturn. In the search for