Educational Product

Educators Grades K–12+

LS-2005-12-003-HQ

JPL 400-1253A 12/05

National Aeronautics and Space Administration

www.nasa.gov

This set contains the following lithographs:

• Our Solar System • Mars • Jupiter • Neptune

• Our Star – The Sun • Asteroids • Galilean Moons • Pluto and Charon

of Jupiter • Mercury • Meteors and • Comets

Meteorites • Venus • Saturn • Kuiper Belt 

• Moons of the • Moons of Saturn and Oort Cloud • Earth Solar System • Uranus • Earth’s Moon
www .

.

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National Aeronautics and Space Administration Solar System Lithograph Set 

www.nasa.gov

NASA EDUCATIONAL RESOURCES 

NASA’s Central Operation of Resources for Educators (CORE) was established for the national and international distribution of NASA-produced 

educational materials in multimedia format. Educators can obtain a catalogue and an order form by one of the following methods: 

NASA CORE 

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Phone: (440) 775-1400 

FAX: (440) 775-1460 

E-mail: nasaco@leeca.org 

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NASA Educator Resource Center (ERC) network. Educators may preview, 

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you are unable to visit the ERC that serves your geographic area. A list of 

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Phone: (757) 824-2214 

Regional Educator Resource Centers offer more educators access 

to NASA educational materials. NASA has formed partnerships with 

universities, museums, and other educational institutions to serve as 

regional ERCs in many states. A complete list of regional ERCs is available through CORE, or electronically via the NASA Portal at education. 

nasa.gov/ercn. 

The NASA Portal serves as the gateway for information regarding 

content, programs, and services offered by NASA for the general public 

and, specifically, for the education community with the goals to inform, 

involve, and inspire. NASA’s goal is to improve interactions for students, 

educators, and families with NASA and its education resources. Visit the 

NASA Portal and begin a journey of personal discovery — www.nasa.gov.. 

NASA’s Education Home Page serves as the education portal for information regarding educational programs and services offered by NASA for 

the American education community. This high-level directory of information provides specific details and points of contact for all of NASA’s educational efforts, Field Center offices, and points of presence within each 

state. Visit this resource at education.nasa.gov.. 

NASA features electronic resources specifically developed for the educational community. To locate these resources, visit the NASA home page 

at www.nasa.gov; click on the “For Educators” bar, then click on the grade 

level in which you are interested. 

NASA Television (NTV) features International Space Station (ISS) and 

Space Shuttle mission coverage, live special events, interactive educational live shows, electronic field trips, aviation and space news, and historical NASA footage. Programming includes the Video (News) File, NASA 

Gallery, and Education File, beginning at noon Eastern and repeated four 

more times throughout the day. Check the Internet for program listings at 

www.nasa.gov/multimedia/nasatv/. 

For more information on NTV, contact: NASA TV, NASA Headquarters, 

Washington, DC 20546-0001, phone: (202) 358-1308.

NTV Weekday Programming Schedules (Eastern Times):

Video File — 6–7 a.m., 10–11 a.m., 12–1 p.m., 3–4 p.m., 6–7 p.m., 

10–11 p.m., 12–1 a.m.

NASA Gallery — 1–6 a.m., 1–3 p.m. 

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ISS Mission Coverage — 7–8 a.m., 11 a.m.–12 p.m., 7–8 p.m., 

11 p.m.–12 a.m.

Saturday and Sunday: The weekday schedule repeats on Saturday and 

Sunday with one exception: the 8 a.m. Education File runs an additional 

two hours, until noon instead of 10 a.m. Live feeds preempt regularly 

scheduled programming.

How to Access Information on NASA’s Education Program, Materials, 

and Services (EP-2002-07-345-HQ) — This brochure serves as a guide 

to accessing a variety of NASA materials and services for educators. Copies are available through the ERC network or through the NASA Education home page at education.nasa.gov.

EDUCATORS

Please take a moment to evaluate this product at 

ehb2.gsfc.nasa.gov/edcats/lithograph_set/ 

Your evaluation and suggestions are vital to 

continually improving NASA educational materials. Thank you.

LS-2005-12-003-HQ — JPL 400-1253A 12/05
National Aeronautics and Space Administration Our Solar System 

www.nasa.gov

Mercury

Venus

Earth

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto
planetarynames.wr .

Mean Distance 

Equatorial from the Sun 

Body 

Radius km, mi, 

km mi millions millions Moons* 

Sun 695,500 432,200 — — — 

Mercury 2,440 1,516 57.91 35.98 0 

Venus 6,052 3,760 108.21 67.24 0 

Earth 6,378 3,963 149.60 92.96 1 

Moon 1,737.4 1,079.6 ** ** — 

Mars 3,397 2,111 227.94 141.63 2 

Jupiter 71,492 44,423 778.41 483.68 62 

Saturn 60,268 37,449 1,426.73 886.53 47 

Uranus 25,559 15,882 2,870.97 1,783.94 27 

Neptune 24,764 15,388 4,498.25 2,795.08 13 

Pluto 1,180 733 5,906.38 3,670.05 1*** 

Our Solar System 

.

National Aeronautics and Space Administration

www.nasa.gov

Humans have gazed upon and tried to understand the cosmos for 

thousands of years. Ancient civilizations placed great emphasis 

on careful astronomical observation, building impressive monuments that were used to observe the skies. Stories were created 

about the objects in the heavens and many different names were 

given to them. Ancient Greek astronomers were among the first 

to leave a written record of their attempts to explain the cosmos. 

For them, the universe was Earth, the Sun, the Moon, the stars, 

and five glowing points of light that move among the stars. 

The Greeks named the five points of light after their gods. The 

later Romans named the five bright objects after their own gods 

— Mercury, Venus, Mars, Jupiter, and Saturn — and these are 

the names astronomers use today. Planetary features are named 

by the International Astronomical Union, founded in 1919. For 

more information about names, consult the Gazetteer of Planetary 

Nomenclature website at planetarynames.wr.usgs.gov.. 

Ancient observers believed that the Sun and all the other stars 

revolved around Earth. Astronomers came to realize that this 

Earth-centered model did not account for the motions of the 

planets. With the development of a Sun-centered model, our understanding of the solar system and the universe deepened. In the 

early 17th century, Galileo Galilei’s discoveries using the recently 

invented telescope strongly supported the Sun-centered model. 

With the telescope as a tool, planetary moons, the rings of Saturn, 

and three more planets were eventually discovered: Uranus (in 

1781), Neptune (1846), and Pluto (1930). Telescopes also aided 

in the discovery of asteroids and the study of comets. 

The four planets closest to the Sun — Mercury, Venus, Earth and 

Mars — are called the terrestrial planets because they have solid 

rocky surfaces. The four outer planets beyond the orbit of Mars 

— Jupiter, Saturn, Uranus, and Neptune — are called gas giants. 

Tiny, distant Pluto has a solid surface covered with ice. 

Earth’s atmosphere is primarily nitrogen and oxygen. Mercury has 

no atmosphere, while Venus has a thick atmosphere of mainly 

carbon dioxide. Mars’ carbon dioxide atmosphere is extremely 

thin. The gas giants Jupiter, Saturn, Uranus, and Neptune are 

mostly composed of hydrogen and helium. When Pluto is near 

the Sun, it has a thin atmosphere, but when Pluto travels to the 

outer parts of its orbit, the atmosphere freezes. 

Moons, rings, and magnetic fields characterize the planets. There 

are 153 known natural satellites (moons) orbiting the planets, 

and they are not all alike. One moon (Saturn’s Titan) has a thick 

atmosphere; another has active volcanoes (Jupiter’s Io). Rings are 

an intriguing planetary feature. From 1659 to 1977, Saturn was 

thought to be the only planet with rings. NASA’s Voyager missions to the outer planets showed that Jupiter, Uranus, and Neptune also have ring systems. Most of the planets have magnetic 

fields that extend into space and form a magnetosphere around 

each planet. These magnetospheres rotate with the planet, 

sweeping charged particles with them. 

How big is our solar system? To think about the large distances 

involved, we use a cosmic ruler based on the astronomical unit 

(AU). One AU is the distance from Earth to the Sun, which is 

about 150 million kilometers or 93 million miles. To envision 

the Sun in the Milky Way galaxy, think of a ship moving through 

the ocean. The area of the Sun’s influence stretches far beyond 

the planets, forming a giant bubble called the heliosphere. The 

enormous bubble of the heliosphere is created by the solar wind, 

a stream of charged gas blowing outward from the Sun. As the 

Sun orbits the center of the Milky Way, the bubble of the heliosphere moves also, creating a bow shock — like a ship in water 

— ahead of it in interstellar space as it crashes into the interstellar gases. The area where the solar wind is suddenly slowed by 

pressure from gas between the stars is called the termination 

shock. 

A spacecraft that reached the termination shock would be able 

to measure the slowing effect, and that is exactly what happened 

when Voyager 1, traveling at a speed of 3.6 AU per year, began 

sending unusual data to Earth in November and December 2003. 

In May 2005, NASA confirmed that Voyager 1, 26 years after its 

launch, had entered the vast, turbulent expanse where the Sun’s 

influence wanes — at about 94 AU, approximately 13 billion 

kilometers (8.7 billion miles) from the Sun. This is the solar 

system’s final frontier. Voyager 1 is anticipated to have electrical 

power to continue to send data until at least 2020, when it may 

actually have traveled all the way through the last of the Sun’s 

magnetic region and will enter an interstellar environment. It will 

be thousands of years before Voyager exits the region of the solar 

system’s enormous Oort Cloud. 

In 2004, President George W. Bush announced an exciting new 

Vision for Space Exploration that includes sustained robotic and 

human exploration of the solar system and beyond. It begins 

with robotic exploration of Earth’s Moon with an orbiter and then 

a lander, with a human return to the Moon by 2018. The Moon 

would be a testbed for technologies to support human exploration of the Moon, Mars, and beyond. 

As we explore the universe, we wonder: Are there other planets 

where life might exist? Are we alone? Only recently have astronomers had the tools to detect large planets around other stars in 

other solar systems using ground- and space-based telescopes. 

FAST FACTS 

*Known moons as of November 2005. 

**Mean Earth–Moon distance: 384,400 kilometers or 238,855 miles.

***In October 2005, NASA announced that Hubble Space Telescope scientists had found two possible additional moons of Pluto.

ABOUT THE ILLUSTRATION 

The planets are shown in the correct order of distance from the 

Sun, the correct relative sizes, and the correct relative orbital 

distances. The sizes of the bodies are greatly exaggerated relative 

to the orbital distances. The faint rings of Jupiter, Uranus, and 

Neptune are not shown. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys 

solarsystem.nasa.gov/education/ 

LG-2005-12-563-HQ — JPL 400-1253B 12/05
National Aeronautics and Space Administration Our Star — The Sun 

www.nasa.gov
3 onal mass ejection from the 

Our Star — The Sun 

5

National Aeronautics and Space Administration

www.nasa.gov

2 elds 

4 s corona taken in three wave5 ge sunspots in the photosphere were associated 

Our solar system’s star, the Sun, has inspired mythological stories 

in cultures around the world, including those of the ancient 

Egyptians, the Aztecs of México, Native American tribes of North 

America and Canada, the Chinese, and many others. A number 

of ancient cultures built stone structures or modified natural rock 

formations to observe the Sun and Moon — they charted the seasons, created calendars, and monitored solar and lunar eclipses. 

These architectural sites show evidence of deliberate alignments 

to astronomical phenomena: sunrises, moonrises, moonsets, even 

stars or planets. 

The Sun is the closest star to Earth, at a mean distance from our 

planet of 149.60 million kilometers (92.96 million miles). This 

distance is known as an astronomical unit (abbreviated AU), and 

sets the scale for measuring distances all across the solar system. 

The Sun, a huge sphere of mostly ionized gas, supports life on 

Earth. It powers photosynthesis in green plants, and is ultimately 

the source of all food and fossil fuel. The connection and interactions between the Sun and Earth drive the seasons, ocean currents, weather, and climate. 

The Sun is 332,900 times more massive than Earth and contains 

99.86 percent of the mass of the entire solar system. It is held 

together by gravitational attraction, producing immense pressure 

and temperature at its core. The Sun has six regions — the core, 

the radiative zone, and the convective zone in the interior; the 

visible surface, known as the photosphere; the chromosphere; 

and the outermost region, the corona. 

At the core, the temperature is about 15 million degrees Celsius 

(about 27 million degrees Fahrenheit), which is sufficient to 

sustain thermonuclear fusion. The energy produced in the core 

powers the Sun and produces essentially all the heat and light we 

receive on Earth. Energy from the core bounces around the radiative zone, taking about 170,000 years to get to the convective 

zone. The temperature drops below 2 million degrees Celsius 

(3.5 million degrees Fahrenheit) in the convective zone, where 

large bubbles of hot plasma (a soup of ionized atoms) move 

upwards. 

The Sun’s “surface” — the photosphere — is a 500-kilometerthick (300-mile-thick) region, from which most of the Sun’s radiation escapes outward and is detected as the sunlight we observe 

here on Earth about eight minutes after it leaves the Sun. Sunspots in the photosphere are areas with strong magnetic fields 

that are cooler, and thus darker, than the surrounding region. 

The temperature of the photosphere is about 5,500 degrees Celsius (10,000 degrees Fahrenheit). Above the photosphere lie the 

tenuous chromosphere and the corona. Visible light from these 

top regions is usually too weak to be seen against the brighter 

photosphere, but during total solar eclipses, when the Moon covers the photosphere, the chromosphere can be seen as a red rim 

around the Sun and the corona forms a beautiful white halo. 

Above the photosphere, the temperature increases with altitude, 

reaching temperatures as high as 2 million degrees Celsius 

(3.5 million degrees Fahrenheit). The source of coronal heating has been a scientific mystery for more than 50 years. Likely 

solutions have emerged from observations by the Solar and 

Heliospheric Observatory (SOHO) and the Transition Region 

and Coronal Explorer (TRACE) missions, which found patches 

of magnetic field covering the entire solar surface. Scientists 

now think that this magnetic “carpet” is probably a source of the 

corona’s intense heat. The corona cools rapidly, losing heat as radiation and in the form of the solar wind — a stream of charged 

particles that flows to the edge of the solar system. 

FAST FACTS 

Spectral Type of Star G2V 

Age 4.6 billion years 

Mean Distance to Earth 149.60 million km (92.96 million mi) 

(1 astronomical unit) 

Rotation Period at Equator 26.8 days 

Rotation Period at Poles 36 days 

Equatorial Radius 695,500 km (432,200 mi) 

Mass 1.989 × 1030 kg 

Density 1.409 g/cm3 

Composition 92.1% hydrogen, 7.8% helium 

Surface Temperature (Photosphere) 5,500 deg C (10,000 deg F) 

Luminosity* 3.83 × 1033 ergs/sec 

*Luminosity measures the total energy radiated by the Sun (or 

any star) per second at all wavelengths. 

SIGNIFICANT DATES 

150 AD — Greek scholar Claudius Ptolemy launches a millennium of misconception when he writes that the Sun and planets 

revolve around Earth. 

1543 — Nicolaus Copernicus publishes On the Revolutions of the 

Celestial Spheres describing his heliocentric (Sun-centered) model 

of the solar system, beginning a new age of astronomy. 

1645–1715 — Sunspot activity declines to almost zero, possibly 

causing a “Little Ice Age” on Earth. 

1860 — Eclipse observers see a massive burst of material from 

the Sun; it is the first recorded coronal mass ejection. 

1994 — The Ulysses spacecraft makes history as it makes the 

first observations of the Sun’s polar regions, which cannot be 

studied from Earth. 

ABOUT THE IMAGES 

1 2

3

4 5 Magnetic fi 2 Magnetic fiMagnetic fiMagnetic fields 

are believed to cause 

huge, superhot coro5

1 Two huge clouds 

of plasma erupt from 

the chromosphere 

of the Sun (SOHO 

image).

nal loops to tower above sunspots visible in the photosphere and 

chromosphere (TRACE image).

 This illustration shows a cor 3 This illustration shows a cor This illustration shows a cor This illustration shows a coronal mass ejection fr onal mass ejection fr onal mass ejection from the 

chromosphere and interaction with Earth’s magnetic field (not 

to scale).

 A composite image of the Sun’

 These lar

4 A composite image of the Sun’ A composite image of the Sun’ A composite image of the Sun’s corona taken in thr ona taken in thr ona taken in three wave-ee wave-ee wavelengths emitted at different temperatures shows a very active star 

(SOHO image).

5 These larThese larThese large sunspots in the photospher ge sunspots in the photospher ge sunspots in the photosphere were associatede associatede associated 

with several powerful solar flares in 2003 (SOHO image). 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Sun 

LG-2005-12-564-HQ — JPL 400-1253C 12/05
National Aeronautics and Space Administration Mercury

www.nasa.gov
 Mer 1 cury’s south-

 T 2 erraces and a 

 A scarp (clif

 A Mariner 10 photomosaic of a portion of the Caloris Basin.

4 f) more than 300 kilometers (185 miles) long 

 A close-up of Mer 5 cury’s south pole taken by Mariner 10 in 

3

National Aeronautics and Space Administration Mercury

www.nasa.gov

Mercury’s elliptical orbit takes the small planet as close as 47 million kilometers (29 million miles) and as far as 70 million kilometers (43 million miles) from the Sun. If one could stand on the 

scorching surface of Mercury when it is at its closest point to the 

Sun, the Sun would appear almost three times as large as it does 

when viewed from Earth. Temperatures on Mercury’s surface 

can reach 430 degrees Celsius (800 degrees Fahrenheit). Because 

the planet has no atmosphere to retain that heat, nighttime 

temperatures on the surface can drop to –170 degrees Celsius 

(–280 degrees Fahrenheit). 

Because Mercury is so close to the Sun, it is hard to directly 

observe from Earth except during twilight. Mercury makes an 

appearance indirectly, however — 13 times each century, Earth 

observers can watch Mercury pass across the face of the Sun, an 

event called a transit. These rare transits fall within several days 

of May 8 and November 10. The first two transits of Mercury in 

the 21st century occur May 7, 2003, and November 8, 2006. 

Scientists used to think that the same side of Mercury always 

faces the Sun, but in 1965 astronomers discovered that the planet rotates three times during every two orbits. Mercury speeds 

around the Sun every 88 days, traveling through space at nearly 

50 kilometers (31 miles) per second — faster than any other 

planet. One Mercury day equals 175.97 Earth days. 

Rather than an atmosphere, Mercury possesses a thin “exosphere” made up of atoms blasted off its surface by the solar 

wind and striking micrometeoroids. Because of the planet’s extreme surface temperature, the atoms quickly escape into space. 

With the thin exosphere, there has been no wind erosion of the 

surface and meteorites do not burn up due to friction as they do 

in other planetary atmospheres. 

Mercury’s surface resembles that of Earth’s Moon, scarred by 

many impact craters resulting from collisions with meteoroids 

and comets. While there are areas of smooth terrain, there are 

also lobe-shaped scarps or cliffs, some hundreds of miles long 

and soaring up to a mile high, formed by early contraction of the 

crust. The Caloris Basin, one of the largest features on Mercury, is 

about 1,300 kilometers (800 miles) in diameter. It was the result 

of an asteroid impact on the planet’s surface early in the solar 

system’s history. Over the next half-billion years, Mercury shrank 

in radius about 1 to 2 kilometers (0.6 to 1.2 miles) as the planet 

cooled after its formation. The outer crust contracted and grew 

strong enough to prevent magma from reaching the surface, ending the period of geologic activity. 

Mercury is the second smallest planet in the solar system, larger 

only than Pluto. Mercury is the second densest planet after Earth, 

with a large iron core having a radius of 1,800 to 1,900 kilometers (1,100 to 1,200 miles), about 75 percent of the planet’s 

radius. Mercury’s outer shell, comparable to Earth’s outer shell 

(called the mantle), is only 500 to 600 kilometers (300 to 400 

miles) thick. Mercury’s magnetic field is thought to be a miniature version of Earth’s, but scientists are uncertain of the strength 

of the field. 

Only one spacecraft has ever visited Mercury: Mariner 10, which 

imaged about 45 percent of the surface. In 1991, astronomers 

using radar observations showed that Mercury may have water 

ice at its north and south poles inside deep craters that are perpetually cold (below –212 degrees Celsius or –350 degrees Fahrenheit). Infalling comets or meteorites might have brought ice to 

these regions of Mercury, or water vapor might have outgassed 

from the interior and frozen out at the poles. 

A new NASA mission to Mercury called MErcury Surface, Space 

ENvironment, GEochemistry, and Ranging (MESSENGER) will 

begin orbiting Mercury in March 2011 to investigate key scientific areas such as the planet’s composition, the structure of the 

core, the magnetic field, and the materials at the poles. 

FAST FACTS 

Namesake Messenger of the Roman gods 

Mean Distance from the Sun 57.91 million km 

(35.98 million mi) 

Orbit Period 87.97 Earth days 

Orbit Eccentricity (Circular Orbit = 0) 0.206 

Orbit Inclination to Ecliptic 7 deg 

Inclination of Equator to Orbit 0 deg 

Rotation Period 58.65 Earth days 

Successive Sunrises 175.97 days 

Equatorial Radius 2,440 km (1,516 mi) 

3

Mass 0.055 of Earth’s

Density 5.43 g/cm3 (0.98 of Earth’s) 

Gravity 0.38 of Earth’s 

Exosphere Components hydrogen, helium, oxygen, sodium, 

potassium, calcium 

Temperature Range –170 to 430 deg C (–280 to 800 deg F) 

Known Moons 0 

Rings 0 

SIGNIFICANT DATES 

1631 — Pierre Gassendi uses a telescope to watch from Earth as 

Mercury crosses the face of the Sun. 

1965 — Though it was thought for centuries that the same side 

of Mercury always faced the Sun, astronomers find the planet rotates three times for every two orbits. 

1974–1975 — Mariner 10 photographs roughly half of Mercury’s 

surface in three flybys. 

1991 — Scientists using Earth-based radar find signs of ice 

tucked in permanently shadowed areas of craters in Mercury’s 

polar regions. 

2004 — MESSENGER launches on a mission to make the most 

comprehensive study yet of the innermost planet. 

ABOUT THE IMAGES

1 2

3

4

5

1 Mercury’s southern hemisphere imaged by Mariner 10.1

2 Terraces and a erraces and a 

central peak mark this 

as a complex impact 

crater. 

 3 A Mariner 10 photomosaic of a portion of the Caloris Basin.

4 A scarp (clif A scarp (cliff) more than 300 kilometers (185 miles) long e than 300 kilometers (185 miles) long 

extends from upper left to lower right in this image.

5 A close-up of Mer A close-up of Mercury’s south pole taken by Mariner 10 in s south pole taken by Mariner 10 in 

1974. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Mercury

LG-2005-12-565-HQ — JPL 400-1253D 12/05
National Aeronautics and Space Administration

Venus www.nasa.gov
 A 1979 Pioneer 1

 Impact craters ar

 Scientists used Magellan radar images to cr

 This false-color image taken by Galileo’ 5

3 e revealed in this Magellan radar image.

4 eate this three5 s 

 This view of the transit of V 6 enus of 2004 was taken in ultra-

 This Magellan 2

National Aeronautics and Space Administration Venus

www.nasa.gov

Venus and Earth are similar in size, mass, density, composition, 

and distance from the Sun. There, however, the similarities 

end. Venus is covered by a thick, rapidly spinning atmosphere, 

creating a scorched world with temperatures hot enough to melt 

lead and surface pressure 90 times that of Earth. Because of its 

proximity to Earth and the way its clouds reflect sunlight, Venus 

appears to be the brightest planet in the sky. Although we cannot 

normally see through Venus’ thick atmosphere, NASA’s Magellan 

mission to Venus used radar to image the surface, and Galileo 

used infrared mapping to view mid-level cloud structure. 

Like Mercury, Venus can be seen periodically passing across the 

face of the Sun. These “transits” of Venus occur in pairs with 

more than a century separating each pair. Since the telescope was 

invented, transits were observed in 1631, 1639; 1761, 1769; and 

1874, 1882. On June 8, 2004, astronomers worldwide saw the 

tiny dot of Venus crawl across the Sun; the second in this pair of 

early 21st-century transits occurs June 6, 2012. 

The atmosphere consists mainly of carbon dioxide, with clouds 

of sulfuric acid droplets. Only trace amounts of water have been 

detected in the atmosphere. The thick atmosphere traps the Sun’s 

heat, resulting in surface temperatures over 470 degrees Celsius 

(880 degrees Fahrenheit). Probes that have landed on Venus have 

not survived more than a few hours before being destroyed by 

the incredible temperatures. 

The Venusian year (orbital period) is about 225 Earth days long, 

while the planet’s rotation period is 243 Earth days, making a 

Venus day about 117 Earth days long. Venus rotates retrograde 

(east to west) compared with Earth’s prograde (west to east) rotation. Seen from Venus, the Sun would rise in the west and set in 

the east. As Venus moves forward in its solar orbit while slowly 

rotating “backwards” on its axis, the cloud-level atmosphere zips 

around the planet in the opposite direction from the rotation 

every four Earth days, driven by constant hurricane-force winds. 

How this atmospheric “superrotation” forms and is maintained 

continues to be a topic of scientific investigation. 

About 90 percent of the surface of Venus appears to be recently 

solidified basalt lava; it is thought that the planet was completely 

resurfaced by volcanic activity 300 to 500 million years ago. 

Sulfur compounds, possibly attributable to volcanic activity, are 

abundant in Venus’ clouds. The corrosive chemistry and dense, 

moving atmosphere cause significant surface weathering and 

erosion. Radar images of the surface show wind streaks and sand 

dunes. Craters smaller than 1.5 to 2 kilometers (0.9 to 1.2 miles) 

across do not exist on Venus, because small meteors burn up in 

the dense atmosphere before they can reach the surface. 

More than 1,000 volcanoes or volcanic centers larger than 

20 kilometers (12 miles) in diameter dot the surface of Venus. 

Volcanic flows have produced long, sinuous channels extending 

for hundreds of kilometers. Venus has two large highland areas 

— Ishtar Terra, about the size of Australia, in the north polar 

region; and Aphrodite Terra, about the size of South America, 

straddling the equator and extending for almost 10,000 kilometers (6,000 miles). Maxwell Montes, the highest mountain on 

Venus and comparable to Mount Everest on Earth, is at the eastern edge of Ishtar Terra. 

Venus has an iron core about 3,000 kilometers (1,200 miles) in 

radius. Venus has no global magnetic field — though its core 

iron content is similar to that of Earth, Venus rotates too slowly 

to generate the type of magnetic field that Earth has. 

FAST FACTS 

Namesake Roman goddess of love and beauty 

Mean Distance from the Sun 108.21 million km 

(67.24 million mi) 

Orbit Period 224.70 Earth days 

Orbit Eccentricity (Circular Orbit = 0) 0.0068 

Orbit Inclination to Ecliptic 3.39 deg 

Inclination of Equator to Orbit 177.3 deg 

Rotation Period 243.02 Earth days (retrograde) 

Successive Sunrises 116.75 days 

Equatorial Radius 6,052 km (3,760 mi) 

Mass 0.815 of Earth’s 

Density 5.204 g/cm3 (0.95 of Earth’s) 

Gravity 0.91 of Earth’s 

Atmosphere Primary Component carbon dioxide 

Temperature at Surface 470 deg C (880 deg F) 

Known Moons 0 

Rings 0 

SIGNIFICANT DATES 

650 AD — Mayan astronomers make detailed observations of 

Venus, leading to a highly accurate calendar. 

1761–1769 — Two European expeditions to watch Venus cross 

in front of the Sun lead to the first good estimate of the Sun’s 

distance from Earth. 

1962 — Mariner 2 reaches Venus and reveals the planet’s extreme surface temperatures. It is the first spacecraft to send back 

information from another planet. 

1970 — Venera 7 sends back 23 minutes of data from the surface 

of Venus. It is the first spacecraft to successfully land on another 

planet. 

1990–1994 — The Magellan spacecraft, in orbit around Venus, 

uses radar to map 98 percent of the planet’s surface. 

ABOUT THE IMAGES

1 2 3

4

5 6

1 A 1979 Pioneer A 1979 Pioneer A 1979 Pioneer 

Venus image of Venus’ clouds seen in 

ultraviolet. 

2 This Magellan This Magellan 

radar image is colorcoded to represent 

varying elevations. 

4

 3 Impact craters are revealed in this Magellan radar image.

 4 Scientists used Magellan radar images to cr Scientists used Magellan radar images to create this threedimensional view of Venus’ Maat Mons volcano.

 This false-color image taken by Galileo’ This false-color image taken by Galileo’s infrared mapping 

instrument shows details of Venus’ mid-level cloud structure. 

6 This view of the transit of Venus of 2004 was taken in ultraviolet light by NASA’s Transition Region and Coronal Explorer 

spacecraft. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Venus

LG-2005-12-566-HQ — JPL 400-1253E 12/05
National Aeronautics and Space Administration Earth

www.nasa.gov
 A true-color 1

 A view fr 2 om 

 Astr 4 onauts aboard the space shuttle took this photograph of 

 Satellites track El Niño in the Pacifi 5 c; red and yellow in this 

 A false-color image generated fr 6 om radar data. Orange indi-

 The 2003 Antar 3 ctic ozone hole was one of the largest ever 

National Aeronautics and Space Administration Earth

www.nasa.gov

Earth, our home planet, is the only planet in our solar system 

known to harbor life — life that is incredibly diverse. All the 

things we need to survive exist under a thin layer of atmosphere 

that separates us from the cold, airless void of space. Earth is 

made up of complex, interactive systems that are often unpredictable. Air, water, land, and life — including humans — combine forces to create a constantly changing world that we are 

striving to understand. 

From the vantage point of space we are able to observe our 

planet globally, as we do other planets, using sensitive instruments to understand the delicate balance among its oceans, air, 

land, and life. 

Some facts are well known. Earth is the third planet from the 

Sun and the fifth largest in the solar system. Earth’s diameter 

is just a few hundred kilometers larger than that of Venus. The 

four seasons are a result of Earth’s axis of rotation being tilted 

23.45 degrees with respect to the plane of Earth’s orbit around 

the Sun. During part of the year the northern hemisphere is tilted 

toward the Sun and the southern hemisphere is tilted away, producing summer in the north and winter in the south. Six months 

later, the situation is reversed. During March and September, 

when spring and fall begin, both hemispheres receive roughly 

equal amounts of illumination from the Sun. 

The ocean, which covers nearly 70 percent of Earth’s surface, has 

an average depth of about 4 kilometers (2.5 miles). Fresh water 

exists in the liquid phase only within a narrow temperature span 

— 0 to 100 degrees Celsius (32 to 212 degrees Fahrenheit). This 

temperature span is especially narrow when contrasted with the 

full range of temperatures found within the solar system. The 

presence and distribution of water vapor in the atmosphere is 

responsible for much of Earth’s weather. 

Near the surface, an ocean of air that consists of 78 percent 

nitrogen, 21 percent oxygen, and 1 percent other ingredients 

envelops us. This atmosphere affects Earth’s long-term climate 

and short-term local weather, shields us from nearly all harmful 

radiation coming from the Sun, and protects us from meteors as 

well — most of which burn up before they can strike the surface 

as meteorites. Earth-orbiting satellites have revealed that the upper atmosphere actually swells by day and contracts by night due 

to solar heating during the day and cooling at night. 

Our planet’s rapid spin and molten nickel–iron core give rise to 

a magnetic field, which the solar wind distorts into a teardrop 

shape. (The solar wind is a stream of charged particles continuously ejected from the Sun.) Earth’s magnetic field does not 

fade off into space, but has definite boundaries. When charged 

particles from the solar wind become trapped in Earth’s magnetic 

field, they collide with air molecules above our planet’s magnetic 

poles. These air molecules then begin to glow and are known as 

the aurorae, or the northern and southern lights. 

Earth’s lithosphere, which includes the crust (both continental 

and oceanic) and the uppermost, rigid mantle, is divided into 

huge plates that are constantly moving. For example, the North 

American plate moves west over the Pacific Ocean basin, roughly 

at a rate equal to the growth of our fingernails. Earthquakes result when plates grind past one another, ride up over one another, collide to make mountains, or split and separate. The theory 

of motion of the large plates of the lithosphere is known as plate 

tectonics. Developed within the last 40 years, this explanation 

has unified the results of centuries of study of our planet. 

FAST FACTS 

Mean Distance from the Sun 149.60 million km 

(92.96 million mi) (1 astronomical unit) 

Orbit Period 365.26 days 

Orbit Eccentricity (Circular Orbit = 0) 0.0167 

Orbit Inclination to Ecliptic 0.00005 deg 

Inclination of Equator to Orbit 23.45 deg 

Rotation Period 23.93 hr 

Successive Sunrises 24.00 hr 

Equatorial Radius 6,378 km (3,963 mi) 

Mass 5.9737 24 × 10 kg 

Density 5.515 g/cm3 

Gravity (Global Average) 9.8 m/sec2 (32.15 ft/sec2) 

Atmosphere Primary Components nitrogen, oxygen 

Surface Temperature Range –88 to 58 deg C 

(–126 to 136 deg F) 

Known Moons 1 

Rings 0 

SIGNIFICANT DATES 

1960 — NASA launches Tiros, the first weather satellite. 

1972 — Landsat 1 is launched, the first in a series that continues 

today. The images are used in agriculture, geology, forestry, regional planning, mapping, and global change research. 

1992 — Topex/Poseidon, a U.S.–France mission, begins measuring sea-surface height; the data revolutionize understanding of 

the ocean’s role in weather and climate. 

1997 — The U.S.–Japan Tropical Rainfall Measuring Mission is 

launched. It includes the first spaceborne instrument to provide 

three-dimensional maps of storm structure. 

1999–2005 — A series of Earth-observing satellites is launched 

to provide complementary data sets on Earth’s system: Terra 

(land, oceans, atmosphere), Aqua (water cycle), Aura (atmospheric chemistry), and Gravity Recovery and Climate Experiment (gravity fields). Planned are CloudSat (clouds) and the 

Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations mission (aerosols, clouds). 

ABOUT THE IMAGES

2

4

5

1

6

3 1 A true-color A true-color A true-color 

NASA satellite 

mosaic of Earth.

2 A view fr A view from 

orbit of the 2002 

eruption of Mt. 

Etna, Sicily. 

3 The 2003 Antarctic ozone hole was one of the largest ever gest ever 

recorded.

4 Astronauts aboard the space shuttle took this photograph of d the space shuttle took this photograph of 

the Aurora Australis. Aurorae are caused when charged particles 

from the Sun interact with Earth’s magnetic field.

5 Satellites track El Niño in the Pacifi Satellites track El Niño in the Pacific; red and yellow in this ed and yellow in this 

color-coded image indicate warmer ocean water. 

6 A false-color image generated from radar data. Orange indicates fastest wind speed; arrows indicate wind direction. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Earth

LG-2005-12-567-HQ — JPL 400-1253F 12/05
National Aeronautics and Space Administration Earth’s Moon 

www.nasa.gov
National Aeronautics and Space Administration Earth’s Moon 

www.nasa.gov

The regular daily and monthly rhythms of Earth’s only natural 

satellite, the Moon, have guided timekeepers for thousands of 

years. Its influence on Earth’s cycles, notably tides, has also been 

charted by many cultures in many ages. The presence of the 

Moon stabilizes Earth’s wobble on its axis, leading to a stable 

climate over billions of years, which may have affected the course 

of the development and growth of life on Earth. From Earth, we 

see the same face of the Moon all the time because the Moon rotates just once on its own axis in very nearly the same time that 

it travels once around Earth (called synchronous rotation). Patterns of dark and light features on the near side have given rise 

to the fanciful “Man in the Moon” description. The light areas are 

lunar highlands. The dark features, called maria (Latin for seas), 

are impact basins that were filled with dark lava between 4 and 

2.5 billion years ago. 

How did the Moon come to be? The leading theory, based on research, is that a Mars-sized body collided with Earth and the resulting debris from both Earth and the impactor accumulated to 

form our natural satellite approximately 4.5 billion years ago (the 

age of the oldest collected lunar rocks). When the Moon formed, 

its outer layers melted under very high temperatures, forming the 

lunar crust, probably from a global “magma ocean” — a sea of 

molten rock. The lunar highlands contain the remnants of rocks 

that floated to the surface of the magma ocean. 

After the ancient time of volcanism, the Moon cooled and has 

since been nearly unchanged, except for a steady rain of “hits” 

by meteorites and comets. Impacts over billions of years have 

ground up surface areas into powder. Because the Moon has essentially no atmosphere, even the tiniest meteorites strike the 

surface. The Moon’s surface is charcoal gray and sandy, with 

much fine soil. This powdery blanket of lunar soil is called 

the lunar regolith. The regolith is thin, ranging from about 

2 meters (7 feet) on the youngest maria to perhaps 20 meters 

(70 feet) on the oldest surfaces in the highlands. 

Four impact craters are used to date objects on the Moon: Nectaris, Imbrium, Eratosthenes, and Copernicus. Lunar history is 

carved up into time segments associated with the date of each 

crater. A Copernican feature, for example, is associated with a 

crater that is similar in age to the Copernicus impact crater, that 

is, 1 billion years old or less. 

The Moon was first visited by the U.S.S.R. spacecraft Luna 2 in 

1959, and a number of U.S. and U.S.S.R. robotic spacecraft followed. The U.S. first sent a series of Ranger spacecraft, designed 

to relay images and data and then crash-land onto the surface. 

This series was followed by the Surveyors, the first U.S. spacecraft to make lunar soft-landings. The first human landing on the 

Moon was on July 20, 1969. Twelve American astronauts walked 

upon its surface and brought back 382 kilograms (842 pounds) 

of lunar rock and soil to Earth during the Apollo lunar surface 

exploration missions of 1969 to 1972. The chief repository of the 

Apollo samples is NASA’s Johnson Space Center in Houston. 

Startling results from the Clementine and Lunar Prospector 

spacecraft indicate that there may be water ice on the Moon. 

Though a controlled crash of the Lunar Prospector in 1999 

produced no observable signature of water, the issue of whether 

ancient cometary impacts delivered ice that is harbored in dark, 

cold areas of the Moon is still an open question. 

In 2004, President George W. Bush announced a new Vision for 

Space Exploration that includes sustained robotic and human exploration of the solar system and beyond. It begins with robotic 

exploration of the Moon with an orbiter and then a lander, with a 

human return to the Moon by 2018. The Moon would be a testbed for technologies to support human exploration of the Moon, 

Mars, and beyond. 

FAST FACTS 

Mean Distance from Earth 384,400 km (238,855 mi) 

Orbit Period 27.32 Earth days 

Orbit Eccentricity (Circular Orbit = 0) 0.05490 

Orbit Inclination to Ecliptic 5.145 deg 

Inclination of Equator to Orbit 6.68 deg 

Rotation Period 27.32 Earth days 

Equatorial Radius 1,737.4 km (1,079.6 mi)

Mass 0.0123 of Earth’s

Density 3.341 g/cm3

 (0.61 of Earth’s)

Gravity 0.166 of Earth’s

Temperature Range –233 to 123 deg C (–387 to 253 deg F)

SIGNIFICANT DATES 

1610 — Galileo Galilei is the first to use a telescope to make 

scientific observations of the Moon.

1959–1960 — Luna 1, 2, and 3 are the first to fly by, impact, 

and photograph the far side of the Moon.

1966 — Surveyor 1 makes the first soft landing on the Moon.

1969 — Astronaut Neil Armstrong is the first of 12 humans to 

walk on the lunar surface.

1996 — Clementine data indicate water ice at the south pole.

1998 — Lunar Prospector data indicate that ice exists at both 

lunar poles.

ABOUT THE IMAGES

 The dark ar 1 eas 

of this lunar image 

are lava-filled impact 

basins. The bright 

ray crater on the 

bottom is the Tycho 

impact basin. 

 Apollo 12 astr 2 onaut Charles Conrad visits Surveyor 3, a 

robotic spacecraft that landed on the Moon three years earlier. 

3 This boot print marks one of the first steps human beings 

took on the Moon in July 1969. 

4 False-color images help scientists identify different types of 

soil on the Moon’s surface.

 An illustration of futur 5 e astronauts investigating a lava cave 

on the Moon. 

6 The Apollo 8 crew took this picture of Earth rising over the 

surface of the Moon in 1968.

 Coper 7 nicus Crater is part of the youngest assemblage of 

lunar rocks. The photo was taken by Lunar Orbiter 2 in 1966. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Moon

LG-2005-12-568-HQ — JPL 400-1253G 12/05
National Aeronautics and Space Administration Mars

www.nasa.gov
National Aeronautics and Space Administration Mars

www.nasa.gov

Though details on Mars’ surface are difficult to see from Earth, 

telescope observations show seasonally changing features and 

white patches at the poles. For decades, people speculated that 

bright and dark areas on Mars were patches of vegetation, that 

Mars could be a likely place for life forms, and that water might 

exist in the polar caps. When the Mariner 4 spacecraft flew by 

Mars in 1965, many were shocked to see photographs of a bleak, 

cratered surface. Mars seemed to be a dead planet. Later missions, however, have shown that Mars is a complex member of 

the solar system and holds many mysteries yet to be solved. 

Mars is a rocky body about half the size of Earth. Like the other 

terrestrial planets — Mercury, Venus, and Earth — its surface has 

been altered by volcanism, impacts, crustal movement, and atmospheric effects such as dust storms. Mars’ polar ice caps grow 

and recede with the seasons; layered areas near the poles suggest 

that the planet’s climate has changed more than once. 

Mars has no global magnetic field as does Earth. However, 

NASA’s Mars Global Surveyor orbiter found that areas of the 

martian crust in the southern hemisphere are highly magnetized 

— evidently traces of Mars’ magnetic field remained in the 

planet’s crust from about 4 billion years ago. 

Scientists believe that Mars experienced huge floods about 

3.5 billion years ago. Though we do not know where the ancient 

flood water came from, how long it lasted, or where it went, recent missions to Mars have uncovered intriguing hints. 

In 2002, NASA’s Mars Odyssey orbiter detected hydrogen-rich 

polar deposits, indicating large quantities of water ice close to the 

surface. Further observations found hydrogen in other areas as 

well. If water ice permeated the entire planet, Mars could have 

substantial subsurface layers of frozen water. In 2004, the Mars 

Exploration Rover named Opportunity found structures and 

minerals indicating that its landing site was once the shoreline of 

a salty martian sea. The rover’s twin, Spirit, also found the signature of ancient water at its landing site halfway around Mars from 

Opportunity’s location. 

Many questions remain. The cold temperatures and thin atmosphere on Mars don’t allow liquid water to exist at the surface 

for long, and the quantity of water required to carve Mars’ great 

channels and flood plains is not evident today. Unraveling the 

story of water on Mars is important to unlocking its climate history, which will help us understand the evolution of all the planets. Water is also believed to be an essential ingredient for life; 

evidence of past or present water on Mars is expected to hold 

clues about whether Mars could ever have been a habitat for life. 

Volcanism in the highlands and plains stopped 3 billion years 

ago, but some of the giant shield volcanoes are younger, forming 

between 1 and 2 billion years ago. Mars has the largest volcanic 

mountain in the solar system, Olympus Mons, as well as a spectacular equatorial canyon system, Valles Marineris. The length of 

Valles Marineris is equivalent to the distance from New York to 

Los Angeles. 

Mars has two small moons, Phobos and Deimos. Although it is 

not known how they formed, they may be asteroids snared by 

Mars’ gravity. 

FAST FACTS 

Namesake Roman god of war 

Mean Distance from the Sun 227.94 million km 

(141.63 million mi) 

Orbit Period 1.8807 Earth years 

(686.98 Earth days) 

Orbit Eccentricity (Circular Orbit = 0) 0.0934 

Orbit Inclination to Ecliptic 1.8 deg 

Inclination of Equator to Orbit 25.19 deg 

Rotation Period 24.62 hr 

Successive Sunrises 24.660 hr 

Equatorial Radius 3,397 km (2,111 mi) 

Mass 0.10744 of Earth’s 

Density 3.934 g/cm3 (0.714 of Earth’s) 

Gravity 0.38 of Earth’s 

Atmosphere Primary Components carbon dioxide, 

nitrogen, argon 

Temperature Range –143 to 17 deg C (–225 to 63 deg F) 

Known Moons* 2 

Rings 0 

*As of November 2005. 

SIGNIFICANT DATES 

1877 — Asaph Hall discovers the two moons of Mars, Phobos 

and Deimos. 

1965 — NASA’s Mariner 4 sends back 22 photos of Mars, the 

world’s first close-up photos of a planet beyond Earth.

1976 — Viking 1 and 2 make the first, and still one of the few, 

successful landings on the surface of Mars.

1997 — Mars Pathfinder lands and dispatches Sojourner, the 

first wheeled rover to explore the surface of another planet.

2004 — Twin Mars Exploration Rovers named Spirit and Opportunity land on Mars and find the strongest evidence yet obtained 

that the red planet once had long-standing bodies of water.

ABOUT THE IMAGES

 A lar 1 ge water ice 

cloud hangs over 

Olympus Mons (upper left in the image).

2 Gullies may be a 

sign that water has 

recently flowed on 

Mars. 

 Spher 3 e-like grains that once may have formed in water appear blue in this false-color image taken by Mars rover Opportunity near its landing site. 

 False color (blue) shows wher 4 e water ice is buried beneath 

the martian surface in this Mars Odyssey image.

 A view of “Endurance Crater 5 ,” near where Mars rover 

Opportunity landed in Meridiani Planum.

 Mars r 6 over Spirit uses its robotic arm to examine a rock 

named “Adirondack.”

 Mars r 7 over Spirit photographed its empty landing platform 

after it rolled onto the surface of Mars at Gusev Crater. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Mars 

LG-2005-12-569-HQ — JPL 400-1253H 12/05
National Aeronautics and Space Administration

www.nasa.gov

Asteroids
FAST FACTS 433 Eros 951 Gaspra 4 Vesta 1 Ceres 243 Ida

 This computer 4 -generated model of asteroid Golevka was 

 A false-color view of a lar 5 ge crater on Eros. Redder hues indi-

 A four 1 -image 

National Aeronautics and Space Administration Asteroids

FAST FACTS

www.nasa.gov

Asteroids, sometimes called minor planets, are small, rocky fragments left over from the formation of the solar system about 4.6 

billion years ago. Most of this ancient space rubble can be found 

orbiting the Sun between Mars and Jupiter. Asteroids range in 

size from Ceres, about one-quarter the diameter of Earth’s Moon, 

to bodies that are less than 1 kilometer (0.6 mile) across. The 

total mass of all the asteroids is less than that of the Moon. 

Early in the history of the solar system, the formation of Jupiter 

brought an end to the formation of planetary bodies in the gap 

between Mars and Jupiter and caused the small bodies that 

occupied this region to collide with one another, fragmenting 

them into the asteroids we observe today. This region, called the 

asteroid belt or simply the main belt, may contain millions of 

asteroids. Because asteroids have remained mostly unchanged for 

billions of years, studies of them could tell us a great deal about 

the early solar system. 

Most asteroids are irregularly shaped, though a few are nearly 

spherical, and are often pitted or cratered. As they revolve 

around the Sun in elliptical orbits, the asteroids also rotate, 

sometimes quite erratically, tumbling as they go. A few asteroids 

are known to have a small companion moon, and there are even 

some binary asteroids, in which two rocky bodies of roughly 

equal size orbit each other. 

There are three broad composition classes of asteroids: C-, S-, 

and M-types. The C-type asteroids are most common, probably 

consist of clay and silicate rocks, and are dark in appearance. 

They are among the most ancient objects in the solar system. The 

S-types (“stony”) are made up of silicate materials and nickeliron. The M-types are metallic (nickel–iron). Their compositional 

differences are related to how far from the Sun asteroids of different types formed. Some of the asteroids experienced high temperatures after they formed and partly melted, with iron sinking 

to the center and forcing basaltic (volcanic) lava to the surface. 

One such asteroid, Vesta, survives to this day. 

Jupiter’s gravity and occasional close encounters with Mars or 

with another asteroid change the asteroids’ orbits, knocking 

them out of the main belt and hurling them into space in both 

directions across the orbits of the planets. Stray asteroids or asteroid fragments slammed into Earth and the other planets in the 

past, playing a major role in altering the geological history of the 

planets and in the evolution of life on Earth. Scientists monitor 

asteroids whose paths intersect Earth’s orbit, called Earth-crossing asteroids. Some of these come so close to Earth that they are 

further classified as near-Earth asteroids. 

Radar observations that bounce signals off asteroids can tell scientists a great deal about an asteroid’s size, shape, spin, and metal 

concentration. Radar is used to track asteroids that pass close to 

Earth; sometimes it detects small companion asteroids. 

A few space missions have flown by and observed asteroids close 

up. The Galileo spacecraft flew by asteroids Gaspra in 1991 and 

Ida in 1993; the Near-Earth Asteroid Rendezvous (NEAR) mission studied asteroids Mathilde and Eros; and Deep Space 1 

and Stardust have both had close encounters with asteroids. 

NASA’s Dawn mission is planned to orbit asteroids Vesta and 

Ceres. Vesta and Ceres are considered “baby planets” — their 

growth was interrupted by the formation of Jupiter, and they 

followed different evolutionary paths. Scientists hope to characterize the conditions and processes of the solar system’s earliest 

epoch by studying these two very different large asteroids. 

SIGNIFICANT DATES 

1801 — Giuseppe Piazzi discovers the first asteroid, Ceres. 

1898 — Gustav Witt discovers Eros, one of the largest near-Earth 

asteroids. 

1991–1994 — On its way to Jupiter, the Galileo spacecraft takes 

the first close-up images of an asteroid (Gaspra) and discovers the 

first moon (later named Dactyl) orbiting an asteroid (Ida). 

1997–2000 — NEAR Shoemaker spacecraft flies by Mathilde and 

orbits and lands on Eros. 

ABOUT THE IMAGES 

A four-image 

mosaic of asteroid 

Eros taken by the 

NEAR spacecraft. 

1

2 A Galileo image 

of asteroid Ida and 

its moon Dactyl. 

3 Elevation mapping using imagery from the Hubble Space 

Telescope reveals a giant crater (the blue ring) on asteroid Vesta.

 This computer This computer-generated model of asteroid Golevka was oid Golevka was 

created from radar data. Tiny Golevka is just 0.5 kilometer 

4

(0.33 mile) across.

5 A false-color view of a lar A false-color view of a large crater on Eros. Redder hues indicate rock and soil altered by exposure to the solar wind. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids 

FAST FACTS 433 Eros 951 Gaspra 951 Gaspra 4 Vesta 1 Ceres 243 Ida

Mean Distance from the Sun (AU*) 1.46 2.21 2.36 2.77 2.86 

Orbit Period (years) 1.76 3.29 3.63 4.60 4.84 

Orbit Eccentricity (Circular = 0) 0.22 0.17 0.09 0.08 0.05 

Orbit Inclination to Ecliptic (deg) 10.83 4.10 7.13 10.58 1.14 

Rotation Period 5 hr, 16 min 7 hr, 2 min 5 hr, 20 min 9 hr, 4 min 4 hr, 38 min 

Dimensions (km) 34 × 11 × 11 20 × 12 × 11 578 × 560 × 458 960 × 932 60 × 25 × 19 

Dimensions (mi) 21 × 7 × 7 12 × 7 × 7 359 × 348 × 285 597 × 579 37 × 15 × 12 

*AU = astronomical unit, the mean distance from Earth to the Sun: 149.60 million km or 92.96 million mi. 

LG-2005-12-570-HQ — JPL 400-1253I 12/05
National Aeronautics and Space Administration Meteors and Meteorites 

www.nasa.gov
National Aeronautics and Space Administration Meteors and Meteorites 

www.nasa.gov

“Shooting stars,” or meteors, are bits of interplanetary material 

falling through Earth’s atmosphere and heated to incandescence 

by friction. These objects are called meteoroids as they are hurtling through space, becoming meteors for the few seconds they 

streak across the sky and create glowing trails. Chunks of these 

extraterrestrial visitors that survive their journey through the 

atmosphere and fall to the ground are called meteorites. 

Several meteors per hour can usually be seen on any given night. 

Sometimes the number increases dramatically — these events 

are termed meteor showers. Some occur annually or at regular 

intervals as the Earth passes through the trail of dusty debris left 

by a comet. Meteor showers are usually named after a star or 

constellation that is close to where the meteors appear in the sky. 

Perhaps the most famous are the Perseids, which peak around 

August 12 every year. Every Perseid meteor is a tiny piece of the 

comet Swift–Tuttle, which swings by the Sun every 135 years. 

Other meteor showers and their associated comets are the Leonids (Tempel–Tuttle), the Aquarids and Orionids (Halley), and 

the Taurids (Encke). Comet dust in meteor showers burns up in 

the atmosphere before reaching the ground. 

Most meteorites are no bigger than an average Earth rock, but 

some have been quite large, especially in Earth’s early history. 

Large meteorites can cause extensive destruction when they 

strike. One of the most distinct impact craters is the Barringer 

Meteor Crater in Arizona, about 1,000 meters (3,300 feet) across. 

It is only 50,000 years old and so well preserved that it has been 

used to study impact processes. Since this feature was recognized 

as an impact crater in the 1920s, about 160 impact craters have 

been identified on Earth. A very large asteroid impact 65 million 

years ago, which created the 300-kilometer (180-mile) wide 

Chicxulub crater on the Yucatán Peninsula, is thought to have 

contributed to the extinction of about 75 percent of marine and 

land animals on Earth at the time, including the dinosaurs. Welldocumented stories of meteorite-caused injury or death are rare, 

but in November 1954, Ann Hodges of Sylacauga, Alabama, was 

severely bruised by a 3.6-kilogram (8-pound) stony meteorite 

that crashed through her roof. 

Meteorites may resemble Earth rocks, but they usually have a 

“burned” exterior. This fusion crust is formed as the meteorite 

is melted by friction as it passed through the atmosphere. There 

are three major types of meteorites: the “irons,” the “stones,” and 

the “stony-irons.” Although the majority of meteorites that fall 

to Earth are stony, more of the meteorites that are discovered 

long after they fall are “irons” — these heavy objects are easier to 

distinguish from Earth rocks than stony meteorites. Meteorites 

also fall on other planets. Imagine the excitement when Mars 

Exploration Rover Opportunity found an iron meteorite on Mars! 

More than 30,000 meteorites have been found on Earth. 

Of these, 99.8 percent are thought to come from asteroids. 

Evidence for an asteroid origin includes: orbits calculated from 

photographic observations of meteorite falls project back to the 

asteroid belt; spectra of several classes of meteorites match those 

of some asteroid classes; and all but the rare lunar and martian 

meteorites are very old, 4.5 to 4.6 billion years. However, we can 

only match one group of meteorites to a specific asteroid. The 

eucrite, diogenite, and howardite igneous meteorites come from 

the third largest asteroid, Vesta. Asteroids and the meteorites 

that fall to Earth are not pieces of a planet that broke apart, but 

instead the original diverse materials from which the planets 

formed. The study of meteorites tells us much about the conditions and processes during the formation and earliest history of 

the solar system. 

The remaining 0.2 percent of meteorites is split roughly equally 

between meteorites from the Moon and Mars. The current 35 

known martian meteorites were blasted off Mars by meteoroid 

impacts. All are igneous rocks crystallized from magma, with distinctive composition indicating martian origin. Controversy continues about whether structures found in the meteorite known 

as ALH84001 might be evidence of fossil martian bacteria. The 

36 lunar meteorites are similar in mineralogy and composition to 

Apollo Moon rocks, but distinct enough to show that they have 

come from other parts of the Moon. Studies of lunar and martian 

meteorites complement studies of Apollo Moon rocks and the 

robotic exploration of Mars. 

SIGNIFICANT DATES 

4.55 billion years ago — Formation age of most meteorites, 

taken to be the age of the solar system. 

65 million years ago — Chicxulub impact that leads to the death 

of dinosaurs and 75 percent of animals on Earth.

50,000 years — Age of Barringer Meteor Crater in Arizona.

1478 BC — First recorded observation of meteors.

1794 AD — Ernst Friedrick Chladni publishes the first book on 

meteorites.

1908 (Tunguska), 1947 (Sikote Alin), 1969 (Allende and Murchison), 1976 (Jilin) — Important 20th-century meteorite falls.

1969 — Discovery of meteorites in a small area of Antarctica 

leads to annual expeditions by U.S. and Japanese teams.

1982–1983 — Meteorites from the Moon and Mars are identified 

in Antarctic collections.

1996 — A team of NASA scientists suggests that martian meteorite ALH 84001 may contain evidence of microfossils from Mars.

2005 — NASA’s Mars Exploration Rover Opportunity finds an 

iron meteorite on Mars.

ABOUT THE IMAGES 

1 A basketballsize iron meteorite 

found on Mars by 

Opportunity rover.

 A burst of me 2 -

teors photographed 

by NASA Ames 

Research Center 

scientists in 1995. 

3 The glassy black patches in this martian meteorite contain 

atmospheric gases.

4 Barringer Meteor Crater in Arizona.

 A stony meteorite found in Antar 5 ctica.

 A scientist working in the Meteorite Pr 6 ocessing Lab at NASA 

Johnson Space Center.

 An ir 7 on meteorite from Barringer Meteor Crater. 

 A lunar meteorite found in Antar 8 ctica similar in composition 

to lunar rocks brought back by Apollo astronauts. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profilecfm?Object=Meteoroids

LG-2005-12-571-HQ — JPL 400-1253J 12/05
National Aeronautics and Space Administration Moons of the Solar System 

www.nasa.gov

Triton Dione

Tethys

Earth

Mimas

Iapetus

Rhea

Enceladus

Titan

Earth’s Moon

Titania

Europa

Miranda

Oberon

Io

Charon

Callisto

Ganymede
National Aeronautics and Space Administration Moons of the Solar System 

www.nasa.gov

Moons come in many shapes, sizes, and types. They are generally solid bodies, and few have atmospheres. Most of the moons 

probably formed from the discs of gas and dust circulating 

around planets in the early solar system. As of November 2005, 

astronomers have found at least 153 moons orbiting planets in 

our solar system. The moon champion, Jupiter, has a total of 

62 known moons, including the largest moon in the solar system, Ganymede. Many of Jupiter’s smallest outer moons have 

highly elliptical orbits and orbit “backwards” (opposite to the 

spin of the planet). Saturn, Uranus, and Neptune also have some 

“irregular” moons, which orbit far from their respective planets. 

Usually the term “moon” brings to mind a spherical object, like 

Earth’s Moon. The two moons of Mars, Phobos and Deimos, are 

somewhat different. While both have nearly circular orbits and 

travel close to the plane of the planet’s equator, they are lumpy 

and dark. Phobos is slowly drawing closer to Mars, and could 

crash into Mars in 40 or 50 million years, or the planet’s gravity 

might break Phobos apart, creating a thin ring around Mars. 

Of the terrestrial (rocky) planets of the inner solar system, neither Mercury nor Venus has any moons at all, Earth has one, and 

Mars has its two small moons. In the outer solar system, the gas 

giants (Jupiter, Saturn, Uranus, and Neptune) have many moons. 

As these planets grew in the early solar system, they were able to 

capture objects with their large gravitational fields. 

Earth’s Moon is another story. The Moon probably formed when 

a large body about the size of Mars collided with Earth, ejecting 

a lot of material from our planet into orbit. Debris from the early 

Earth and the impacting body accumulated to form the Moon 

approximately 4.5 billion years ago (the age of the oldest collected lunar rocks). Twelve American astronauts visited the Moon 

during the Apollo lunar landings in 1969 to 1972. 

Saturn has more than 40 known moons. The chunks of ice 

and rock in Saturn’s rings (and the particles in the rings of the 

other outer planets) are not considered moons, yet embedded in 

Saturn’s rings are distinct moons. Small “shepherd” moons help 

keep the rings in line. Saturn’s moon Titan, the second largest in 

the solar system, is the only moon with a thick atmosphere. 

Uranus has 27 known moons. The inner moons appear to be 

about half water ice and half rock. Miranda is the most unusual; 

its chopped-up appearance shows the scars of impacts of large 

rocky bodies. Neptune’s moon Triton is as big as Pluto, and orbits backwards compared with Neptune’s direction of rotation. 

Neptune has 13 known moons. 

Pluto’s single moon, Charon, is about half the size of Pluto. Some 

astronomers think of them as forming a binary, or double, planet 

system. Like Earth’s Moon, Charon may have formed from debris 

resulting from an early collision of an impactor with Pluto. In 

October 2005, scientists using the Hubble Space Telescope to 

study this distant planet announced that Pluto may have two 

additional moons. The candidate moons are about two to three 

times as far from Pluto as Charon and roughly 5,000 times 

fainter than Pluto. 

Saturn Mimas 198.6 123.4 

Saturn Enceladus 249.4 154.9 

Saturn 

Saturn 

Tethys 

Dione 

529.9 

560 

329.3 

348 

Saturn Rhea 764 475 

Saturn 

Saturn 

Titan 

Iapetus 

2,575 

718 

1,600 

446 

FAST FACTS 

Mean Radius Mean Radius 

Planet Moon (km) (mi) 

Earth Moon 1,737.4 1,079.6 

Mars Phobos 11.1 6.9 

Mars Deimos 6.2 3.9 

Jupiter Io 1,821.6 1,131.9 

Jupiter Europa 1,560.8 969.8 

Jupiter Callisto 2,410 1,498 

Jupiter Ganymede 2,631 1,635 

Uranus 

Uranus 

Uranus 

Uranus 

Uranus 

Miranda 

Ariel 

Umbriel 

Titania 

Oberon 

235.8 

578.9 

584.7 

788.9 

761.4 

146.5 

359.7 

363.3 

490.2 

473.1 

Neptune 

Neptune 

Triton 

Nereid 

1,353.4 

170 

841 

106 

Pluto Charon 600 373 

SIGNIFICANT DATES 

1610 — Galileo Galilei and Simon Marius independently discover 

four moons orbiting Jupiter. Galileo is credited and the moons are 

called “Galilean.” 

1877 — Asaph Hall discovers Mars’ moons Phobos and Deimos. 

1969 — Astronaut Neil Armstrong is the first of 12 humans to 

walk on the surface of the Moon. 

1980 — Voyager 1 instruments detect signs of surface features 

beneath the hazy atmosphere of Saturn’s largest moon, Titan. 

2000–2005 — Since the beginning of the year 2000, 84 of the 

153 known moons of the solar system have been discovered. 

Jupiter’s known moons have increased from 18 to 62. Saturn’s 

known moons have jumped from 18 to 47. Uranus’ moon count 

has gone from 21 to 27, and Neptune’s known moons have increased in number from 8 to 13. 

ABOUT THE IMAGES 

1 Selected solar 

system moons, 

displaying a variety 

of surface features, 

are shown at correct 

relative sizes to each 

other and to planet 

Earth. 

2 Miranda, a moon of Uranus, has many rugged features.

 This false-color image of Neptune’ 3 s moon Triton shows what 

appear to be volcanic deposits.

 This V 4 oyager 1 close-up of Saturn’s moon Rhea shows the 

moon’s ancient, cratered surface.

 A portion of a Cassini radar image of Satur 5 n’s largest moon, 

Titan, showing the complexity of the surface. 

 Cassini imaged Satur 6 n’s outer moon Phoebe when the spacecraft was inbound for orbit insertion in June 2004. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/index.cfm 

LG-2005-12-572-HQ — JPL 400-1253K 12/05
National Aeronautics and Space Administration Jupiter

www.nasa.gov
National Aeronautics and Space Administration Jupiter

www.nasa.gov

The most massive planet in our solar system, with four planetsized moons and many smaller moons, Jupiter forms a kind of 

miniature solar system. Jupiter resembles a star in composition. 

In fact, if it had been about eighty times more massive, it would 

have become a star rather than a planet. 

On January 7, 1610, using his primitive telescope, astronomer 

Galileo Galilei saw four small “stars” near Jupiter. He had discovered Jupiter’s four largest moons, now called Io, Europa, Ganymede, and Callisto. Collectively, these four moons are known 

today as the Galilean satellites. 

Galileo would be astonished at what we have learned about 

Jupiter and its moons in the past 30 years. Io is the most volcanically active body in our solar system. Ganymede is the largest 

planetary moon and is the only moon in the solar system known 

to have its own magnetic field. A liquid ocean may lie beneath 

the frozen crust of Europa. Icy oceans may also lie deep beneath 

the crusts of Callisto and Ganymede. In 2003 alone, astronomers 

discovered 23 new moons orbiting the giant planet, giving Jupiter a total moon count of 62 — the most in the solar system. The 

numerous small outer moons may be asteroids captured by the 

giant planet’s gravity. 

Jupiter’s appearance is a tapestry of beautiful colors and atmospheric features. Most visible clouds are composed of ammonia. 

Water exists deep below and can sometimes be seen through 

clear spots in the clouds. The planet’s “stripes” are dark belts and 

light zones created by strong east–west winds in Jupiter’s upper 

atmosphere. Within these belts and zones are storm systems that 

have raged for years. The Great Red Spot, a giant spinning storm, 

has been observed for more than 300 years. 

The composition of Jupiter’s atmosphere is similar to that of the 

Sun — mostly hydrogen and helium. Deep in the atmosphere, 

the pressure and temperature increase, compressing the hydrogen gas into a liquid. At depths about a third of the way down, 

the hydrogen becomes metallic and electrically conducting. In 

this metallic layer, Jupiter’s powerful magnetic field is generated 

by electrical currents driven by Jupiter’s fast rotation. At the center, the immense pressure may support a solid core of ice–rock 

about the size of Earth. 

Jupiter’s enormous magnetic field is nearly 20,000 times as powerful as Earth’s. Trapped within Jupiter’s magnetosphere (the area 

in which magnetic field lines encircle the planet from pole to 

pole) are swarms of charged particles. Jupiter’s rings and moons 

are embedded in an intense radiation belt of electrons and ions 

trapped in the magnetic field. The jovian magnetosphere, comprising these particles and fields, balloons 1 to 3 million kilometers (600,000 to 2 million miles) toward the Sun and tapers into 

a windsock-shaped tail extending more than 1 billion kilometers 

(600 million miles) behind Jupiter as far as Saturn’s orbit. 

Discovered in 1979 by NASA’s Voyager 1 spacecraft, Jupiter’s 

rings were a surprise: a flattened main ring and an inner cloudlike ring, called the halo, are both composed of small, dark 

particles. A third ring, known as the gossamer ring because of its 

transparency, is actually three rings of microscopic debris from 

three small moons: Amalthea, Thebe, and Adrastea. Jupiter’s ring 

system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet’s four small inner moons. The 

main ring probably comes from the moon Metis. Jupiter’s rings 

are only visible when backlit by the Sun. 

In December 1995, NASA’s Galileo spacecraft dropped a probe 

into Jupiter’s atmosphere, which collected the first direct measurements of Jupiter’s atmosphere. Following the release of the 

probe, the Galileo spacecraft began a multiyear study of Jupiter 

and the largest moons. As Galileo began its 29th orbit, the Cassini–Huygens spacecraft was nearing Jupiter for a gravity-assist 

maneuver on the way to Saturn. The two spacecraft made simultaneous observations of the magnetosphere, solar wind, rings, 

and Jupiter’s auroras. 

FAST FACTS 

Namesake King of the Roman gods 

Mean Distance from the Sun 778.41 million km 

(483.68 million mi) 

Orbit Period 11.8565 Earth years 

(4,330.6 Earth days) 

Orbit Eccentricity (Circular Orbit = 0) 0.04839 

Orbit Inclination to Ecliptic 1.305 deg 

Inclination of Equator to Orbit 3.12 deg 

Rotation Period 9.92 hr 

Equatorial Radius 71,492 km (44,423 mi) 

Mass 317.82 of Earth’s 

Density 1.33 g/cm3 

Gravity 20.87 m/sec2

 (68.48 ft/sec2) 

Atmosphere Primary Components hydrogen, helium 

Effective Temperature –148 deg C (–234 deg F) 

Known Moons* 62 

Rings 1 (three major components) 

*As of November 2005. 

SIGNIFICANT DATES 

1610 — Galileo Galilei makes the first detailed observations of 

Jupiter using a telescope.

1973 — Pioneer 10 becomes the first spacecraft to cross the asteroid belt and fly past Jupiter. 

1979 — Voyager 1 and 2 discover Jupiter’s faint rings, several 

new moons, and volcanic activity on Io’s surface.

1994 — Astronomers watch pieces of comet Shoemaker–Levy 9 

collide with Jupiter.

1995–2003 — The Galileo spacecraft conducts extended observations of Jupiter and its moons and rings. 

ABOUT THE IMAGES 

A detailed, 

true-color image of 

Jupiter taken by the 

Cassini spacecraft. 

The Galilean moon 

Europa casts a shadow on the planet. 

1

2 A Voyager 1 image of Jupiter’s Great Red Spot.

 An image of Jupiter’ 3 s aurora, a sign of the interaction between Jupiter’s magnetic field and energy from the Sun.

 A schematic of the components of Jupiter’ 4 s ring system. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter 

LG-2005-12-573-HQ — JPL 400-1253L 12/05
National Aeronautics and Space Administration Galilean Moons of Jupiter 

www.nasa.gov
National Aeronautics and Space Administratio Galilean Moons of Jupiter 

www.nasa.gov

n

The planet Jupiter’s four largest moons are called the Galilean 

satellites, after Italian astronomer Galileo Galilei, who observed 

them in 1610. The German astronomer Simon Marius claimed to 

have seen the moons around the same time, but he did not publish his observations and so Galileo is given the credit for their 

discovery. These large moons, named Io, Europa, Ganymede, and 

Callisto, are each distinctive worlds. 

Io is the most volcanically active body in the solar system. Its 

surface is covered by sulfur in different colorful forms. As Io travels in its slightly elliptical orbit, Jupiter’s immense gravity causes 

“tides” in the solid surface 100 meters (300 feet) high on Io, generating enough heat to give rise to the volcanic activity and drive 

off any water. Io’s volcanoes are driven by hot silicate magma. 

Europa’s surface is mostly water ice, and there is evidence that 

it may be covering an ocean of water or slushy ice. Europa is 

thought to have twice as much water as does Earth. This moon 

intrigues astrobiologists because of its potential for having a 

“habitable zone.” Life forms have been found thriving near 

subterranean volcanoes on Earth and in other extreme locations 

that may be analogues to what may exist on Europa. Ganymede 

is the largest moon in the solar system (larger than the planet 

Mercury), and is the only moon known to have its own internally 

generated magnetic field. Callisto’s surface is extremely heavily 

cratered and ancient — a record of events from the early history 

of the solar system. However, the very few small craters on Callisto indicate a small degree of current surface activity. 

The interiors of Io, Europa, and Ganymede have a layered 

structure (as does Earth). Io has a core, and a mantle of at least 

partially molten rock, topped by a crust of solid rock coated with 

sulfur compounds. Europa and Ganymede both have a core; a 

rock envelope around the core; a thick, soft ice layer; and a thin 

crust of impure water ice. In the case of Europa, a global subsurface water layer probably lies just below the icy crust. Layering at 

Callisto is less well defined and appears to be mainly a mixture 

of ice and rock. 

Three of the moons influence each other in an interesting way. Io 

is in a tug-of-war with Ganymede and Europa, and Europa’s orbital period (time to go around Jupiter once) is twice Io’s period, 

and Ganymede’s period is twice that of Europa. In other words, 

every time Ganymede goes around Jupiter once, Europa makes 

two orbits, and Io makes four orbits. The moons all keep the 

same face towards Jupiter as they orbit, meaning that each moon 

turns once on its axis for every orbit around Jupiter. 

Pioneers 10 and 11 (1973 to 1974) and Voyagers 1 and 2 (1979) 

offered striking color views and global perspectives from their flybys of the Jupiter system. From 1995 to 2003, the Galileo spacecraft made observations from repeated elliptical orbits around 

Jupiter, passing as low as 261 kilometers (162 miles) over the 

surfaces of the Galilean moons. These close approaches resulted 

in images with unprecedented detail of selected portions of the 

surfaces. 

Close-up images taken by the Galileo spacecraft of portions of 

Europa’s surface show places where ice has broken up and moved 

apart, and where liquid may have come from below and frozen 

smoothly on the surface. The low number of craters on Europa 

leads scientists to believe that a subsurface ocean has been present in recent geologic history and may still exist today. The heat 

needed to melt the ice in a place so far from the Sun is thought 

to come from inside Europa, resulting primarily from the same 

type of tidal forces that drive Io’s volcanoes. 

FAST FACTS 

Moon Distance from Jupiter 

Io 422,000 km (262,200 mi)

 Europa 671,000 km (417,000 mi)

 Ganymede 1,070,000 km (665,000 mi)

 Callisto 1,883,000 km (1,170,000 mi) 

Moon Mean Radius

 Io 1821.6 km (1,131.9 mi)

 Europa 1,560.8 km (969.8 mi)

 Ganymede 2,631 km (1,635 mi)

 Callisto 2,410 km (1,498 mi) 

Moon Orbital Period (Earth Days)

 Io 1.769

 Europa 3.551

 Ganymede 7.155

 Callisto 16.689 

Moon Density (g/cm3)

 Io 3.528

 Europa 3.013

 Ganymede 1.942

 Callisto 1.834 

SIGNIFICANT DATES 

1610 — Galileo Galilei and Simon Marius independently discover four moons orbiting Jupiter. 

1979 — Voyager 1 photographs an erupting volcano on Io: the 

first ever seen anywhere but on Earth. 

1979–2000 — Using data from the Voyager and Galileo spacecraft, scientists gather strong evidence of an ocean beneath the 

icy crust of Europa; Galileo data indicate possible oceans on 

Ganymede and Callisto. 

2003 — The Galileo mission ends with the spacecraft deliberately descending into Jupiter’s atmosphere. In the same year, 

scientists discover 23 new moons of Jupiter. 

ABOUT THE IMAGES 

1 A composite 

“portrait” of Jupiter’s 

four Galilean moons 

Io, Europa, Ganymede, and Callisto 

(Jupiter is not at the 

same scale as the 

satellites). 

2 During one flyby of Io, the Galileo spacecraft photographed 

Tvashtar Catena, an area of giant erupting volcanoes.

 This false-color image of Eur 3 opa shows the icy crust broken 

up into blocks that “rafted” into new positions.

 Fr 4 esh, bright material was thrown out of an impact crater on 

Ganymede.

 Ice on Callisto excavated by younger impact craters con 5 -

trasts with darker, redder coatings on older surfaces. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter 

LG-2005-12-574-HQ — JPL 400-1253M 12/05
National Aeronautics and Space Administration Saturn

www.nasa.gov
National Aeronautics and Space Administration Saturn

www.nasa.gov

Saturn was the most distant of the five planets known to the 

ancients. In 1610, Italian astronomer Galileo Galilei was the first 

to gaze at Saturn through a telescope. To his surprise, he saw a 

pair of objects on either side of the planet. He sketched them 

as separate spheres and wrote that Saturn appeared to be triplebodied. Continuing his observations over the next few years, 

Galileo drew the lateral bodies as arms or handles attached to 

Saturn. In 1659, Dutch astronomer Christiaan Huygens, using a 

more powerful telescope than Galileo’s, proposed that Saturn was 

surrounded by a thin, flat ring. In 1675, Italian-born astronomer 

Jean-Dominique Cassini discovered a “division” between what 

are now called the A and B rings. It is now known that the gravitational influence of Saturn’s moon Mimas is responsible for the 

Cassini Division, which is 4,800 kilometers (3,000 miles) wide. 

Like Jupiter, Saturn is made mostly of hydrogen and helium. 

Its volume is 755 times greater than that of Earth. Winds in the 

upper atmosphere reach 500 meters (1,600 feet) per second in 

the equatorial region. (In contrast, the strongest hurricane-force 

winds on Earth top out at about 110 meters, or 360 feet, per 

second.) These super-fast winds, combined with heat rising from 

within the planet’s interior, cause the yellow and gold bands visible in the atmosphere. 

Saturn’s ring system is the most extensive and complex in the solar system, extending hundreds of thousands of kilometers from 

the planet. In the early 1980s, NASA’s two Voyager spacecraft revealed that Saturn’s rings are made mostly of water ice, and they 

found “braided” rings, ringlets, and “spokes” — dark features in 

the rings that circle the planet at different rates from that of the 

surrounding ring material. Material in the rings ranges in size 

from a few micrometers to several tens of meters. Two of Saturn’s 

small moons orbit within gaps in the main rings. 

Saturn’s largest satellite, Titan, is a bit bigger than the planet 

Mercury. (Titan is the second-largest moon in the solar system; 

only Jupiter’s moon Ganymede is bigger.) Titan is shrouded in 

a thick, nitrogen-rich atmosphere that might be similar to what 

Earth’s was like long ago. Further study of this moon promises to 

reveal much about planetary formation and, perhaps, about the 

early days of Earth. Saturn also has many smaller “icy” satellites. 

From Enceladus, which shows evidence of recent (and ongoing) 

surface changes, to Iapetus, with one hemisphere darker than 

asphalt and the other as bright as snow, each of Saturn’s satellites 

is unique. 

Though Saturn’s magnetic field is not as huge as Jupiter’s, it 

is still 578 times as powerful as Earth’s. Saturn, the rings, and 

many of the satellites lie totally within Saturn’s enormous magnetosphere, the region of space in which the behavior of electrically charged particles is influenced more by Saturn’s magnetic 

field than by the solar wind. Hubble Space Telescope images 

show that Saturn’s polar regions have aurorae similar to Earth’s. 

Aurorae occur when charged particles spiral into a planet’s atmosphere along magnetic field lines. 

Voyagers 1 and 2 flew by and photographed Saturn in 1981. 

The next chapter in our knowledge of Saturn is underway, as 

the Cassini–Huygens spacecraft continues its exploration of the 

Saturn system. The Huygens probe descended through Titan’s 

atmosphere in January 2005, collecting data on the atmosphere 

and surface. Cassini will orbit Saturn more than 70 times during 

a four-year study of the planet and its moons, rings, and magnetosphere. Cassini–Huygens is sponsored by NASA, the European 

Space Agency, and the Italian Space Agency. 

FAST FACTS 

Namesake Roman god of agriculture 

Mean Distance from the Sun 1,426.73 million km 

(886.53 million mi) 

Orbit Period 29.4 Earth years 

(10,755.7 Earth days) 

Orbit Eccentricity (Circular Orbit = 0) 0.0541506 

Orbit Inclination to Ecliptic 2.484 deg 

Inclination of Equator to Orbit 26.73 deg 

Rotation Period 10.656 hours 

Equatorial Radius 60,268 km (37,449 mi) 

Mass 95.16 of Earth’s 

Density 0.70 g/cm3 

Gravity 7.207 m/sec2 (23.64 ft/sec2

) 

Atmosphere Primary Components hydrogen, helium 

Effective Temperature –178 deg C (–288 deg F) 

Known Moons* 47 

Rings 7 main rings (C, B, A, D, F, G, E) 

*As of November 2005. 

SIGNIFICANT DATES 

1610 — Galileo Galilei reports seeing odd appendages on either 

side of Saturn. 

1979 — Pioneer 11 is the first spacecraft to reach Saturn, flying 

within 22,000 kilometers (13,700 miles) of the ringed planet’s 

cloud tops. 

1981 — Using Saturn’s powerful gravity as an interplanetary 

“slingshot,” Voyager 2 is placed on a path toward Uranus, then 

Neptune, then out of the solar system. 

1994 — The Hubble Space Telescope finds evidence of surface 

features beneath the hazy atmosphere of Saturn’s largest moon, 

Titan. 

2004 — After a seven-year journey, the Cassini–Huygens spacecraft becomes the first spacecraft to orbit Saturn; 76 orbits of 

Saturn and 45 Titan flybys are planned for the four-year mission. 

2005 — The Huygens probe successfully lands on Titan, returning images of the complex surface. 

ABOUT THE IMAGES 

 Cassini captur 1 ed 

this true-color image 

of Saturn and its 

magnificent rings.

2 A natural-color 

close-up view of 

Saturn’s rings. 

3 This Cassini image shows a bright storm in a region of high 

atmospheric activity.

 Satur 4 n’s bright equatorial region displays swirls and eddies 

in this Cassini image.

 An ultraviolet image of Satur 5 n’s rings shows more ice (turquoise colors) near the outer edge of the rings. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn

LG-2005-12-575-HQ — JPL 400-1253N 12/05
National Aeronautics and Space Administration Moons of Saturn 

www.nasa.gov
National Aeronautics and Space Administration Moons of Saturn 

www.nasa.gov

Saturn, the sixth planet from the Sun, is home to a vast array of 

intriguing and unique worlds. From the cloud-shrouded surface 

of Titan to crater-riddled Phoebe, each of Saturn’s moons tells 

another piece of the story surrounding the Saturn system. 

Christiaan Huygens discovered the first known moon of Saturn. 

The year was 1655 and the moon is Titan. Jean-Dominique Cassini made the next four discoveries: Iapetus (1671), Rhea (1672), 

Dione (1684), and Tethys (1684). Mimas and Enceladus were 

both discovered by William Herschel in 1789. The next two 

discoveries came at intervals of 50 or more years — Hyperion 

(1848) and Phoebe (1898). 

As telescopic resolving power increased through the 19th century, Saturn’s family of known moons grew. In 1966 Epimetheus 

and Janus were discovered. By the time Cassini–Huygens was 

launched in 1997, Saturn’s moon count had reached 18. The 

number of known moons soon increased with high-resolution 

imaging techniques used on ground-based telescopes. Cassini 

discovered four more moons after its arrival at Saturn and may 

find even more during its mission. 

Each of Saturn’s moons bears a unique story. Two of the moons 

orbit within gaps in the main rings. Some, such as Prometheus 

and Pandora, interact with ring material, shepherding the ring 

in its orbit. Some small moons are trapped in the same orbits as 

Tethys or Dione. Janus and Epimetheus occasionally pass close to 

each other, causing them to periodically exchange orbits. Here’s a 

sampling of some of the unique aspects of the moons: 

• Titan — Titan is so large that it affects the orbits of other nearby moons. At 5,150 kilometers (3,200 miles) across, it is the second-largest moon in the solar system. Titan hides its surface with 

a thick nitrogen-rich atmosphere. Titan’s atmosphere is similar to 

Earth’s atmosphere of long ago, before biology took hold on our 

home planet. Titan’s atmosphere is approximately 95% nitrogen 

with traces of methane. While Earth’s atmosphere extends about 

60 kilometers (37 miles) into space, Titan’s extends nearly 600 

kilometers (ten times that of Earth’s atmosphere) into space. 

• Iapetus has one side as bright as snow and one side as dark as 

black velvet, with a huge ridge running around most of its darkside equator. 

• Phoebe orbits the planet in a direction opposite that of Saturn’s 

larger moons, as do several of the recently discovered moons. 

• Mimas has an enormous crater on one side, the result of an 

impact that nearly split the moon apart. 

• Enceladus displays evidence of active ice volcanism: Cassini 

observed warm fractures where evaporating ice evidently escapes 

and forms a huge cloud of water vapor over the south pole. 

• Hyperion has an odd flattened shape and rotates chaotically, 

probably due to a recent collision. 

• Pan orbits within the main rings and helps sweep materials out 

of a narrow space known as the Encke Gap. 

• Tethys has a huge rift zone called Ithaca Chasma that runs 

nearly three-quarters of the way around the moon. 

• Four moons orbit in stable places around Saturn called Lagrangian points. These places lie 60 degrees ahead of or behind 

a larger moon and in the same orbit. Telesto and Calypso occupy 

the two Lagrangian points of Tethys in its orbit; Helene and Polydeuces occupy the corresponding Lagrangian points of Dione. 

• Sixteen of Saturn’s moons keep the same face toward the planet 

as they orbit. Called “tidal locking,” this is the same phenomenon that keeps our Moon always facing toward Earth. 

The Cassini spacecraft will fly past Titan 45 times during its fouryear primary mission. In addition, Cassini will gather data about 

many of the other satellites in an effort to fully understand the 

nature, formation, and dynamics of Saturn’s moons. 

FAST FACTS 

• Largest Moon of Saturn Titan 

Titan’s Diameter 5,150 km (3,200 mi) 

• Farthest Moon from Saturn Ymir 

Ymir’s Distance from Saturn 23,096,000 km (14,354,164 mi) 

• Closest Moon to Saturn Pan 

Pan’s Distance from Saturn 133,583 km (83,022 mi) 

• Fastest Orbit Pan 

Pan’s Orbit Around Saturn 13.8 hours 

• Number of Moons Discovered by Voyager 3 

(Atlas, Prometheus, and Pandora) 

• Number of Moons Discovered by Cassini (So Far) 4 

(Methone, Pallene, Polydeuces, 

ABOUT THE IMAGES

nd the moonlet 2005S1) 

1 An ultraviolet 

(blue) and infrared 

(red and green) 

image of Titan. 

2 False color 

(blue) emphasizes 

icy walls of fractures 

on Enceladus. 

a

f a large impact that 

of Titan taken by the 

 ultraviolet image.

 The Herschel crater on Mimas is a r 3 elic o

nearly destroyed this moon. 

4 One of the first images from the surface 

Huygens probe.

 T 5 itan’s layers of haze are revealed in this

 A mosaic of high-r 6 esolution images of Phoebe taken by 

Cassini during its historic close encounter in June 2004. 

7 This image of Iapetus, the two-toned moon, shows the equatorial ridge as well as the icy-bright and dark regions.

 Cassini 8 ’s false-color image of Rhea enhances the slight differences in natural color across the moon’s face. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn 

Some of Saturn’s moons are shown at relative distances to the planet.

LG-2005-12-576-HQ — JPL 400-1253O 12/05
National Aeronautics and Space Administration Uranus

www.nasa.gov
National Aeronautics and Space Administration Uranus

www.nasa.gov

Once considered one of the blander-looking planets, Uranus 

(pronounced YOOR un nus) has been revealed as a dynamic 

world with some of the brightest clouds in the outer solar system 

and 11 rings. The first planet found with the aid of a telescope, 

Uranus was discovered in 1781 by astronomer William Herschel. 

The seventh planet from the Sun is so distant that it takes 84 

years to complete one orbit. Uranus, with no solid surface, is 

one of the gas giant planets (the others are Jupiter, Saturn, and 

Neptune). 

The atmosphere of Uranus is composed primarily of hydrogen 

and helium, with a small amount of methane and traces of water 

and ammonia. Uranus gets its blue-green color from methane 

gas. Sunlight is reflected from Uranus’ cloud tops, which lie 

beneath a layer of methane gas. As the reflected sunlight passes 

back through this layer, the methane gas absorbs the red portion 

of the light, allowing the blue portion to pass through — resulting in the blue-green color that we see. The planet’s atmospheric 

details are very difficult to see in visible light. The bulk (80 percent or more) of the mass of Uranus is contained in an extended 

liquid core consisting primarily of “icy” materials (water, methane, and ammonia), with higher-density material at depth. 

In 1986, Voyager 2 observed faint cloud markings in the southern latitudes blowing westward between 100 and 600 kilometers 

(60 and 400 miles) per hour. In 2004, the Keck Observatory in 

Hawaii used advanced optics to capture highly detailed images of 

Uranus as the planet approached its southern autumnal equinox, 

when the equator will be vertically illuminated by the Sun. 

Uranus’ rotation axis is nearly horizontal with respect to the 

ecliptic plane — the imaginary plane containing Earth’s orbit, as 

well as the orbits of most of the planets — as though Uranus had 

been knocked on its side. This unusual orientation may be the 

result of a collision with a planet-sized body early in the planet’s 

history, which apparently radically changed Uranus’ rotation. Additionally, while magnetic fields are typically in alignment with a 

planet’s rotation, Uranus’ magnetic field is tipped over: instead of 

aligning along the rotational axis, the magnetic axis is tilted nearly 60 degrees from the planet’s axis of rotation, and is also offset 

from the center of the planet by one-third of the planet’s radius. 

Unlike the magnetic fields of Earth, Jupiter, and Saturn, which 

can be thought of as acting like dipole bar magnets, the fields of 

Uranus (and Neptune also) are very irregular. Uranus’ magnetic 

field is about 48 times more powerful than Earth’s. 

Even though Uranus is tipped on its side and experiences seasons that last over 20 years, the temperature differences on the 

summer and winter sides do not differ greatly because the planet 

is so far from the Sun. Near the cloud tops, the temperature of 

Uranus is –216 degrees Celsius (–357 degrees Fahrenheit). 

Because of the planet’s unusual orientation, Uranus’ rings are 

perpendicular to its orbital path about the Sun. The 10 outer 

rings are dark, thin, and narrow, while the 11th ring is inside the 

others and is broad and diffuse. 

Uranus has 27 known moons, named mostly for characters from 

the works of William Shakespeare and Alexander Pope. Miranda 

is the strangest-looking Uranian moon, appearing as though it 

were made of spare parts. The high cliffs and winding valleys of 

the moon may indicate partial melting of the interior, with icy 

material occasionally drifting to the surface. 

FAST FACTS 

Namesake Greek supreme sky god 

Mean Distance from the Sun 2,870.97 million km 

(1,783.94 million mi) 

Orbit Period 84.02 Earth years 

(30,687.2 Earth days) 

Orbit Eccentricity (Circular Orbit = 0) 0.047168 

Orbit Inclination to Ecliptic 0.770 deg 

Inclination of Equator to Orbit 97.86 deg 

Rotation Period 17.24 hours (retrograde) 

Equatorial Radius 25,559 km (15,882 mi) 

Mass 14.371 of Earth’s 

Density 1.32 g/cm3 

Gravity 8.43 m/sec2

 (27.7 ft/sec2) 

Atmosphere Primary Components hydrogen, helium, methane 

Effective Temperature –216 deg C (–357 deg F) 

Known Moons* 27 

Rings 11 (1986U2R, 6, 5, 4, Alpha, Beta, Eta, 

Gamma, Delta, Lambda, Epsilon) 

*As of November 2005. 

SIGNIFICANT DATES 

1781 — Astronomer William Herschel discovers Uranus, increasing the number of known planets to seven. 

1787–1851 — Four Uranian moons are discovered and named 

Titania, Oberon, Ariel, and Umbriel.

1948 — A new moon, Miranda, is discovered.

1977 — Scientists discover Uranus’ faint ring system while 

watching a distant star pass behind the planet.

1986 — Voyager 2 becomes the first, and still the only, spacecraft 

to visit Uranus, discovering 10 moons and several additional 

rings during its flyby.

1997–1999 — Astronomers discover more tiny moons orbiting 

Uranus.

ABOUT THE IMAGES

1 A true-color 

composite image by 

Voyager 2.

 A V 2 oyager 2 

computer-enhanced 

image emphasizing 

high-level haze in 

the atmosphere. 

 V 3 oyager 2 imaged “lanes” of fine dust particles in the rings.

 An infrar 4 ed composite image taken by the Keck Observatory 

in November 2004 shows atmospheric details and turns the rings 

a reddish color. 

 As it departed Uranus for Neptune, V 5 oyager 2 looked back 

and captured this crescent view of the planet.

 Slight contrasts ar 6 e exaggerated in this false-color image, 

showing Uranus’ dark “polar hood” — a veil of ice and dust.

 Exaggerated contrast and false color r 7 eveal some details of 

Uranus’ clouds. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Uranus

LG-2005-12-577-HQ — JPL 400-1253P 12/05
National Aeronautics and Space Administration Neptune

www.nasa.gov
13 

National Aeronautics and Space Administration Neptune

www.nasa.gov

The eighth planet from the Sun, Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. (Galileo had recorded it as a fixed star 

during observations with his small telescope in 1612 and 1613.) 

When Uranus didn’t travel exactly as astronomers expected it to, 

a French mathematician, Urbain Joseph Le Verrier, proposed the 

position and mass of another as yet unknown planet that could 

cause the observed changes to Uranus’ orbit. After being ignored 

by French astronomers, Le Verrier sent his predictions to Johann 

Gottfried Galle at the Berlin Observatory, who found Neptune on 

his first night of searching in 1846. Seventeen days later, its largest moon, Triton, was also discovered. 

Nearly 4.5 billion kilometers (2.8 billion miles) from the Sun, 

Neptune orbits the Sun once every 165 years. It is invisible to the 

naked eye because of its extreme distance from Earth. Interestingly, due to Pluto’s unusual elliptical orbit, Neptune is actually 

the farthest planet from the Sun for a 20-year period out of every 

248 Earth years. 

The main axis of Neptune’s magnetic field is “tipped over” by 

about 47 degrees compared with the planet’s rotation axis. Like 

Uranus, whose magnetic axis is tilted about 60 degrees from the 

axis of rotation, Neptune’s magnetosphere undergoes wild variations during each rotation because of this misalignment. The 

magnetic field of Neptune is about 27 times more powerful than 

that of Earth. 

Neptune’s atmosphere extends to great depths, gradually merging 

into water and other “melted ices” over a heavier, approximately 

Earth-sized solid core. Neptune’s blue color is the result of methane in the atmosphere. Uranus’ blue-green color is also the result 

of atmospheric methane, but Neptune is a more vivid, brighter 

blue, so there must be an unknown component that causes the 

more intense color that we see. The cause of Neptune’s bluish 

tinge remains a mystery. 

Despite its great distance from the Sun and lower energy input, 

Neptune’s winds are three times stronger than Jupiter’s and nine 

times stronger than Earth’s. In 1989, Voyager 2 tracked a large 

oval dark storm in Neptune’s southern hemisphere. This hurricane-like “Great Dark Spot” was observed to be large enough 

to contain the entire Earth, spun counterclockwise, and moved 

westward at almost 1,200 kilometers (750 miles) per hour. (Subsequent images from the Hubble Space Telescope showed no sign 

of the Great Dark Spot photographed by Voyager. A comparable 

spot appeared in 1994 in Neptune’s northern hemisphere but 

had disappeared by 1997.) Voyager 2 also photographed clouds 

casting shadows on a lower cloud deck, enabling scientists to 

visually measure the altitude differences between the upper and 

lower cloud decks. 

The planet has six rings of varying thicknesses, confirmed by 

Voyager 2’s observations in 1989. Neptune’s rings are believed to 

be relatively young and relatively short-lived. 

Neptune has 13 known moons, six of which were discovered 

by Voyager 2. The largest, Triton, orbits Neptune in a direction 

opposite to the direction of the planet’s rotation. Triton is the 

coldest body yet visited in our solar system — temperatures on 

its surface are about –235 degrees Celsius (–391 degrees Fahrenheit). Despite this deep freeze at Triton, Voyager 2 discovered 

geysers spewing icy material upward more than 8 kilometers 

(5 miles). Triton’s thin atmosphere, also discovered by Voyager, 

has been seen from Earth several times since, and is growing 

warmer — although scientists do not yet know why. 

FAST FACTS 

Namesake Roman god of the sea 

Mean Distance from the Sun 4,498.25 million km 

(2,795.08 million mi) 

Orbit Period 164.79 Earth years 

(60,190 Earth days) 

Orbit Eccentricity (Circular Orbit = 0) 0.00859 

Orbit Inclination to Ecliptic 1.769 deg 

Inclination of Equator to Orbit 29.58 deg 

Rotation Period 16.11 hours 

Equatorial Radius 24,764 km (15,388 mi) 

Mass 17.147 of Earth’s 

Density 1.64 g/cm3 

Gravity 10.71 m/sec2

 (35.14 ft/sec2)

Atmosphere Primary Components hydrogen, helium, methane

Effective Temperature –214 deg C (–353 deg F)

Known Moons* 

Rings 6 (Galle, Arago, Lassell, Le Verrier, 

the ring coorbital with the moon Galatea, Adams) 

*As of November 2005. 

SIGNIFICANT DATES 

1846 — Using mathematical calculations, astronomers discover 

Neptune, increasing the number of known planets to eight. 

Neptune’s largest moon, Triton, is found the same year.

1984 — Astronomers find evidence for the existence of a ring 

system, but the rings are clearly not uniform in density.

1989 — Voyager 2 becomes the first and only spacecraft to visit 

Neptune, passing about 4,800 kilometers (2,983 miles) above 

the planet’s north pole.

2003 — Using improved observing techniques, astronomers discover five new moons orbiting Neptune.

2001 — Neptune completes its first 165-year orbit of the Sun 

since its discovery in 1846.

ABOUT THE IMAGES 

 V 1 oyager 2 captured this image of 

Neptune in 1980.

2 A detailed look 

at Neptune’s thin 

rings.

 The cr 3 escents of 

Neptune and Triton 

were imaged by Voyager 2 shortly after the spacecraft’s closest approach to the planet.

 V 4 oyager 2’s view of Neptune’s south pole as the spacecraft 

sped away from its encounter.

 The clouds in this image ar 5 e estimated to be about 50 kilometers (31 miles) tall.

 V 6 oyager 2’s closest image of Neptune’s mysterious Great 

Dark Spot. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune

LG-2005-12-578-HQ — JPL 400-1253Q 12/05
National Aeronautics and Space Administration Pluto and Charon 

www.nasa.gov
 Pluto is mostly 1

 Astr 2 onomers 

4 eated fr eated from four Hubble Space T om four Hubble Space Tele-

 The Hubble Space T 3 elescope resolved Pluto and Charon as 

Pluto and Charon

3

National Aeronautics and Space Administration

1 

www.nasa.gov

The smallest, coldest, and most distant planet from the Sun, Pluto has a dual identity — it is also a member of a group of objects 

that orbit in a disc-like zone beyond the orbit of Neptune called 

the Kuiper Belt. Pluto and its companion moon, Charon, orbit 

the Sun in this region. This distant realm is populated with thousands of miniature icy worlds, which formed early in the history 

of the solar system. While Pluto retains its position as the only 

recognized planet beyond Neptune, recent discoveries of a number of icy objects orbiting the Sun in Pluto’s realm — at least one 

of which may be as large or larger than Pluto — have prompted a 

re-examination of how a planet should be defined. 

Discovered by American astronomer Clyde Tombaugh in 1930, 

Pluto takes 248 years to orbit the Sun in a highly elliptical orbit. 

Pluto’s closest approach to the Sun was in 1989, when its path 

carried it inward from its usual distance of about 39 astronomical units (AU) to within 29.7 AU of the Sun. (One AU is the 

mean distance between Earth and the Sun: about 150 million 

kilometers or 93 million miles.) Between 1979 and 1999, Pluto’s 

orbit brought it closer to the Sun than Neptune (Neptune’s mean 

distance is about 30 AU), providing rare opportunities to study 

this small, cold, distant world and its moon. 

Pluto is about two-thirds the diameter of Earth’s Moon and 

probably has a rocky core surrounded by a mantle of water ice. 

More exotic ices like methane and nitrogen frost coat its surface. 

Owing to its lower density, Pluto’s mass is about one-sixth that of 

the Moon. While it is close to the Sun, these ices thaw, rise, and 

temporarily form a thin atmosphere. Pluto’s low gravity (about 

6 percent of Earth’s) causes the atmosphere to be much more extended in altitude than our planet’s atmosphere. Because Pluto’s 

orbit is so elliptical, Pluto becomes much colder during the part 

of each orbit when it is traveling away from the Sun. During this 

time, the bulk of the planet’s atmosphere may freeze. 

In 1978, astronomers discovered that Pluto has a large moon, 

which was named Charon. Charon is almost half the size of 

Pluto and shares the same orbit; thus, Pluto and Charon are 

essentially a double planet system. The distance between the 

two is 19,640 kilometers (12,200 miles). The Hubble Space 

Telescope photographed Pluto and Charon in 1994 when Pluto 

was about 30 AU from Earth. These photos showed that Charon 

is bluer than Pluto, indicating that they have different surface 

compositions and structure. Charon is known to have water ice 

on its surface. 

Charon’s orbit around Pluto takes about 6-1/2 Earth days. One 

Pluto rotation (a Pluto day) takes 6-1/2 Earth days, so Charon 

neither rises nor sets but “hovers” over the same spot on Pluto’s 

surface, and the same side of Charon always faces Pluto — this 

is called tidal locking. Compared with most of the planets and 

moons, the Pluto–Charon system is tipped on its side. Pluto’s 

axis of rotation is highly tilted the planet’s rotation is retrograde 

(Uranus and Venus also have retrograde rotation). It isn’t known 

whether Pluto has a magnetic field, but its small size and slow 

rotation suggest little or no magnetic field. 

Because Pluto and Charon are so small and far away, they are 

extremely difficult to observe from Earth. In the late 1980s, Pluto 

and Charon passed in front of each other repeatedly for several 

years. Observations of these rare events allowed astronomers to 

make rudimentary maps of each body showing areas of relative 

brightness and darkness. 

No spacecraft has ever visited Pluto, but NASA is preparing a 

mission called New Horizons that would explore both Pluto and 

the Kuiper Belt region. Expected to launch in 2006, the spacecraft would reach Pluto about 2015. 

FAST FACTS 

Namesake Roman god of the underworld 

Mean Distance from the Sun 5,906.38 million km 

(3,670.05 million mi) 

Orbit Period 247.92 Earth years 

(90,553 Earth days) 

Orbit Eccentricity (Circular Orbit = 0) 0.2488 

Orbit Inclination to Ecliptic 17.14 deg 

Inclination of Equator to Orbit 119.61 deg 

Rotation Period 6.387 Earth days 

Equatorial Radius (Pluto) 1,180 km (733 mi) 

Equatorial Radius (Charon) 600 km (373 mi) 

Mass 0.0022 of Earth’s 

Density 2.03 g/cm3 

Gravity 0.65 m/sec2 (2.1 ft/sec2) 

Atmosphere Primary Components nitrogen, 

carbon monoxide, methane 

Surface Temperature –233 to –223 deg C (–387 to –369 deg F) 

Known Moons* 

Rings None known 

*As of November 2005. In late October 2005, NASA announced 

that scientists using the Hubble Space Telescope had discovered 

that Pluto may have two additional moons. If the finding is confirmed, Pluto will be the only object beyond Neptune known to 

have more than one mooon. 

SIGNIFICANT DATES 

1930 — Clyde Tombaugh discovers Pluto.

1977–1999 — Pluto’s lopsided orbit brings it slightly closer to 

the Sun than Neptune. It will be at least 230 years before Pluto 

again gets another 20-year stint as the eighth planet.

1978 — American astronomers James Christy and Robert Harrington discover Pluto’s only moon, Charon.

1988 — Astronomers discover that Pluto has an atmosphere.

2010–2025 — Pluto’s atmosphere may freeze and collapse as the 

planet moves farther from the Sun.

ABOUT THE IMAGES 

1

4

1 Pluto is mostly Pluto is mostly 

brown and is probably covered with 

methane frost.

2 Astronomers onomers 

investigating the 

sometimes-elongated 

shape of Pluto in

some images eventually determined that Pluto had a companion 

— a large moon nearly half Pluto’s size.

3 The Hubble Space T The Hubble Space Telescope resolved Pluto and Charon as 

separate disks, enabling better measurements of both bodies. 

4 A surface map of Pluto cr A surface map of Pluto created from four Hubble Space Telescope images, showing distinct areas of brightness.

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Pluto 

LG-2005-12-579-HQ — JPL 400-1253R 12/05
National Aeronautics and Space Administration Comets

www.nasa.gov
1 The hyperspeed 

2 Stardust revealed the nucleus of comet Wild 2 during a 2004 

 The tail of comet C/2001 Q4 (NEA 3 T) expands as it brushes 

 A beautiful cloud of dust and gas surr 4 ound comet NEAT as 

National Aeronautics and Space Administration Comets

www.nasa.gov

Throughout history, people have been both awed and alarmed 

by comets, perceiving them as “long-haired” stars that appeared 

in the sky unannounced and unpredictably. To many ancient 

observers, an elongated comet looked like a fiery sword or some 

other symbol of death and destruction blazing across the night 

sky. Chinese astronomers kept extensive records for centuries, 

including illustrations of characteristic types of tails. As well as 

noting associated disasters, they recorded the times of cometary 

appearances and disappearances in addition to celestial positions. 

These comet annals have proven to be a valuable resource for 

later astronomers. 

We now know that comets are “dirty-ice” leftovers from the dawn 

of the solar system around 4.6 billion years ago. They are among 

the least-changed objects and, as such, may yield important 

clues about the formation of our solar system. Comets may have 

brought water and organic compounds, the building blocks of 

life, to the early Earth and other parts of the solar system. 

As theorized by astronomer Gerard Kuiper in 1951, a disc-like 

belt of icy bodies exists just beyond Neptune, where a population of dark comets orbits the Sun in the realm of Pluto. These 

icy objects occasionally fall towards the Sun and become the socalled short-period comets. Short-period comets take less than 

200 years to orbit the Sun, and in many cases their appearance is 

predictable because they have passed by before. 

Less predictable are long-period comets, many of which arrive 

from a region called the Oort Cloud about 100,000 astronomical 

units (that is, 100,000 times the distance between Earth and the 

Sun) from the Sun. These Oort Cloud comets can take as long as 

30 million years to complete one trip around the Sun. 

Each comet has only a tiny solid part, called a nucleus, often no 

bigger than a few kilometers across. The nucleus contains icy 

chunks and frozen gases with bits of embedded rock and dust. At 

its center, the nucleus may have a small rocky core. 

Traveling in a highly elliptical orbit, a comet warms up as it nears 

the Sun and develops an atmosphere, or coma. The Sun’s heat 

causes ices on the comet’s surface to change to gases so that its 

coma gets larger. This coma may be hundreds of thousands of 

kilometers in diameter. The pressure of sunlight and high-speed 

solar particles blow the coma materials away from the Sun, forming the comet’s long, and sometimes bright, tails. These tails 

point away from the Sun’s direction. 

Most comets travel a safe distance from the Sun — comet Halley 

comes no closer than 89 million kilometers (55 million miles). 

However, some comets, called sun-grazers, crash straight into the 

Sun or get so close that they break up and evaporate. 

Scientists have long wanted to study comets in some detail, tantalized by the few 1986 images of comet Halley’s nucleus. NASA’s 

Deep Space 1 spacecraft flew by comet Borrelly in 2001 and 

photographed its nucleus, which is about 8 kilometers (5 miles) 

long. Another NASA mission, called Stardust, was designed to 

approach a comet, photograph the nucleus, then capture dust 

samples from the coma and return them to Earth for analysis. 

Stardust successfully flew within 236 kilometers (147 miles) of 

the nucleus of Comet Wild 2 (pronounced “Vilt 2”) in January 

2004, collecting cometary particles as well as interstellar dust 

particles for a later sample return to Earth. 

Another NASA mission, called Deep Impact, consisted of a flyby 

spacecraft and a small “impactor.” In July 2005, the impactor 

was released into the path of the nucleus of comet Tempel 1 in 

a planned collision, which vaporized the impactor and ejected 

massive amounts of fine, powdery material from beneath the 

comet’s surface. A camera on the impactor and two cameras and 

a spectrometer on the flyby spacecraft showed dramatic brightening that revealed the interior composition and structure of the 

nucleus. 

SIGNIFICANT DATES 

1070–1080 — The comet later designated Halley’s Comet is 

pictured in the Bayeux Tapestry, which chronicles the Battle of 

Hastings of 1066. 

1449–1450 — Astronomers make one of the first known efforts 

to record the paths of comets across the night sky. 

1705 — Edmond Halley determines that the comets of 1531, 

1607, and 1682 are the same comet and predicts its return in 

1758. The comet arrives on schedule and is later named Halley’s 

Comet. 

1986 — An international fleet of five spacecraft converges on 

comet Halley as it makes its regular (about every 76 years) pass 

through the inner solar system. 

1994 — In the first observed planetary impact by a comet, awed 

scientists watch as fragments of comet Shoemaker–Levy 9 smash 

into Jupiter’s atmosphere. 

2001 — Deep Space 1 flies by and photographs comet Borrelly. 

2004 — NASA’s Stardust spacecraft snaps photos and collects 

dust samples from comet Wild 2 during the closest-ever flyby of 

a comet nucleus. The photographs show jets of dust and a rugged, textured surface. 

2005 — The Deep Impact impactor collides with comet Tempel 1 to reveal the interior of the nucleus. 

ABOUT THE IMAGES 

1 2

3

4

1 Thehyperspeed hyperspeed 

collision of the 

Deep Impact impactor with comet 

Tempel 1 generated 

a huge cloud of 

dust that reflected 

sunlight. 

1

2 Stardust revealed evealed evealed the nucleus nucleus nucleus of comet Wild 2 during a 2004 

flyby. Tiny cometary and interstellar dust particles were captured 

for return to Earth for analysis.

3 The tail of comet C/2001 Q4 (NEA The tail of comet C/2001 Q4 (NEAT) expands as it brushes T) expands as it brushes 

close to a coronal mass ejection from the Sun in 2003. 

4 A beautiful cloud of dust and gas surround comet NEAT as 

it passes through the inner solar system in 2004. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=Comets 

LG-2005-12-580-HQ — JPL 400-1253S 12/05 
National Aeronautics and Space Administration Kuiper Belt and Oort Cloud 

www.nasa.gov

Pluto

Typical KBO

Oort Cloud

2003 UB313
 Artist’ 1 s concept 

 An illustration 2

 Artist’ 3 s concept of Sedna and a hypothetical moon.

 A diagram showing the highly tilted orbit of 2003 UB313. 4

National Aeronautics and Space Administration Kuiper Belt and Oort Cloud 

www.nasa.gov

In 1950, Dutch astronomer Jan Oort proposed that certain comets came from a vast spherical shell of icy bodies near the edge of 

the solar system. This giant swarm of objects is now named the 

Oort Cloud, occupying space at a distance between 5,000 and 

100,000 astronomical units. (One astronomical unit, or AU, is 

the mean distance of Earth from the Sun: about 150 million kilometers or 93 million miles.) 

The Oort Cloud contains billions of icy bodies in solar orbit. 

Occasionally, passing stars disturb the orbit of one of these bodies, causing it to come streaking into the inner solar system as 

a long-period comet. These comets have very large orbits and 

are observed in the inner solar system only once. In contrast, 

short-period comets take less than 200 years to orbit the Sun and 

they travel along the plane in which most of the planets orbit. 

They come from a region beyond Neptune called the Kuiper Belt, 

named for astronomer Gerard Kuiper, who proposed its existence 

in 1951. The Kuiper Belt, extending out to about 50 AU around 

the Sun, is populated with thousands of small icy bodies. 

In 1992, astronomers detected a reddish speck about 42 AU 

from the Sun — the first time a Kuiper Belt object (or KBO 

for short) had been sighted. More than 1,000 KBOs have been 

identified since 1992. (They are sometimes called Edgeworth– 

Kuiper Belt objects, acknowledging another astronomer who 

also is credited with the idea, or they are simply called transneptunian objects — TNOs.) 

One of the largest KBOs is Quaoar (2002 LM60), named by its 

discoverers after the mythical creation-force figure of the Tongva 

tribe of the Los Angeles basin. Quaoar orbits the Sun every 288 

years about a billion miles beyond the orbit of Pluto (somewhere 

around 42 AU). Quaoar was photographed in 1980, but was not 

recognized as a KBO until 2002. An even larger KBO (2004 DW, 

now officially named Orcus) was found at a distance of about 

45 AU from the Sun. It is considered a “plutino” or “little Pluto” 

because of its similar orbit with the planet. 

In March 2004, a team of astronomers announced the discovery 

of a planet-like object, or planetoid, orbiting the Sun at an extreme distance, in the coldest known region of our solar system. 

The planetoid (2003 VB12), since named Sedna for an Inuit 

goddess who lives at the bottom of the frigid Arctic ocean, approaches the Sun only briefly during its 10,500-year solar orbit. 

Sedna is about one-quarter to three-eighths the size of the planet 

Pluto. At the farthest point in its long, elliptical orbit, Sedna is 

130 billion kilometers (84 billion miles) from the Sun — that’s 

about 86 AU, compared with the mean distances of Neptune 

(about 30 AU) and Pluto (about 39 AU). 

The discoverers of Sedna describe it as an inner Oort Cloud 

object, because it never enters the Kuiper Belt — Sedna never 

comes closer to the Sun than 76 AU. Sedna is quite an oddity: 

nobody expected to find an object like it in the largely empty 

space between the Kuiper Belt and the Oort Cloud. Possibly the 

Oort Cloud extends much farther in towards the Sun than previously thought, or perhaps Sedna is yet another type of object 

from the very early solar system, trapped between the Kuiper 

Belt and the Oort Cloud. 

Because KBOs are so distant, their sizes are difficult to measure. 

The given diameter of a KBO depends on assumptions about 

how its brightness relates to its size. To estimate size based on 

brightness, one assumes what percentage of sunlight the object’s 

surface reflects; this percentage is known as the albedo. Thinking 

that the albedo of an average KBO is similar to that of comets, 

astronomers calculated the sizes of KBOs based on the reflectivity 

of comets, which is about 4 percent. An efficient way to calculate 

an object’s albedo is to measure the heat it radiates in the infrared. In 2004, astronomers using the Spitzer Space Telescope did 

a survey of KBOs at infrared wavelengths and found that they 

averaged about 12 percent; thus, KBOs might be smaller objects 

than astronomers originally thought. However, new discoveries 

may alter this perception. 

Until recently, all the KBOs found were judged to be significantly 

smaller than Pluto. In July 2005, a team of scientists announced 

the discovery of a body that appears to be as large as, or possibly 

even larger, than Pluto. The object, provisionally designated 

2003 UB313, orbits the Sun about every 560 years, its distance 

varying from about 38 to 98 AU. Initial analysis indicates that 

the surface is covered with methane ice, similar to Pluto. In September 2005, the team found that 2003 UB313 even has a small 

moon, thought to be made of water ice. 

The discovery of large objects in this frozen outer realm may 

eventually affect the planetary status of Pluto. This is the nature of 

planetary astronomy, to reconsider past conclusions in the light of 

new discoveries and change our theories or definitions to respond 

to those discoveries. No spacecraft has ever traveled to the Kuiper 

Belt, but NASA’s New Horizons mission, planned to arrive at Pluto 

in 2015, might be able to penetrate farther into the Kuiper Belt to 

study one of these mysterious objects. 

SIGNIFICANT DATES 

1943 — Astronomer Kenneth Edgeworth suggests that a reservoir 

of comets exists beyond the planets.

1950 — Astronomer Jan Oort theorizes that a vast population of 

comets may exist on the distant edges of our solar system.

1951 — Astronomer Gerald Kuiper predicts the existence of a 

belt of icy objects just beyond the orbit of Neptune. 

1992 — After five years of searching, astronomers David Jewitt 

and Jane Luu discover the first Kuiper Belt object, 1992 QB1.

2002 — Scientists using the 48-inch Oschin telescope at Palomar 

Observatory find Quaoar. 

2004 — Astronomers using the Palomar 48-inch Oschin telescope announce the discovery of Sedna (2003 VB12).

2005 — Astronomers announce the discovery of 2003 UB313, 

a distant icy solar system body that has a small moon.

ABOUT THE IMAGES

3

2

4

 Artist’ Artist’s concept s concept 

of 2003 UB313 and 

its moon. The Sun 

appears in the distance at right.

1

2 An illustration An illustration An illustration 

of the Kuiper Belt 

and the Oort Cloud. 

3 Artist’s concept of Sedna and a hypothetical moon.

4 A diagram showing the highly tilted orbit of 2003 UB313. A diagram showing the highly tilted orbit of 2003 UB313. A diagram showing the highly tilted orbit of 2003 UB313. 

FOR MORE INFORMATION 

solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs 

LG-2005-12-581-HQ — JPL 400-1253T 12/05