Solar System Exploration: Planet Selector: Comets

PLANET SELECTOR

ABOUT COMETS

Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town. When a comet's orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of kilometers.
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Featured Mission: Stardust
Stardust will collect samples from Comet Wild 2 on Jan. 2, 2004 and return them to Earth for study. The samples will land on Earth in 2006.
Read More About Stardust >>

COMET HALLEY: VITAL STATISTICS

Closest Approach to the Sun:	88,000,000 km (55,000,000 miles)
Farthest Distance from the Sun:	5,300,000,000 km (3,300,000,000 miles)
Diameter:	16 x 8 x 7 km (10 x 5 x 4 miles)
Mass:	100,000,000,000,000 kg (0.000000000016 x Earth's)
Length of Day:	2.2-7.4 Earth days
Length of Year:	76 Earth years

SIZE & DISTANCE

Frozen in deep space, Comet Halley is about the size of the Island of Manhattan in New York City. When it gets close the Sun, Halley spews dust and gas that form a glowing head - a coma - and forms a tail that extends for millions of kilometers. The coma can get bigger than Jupiter. Only our Sun is larger. If Halley's Manhattan-sized nucleus were the size of a pinhead, the tail alone would extend for five miles.

Average Distance from the Sun: 3,300,000 km (22.06 A.U.)

EXTREME SPACE

Swell Comet
When they are far from the Sun, most comets are insignificant specks less than 10 km (6 miles) across. But when a comet gets close to the Sun, the cloud of gases surrounding it can swell larger than the size of Jupiter - more than 10 times the diameter of Earth. Comets also sprout beautiful tails that stretch for millions of kilometers away from the Sun.

One Tough Robot
No one expected Europe's Giotto spacecraft to survive the beating it took from dust and rocks when it made a close pass by Comet Halley in 1986. Damaged, but far from dead, Giotto flew on to study a second comet.

Thar She Blows
Comet Hale-Bopp spewed out about 250 tons of gas and dust per second - more than 50 times what most other comets churn out. It made for a spectacular show as it passed through the inner solar system in 1997.

Don't Wait Up
Comet Hale-Bopp won't return to our skies for about 2,740 years - and that's not even close to the longest orbit comets make around our Sun. Some far travelers take millions of years to make one orbit. Others, such as Comet Encke, run by the Sun every few years.

Hang On
If you weigh 45 kilograms (100 pounds) on Earth, you'd weigh only about 0.005 kilograms (0.01 pounds) on a comet. Although we don't recommend it, you could easily jump right off into space. If you rode a comet close to the Sun, you'd probably get blown into space on a jet of dust and gas.

Killer Comet?
Were the dinosaurs wiped out by a comet impact 65 million years ago? It's possible. In the earliest days of the solar system, comets regularly bombarded the planets. And a comet smashed into Jupiter as recently as 1994.

TIMELINE

1449-1450 - Italian Paolo Toscanelli makes one of the first efforts to record the paths of comets across the night sky.

1705 - Edmond Halley determines the comets of 1533, 1607 and 1682 are the same comet and predicts its return in 1758. The comet arrives on schedule and is named Halley's Comet.

1985 - NASA's International Comet Explorer is the first spacecraft to visit a comet. The spacecraft studied Comet Giacobini-Zinner.

1986 - A fleet of five international spacecraft converge on Comet Halley as it makes its regular 76-year pass through the inner solar system.

1994 - Awed scientists watch as fragments of Comet Shoemaker-Levy 9 smash into Jupiter's atmosphere - the first observed planetary impact by a comet.

2002-now - NASA's Deep Space 1 returns images and science data from comet Borelly in 2002. In 2005, Deep Impact completes its mission by impacting with comet Tempel 1, becoming he first experiment to discover water ice on the surface of a comet. The Stardust mision returns samples to Earth from comet Wild 2 in 2006.

SLIDE SHOW

Comet Fragments Hit Jupiter
Comet Shoemaker-Levy 9 Fragments
Comet Impact on Jupiter
Comet Borrelly (False Color)
Comet Borrelly's Nucleus
Comet Halley
Comet C/2002 V1 (NEAT)
Comet NEAT at the Sun
Two Sungrazing Comets
Comet Hale-Bopp

MOONS

We don't know whether comets have moons. None has ever been seen. Any potential moon would most likely be a chunk of the nucleus. As the comet closes in on the Sun, those chunks would probably be pushed away by jets of gas erupting as the nucleus heats. If a comet did have a moon, it is unlikely it would stick around more than a few months or years. Most of the moons orbiting planets in our solar system have been there for billions of years.