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Icy Travelers
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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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.
Swell Comet
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.
One Tough Robot
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.
Thar She Blows
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.
Don't Wait Up
Hang On
If you weigh 45 kg (100 lb.) on Earth, you'd weigh only about 0.005 kg (0.01 lb.) on a comet. At that weight, you could easily jump right off into space (although we don't recommend it ). If you rode on a comet close to the sun, you'd probably get blown off into space on a jet of dust and gas.
Hang On
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.
Killer Comet?
Darkest
A comet's nucleus is very dark -- as dark as coal or asphalt, which are some of the darkest materials on Earth. This makes a comet's nucleus one of the darkest things in our solar system.
Darkest
Survivor
Sungrazer comets are comets that come so close to the sun that they vaporize and disappear -- never to be seen again. However, in December of 2011 one comet (Lovejoy) was the first spacecraft-viewed comet to graze the sun and survive to tell the tale.
Survivor
Comet Collision
Creating space fireworks just in time for the Fourth of July (2005), Deep Impact flew by and sent an impactor into the path of the nucleus of comet Tempel 1. Deep Impact was the first mission to collide with and eject material from a comet.
Comet Collision
Small Town / Big State
The size of a comet is no small matter: Did you know that the average size of a comet is as big as an entire town? Now, imagine a comet as an average town and the Kuiper Belt (where comets reside and come from) as a state, but this state would be gigantic. The Kuiper Belt expands from about 30 to 55 AU, which is about 3.7 billion km (2.3 billion miles) long. The circumference of the entire Earth is only about 40 thousand km (25 thousand miles).
Small Town / Big State
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.
Swell Comet
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.
One Tough Robot
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.
Thar She Blows
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.
Don't Wait Up
Hang On
If you weigh 45 kg (100 lb.) on Earth, you'd weigh only about 0.005 kg (0.01 lb.) on a comet. At that weight, you could easily jump right off into space (although we don't recommend it ). If you rode on a comet close to the sun, you'd probably get blown off into space on a jet of dust and gas.
Hang On
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.
Killer Comet?
Darkest
A comet's nucleus is very dark -- as dark as coal or asphalt, which are some of the darkest materials on Earth. This makes a comet's nucleus one of the darkest things in our solar system.
Darkest
Survivor
Sungrazer comets are comets that come so close to the sun that they vaporize and disappear -- never to be seen again. However, in December of 2011 one comet (Lovejoy) was the first spacecraft-viewed comet to graze the sun and survive to tell the tale.
Survivor
Comet Collision
Creating space fireworks just in time for the Fourth of July (2005), Deep Impact flew by and sent an impactor into the path of the nucleus of comet Tempel 1. Deep Impact was the first mission to collide with and eject material from a comet.
Comet Collision
Small Town / Big State
The size of a comet is no small matter: Did you know that the average size of a comet is as big as an entire town? Now, imagine a comet as an average town and the Kuiper Belt (where comets reside and come from) as a state, but this state would be gigantic. The Kuiper Belt expands from about 30 to 55 AU, which is about 3.7 billion km (2.3 billion miles) long. The circumference of the entire Earth is only about 40 thousand km (25 thousand miles).
Small Town / Big State