Stardust is the first space mission to capture dust from a comet and return it to Earth. Stardust was launched on February 7, 1999. On January 2, 2004, Stardust met up with Comet Wild 2 (pronounced "Vilt 2"). Before it got there, the spacecraft had to make two trips around the Sun. Then it captured particles of dust using some very weird stuff called aerogel. Aerogel looks like frozen smoke. It is so light and wispy you can see right through it. It is the lightest known solid material. The comet particles were trapped gently inside the aerogel and stored safely for the trip home. See more pictures of aerogel.
Stardust made one more trip around the Sun to catch up with Earth again. The samples inside the aerogel, stored in a special reentry capsule, parachuted safely to Earth on January 15, 2006. Scientists will soon learn what Stardust has brought home.
But why take all this time and trouble to bring home comet samples?
Comets are believed to be a very old part of our solar system. They are made of the leftover materials that didn't become part of the Sun, the planets, or the moons. If we knew more about comets, we would know more about how our solar system formed over four billion years ago!
Most comets live in a cloud of comets far outside the orbit of Pluto. This comet cloud is called the Oort Cloud. Comets in the Oort Cloud may take 30 million years to make one trip around the Sun!
Most of the comets we see in our part of the solar system, though, come from just outside the orbit of Neptune in a region called the Kuiper Belt. These comets usually take 200 years or less to make one orbit of the Sun. They are called short-period comets. It's when one of them gets in close to the Sun that we can see it in our skies.
The nucleus, or solid part of a comet, is usually less than 10 kilometers (about 6 miles) across. The nucleus is like a dirty snowball. Nobody knows for sure what any comet is like inside. Maybe they are not all similar.
Comets seem to contain a lot of ice, some rocks and dust, and some gas. As they get closer to the Sun and start to heat up, some of their materials starts to boil off. This material forms a cloud around the nucleus. The cloud is called the coma and may be hundred of thousands of kilometers in diameter. And trailing out for oftentimes millions of kilometers are the comet's tails.
The tails appear as the comet approaches the Sun. Sunlight pushes on things, but very gently. Because the comet dust particles are so small, they are pushed away from the Sun into a long tail. Another tail is made of electrically charged molecules of gas (called ions). And a third tail is made of sodium, which we usually don't see with our unaided eyes.
In the early time of our solar system, Earth was often hit by comets. Scientists believe comets may have contributed some of the water for our oceans or even some of the molecules from which life eventually evolved.
Some believe it may have been a comet hitting Earth that caused the dinosaurs to become extinct.
The Stardust mission, as well as other comet missions NASA has planned, will teach us much more about these fascinating solar system objects. Read about the discoveries that another comet mission has made. |
