Comets usually have two tails — one of gas and one of dust. The gas tail is normally blue, and is pushed away from the Sun by particles in the solar wind. The dust tail is yellow or white, and although it too is pushed away from the Sun, the star’s gravity causes the tail to curve.
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). Very rarely a comet will have a third tail 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.
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! 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 start to boil off. This material forms a cloud around the nucleus. The cloud is called the coma and may be hundreds of thousands of kilometers in diameter. And trailing out for oftentimes millions of kilometers are the comet’s tails.
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. 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.
Picture Credit : Google