At the cold outer edge of the solar system, trillions of celestial bodies speed through space. These objects are remnants of early planet formation. On occasion, one of these objects is coaxed out of its orbit by the gravitational tug of a planet or star. It begins an inward journey towards the Sun. As it approaches the Sun, it speeds up. The intensifying solar heat causes some of the object’s solid materials to sublimate, or change directly to gas. As jets of gas and dust stream from the object, a thin atmosphere, called a coma, forms around its nucleus, or core, and a long and sometimes bright tail extends from it. This tail, which can be as long as the distance between Earth and the Sun, always points away from the Sun. When the object rounds the Sun and begins its outward journey, its contents refreeze and its coma and tail disappear.

Orbiting objects that show a coma and tail are classified as comets. Made of icy chunks, frozen gases, and bits of embedded rock and dust, comets differ from asteroids, which are mostly rock or metal. Compared with planets, comets are small. They range in diameter from a few hundred to a few thousand meters. But like the planets, each comet travels on a regular path, or orbit, around the Sun. While Earth's orbit is close to circular, many comets orbit along a more stretched-out, or elliptical, path, with the Sun nearer to one end of the orbital path than to the center.

Comets are further classified according to the time they take to complete their orbits. Short-period comets complete their orbits in as little as a few years. These comets originate in the Kuiper belt, a disc-shaped region of space beyond the planet Neptune. Objects that orbit in this region are called trans-Neptunian objects. Long-period comets may take 100,000 to as many as 30 million years to complete a single orbit. These comets originate in the Oort cloud, a vast cloud of ice and dust that is believed to exist far beyond Pluto.

Comets change very little over time. Thus they contain a record of the conditions of the early solar system. By studying comets, scientists hope to learn how planets formed and even how life may have evolved on Earth. Samples of cometary dust from NASA’s Stardust mission revealed the presence of certain minerals. The minerals suggest that materials from the inner regions of the solar system traveled to the outer regions, where comets formed. The mission also revealed in the dust the presence of glycine, an amino acid. Amino acids are molecules used to build proteins and are essential to life. The presence of glycine supports the hypothesis that comets could have delivered raw ingredients for life to Earth about 4 billion years ago, during a period of instability in the solar system that caused comets (and asteroids) to pummel all the planets and moons.

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