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Comets have tails and elongated orbits, while asteroids have circular orbits and merge into belts. Comets’ tails and coma are generated by solar heating, while asteroids are dry. Most comets originate from the Oort cloud, while asteroids are within the solar system’s “snow line.”
There are several important differences between comets and asteroids, although the distinction between the two is not absolute. The main difference is that comets have tails, while asteroids don’t. Equally important is that comets tend to have extremely elongated orbits, sometimes traveling as far as 50,000 AU (astronomical units or Earth-Sun distances) or farther from the Sun, although short-period comets only travel as far as the outer planets before returning to the Sun. inner solar system. Asteroids tend to have more circular orbits and merge into belts, such as the asteroid belt between Mars and Jupiter or the Kuiper belt beyond the orbit of Neptune.
The tail and coma (atmosphere) of comets are generated by solar heating which vaporizes volatiles (substances with a low boiling point) on a comet’s surface, particularly ice, and causes them to be expelled all around the comet. Then, the solar wind carries the vaporized materials away, forming the tail. While one could imagine the tail extending behind the comet in the direction of its travel, space is a vacuum, so there is no wind resistance to make this happen. Instead, the tail always points away from the Sun, changing its orientation as the comet orbits the Sun and is drawn back into the outer solar system.
Many more asteroids are known than comets. As of 2008, only about 3,572 comets are known, while many millions of asteroids are known to exist. Most comets are believed to originate in places very distant from the Sun, most notably the Oort cloud, a hypothetical belt of orbiting material located about 50,000 AU from the Sun. Hence, comets consist of those very few objects in the outer solar system that have orbits that bring them closer to the Sun. The real reason ice exists on them in such large quantities is that they spend most of their time far away from the Sun, where ice is common and rays from the sun are not intense enough to burn it.
In contrast, most of the asteroids in the asteroid belt are located within the solar system’s “snow line,” meaning their surfaces are dry as a bone — all of the ice has long since evaporated. Nearly everything within this snowline is dry, with Earth (and ancient Mars) being a big exception. Since the Earth lies within the snow line, it is thought that much of its initial water may have been deposited via comet impacts. Further water was produced by chemoautrophic bacteria, which can synthesize water from hydrogen sulfide and atmospheric carbon dioxide.
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