Asteroids and comets are two categories of small space objects which, together, make up the majority of the objects in our solar system - although, since they are small, none of these are as visible or as distinctive as the solar system's eight planets (including Earth) or dwarf planets (such as Pluto). Although similar in size, and in the threat they can pose to life on Earth in the event of a collision, the essential differences between comets and asteroids are where they formed in the solar system, and what they are made of. Traditionally, this distinction was made on the basis of visual appearance: a comet was a space object with a visible trail of gas and debris, referred to as a "coma." There are perhaps trillions of asteroids and comets in the solar system, though only a fraction of these are large enough to be noticeable or significant.
- Location -
Asteroids are dispersed throughout the solar system, but a large proportion can be found in the asteroid belt, the region of space lying between Mars and Jupiter. They formed from the same chaotic series of collisions in the early solar system which, eventually, led to the planets. In one sense, in terms of the evolution of the solar system, the planets are simply enormous asteroids, the result of millions of collisions and accretions between smaller bodies, which coalesced into spherical planets under the force of gravity. Asteroids range tremendously in size; most are very small, perhaps metres across, but they include such massive bodies as Ceres, about six hundred miles wide. In fact, more than half of the total mass of the objects in the entire asteroid belt is contained in just four huge bodies: Ceres, Vesta, Pallas, and Hygiea.
However, asteroids are not confined solely to the asteroid belt. Small and dispersed populations of these rocky bodies can be found drifting throughout the solar system. There are a number of asteroids with orbits which come close to that of the Earth, for example.
Comets, by contrast, tended to form much farther away from the Sun. There are several billion comets, contained mostly within the Kuiper belt beyond the orbit of Neptune, and possibly within the Oort Cloud, beyond that. The Oort Cloud is theorized to be as much as 100 000 times as far from the Sun as Earth, but its existence at this point is largely theoretical; in contrast, the existence of the several-billion-object Kuiper Belt has been confirmed, and Pluto is even believed to be an exceptionally large inner member of this belt.
Comets visible to us on Earth, typically, are those which the Sun's gravity has pulled out of their normal, distant orbit. Many of these take the form of extremely long, highly elliptical (oval-shaped) orbits, resulting in comets plunging deep into the inner solar system, circling the Sun, and then travelling far into the outer reaches of the solar system again. For example, Halley's Comet - one of the best-known comets - follows a roughly 75-year orbit which takes it from roughly the orbit of Pluto, at its farthest point, to somewhere between Mercury and Venus, at its closest point to the Sun. However, comets often have much longer orbits because of the vast distances at which they were formed; so-called long-period comets have orbits which last thousands of years.
- Composition -
Asteroids tend to be mostly or entirely rocky - much the same as planets like Earth, except far smaller. This is because they formed relatively close to the Sun, in the same series of collisions between dust, rocks, and other debris as the planets themselves did. The largest concentration of asteroids, the asteroid belt between Mars and Jupiter, was once thought to have been the result of a massive cataclysm, the breaking up of a tenth planet destroyed either by some sort of internal explosion or by an enormous impact event, far larger than anything Earth has ever experienced. However, the most commonly accepted hypothesis today is that the asteroid belt today resembles what the orbits of the rocky planets once were like, but, unlike those planets, the debris in the asteroid belt never accumulated together to form a planet.
Comets, in contrast, are more than just balls of rock. Because of where and how they formed, according to the European Space Agency, comets are especially interesting because they contain the earliest material in the solar system, without the long history of collisions and chemical changes which the inner asteroids and the planets have undergone. Because they formed at great distances from the Sun, they contain relatively high concentrations of ices, including frozen methane and - much more interesting from a human perspective - water. Indeed, it is speculated that large numbers of comet impacts delivered at least some of our current surface water, since much of Earth's original water may have been lost during the process of planetary formation.
It is because of this relatively high concentration of ices that comets develop their telltale smudge, stream, or trail of vapour as they pass by the Earth. Approaching the Sun heats up the comet, causing the vaporization of some of the ice on its outer layers. This then results in a visible "tail" of gas and dust, ejected from the comet. In theory, enough such orbits should result in the loss of all volatile material, rendering the comet indistinguishable from an asteroid.
- An Arbitrary Distinction -
Ultimately, the distinction between asteroids and comets is really significant only in stargazing: an asteroid, on the rare occasions that one is visible, would simply appear as a star-like prick of light, but a comet has a distinguishable halo or tail. In practice, these categories are somewhat arbitrary and cannot describe all of the enormous variety of small objects in the solar system. The Centaurs, for example, are a group of small bodies orbiting between Jupiter and Neptune, which have some of the traditional characteristics of both asteroids and comets.