Asteroids are small rocky objects which are dispersed throughout the Solar System, though principally found within the asteroid belt between Mars and Jupiter. Similar but icier objects beyond the orbit of Neptune are referred to as Kuiper Belt objects (including dwarf planet Pluto) and, beyond that, Scattered Disk objects (including dwarf planet Eris). There are many millions of asteroids within the solar system.
There is no specific definition of asteroid, just as, until 2006, there was no specific definition of a planet (resulting in the Pluto debate of that year). Traditionally, astronomers had a threefold definition of small space objects: a comet was an object with a coma or halo of water particles and debris; an asteroid was an object which did not have one; and a meteoroid was any object less than ten metres across. Until the 1970s, they were believed to be wholly confined to the asteroid belt and the vicinity of Jupiter, but it is now known that the entire solar system contains varying densities of small objects, ranging to the massive population of icy bodies beyond the orbits of Neptune and Pluto. "Asteroid" conventionally still refers only to those rocky bodies which are located inside the orbit of Neptune, and especially those of the asteroid belt itself.
- Composition and Origins -
Even just within the asteroid belt, asteroids range from moon-sized (like Ceres, about 1000 kilometres in diameter) to tiny rocks, to mere space dust. They are generally composed of a metal core, such as iron, sometimes surrounded by a mantle of ice and loose debris. A few are known to contain complex organic compounds such as amino acids, which has fueled the speculation that life on Earth was originally seeded by asteroid or comet impacts.
The currently accepted theoretical model of social system formation suggests that asteroids were small planetoid objects originally formed during the gradual coalescence of the giant molecular cloud, but which never underwent further collisions and accretions. (The planets, according to the same model, are simply the result of an extremely large number of asteroid impacts, which collided and fused together, growing larger and larger until they became the planets we recognize today.) Although the density of asteroids is now relatively low, even within the asteroid belt, collisions do continue. However, they are not continuing at the frequency necessary for further planetary formation to occur.
- Locations of Asteroids -
The majority of asteroids - as many as 2 million large objects, and many millions more less than one kilometre in diameter - are located within the Asteroid Belt, a region of space lying between Mars and Jupiter. Four minor planets - Ceres, Vesta, Pallas, and Hygeia - are hundreds of kilometres in diameter and account for most of the actual rocky material of the belt. The remainder is dispersed over millions of other objects. However, the actual region of space is very large, so it is mostly empty beyond fine dust matter. Collisions between the larger bodies of the asteroid belt are therefore quite rare, rather than the chaotic environment often pictured in popular science fiction.
Asteroids are also divided into families or clusters, which orbit relatively closely together. Outside of the asteroid belt, Vulcanoids are a category of asteroids believed to exist near Mercury (so far, none have been found, so the category remains empty). Near-Earth asteroids are those which are located near the Earth. Trojans are a large population of asteroids which roughly share the same region of the solar system as Jupiter. Centaurs are asteroids scattered between the orbits of Saturn and Uranus.
- Human Exploration -
Asteroids have been known for centuries - the largest of them, named above, were initially believed to be actual planets. The asteroid belt has also been traversed several times by unmanned spacecraft, beginning with Pioneer 10 in 1972, and then the Voyager twins, Ulysses, Galileo, and recently New Horizons.
So far, there have been no dedicated space missions sent to any of the objects within the asteroid belt. However, NEAR Shoemaker landed on Eros, a near-Earth asteroid, in 2000, and the Japanese probe Hayabusa attempted a landing on Itokawa in 2005 but then went silent, for unknown reasons. The most optimistic and advanced mission yet, Dawn, is currently en route to Vesta and Ceres, which it will reach in 2011 and 2015 respectively. This will give us the first close-up images of dwarf planet candidates.
- The Danger of Asteroid Impacts -
From time to time, asteroids collide with planets. They, along with comets, pose the greatest risk of a catastrophic large explosion: should a sufficiently large object collide with the Earth, it could cause enormous explosions, sudden and catastrophic clouds and climate change, massive tsunamis, and other effects, depending upon the location of the impact. The most recent serious impact event, the Tunguska explosion of 1908 in Siberia, involved a blast equivalent to a large thermonuclear bomb (about 5-10 megatons of TNT), unleashed by an object just tens of metres across. (Had it occurred over a densely populated area of Europe, the consequences could have been much more devastating.)
As a result, the U.S. government maintains an office - albeit an under-funded one - to identify and track as many near-Earth asteroids as possible and predict their future close approaches. It is hoped that if a future major asteroid impact could be identified far enough in advance, technology could be developed to deflect the asteroid before it impacted. So far, however, there is still a meaningful chance that an asteroid on a collision course would not be detected until shortly before impact.