About the Asteroids

Ceres seen from the Hubble Space Telescope

Wikipedia - Asteroid Belt

At the beginning of the 19th century, astronomers discovered an object moving in an orbit between Mars and Jupiter. It was at first classified as a planet and given the name Ceres. Later, as other object were discovered in the same region, astronomers came to realize that there was an entire belt of objects, mostly orbiting between Mars and Jupiter. These were given the name "asteroids". Today, the number of known asteroids numbers in the hundreds of thousands. Most are merely small, irregular chunks of rock, although Ceres is almost spherical, and the largest three—Vesta, Pallas and Hygiea—resemble rounded ellipsoids.

Most asteroids fall within three basic types: S-type, or silicate asteroids, mostly found in the inner portion of the belt; M-type, or metallic asteroids, distributed near the middle of the belt; and C-type, or carbonaceous, which dominate in the outer regions. Astronomers are puzzled over the relative lack of basaltic asteroids. At least one asteroid, Ceres, is known to have water ice on its surface. It is also possible that it has a subsurface ocean, similar to Jupiter's moon Europa.

Recent studies of asteroid rotational rates, combined with models involving collisions, imply that the majority of asteroids with diameters greater than 100 meters are most likely "rubble piles", that is, aggregations of smaller rocks loosely held together by gravity.

Although most asteroids are confined to the belt, not all are. Many have eccentric orbits that bring them within the orbit of Mars. There is one, Icarus, which comes closer to the Sun than Mercury. Sometimes it can pass within 4 million miles of the Sun.

Asteroids that cross Earth's orbit or pass near Earth are classified as near-Earth asteroids. In fact, typical meteors are merely small near-Earth asteroids that happen to hit the planet. (There are also meteors that are of cometary origin.) Small asteroids—also called meteoroids—do not present a hazard to Earth, since they either burn up in the atmosphere, or else simply land somewhere, usually without causing damage. Due to the size distribution of asteroids, the chances of impact diminish as diameter increases, which is fortunate, since if an asteroid merely a hundred meters in diameter were to hit Earth, the effect would be as devastating as that of a large nuclear weapon, only without the radioactivity. In 1908 an explosion estimated at 10 megatons occurred in Siberia, probably caused by an asteroid less than a hundred meters in diameter disintegrating and exploding in the lower atmosphere. In 1994 a comet fragment only two kilometers in diameter blew a hole through Jupiter's cloud deck that was as big across as the entire Earth.

Two groups of asteroids share Jupiter's orbits, one group 60° ahead of the planet, the other 60° behind, in the so-called "Trojan points". These are points that are gravitationally stable; an object in one of these orbital positions will tend to return to that point if perturbed, unlike with most orbits. Today, the group that trails Jupiter is referred to as the Trojans, while the group that leads is called the Greeks.

Because of their small size and low gravity, asteroids have often been considered prime candidates for eventual mining and colonization. M-type asteroids are rich in minerals and ores useful in an industrial society. Carbonaceous asteroids probably contain all the elements necessary to support life; it would merely require the appropriate technology to mine them and convert them into water (probably already present in many asteroids), oxygen, nitrogen, etc.

Observation from Earth

Asteroids were unknown to the ancients. Ceres, the first discovered, can only be seen through a telescope, as with all the others. Nevertheless, the asteroid belt is not without effect on our skies. Zodiacal light—a faint, trinagular patch of luminescence extending upward from the Sun just after sunset and just before sunrise—is caused by reflection of sunlight from cosmic dust in the solar system, and one of the primary sources of this dust is believed to be collisions between asteroids.

Exploration of the Asteroids

Early in space exploration history, the asteroid belt was perceived as a place of rocks and shoals, where spacecraft would be hard put to pass through without suffering collisions and being damaged or destroyed. However, in 1972 Pioneer 10 entered the belt and passed through without incident. Since then, a number of probes have successfully traversed the belt and there has yet to be a collision. Most of these craft were merely passing through on their way to the outer solar system, but there have been a few sent out specifically to study asteroids. And even some of those on their way to places like Jupiter and Saturn sometimes flew past particular asteroids and took data.

Galileo imaged Gaspra and Ida on its way to Jupiter. Similarly, Cassini flew past Masursky on its way to Saturn. NEAR and Japan's Hayabusa both studied near-Earth asteroids, orbiting them and—in the case of Hayabusa—even landing on one. Currently, Dawn is on its way to the main belt to study both Vesta and Ceres.