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the relatively small rocky bodies that occupy the orbital path between Mars and Jupiter

a montage of typical asteroids

On New Year's Day of 1801, an Italian priest by the name of Giuseppe Piazzi was in the midst of making some observations in order to correct an error in a just-published star catalog. While doing so he noticed a "star" that was just bright enough to be seen by the naked eye, yet it was not recorded on star maps. He observed the "star" for several weeks and then, due to illness, stopped short of making his final calculations and conclusions. When he later attempted to resume his observations of the "star" he found that it had "vanished." Piazzi guessed that what he had seen was either a comet or a new planet, and sent accounts of his findings to several astronomers in France and Germany.

A young mathematician in Germany, Karl Friedrich Gauss, used Piazzi's observations to produce a calculation of an elliptical orbit for the new celestial body. Gauss circulated his calculations to several noted astronomers, and one who picked up on them was a well-known physician in Bremen named Heinrich W.M. Olbers. Exactly one year to the day after Piazzi's first sighting of the body, Olbers recorded the rediscovery of the "planet," which he named Ceres.

Gauss' calculations of the orbit of Ceres placed it within the large gap between Mars and Jupiter which had puzzled astronomers for over two hundred years. After Olbers announced the discovery of another "planet" within that gap, which he called Pallas, on March 28, 1802, astronomers were elated that they had found the "missing link" of the Solar System. In 1804, Ludwig Harding observed a third "planet" in the gap, which he named Juno. In 1807, Olbers found a fourth, named it Vesta, and stated that for unknown reasons the original planet which had orbited within the "gap" must have broken into four pieces.

Astraea and Hebe were discovered in 1845 and 1847, respectively, and since then more than 1,500 pieces have been named, and it is believed that there may be over 100,000 pieces circling around in the "gap." These pieces are now known as asteroids (meaning star splinters), or, more properly, planetoids (planet splinters). The word "asteroid" was coined by Sir William Herschel in 1802.

Few scientists now believe that asteroids are the remnants of a former planet. It is more likely that asteroids occupy a place in the Solar System where a sizable planet could have formed, but was prevented from doing so by the disruptive gravitational influences of the nearby giant planet Jupiter.

The planetoids range in size from the largest, Ceres, which is about 592 miles in diameter, down to just a few feet across. If all of the known planetoids could be assembled, they would still be somewhat smaller in size than the planet Mercury.

Classification by Orbit

Main Belt asteroids are those orbiting between Mars and Jupiter.

Near-Earth Asteroids have orbits that closely approach the Earth. Approximately 75 Amor asteroids are known to intersect the orbit of Mars, about 50 Apollo asteroids intersect the orbit of Earth, and less than 10 Aten asteroids have orbits smaller than the Earth's orbit.

Trojan asteroids lie in two clouds, one moving 60 ahead of Jupiter in its orbit and the other 60 behind.

There are also a few asteroids -- designated as Centaurs -- in the outer Solar System.

Classification by Surface Composition

Three-quarters of the known asteroids belong to the C type, which appear to be related to a class of stony meteorites known as carbonaceous chondrites. These are considered to be the oldest materials in the Solar System. Extremely dark in color, probably because of their hydrocarbon content, they show evidence of having absorbed water from hydration.

Asteroids of the S type, related to the stony iron meteorites, make up about 15 percent of all known asteroids. Much rarer are the M-type objects, corresponding in composition to the meteorites known as "irons." Consisting of an iron-nickel alloy, they may represent the cores of melted, differentiated planetary bodies whose outer layers were removed by impact cratering.

A very few asteroids are probably related to the rarest meteorite class of all -- the achondrites. These asteroids appear to have an igneous surface composition like that of many lunar and terrestrial lava flows.

10 Largest Named Asteroids
Diameter (in miles)

Sir William Herschel

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This page was last updated on 03/21/2017.