我想知道有關asteroid既野!急啊(要英文噶)

Asteroids, also called minor planets or planetoids, are a class of astronomical object. The term asteroid is generally used to indicate a diverse group of small celestial bodies that drift in the solar system in orbit around the Sun. Asteroid (Greek for "star-like") is the word used most in the English literature for minor planets, which has been the term preferred by the International Astronomical Union; some other languages prefer planetoid (Greek: "planet-like"), because it more accurately describes what they are. In late August 2006, the IAU introduced the term "small solar system bodies" (SSSBs), which includes most objects thusfar classified as minor planets, as well as comets. At the same time they introduced the term dwarf planet for the largest minor planets. This article deals specifically with the minor planets that orbit in the inner solar system (roughly up to the orbit of Jupiter). For other types of objects, such as comets, Trans-Neptunian objects, and Centaurs, see Small solar system body.

The first asteroid to be discovered, Ceres, is the largest asteroid known to date and is now classified as a dwarf planet. All others are currently classified as small solar system bodies. The vast majority of asteroids are found within the main asteroid belt, with elliptical orbits between those of Mars and Jupiter. It is thought that these asteroids are remnants of the protoplanetary disc, and in this region the incorporation of protoplanetary remnants into the planets was prevented by large gravitational perturbations induced by Jupiter during the formative period of the solar system. Some asteroids have moons or are found in pairs known as binary systems.

仲有好多料


http://en.wikipedia.org/wiki/Asteroid

On the first day of January 1801, Giuseppe Piazzi discovered an object which he first thought was a new comet. But after its orbit was better determined it was clear that it was not a comet but more like a small planet. Piazzi named it Ceres, after the Sicilian goddess of grain. Three other small bodies were discovered in the next few years (Pallas, Vesta, and Juno). By the end of the 19th century there were several hundred. Several hundred thousand asteroids have been discovered and given provisional designations so far. Thousands more are discovered each year. There are undoubtedly hundreds of thousands more that are too small to be seen from the Earth. There are 26 known asteroids larger than 200 km in diameter. Our census of the largest ones is now fairly complete: we probably know 99% of the asteroids larger than 100 km in diameter. Of those in the 10 to 100 km range we have cataloged about half. But we know very few of the smaller ones; there are probably considerably more than a million asteroids in the 1 km range. The total mass of all the asteroids is less than that of the Moon. 11 comets and asteroids have been explored by spacecraft so far, as follows: ICE flyby of Comet Giacobini-Zinner. Multiple flyby missions to Comet Halley. Giotto (retarget) to Comet Grigg-Skellerup. Galileo flybys of asteroids Gaspra and Ida (and Ida satellite Dactyl). NEAR-Shoemaker flyby of asteroid Mathilde on the way to orbit and land on Eros. DS-1 flybys of asteroid Braille and Comet Borrelly. Stardust flyby of asteroid Annefrank and recent sample collection from Comet Wild 2. For future we can expect: Hayabusa (MUSES-C) to asteroid Itokawa, Rosetta to Comet Churyumov-Gerasmenko, Deep Impact to Comet Tempel 1, and Dawn to orbit asteroids Vesta and Ceres. 243 Ida and 951 Gaspra were photographed by the Galileo spacecraft on its way to Jupiter. The NEAR mission flew by 253 Mathilde (left) on 1997 June 27 returning many images.NEAR (now renamed "NEAR-Shoemaker") entered orbit around 433 Eros (right) in January 1999 and returned a wealth of images and data. At the end of its mission it actually landed on Eros. The largest asteroid by far is 1 Ceres. It is 933 km in diameter and contains about 25% of the mass of all the asteroids combined. The next largest are 2 Pallas, 4 Vesta and 10 Hygiea which are between 400 and 525 km in diameter. All other known asteroids are less than 340 km across. There is some debate as to the classification of asteroids, comets and moons. There are many planetary satellites that are probably better thought of as captured asteroids. Mars's tiny moons Deimos and Phobos, Jupiter's outer eight moons, Saturn's outermost moon, Phoebe, and perhaps some of the newly discovered moons of Saturn, Uranus and Neptune are all more similar to asteroids than to the larger moons. (The composite image at the top of this page shows Ida, Gaspra, Deimos and Phobos approximately to scale.) Asteroids are classified into a number of types according to their spectra (and hence their chemical composition) and albedo: C-type, includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles; S-type, 17%: relatively bright (albedo .10-.22); metallic nickel-iron mixed with iron- and magnesium-silicates; M-type, most of the rest: bright (albedo .10-.18); pure nickel-iron. There are also a dozen or so other rare types. Because of biases involved in the observations (e.g. the Because of biases involved in the observations (e.g. the dark C-types are harder to see), the percentages above may not be representative of the true distribution of asteroids. (There are actually several classification schemes in use today.)

2007-02-26 16:05:04 補充：
Asteroids are also categorized by their position :Main Belt: located between Mars and Jupiter roughly 2 - 4 AU from the Sun; NEA:that closely approach the Earth

2007-03-04 15:04:28 補充：
仲有好多料???