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The vast majority of the asteroids are within the asteroid belt, with elliptical orbits between that of Mars and Jupiter. Some asteroids have moonss.
The exact definition of an asteroid is unclear. Minor planets whose semi-major axes are beyond that of Jupiter, and which are primarly made of ice are either comets, Centaurss, or Trans-Neptunian objects. Meteoroids are solid objects in interplanetary space that are substantially smaller than asteroids (much less than 1 km in diameter). Meteoroids are typically boulder-sized or smaller. See Solar System for a complete taxonomy of objects in our system.
More than 9000 asteroids have been discovered within Earth's solar system. The largest asteroid in Earth's inner solar system is Ceres, with a diameter of 900-1000km. Two other large asteroids are Pallas and Vesta; both have diameters of ~500km.
See also a list of interesting or noteworthy asteroids in our solar system.
Asteroids are commonly classified into groups based on the characteristics of their orbits and on the details of the spectrum of sunlight they reflect.
Asteroids are divided into groups and families based on their orbital characteristics.
It is customary to name a group of asteroids after the first member of that group to be discovered.
Groups are relatively loose dynamical associations, whereas families are much "tighter" and result from the catastrophic breakup of a large parent asteroid sometime in the past. The only prominent families are Eos asteroids
(mean orbital radius = 3.1 AU, eccentricity = 0.1, inclination = 10°) Themis asteroids (mean orbital radius = 3.1 AU, eccentricity = 0.1, inclination = 1°), and Koronis asteroids (mean orbital radius = 2.87 AU, eccentricity = 0.05, inclination = 1°).
There are relatively few asteroids that orbit close to the Sun. Several of these groups are hypothetical at this point in time, with no members having yet been discovered; as such, the names they have been given are provisional.
A large number of asteroids have orbits between the orbits of Mars and Jupiter, roughly 2 to 4 AU, in a region known as the Main belt. These couldn't form a planet due to the gravitational influence of Jupiter. Jupiter's gravitational influence also results in Kirkwood gaps in the asteroid belt, orbits cleared by orbital resonance. As a result of these gaps the asteroids in this region are divided into a large number of groups. They are:
Most of the asteroids beyond the orbit Jupiter are believed to be composed of ices and other volatiles. Many are similar to comets, differing only in that the perihelia of their orbits are too distant from the Sun to produce a significant tail.
Asteroids are classified into spectral types by their optical spectrum, which corresponds to the composition of the asteroid's surface material. Note that the proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.
Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope. Pairs of photographs were taken, typically one hour apart. Multiple pairs may be taken over a series of days. Second, the two films of the same region were viewed under a stereoscope. Any body in orbit around the sun would move slightly between the pair of films. Under the stereoscope, the image of the body would appear to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.
These first three steps do not constitute asteroid discovery: the observer has only found an apparition. The final step of discovery was to send
the locations and time of observations to Brian Marsden of the Minor Planets Center. Dr. Marsden has computer programs that compute whether an apparition tied together previous apparitions into a single orbit. If so, then the observer of the final apparition is declared a discoverer, and the discoverer got the honor of naming the asteroid (subject to the approval of the International Astronomical Union).
Since 1998, a large majority of the asteroids have been discovered with automated systems that comprise CCD cameras and computers directly connected to telescopes. A list of teams using such automated systems include[1]:
There is increasing interest in identifying asteroids whose orbit crosses Earth's, and that could, given enough time, collide with Earth. The two most important groups of near-Earth asteroids are the Amors, and the Atens. Various asteroid deflection strategies have been proposed.
The first "nearby" photos, of an asteroid, were taken by the Galileo spacecraft of Gaspra and Ida (1991), while NEAR Shoemaker landed on Eros(2001).
When the orbit of an asteroid is confirmed, it is given a number, and later it may also be given a name (e.g. 1 Ceres). The first few are named after figures from Graeco-Roman mythology, but as such names started to run out, others were also used - famous people, the names of the discover's wives, even television characters. A few groups have names with a common theme - for instance Centaurs are all named after legendary Centaurs, and Trojans after heroes from the Trojan War. The Centaurs are of special interest; many of them are massive comets, such as Chiron.Earth's Solar System
Asteroid classification
Orbit groups and families
Groups out to the orbit of Earth
Groups out to the orbit of Mars
Groups out to the orbit of Jupiter
Between the Hildas and the Trojans (roughly 4.05 AU to 5.0 AU), there's a 'forbidden zone'. Aside from Thule and five objects in unstable-looking orbits, Jupiter's gravity has swept everything out of this region.Groups beyond the orbit of Jupiter
Spectral types
There are also a number of rarer asteroid types, the number of types continuing to grow as more asteroids are studied.Asteroid discovery
The LINEAR system alone has discovered 157,920 asteroids as of April, 2002 [1]. Between all of the automated systems, 2298 near-Earth asteroids have been discovered [1].Asteroid deflection
Asteroid exploration