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An asteroid (or minor planet) is a small, solid object in our Solar System, orbiting the Sun. Asteroids are much smaller than planets. They are believed to be remnants of the protoplanetary disc; which were not incorporated into planets during the system's formation.

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.

This picture of Eros, taken on February 14, 2000, shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. In this mosaic, constructed from two images taken after the NEAR spacecraft was inserted into orbit, features as small as 120 feet (35 meters) across can be seen. House-sized boulders are present in several places; one lies on the edge of the giant crater separating the two ends of the asteroid. A bright patch is visible on the asteroid in the top left-hand part of this image, and shallow troughs can be see just below this patch. The troughs run parallel to the asteroid's long dimension. Click on the image to see an enlarged version.

Table of contents
1 Earth's Solar System
2 Asteroid groups and families
3 Asteroid Discovery
4 Asteroid Deflection
5 Asteroid Exploration
6 Asteroid Classification
7 External Links

Earth's Solar 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.

Asteroid groups and families

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 (mean orbital radius = 3.1 AU, eccentricity = 0.1, inclination = 10°) Themis (mean orbital radius = 3.1 AU, eccentricity = 0.1, inclination = 1°), and Koronis (mean orbital radius = 2.87 AU, eccentricity = 0.05, inclination = 1°).

Groups out to the orbit of Earth

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.

Vulcanoids are asteroids with an aphelion < 0.4 AU, ie, they orbit entirely within the orbit of Mercury. A few searches for Vulcanoids have been conducted but there have been none discovered so far.

Apoheles are asteroids whose aphelion is < 1 AU, meaning they orbit entirely within Earth's orbit. "Apohele" is Hawaiian for "orbit". Other proposed names for this group are Inner-Earth Objects (IEOs) and Anons (as in "Anonymous"). Only one candidate member of this group has been detected so far; 1998 DK36, by David Tholen.

Arjuna asteroids are somewhat vaguely defined as having orbits similar to Earth's; ie, with an average orbital radius of around 1 AU and with low eccentricity and inclination. Due to the vagueness of this definition some asteroids belonging to the Apohele, Amor, Apollo or Aten groups can also be classified as Arjunas.

Earth trojanss are asteroids located in the Earth-Sun L4 and L5 pointss. Their location in the sky as observed from Earth's surface would be fixed at about 60 degrees east and west of the Sun, and as people tend to search for asteroids at much greater elongations few searches have been done in these locations. No Earth trojans are currently known.

Groups out to the orbit of Mars

The Aten, Apollo and Amor asteroids orbit between Earth and Mars.

Mars-crosser asteroids have orbits that cross those of Mars.

There are five known Mars trojans; 5261 Eureka, 1998 VF31, 2001 DH47, 2001 FG24, and 2001 FR127 are located in the "trailing" L5 point, and 1999 UJ7 is located in the "leading" L4 point.

Groups out to the orbit of Jupiter

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.

Another important group are the Trojans; they are in the orbit of Jupiter, on two Lagrangian points. Similar sets of asteriods appear to lie in the trojan points in almost all planetary orbits.

Groups beyond the orbit of Jupiter

Another group exists within the Kuiper belt. The Kuiper belt is the source of about half of the comets that come to the inner solar system. Some of these are not much smaller than Pluto and Charon -- the largest found so far is Quaoar, thought to be around 1250 km in diameter, the same size as (or even slightly larger than) Charon - and some astronomers expect that we shall one day find some Trans-Neptunian objects bigger than Pluto. The exact classification of these objects is unclear, since they are probably fairly different from the asteroids of the inner solar system.

The Centaurs orbit the Sun, between the orbits of the giant planets. The first of these to be discovered was 2060 Chiron in 1977. These are generally supposed to be asteroids or comets that were ejected from their original orbits.

Asteroid Discovery

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]:

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

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.

Asteroid Exploration

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.

Asteroid Classification

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.

There are also a number of rarer asteroid types, the number of types continuing to grow as more asteroids are studied.

External Links

The Solar System

Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets - Kuiper belt - Oort cloud