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Theory Observation
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Asteroids
Asteroids began as minor planets in 1801 as Giuseppe Piazzi discovered Ceres. Further similar bodies were found and their number quickly amounted to several hundred by 19th century's end. Most of these minor planets are located between Mars and Jupiter orbits in what is called the asteroid belt. Further objects were later found in other solar system locations. Terms "minor planets" and "asteroid" are similar when used about these bodies. A new usage among astronomers is to name asteroids both by their number and their name (both being attributed and managed by the International Astronomical Union (IAU)), bringing, for example, to "1 Ceres", instead of "Ceres" only. The number of the asteroids or minor planets refers to the chronology of the discovery of those LocationMost asteroids are found in what is called the asteroid belt. The asteroid belt is located between Mars and Jupiter orbits. Asteroids found there are thought to have originated like planetesimals -those blocks of materials which accreted to form planets. Those planetesimals were prevented from accreting further by Jupiter gravitational influence. Further collisions between them eventually produced this asteroids' flurry which is populating the asteroid belt nowaday. Some more asteroids exist. Centaurs are half-asteroids/half-comets (best known is Chiron) orbiting about the gas giants and the Kuiper Belt. Centaurs are objects which were disturbed from the Kuiper Belt and which are slowly changing their orbits into some alike to short-period comets', on time scales of 1-100 million years. They will ultimately crash unto the Sun. Trojans are asteroids located at a planet's two lagrangian points hence preceding and trailing it, each side of it. They mostly exist around Jupiter. More recently, stress has been put about asteroids whose perihelion (nearest point of their orbit) or whose orbit itself is close to Earth, that is Near-Earth Asteroids (NEAs). The general public better knows the NEOs (Near-Earth Objects) which encompass both asteroids and comets with such hazardous trajectories. Generally speaking known asteroids are now numbering about several hundred thousands, and thousands more are discovered each year. This is due to increase as NEOs research programs will spot more (they will be smaller and smaller in size). A question pending is how much other planetoid bodies are located beyond Neptune's orbit, in what is called the Kuiper Belt, another zone of leftovers of solar system formation. Some important bodies have already been found there like Quaoar or Varuna Physical Featuressee a table of data about asteroids Most asteroids have a Moon-like surface with craters and regolith, this rock dust yielded through the impacts at the surface. Some have been found binary or having a moon of their own (Dactyl e.g. was found about Ida). Most asteroids are under 210 mi wide (340 km). Largest and better known are Ceres, Vesta, Pallas, and Juno. Ceres is about 590 mi wide (650 km). These large minor planets are roughly spherical, as smaller asteroids are irregular in shape. Asteroids have been sorted into three spectral classes:
Space MissionsUntil 1991 asteroids were known from observations made on Earth only (optical, radar, Hubble). Galileo mission, a Jupiter bound mission, made first flyby ever of an asteroid (asteroid Gaspra), and grazed another, Ida. NEAR mission made a flyby of Mathilde en-route to asteroid Eros, about which it began orbiting in 1999. NEAR was successfully controlled-crashed on Eros by mission's end. Japanese mission MUSES-C is scheduled to asteroid Itokawa as NASA Dawn is leaving in 2006 to Vesta and Ceres which it will reach in 2010 and 2014
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