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Asteroids have historically been observed from Earth. The first close-up observation of an asteroid was made by the Galileo spacecraft. Several dedicated missions to asteroids were subsequently launched by NASA and JAXA, with plans for other missions in progress. NASA's NEAR Shoemaker studied Eros, and Dawn observed Vesta and Ceres.
The number of asteroids brighter than H = 25, which corresponds to about 40 m (130 ft) in diameter, is estimated at 840,000 ± 23,000 —of which about 1.3 percent had been discovered by February 2016; the number of asteroids brighter than H = 30 (larger than 3.5 m (11 ft)) is estimated at 400 ± 100 million—of which about 0.003 percent had ...
It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, meteoroids, asteroids, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation.
Asteroids number in the hundreds of thousands. For longer lists, see list of exceptional asteroids, list of asteroids, or list of Solar System objects by size. Asteroid moons; A number of smaller groups distinct from the asteroid belt; The outer Solar System with the giant planets, their satellites, trojan asteroids and some minor planets. Jupiter
The greatest concentration of asteroids (main-belt asteroids) have semimajor axes between at 2.06 and 3.27 AU where the strong 4:1 and 2:1 orbital resonances with Jupiter (Kirkwood gaps) lie. Their orbits have eccentricities less than 0.33 and inclinations below 30°. At Jupiter distance are the three specific dynamic groups of asteroids.
Evidence suggests the main growth of asteroids can result from gas-assisted accretion of chondrules, which are millimeter-sized spherules that form as molten (or partially molten) droplets in space before being accreted to their parent asteroids. [34] In the inner Solar System, chondrules appear to have been crucial for initiating accretion. [35]
Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that reaching an asteroid without aiming carefully would be improbable. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff.
In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger body, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of its Lagrangian points L 4 and L 5. Trojans can share the orbits of planets or of large moons. Trojans are one type of co-orbital object.