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Asteroids were also studied by the Hubble Space Telescope, such as tracking the colliding asteroids in the main belt, [135] [136] break-up of an asteroid, [137] observing an active asteroid with six comet-like tails, [138] and observing asteroids that were chosen as targets of dedicated missions. [139] [140]
The first flyby of a comet occurred in 1985, when the International Cometary Explorer (ICE) passed by the comet Giacobini–Zinner, [67] whereas the first flybys of asteroids were conducted by the Galileo space probe, which imaged both 951 Gaspra (in 1991) and 243 Ida (in 1993) on its way to Jupiter.
Over 200 asteroids are known to be larger than 100 km, [63] and a survey in the infrared wavelengths has shown that the asteroid belt has between 700,000 and 1.7 million asteroids with a diameter of 1 km or more. [64] The number of asteroids in the main belt steadily increases with decreasing size.
Initially, astronomers were concerned that the space rock might impact Earth in 2029 and 2068, ... C-type asteroids are made of clay and silicate rocks, while S-types are composed of silicate ...
Created 50,000 years ago by an impactor about 50 metres (160 ft) across, it shows that the accretion of the Solar System is not over. Gravitational disruption from the outer planets' migration would have sent large numbers of asteroids into the inner Solar System, severely depleting the original belt until it reached today's extremely low mass ...
Counting them among the planets became increasingly cumbersome. Eventually, they were dropped from the planet list (as first suggested by Alexander von Humboldt in the early 1850s) and Herschel's coinage, "asteroids", gradually came into common use. [140] Since then, the region they occupy between Mars and Jupiter is known as the asteroid belt.
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]
Ashley King, a researcher in the planetary materials group at the Natural History Museum, said 12% of the sample was made up of water, and, as the least contaminated specimen to be collected, it ...