The asteroid Ida, which is 60km (37 miles) long, has a tiny moon, Dactyl (lower right). Ida spins once every 4.6 hours, so the Galileo probe could take pictures of most of it as it flew by.
missions to asteroids
The first close-up of an asteroid was provided by the Galileo probe en route to Jupiter in 1991. NEAR was the first probe to study an asteroid in depth, orbiting Eros in February 2000. Although not designed as a lander, it made a soft landing on the asteroid one year later. In late 2005, Hayabusa mapped the asteroid Itokawa and collected a surface sample.
surface of eros
NEAR took 160,000 images of potato-shaped asteroid Eros, and revealed a surface pitted by craters, formed when other asteroids smashed into it. The surface has since been sandblasted smooth by dust.
Pieces of asteroid too large to burn up in Earth's atmosphere land on its surface. Over 3,000 such meteorites, each weighing over 2 lb (1 kg), reach Earth every year. Most fall in the sea, the rest hit land. About 160 impact craters have been identified, measuring from just a few yards to about 90 miles (140 km) across. Most were formed more than 100 million years ago.
impact DAMAGE To shuttle window
Even dust-sized pieces of asteroid can form craters in a collision. The Space Shuttle windows are tested to withstand such a potential hazard.
This 0.75-mile- (1.2-km-) wide crater in Arizona was formed when an iron meteorite, probably some 100 ft (30 m) wide, hit Earth about 50,000 years ago.
Meteorites are classified by composition. The most common are the stony. Next are the irons, composed mainly of iron-nickel alloy. A very small number are stony-irons. Their mix is similar to that which formed the rocky planets. Other kinds are fragments of asteroids that have differentiated into metal cores and rock surfaces.
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