ESA Science & Technology05-Jul-2005 16:50:31

Asteroid Encounters

What are Asteroids Like?

Close-up photographs taken by spacecraft and ground-based radar studies of near-Earth objects show that nearly all asteroids are irregular in shape and heavily pockmarked by impact craters. Only a handful of the largest members, which are more than 300 km across, are spherical in shape.

Astronomers believe that all asteroids are derived from around 640 'protoplanets' - each larger than Ceres. These were large enough to melt inside and allow heavy metals to sink to their centres. However, over billions of years, these protoplanets collided and broke up during numerous impacts. A large amount of material was lost, but the remnants form the main belt we see today.

Studies of light reflected from their surfaces suggests that there are several types of asteroids. More than three quarters of them are very dark - blacker than coal. These C-type asteroids seem to be rich in carbon, but may also contain large amounts of water.

S-type asteroids are a mixture of rock and metals such as nickel, iron and magnesium. This implies that they were once hot enough to melt - probably inside a larger parent asteroid or 'protoplanet' which has since been destroyed.

Of the others, P- and D-types are reddish in colour, possibly due to 'primitive' organic compounds, while M-types seem to be entirely made of metal.

Rock or Rubble Piles?

The presence of a moon or a spacecraft, such as Galileo, allows scientists to determine the mass of an asteroid because of the gravitational effects of the primary asteroid on the orbit of its small neighbour. If both the mass and the size of the asteroid are known, researchers can work out its density. The density then gives a clue to the asteroid's makeup - its composition and internal structure.

These studies have led to the surprising discovery that some asteroids are real lightweights - only about 20 percent denser than water. Until recently, most asteroids were thought to be composed primarily of rock, which has a density about three times greater than water.

It seems that these featherweight objects are either highly porous rubble-piles of rock, or mostly made of water ice. If the asteroids are rubble-piles, it tells us that they have undergone numerous, severe collisions over billions of years. If the objects are largely ice, covered with a dark-coating, then these objects may be remnants of 'burned-out' comets.

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