ASTEROIDS

Asteroids are small rocky bodies, whose maximum dimension is 900 km, irregular in shape and much variable. They are also called "minor planets" due to the small dimensions. The whole mass of their set does not reach that of the Moon.
The first asteroid was discovered at the beginning of last century by the astronomer Piazzi, and was called Ceres. It is the largest of all, in fact it is more than 900 km large.

Toutathis. The asteroid is named after a god of the ancient celtic mythology,  patron of Gauls. (AAAS)

Among the members of the Solar System, asteroids are the most numerous population. The number of the known ones is about 5,000, but many of them are still unknown, since they are too small or not cataloged yet. Most of them orbit within a belt comprised between the orbit of Mars and that of Jupiter. These asteroids often present orbital resonances with Jupiter, that is their orbital period over that of Jupiter is an integer number ratio.

In the asteroid belt we find a few of groups of objects, which orbit around the Sun at the same average distance from Jupiter, trailing or leading it by about 60 degrees on its orbit. They are called "Trojan" asteroids. These and other families of asteroids are probably the rests of a larger dimensions body which disrupted due to violent collisions with other bodies.

Vesta in an image of the Hubble Space Telescope. It is the asteroid with the wider morphological variety of the surface. (HST)

Collisions indeed play an important role in the dynamics of these objects. Asteroids move at quite high speeds, about 18,000 km/h, and their orbits often cross. At this speed, an impact between two asteroids causes the formation of craters on their surface, or even disrupts them. Their fate is to  fragment and to become smaller and smaller.

 

Asteroids are very different one from the other, not only in dimensions, but also in chemical composition. Some of them are mostly composed by metals, other by basaltic rocks, others by compounds rich in carbon and maybe water ice.
We can distinguish many types, among which the most important are:
Type C: it includes more than 75% of the asteroids. They are the darker ones, their albedo is 0.03; they have carbon rich composition.
Type S: 17% of the asteroids are part of this class. Their albedo is comprised between 0.1 and 0.22, and are composed by nickel and iron mixed with iron and magnesium silicates.
 
 

Ida and Dactyl. Ida is the first asteroid around which a satellite has been discovered, Dactyl. The discovery was made by the Galileo probe, which, when crossing the asteroid belt, got as close as 2,400 km from Ida. (NASA/JPL)

The origin of the asteroids is not yet known with certainty. It is believed that they are the remnants of a "missed planet" which should have formed between Mars and Jupiter by aggregation of planetesims.  The relative speed of the asteroids are not tot too high since they can merge and form a larger body.

 

Asteroid Gaspra. (USGS/NASA/JPL)

 
 

ANIMATIONS

Rotation of Toutathis, MPEG, 47 Kb (STScI)

Rotation of Vesta, MPEG, 83 Kb (STScI)