Asteroids

The word asteroid means "star-like": these objects appear in the sky as bright, point-like stars. But, unlike stars, asteroids don't emit light on their own. These objects are rocks orbiting the Solar System, which are visible only because they reflect sunlight.

The first asteroid was discovered in 1801 by Giuseppe Piazzi of Italy, who named it "Ceres" after the Roman goddess of grain. Ever since, asteroids have received an official designation of a number (starting with Ceres of number "1"), and most larger ones have received a name based in Roman mythology. If they have a name, then they are usually referred to with the number then the name, such as 951 Gaspra. Currently, asteroids are also referred to by the International Astronomical Union (the only official body that can name astronomical objects) as minor planets.

Asteroids range in size from dust particles to several miles across. Most current theories hold that asteroids are bits of pieces left over from the formation of the solar system. They are also formed from other asteroids as they collide and break apart, as comets disintegrate, or even when the outer moons of the larger planets collide.
Some asteroids come very close to Earth (and are called NEOs, Near earth Asteroids) and can sometimes be captured by the Earth gravity and fall onto the ground. Once they enter the atmosphere, these bodies are called meteors, and once they land, they are termed meteorites.

Where can they be found?

Asteroids are found in different locations around the solar system. Most of them inhabit the Main Belt, between Mars and Jupiter. Near earth Asteroids (NEAs) are asteroids that closely approach the Earth and are divided in different classes such as Atens, Apollos and Amors. Trojans asteroids lie in the orbit of Jupiter, on special positions called lagrangian points. There are a few objects, belonging to the centaurus Family (not visible in the picture) that orbit in the outer solar system, following unstable planet-crossing orbits.

Different types of asteroids

Asteroids are made of rock and metal. They are grouped into three categories: Stony, Iron-Nickel, and a mixture of the two. Most asteroids that we know about (92.8%) fall into the first category, stony asteroids, and are made of Silicates. 5.7% are Iron-Nickel. The balance form the third type. Despite their relative abundance, stony asteroids that have fallen to Earth are the hardest to find because they look like terrestrial rocks.

There are three basic types of asteroids:

1. C: they are carbonaceous---made of silicate materials with a lot of carbon compounds so they appear very dark. They reflect only 3 to 4% of the sunlight hitting them. You can tell what they are made of by analyzing the spectra of sunlight reflecting off of them. This reflectance spectra shows that they are primitive, unchanged since they first solidified about 4.6 billion years ago. A sizable fraction of the asteroids are of this type. The asteroid called Mathilde, recently explored by the NEAR spacecraft is an example of this type.
2. S: they are made of silicate materials without the dark carbon compounds so they appear brighter than the C types. They reflect about 15 to 20% of the sunlight hitting them. Most of them appear to be primitive and they make up a smaller fraction of the asteroids than the C types. Gaspra and Ida, explored by the Galileo spacecraft on its way to Jupiter, are examples of this type.
3. M: they are made of metals like iron and nickel. These rare type of asteroids are brighter than the S and C types. We think they are the remains of the cores of differentiated objects. Large objects were hot enough in the early solar system so that they were liquid. This allowed the dense materials like iron and nickel to sink to the center while the lighter material like ordinary silicate rock floated up to the top. Smaller objects cooled off quicker than larger objects, so they underwent less differentiation. In the early solar system, collisions were much more common and some of the smaller differentiated large asteroids collided with one another, breaking them apart and exposing their metallic cores.

Interesting Facts and Features about asteroids

If all the asteroids in the belt were combined into one, it would form a body less than 1500 km in diameter. Noting the immense size of Ceres, it comprises over 1/3 the total suspected mass of the belt (2.3 x 1021 kg). 26 known asteroids are larger than 200 km (124 miles). We probably know 99% of the asteroids that are greater than 100 km (62 miles), and there are probably literally millions of asteroids that are greater than 1 km (0.62 miles) in diameter. Over 10,000 asteroids have been found.

Asteroids are too small to be spherical in shape. Instead, they are usually ellipsoids, but some are dumbbell-shaped, and others form even stranger ones. Asteroids bear the tale of the violence of the solar system; the larger ones have many sizeable craters pockmarking their surface.

One of the most surprising features of asteroids is that several have been observed to have moons of their own. The first asteroid to be observed with a moon was 243 Ida (58 x 23 km); it's moon is called Dactyl, and measures approximately 0.75 x 0.87 x 1.0 miles. It is now estimated that between 10-30% of asteroids have moons.

When asteroids break apart, the pieces don't always fly off in random directions. Sometimes, they will continue in the same orbit as the original asteroid. When several asteroids are seen in relatively the same place and traveling along similar orbits, they are called orbital families.