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Where They Are

The vast majority of asteroids are grouped in the asteroid belt, which is more like a loose grouping than a belt, and lies between 1.8 and 4.5 A.U. (1 A.U. is the average distance between Earth and the sun) from the sun - between the orbits of Mars and Jupiter. The asteroids are so small and far away that they appear as faint stars, if they even appear at all; no asteroid is bright enough to be seen without some optical aid, except Ceres (see below) on its closest approach to Earth.

The asteroid belt is usually thought of as a defined region where asteroids abound (reference many science fiction movies, with spaceships flying in and out, dodging debris). This is actually very unlike the asteroid belt -- the region is so vast that asteroids are usually hundreds of thousands of kilometers from their closest neighbor. If the asteroid belt were taken to be the width of its densest region (3.2-1.8 A.U.), and it were considered flat, it would have an area of about 6·1017 km2. Even if there are 1000 times more asteroids than we know of today, then on average each asteroid would have over one million km2 to itself.

Asteroid - GaspraBut, there are asteroids all over the solar system -- they are not just confined to the belt. Their location only indicates what kind of orbit they have, as most asteroids look pretty much the same, such as the image of Gaspra on the right.

There are also many asteroids that gallivant around the solar system on highly elliptical orbits like comets. Of note, the asteroid Icarus, when closest to the sun, lies within Mercury's orbit - it comes close enough to the sun that Relativity must be taken into account in order to accurately predict its orbit. Another example is Hildago, whose closest approach is between the orbit of Mars and Earth, and its farthest from the sun between Saturn and Uranus.

It is estimated that the main belt alone contains well over 1 million asteroids. The total number in the solar system is estimated as much higher, especially once trans-Neptunian objects (asteroids beyond Neptune's orbit) are considered.

What They Are

The first asteroid was discovered in 1801 by Giuseppe Piazzi of Italy. He named it "Ceres" after the Roman goddess of grain. Ceres is the largest known asteroid at approximately 950 km (590 miles) in diameter, and it lies in the belt of asteroids between Mars and Jupiter (see the above section) at an average distance from the sun of 2.6 A.U. 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 many miles across. Most current theories hold that asteroids are bits and 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. Past theories have suggested that the asteroids are remnants of a planet that was destroyed early in the solar system's history. However, that theory is no longer held in much regard, for if all of the asteroids in the belt were combined, they would form a body less than 1500 km (932 miles) in diameter -- less than half the size of Earth's moon, and so there is not enough material to make a planet.

Asteroids are made of rock and metal. They are mainly 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, 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 and they weather much faster than the metallic ones.

Asteroids have a confusing system of nomenclature, especially when they are on Earth. While still in orbit, they are asteroids. Once they enter the atmosphere, they are called meteors, and once they land, they are termed meteorites.

Interesting Facts and Features

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 bare a tale of the violence of the solar system; the larger ones have many sizeable craters pockmarking their surface.

Asteroid - Ida and DactylOne 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.

As previously stated, 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 300,000 asteroids have been found.

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.

Some Famous Asteroids


Diameter (kg)
Mass (1015 kg)
Rotation Period (hours)
Distance from Sun (A.U.)
Orbital Period (years)
1 Ceres
960 x 932
2 Pallas
570 x 525 x 482
3 Juno
4 Vesta
45 Eugenia
140 Siwa
243 Ida
58 x 23
433 Eros
33 x 13 x 13
951 Gaspra
19 x 12 x 11
1862 Apollo
2060 Chiron

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