INTRODUCTION TO ASTEROIDS

Asteroid 253 Mathilde, a Near-Earth Asteroid photographed by NASA's NEAR (Near Earth Asteroid Rendezvous) mission in June 1997. Mathilde is about 60 km in diameter and orbits in the asteroid belt between Mars and Jupiter.

Asteroids are rocky or metallic objects, most of which orbit the Sun in the asteroid belt between Mars and Jupiter. A few asteroids approach the Sun more closely. None of the asteroids have atmospheres.

Asteroids are also known as planetoids or minor planets.

THE ASTEROID BELT

The asteroid belt is a doughnut-shaped concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter, closer to the orbit of Mars. Most asteroids orbit from between 186 million to 370 million miles (300 million to 600 million km or 2 to 4 AU) from the Sun. The asteroids have a slightly elliptical orbit. The time for one revolution around the Sun varies from about three to six Earth years.

The strong gravitational forces of the planet Jupiter shepherds the asteroid belt, pulling the asteroids away from the Sun, keeping them from careening into the inner planets.

THE KIRKWOOD GAPS

The asteroid belt is not smooth; there are concentric gaps in it (known as Kirkwood gaps). These gaps are orbital radii where the gravitational forces from Jupiter do not let asteroids orbit (they would be pulled into Jupiter). For example, an orbit in which an asteroid orbited the Sun exactly three times for each Jovian orbit would experience great gravitational forces each orbit, and would soon be pulled out of that orbit. There is a gaps at 3.28 AU (which corresponds to 1/2 of Jupiter's period), another at 2.50 AU (which corresponds to 1/3 of Jupiter's period), etc. The Kirkwood gaps are named for Daniel Kirkwood who discovered them in 1866.

HOW MANY ASTEROIDS ARE THERE?

There are perhaps about 40,000 asteroids that are over 0.5 miles (2.4 km) in diameter in the asteroid belt. About 3,000 asteroids have been cataloged. There are perhaps a million asteroids with diameters about 0.6 mile (1 km). The first one discovered (and the biggest) is named Ceres; it was discovered in 1801.

THE SIZES OF ASTEROIDS

Asteroids range in size from tiny pebbles to about 578 miles (930 kilometers) in diameter (Ceres). 16 of the 3,000-known asteroids are over 150 miles (240 km) in diameter. Some asteroids even have orbiting moons.

CERES: THE LARGEST ASTEROID

Ceres is the largest of the asteroids. It was the first asteroid ever discovered (by the Italian astronomer Giuseppe Piazzi on January 1, 1801). Ceres is the size of the state of Texas! It is so huge in comparison with the other asteroids, that its mass is equal to over one-third of the 2.3 x 10²¹ kg estimated total mass of all the 3,000 cataloged asteroids. Ceres is about 578 miles (930 kilometers) in diameter.

ASTEROIDS BECOMING MOONS

Asteroids can be pulled out of their solar orbit by the gravitational pull of a planet. They would then orbit that planet instead of orbiting the Sun.

Astronomers theorize that the two moons of Mars, Phobos and Deimos, are captured asteroids.

ORIGIN OF THE ASTEROID BELT

The asteroid belt may be material that never coalesced into a planet, perhaps because its mass was too small; the total mass of all the asteroids is only a small fraction of that of our Moon. The total mass of all the asteroids is about 2.3 x 10²¹ kg ); our moon's mass is 7.35 x 10²² kg; the asteroids' mass is about 1/30 of the mass of the Moon. A less satisfactory explanation of the origin of the asteroid belt is that it may have once been a planet that was fragmented by a collision with a huge comet.

Comparison of 253 Mathilde, 951Gaspra, 433 Eros and 243 Ida

• Potentially Hazardous Asteroids (PHA's)

• Near Earth Objects (NEO's) and Orbits

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