Minor Planets - Asteroids

by SpaceHike.com

Space rocks, or asteroids, orbit the Sun inside the solar system. They are referred to as minor planets since each one follows it own orbit around the Sun. They also spin as they travel. Over 90% of these asteroids lie within the Asteroid Belt, or Main Belt, inside the orbits of Jupiter and Mars. This region resembles the shape of a doughnut, and can take from three to six years to make a complete orbit around the Sun.

Asteroids vary in size, shape, and color, but only one is bright enough to be seen without the aid of a telescope, Vesta. The probes that have flown past asteroids revealed only four with an up close view. Asteroid make-up made be of rock, or metal, or a combination of rock and metal. The first rock asteroid to be seen up close was Gaspra, which is around 19 kilometers long, and orbits the Sun every 3.3 years. The Galileo probe made photographs of Gaspra in October 1991.

Ceres was the first asteroid to be discovered, in 1801, and it is also the biggest with a 932 kilometer diameter. An asteroid needs to measure more than 300 kilometers across to have a spherical shape. The majority of asteroids have a smaller diameter than this causing irregular shapes. Only a few measure larger than 250 kilometers in diameter.

The Main Belt region measures from 254 million kilometers to around 598 million kilometers from the Sun. The Main Belt has billions of asteroid, but each moves independently around the same, but spin when traveling in the same direction of the planets. Close to one billion asteroids measure over one kilometer across, but are thousands of kilometers apart from one another.

There are asteroids with orbits that bring them very close to Earth's orbit: Apollo, Amor, and the Aten groups. The group of asteroids received their name from an individual asteroid within the region. The group members follow a particular orbit: Atens stays mainly inside Earth's orbit, Apollo crosses Earth's orbit, and the Amors have orbits going between the orbits of Mars and Earth.

Theory suggests that the Asteroid Belt was the leftovers of an unborn planet, which failed to form when the other planets were beginning around 4.6 billion years ago. There was the formation of over 600 huge protoplanets, but failed in creating one large body. It is thought that the gravity of Jupiter aroused the protoplanets, causing a collision and breaking up to create today's Asteroid Belt.

The Near Earth Asteroid Rendezvous (NEAR) probe in 1997, photographed the 66 kilometer long asteroid called Mathilde. It revealed a surface with giant craters and the interior having holes. NEAR went past Eros in 1998 and showed the surface to be a 33 kilometer long body with pock marks.

Sources:

1. Couper, Heather and Nigel Henbest. Space Encyclopedia DK Publishing, Inc.: NY 1999

2. Editors. Secrets of the Universe. International Master Publishing: US. 1999