The Asteroid Belt

Looking at some maps of the solar system that include the asteroid belt, one might think there is a ring of armour protecting the inner terrestrial planets composed of hundreds of thousands of deadly debris hurtling through space. The truth of the asteroid belt is actually more fascinating.

A Look at The Asteroid Belt

The asteroid belt is a ring of objects composed mostly of ice, rock and metal that fill the space in between the orbits of Mars and Jupiter. Around the time that Sir William Herschel discovered Uranus, Johan Bode suggested there might be a fifth terrestrial planet between Mars and Jupiter. Shortly there after Bode discovered Ceres, now considered a dwarf planet, and the first known object in the asteroid belt.

Ceres is the largest asteroid in the belt at about 950 kilometres in diameter, and indeed is responsible for a third of the belt’s total mass. Three other objects (4 Vesta, 2 Pallas and 10 Hygia) are about 400 km in diameter and, along with Ceres, make up about half the total mass of the belt.

The term asteroid was coined by Herschel, borrowing from the Greek word for star, aster, due to the star like appearance of the objects. They are also called minor planets, and are defined as objects too large to be meteors (which are 10 km in diameter or less) and that are not comets.

Formation of the Asteroid Belt

There are many theories about how the asteroid belt formed, as there are about most things in astronomy. Early theories revolved around a destroyed fifth terrestrial planet, but the truth is more likely to be found at the other end of the spectrum. Rather than the hundreds of thousands of objects coming from a single planet’s destruction (unlikely due to the differences in composition of the various asteroids) it is more likely that they are the results of a planet NOT forming.

Planet formation comes when a plantismal collides with other objects that stick together in a process called accretion, eventually growing to a protoplanet and then a planet. The theory is that the influence of nearby Jupiter was so strong that it kept any single plantismal from growing enough to form a protoplanet despite the many collisions.

Exploring the Asteroid Belt

Despite popular fiction, there is no real danger in traversing the asteroid belt when it comes to collisions. This was not known when the first spacecraft entered the belt on July 16th, 1972, when scientists held their collective breath and waited for impact. Since then it is understood that, while there are hundreds of thousands of known asteroids, they are so thinly spread out in such a large region that the chance of impact is less than one in a billion. Since then the Pioneer 11, Voyager 1, Voyager 2, Galileo, NEAR, Ulysses and New Horizons spacecraft have all managed through with no problems. The Dawn Mission will explore Vesta and Ceres and possibly more in the near future.

Final Thoughts

From the time of the first Asteroid Belt object discovered (Ceres), the discovery of additional asteroids, or minor planets, has grown exponentially. With future plans to spend some appreciable time in the asteroid belt in future space exploration it is exciting to see what else can be found.