Between Mars and Jupiter is a region of space we call the Asteroid Belt. It consists of millions of rocks of all sizes, from the microscopic to the very large. And like the planets, they orbit the Sun. But what is an asteroid?
Quite simply, it is a lump of rock floating in space. The theory on why the asteroid belt was formed has changed over the years. Originally it was believed to have been a planet which was destroyed, perhaps by colliding with another body. But the modern thinking takes the opposite point of view. We now believe that they were the building blocks of a planet which never formed.
When the solar system was very young, such bands of rock debris slowly came together to form the planets. So why didn’t this particular debris field form a planet? We think that it is all down to Jupiter. Its huge mass and gravitational field was so strong that it stopped any planet from forming.
Within the belt, we can find some very large asteroids there. The largest of all is Ceres which is so big at nearly 1,000 kilometres across, that in 2006, it was re-designated as a dwarf planet. Within the belt it is estimated that there are over 100 asteroids that are bigger than 125 miles across and over 1 billion that are over 1.25 miles across.
Another false belief held by many people is that any space-craft flying through the asteroid belt will have to continually dodge the rocks floating around. In practice, this is not the case. The science fiction movies that depict craft trying to weave their way through millions of densely packed asteroids is not correct. Although there are many millions of asteroids, they occupy a region of space which is huge. It is so big that most are separated by very large distances from each other. The final proof we have of this is that to date, a number of space probes have made the journey through the asteroid belt and all have survived without coming to any harm.
The asteroid belt is a long way out, but the constituents can still pose a danger to us on Earth. Many asteroids of differing sizes have broken away from the belt and begun their own individual journey through the solar system. This may be due to collisions with other objects.
If any smaller ones should find their way to the Earth and impact into our atmosphere, we may see them as shooting stars. However, the vast majority are meteors and are just particles which burn up high above the ground. Larger rocks may penetrate further because of their size. But most also burn up before they can reach the ground. Then there are the rare ones that get through and impact our surface. These are known as meteorites once they have reached the surface.
There is also the very small chance that one day a very large asteroid may hit us. It has happened in the past. The most famous example occurred about 66 million years ago and it is believed that it was instrumental in killing off the dinosaurs although this theory is now partially disputed by some. Another such strike could be catastrophic for all of us. However, the chances of it happening are tiny and there are groups of astronomers whose job it is to discover any large wandering asteroids and then log and track them. These asteroids are called Near Earth Objects. Early detection of a possible threat may give us time to put plans into action which could lead to us being able to divert such a danger.
At present, we are aware of a number of such objects but none of them pose a threat to us – yet. But don’t lose any sleep over it; the vast majority will stay within the belt.
John Pullen 