I'm trying to remember the number, but I think 3,800 asteroids are catalogued between the orbits of Mars and Jupiter. Some of them are one-third to one-half the size of our moon; others go all the way down to something the size of a football field in elliptical diameter. Every day they are finding more of these asteroids.
One of the reasons for going onto an asteroid--I'll get esoteric for just one second--is that sometimes they can put you into transfer orbit to somewhere else. There's an orbit taking place, so you match your orbit to when it's going to be in this section and land on the asteroid. Then you can head off to another location in space that allows you to observe a different section of space or look at things that are a bit different.
Why there are so many asteroids is something that is not well understood. Just as there are theories about the origins of the moon, there are theories about the origins of the asteroids, but they're not entirely well understood.
It is important from a scientific point of view to get there. We can do it robotically as well as by having humans go, and we already have gone. The Japanese have gone to an asteroid. The Americans have gone to a comet, which is an agglomeration of dirty rock and ice. It's not the same as an asteroid. An asteroid in theory was once a planetary body, just like the Earth or Mars.
The idea of going to an asteroid is to understand our world beyond low Earth orbit. We do not have a good understanding of this part of space. One of the reasons we fly at 300 or 400 kilometres, as Bob mentioned earlier, is that there is an atmosphere at that altitude. It's almost a vacuum, but there are single atoms and single electrons out there. There isn't an oxygen molecule, but there are oxygen atoms. Here in our space there are 10 to the power of 23. Out in space there are somewhere between 10 to the 10th or 10 to the sixth, depending on your altitude. That gives drag, as Bob said, but it also protects us from the ionizing radiation.
It's important for us to get outside the protective environment of the Earth and see if we can operate beyond that. We have that issue with our geosynchronous satellites that go up to the altitude of 36,000 kilometres. They need to have electronics that are hardened to radiation relative to the ones that are lower.
Your final point was on whether it's really exploitation. That's a tough question to answer because of the costs associated with returning that kind of thing.