Asked by Kyle McDonald
You have probably read about NASA’s plans to capture an asteroid with the intention of dragging it into orbit around our Moon. It is intended that the near-Earth asteroid, which would be towed by a robotic spacecraft, would be around 500 to 1,000 tons in mass. The idea, which is hoped to be carried out by 2025, will allow us to study asteroids in detail and mine material from them.
The main thing to remember is that the mass of the Moon is much greater than that of any asteroid NASA can pull into its orbit. More mass means more of a gravitational attraction and so, in this respect, the force between our Moon and the Earth is much greater than the gravitational attraction between our planet and an asteroid. There is such a relatively smaller Earth-asteroid force that the Earth-Moon relationship overcomes it and remains unhindered.
In order to ensure that the asteroid does not smash into the Earth, yet maintains a position that allows astronauts to visit, it is intended that the asteroid is placed into a high lunar orbit, preferably at Lagrange points 1 or 2. These points are where the gravitational forces of the Earth and Moon cancel out, so anything set there will simply maintain its position, at least for 10 to 50 years before drifting and falling towards the Moon. NASA have decided on small carbonaceous asteroids meaning that if it falls towards the Earth, then it is small enough to burn up in our atmosphere.
Image courtesy of NASA