The rogue asteroid
Stories from Physics for 11-14 14-16
An asteroid passing close to Earth may have its orbit deflected in such a way as to cause a collision in the future; regions which cause this kind of alteration to trajectories are known as gravitational keyholes. For example, in 2029, the asteroid Apophis will pass close to Earth and, should it pass through a 610 m wide keyhole, it will experience a gravitational force that will put it on a trajectory to collide with the Earth in 2036. Calculations suggest the probability of such a collision are vanishingly small.