The asteroid 243 Ida, which poses no danger to Earth at present
A critical emergency – an asteroid or comet is on a collision course with Earth. In principle, there are two ways to avert the impact or collision of an object that is near Earth: destroy the Near Earth Object (NEO) or move it into a safe orbit. But is this realistic?
Destruction is only a possibility for NEOs with a diameter of less than 100 metres, when most of the resulting debris would be small enough to burn up in Earth’s atmosphere. A rocket drive that changes the speed and/or direction of the NEO could be used to change its orbit. However, to do this many tons of fuel would have to be transported to the NEO, which could not be done with the technical capabilities available today.
Alternatively, a missile could slow the NEO. The first experience with this strategy was in 2005 with the US spacecraft 'Deep Impact', which flew directly into a (harmless) comet. The goal of the mission was not, however, disaster control but to research the structure of comets.
There is no lack of ideas – but no viable solution
It would also be an idea to detonate nuclear explosives near, on or underneath the surface of an NEO. Theoretically, a nuclear explosive could transfer a velocity impulse to the NEO that is over 100 000 times greater than would be possible with a rocket. Of course, an action of this sort would have to be carried out as far from the Earth as possible. A missile impact or nuclear explosion could however also cause the NEO to shatter or, if its stone structure is loose enough, it could absorb the impulse like a sandbag. In both cases, the collision course would remain unchanged.
Stationing a mirror system near the NEO, which concentrates the Sun's rays onto the surface of the NEO and thus vaporises the surface material has also been considered. This would generate a thrust in the same way as a rocket motor. The thrust would be small but, with an operating time of several months, would be sufficient to allow the object to drift past the Earth in a free-flight phase of several years.
There is yet another emergency solution for NEOs that are smaller than around one kilometre and which would not have global consequences: evacuating the impact area. This would have to take place quickly, as the exact point and time of impact can only be calculated with sufficient accuracy a few weeks before the impact event.