Astronomers deal with vast distances and spans of time, so on that scale, it was only moments ago the real danger of asteroids and comets came into sharp focus.
In 1994 the Shoemaker-Levy 9 comet collided with Jupiter – an event predicted a year in advance and watched in real-time via the Hubble telescope. Humankind had already seen the craters on the Moon, legacy of numerous asteroid impacts, and photos of the asteroid-blasted Tunguska region of Siberia in 1908. When we watched a comet strike the solar system’s largest planet, though, we saw with absolute clarity the threat asteroids and comets pose to Earth. A potential collision is inevitable – the only unknown is how soon it will happen. If we have enough warning, our planetary defence is most likely to be kinetic impact: sending a spacecraft to collide with the threatening object, slightly changing its momentum and eventually deflecting it from its original path.
We will need years to spare between the spacecraft’s impact with the hazardous object and the object’s close approach to the Earth. The mass of the spacecraft might be 10,000 to 1 million times less than the mass of the object, so even if the spacecraft could travel 10 to 15 kilometres per second, the impact might change the velocity of the object by only 1 centimetre per second – or less.
