There’s a real possibility Earth could be hit by an asteroid, but it’s an avoidable disaster

WORDS ANDREW MAY

I t’s one of Hollywood’s favourite disaster scenarios: a chunk of rock hurtling through space on a collision course with Earth. But this scenario isn’t just science fiction – there really are rocks in space, in the form of asteroids, and some of them do have orbits that cross Earth’s. It’s also no longer science fiction to suggest that a future space mission might be able to avert such a disaster. NASA’s Double Asteroid Redirection Test, or DART, is a mission currently flying through space aiming to test one possible way of doing this. But before we examine it, it’s worth recalling just what asteroids are and why some of them are potentially hazardous to life here on Earth.

Asteroids are essentially just chunks of rock that orbit the Sun like tiny planets. They range in size from a few metres to hundreds of miles across, and there are millions of them in the main asteroid belt between Mars and Jupiter. In smaller numbers, however, asteroids can be found throughout the Solar System. Those that have orbits that come close to Earth’s own orbit around the Sun are called near-Earth asteroids, although that’s a somewhat misleading term as they’re not actually near us all the time. As they travel around their orbits they’ll spend a lot of time at great distances – on the far side of the Sun, for example – but at other times they can pass very close to Earth.

If the closest approach of an asteroid to Earth’s orbit is less than 4.6 million miles and it’s more than 140 metres in size, it’s classified as a Potentially Hazardous Asteroid, or PHA. This doesn’t necessarily mean the asteroid is going to collide with Earth, because even if their orbits intersect they would both have to arrive at the intersection at exactly the same time. But it’s a sensible precaution to keep track of as many PHAs as possible, so we know exactly where they are and where they’re going.