10 INCOMING ASTEROIDS

Discover the near-future encounters Earth will have with passing asteroids - some more hazardous than others

- ANDREW MAY

As scary as an incoming asteroid may sound, it doesn’t have to mean the end of the world. It’s true space rocks sometimes collide with our planet, as happened with devastating effect 66 million years ago when a massive impact wiped out the dinosaurs. But such events are the exception rather than the rule. By astronomical standards, Earth is a very small target. Asteroids move on precisely determined orbits, so the vast majority of them simply whiz past the planet at a safe distance. Such encounters are a good thing for astronomers because they give them a chance to study asteroids at close range, and these small, rocky objects can tell us a lot about the origin and evolution of the Solar System. When the first asteroids were discovered, they were all found to lie in an ‘asteroid belt’ between the orbits of Mars and Jupiter. It’s still true today that most known asteroids are located in this region.

Asteroids can be thought of as miniature planets, a few metres to several thousand metres in size, which move on a range of orbits around the Sun in much the same way planets do. All these orbits have a similar basic shape: a kind of distorted circle called an ellipse. The point on the ellipse that’s furthest from the Sun is called aphelion, while the closest is perihelion. In the case of a planet like Earth, the difference between perihelion and aphelion is very small, which means that the orbit is almost circular. Asteroid orbits, on the other hand, can be much more eccentric than this, and some of them can actually have a perihelion inside Earth’s orbit and an aphelion outside it. In other words, their orbits around the Sun overlap with our own planet’s.