SPECIAL

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

WORDS ANDREW MAY

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DID YOU KNOW? Asteroid Day marks the anniversary of a major impact event that occurred in Siberia on 30 June 1908

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.

ASTEROID TYPES

Asteroids vary in their chemical composition, but these are the three most common types

C-TYPE

C stands for carbonaceous or chondrite, and these asteroids are relatively dark in appearance. Although they’re the most common type of asteroid, they’re more typically found in the outer parts of the asteroid belt rather than close to Earth.

S-TYPE

S stands for stony or silicate, and these asteroids are brighter in appearance than C-types. They become more common in the inner parts of the asteroid belt and account for the majority of near-Earth asteroids.

M-TYPE

M stands for metallic, and these asteroids are mainly composed of a nickeliron mixture. They’re mostly found in the middle of the asteroid belt, and despite their comparative rarity would be a particularly attractive target for asteroid mining.

These three compositions account for most of the known asteroid types in the Solar System