What are NEO s

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Q.� What are the scientists looking for

A.� Large objects that swing out of their predictable and simple orbit, putting them on a collision course with Earth.

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Most asteroids follow simple circular orbits between the Mars and Jupiter, but they can throw each other out of alignment and, worringly, into an orbit that could collide with Earth.

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Q.� When is the next asteroid likely to hit Earth

A.� Asteroids bigger than 50 metres, that's big enough to cause localised flooding or a local disaster, only make it to Earth every hundred years or so.

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Asteroids big enough to cause disaster on a global scale, bigger than one kilometre, only make it to Earth every few hundred thousand years.

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At the moment there are no known asteroids heading our way.

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Q.� How often have asteroids collided with Earth in the past

A.� Asteroid movement hasn't always been so predictable. Up until around 4 million years ago it seems that our solar system was constantly bombarded by cosmic objects, leaving crater marks on the surface of the planets and their moons. Since then things have slowed down significantly and cratering appears to have continued at a much slower and fairly uniform rate.

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Q.� Why isn't the Earth covered in craters

A.� For two reasons. Our atmosphere protects the planet from most cosmic debris, so Earth doesn't suffer as many asteroid hits as the planets without an atmosphere, such as Mercury.

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Secondly because of geological processes like weathering and erosion destroys evidence of any collisions that have occurred.

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Q.� Why aren't asteroids smooth round balls like planets

A.� With much smaller bodies the gravitational pull is too weak to overcome the asteroid's mechanical strength. As a result, these bodies do not form spheres but keep their irregular, rocky shapes.

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Q.� What are asteroids made from

A.� From the debris left over when Mars, Mercury, Venus and Earth formed.

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Do you have a question about our solar system Click here to ask.

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by Lisa Cardy