Why didn't anyone see the asteroid that came close to the Earth last week

asks Woolley:
A. Good question. After all, it's not every day one of the largest asteroids seen by astronomers comes within 450,000km of Earth - that's just one and a half times the distance to the moon. It whizzed by on March 8, but nobody spotted it until March 12.

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According to New Scientist, the asteroid, which has been named 2002 EM7, wasn't spotted because it was moving outwards from the innermost point of its orbit, 87 million km from the Sun. When it passed closest to the Earth it was too close to the Sun to be visible. It wasn't seen until it moved away from us and its bright side came into view. Too small to be seen by the naked eye, it was spotted by an asteroid-hunting telescope.

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Q. So nothing coming from an angle close to the sun can be spotted
A. Yes, it's a bit of a blind spot for astronomers. Any unknown object on a collision course with Earth which passed through this zone would not be spotted until it was too late to do anything about it.

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Q. How potentially dangerous was 2002 EM7
A. It's too small to be classed as a 'potentially hazardous asteroid'. However, it's estimated that it measures between 50 to 100 metres across - that's larger than the object that exploded in 1908 over the Tunguska region of Siberia, flattening trees over 2000 square kilometres.

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Q. Has anything come this close before that we know about
A. It's among the ten closest known approaches - and it's the second largest of them. The biggest was 1996 JA1, which passed only slightly closer to the Earth on 19 May 1996.

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Q. How can we make sure we don't get caught unawares again
A. Astronomers want more money to invest in equipment that would help to keep track of near-Earth objects and their orbits. This would give us early warnings of potential collisions in the future.

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By Sheena Miller