After a million years or so the influence of these large planets, it was thought, caused the fragments to be ejected from the asteroid belt altogether potentially ending up in an Earth-crossing orbit.

ECONOMIST: Celestial mechanics: A rock in a hard place | The

But astronomers have been unable to figure out how they get from the main asteroid belt between Mars and Jupiter to the vicinity of the Earth.

ECONOMIST: Celestial mechanics

Scientists believe it would take an asteroid around 10 million years to travel from the main belt to Earth, so "this doesn't give a Baptistina fragment much time to get into a suitable position", said Dr Mainzer.

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