Rock Farm: Most asteroids lie in the main belt [white] between Mars and Jupiter and pose no threat.

太空岩石场：大部分小行星位于火星和木星之间的小行星带（图中白色部分），对地球没有威胁。

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At first, astronomers thought it might be a so-called "main belt comet" — a rare case of a comet orbiting in the asteroid belt.

起先，天文学家们认为这种景象不过是所谓的“主带彗星”——即较为少见的轨道在小行星主带内的彗星。

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It now appears when the asteroids and planets were first forming in the very early Solar System ice extended far into the Main Belt region.

现在看来，在太阳系初期行星和小行星首次形成时，冰深深地延伸进入主带区域。

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Critically, the Baptistina family of asteroids - those that were formed when the larger asteroid destructively entered the main belt - were only formed around 80 million years ago.

BBC: Asteroid thought to kill dinosaurs may not be guilty

It was once thought that asteroids in the main belt occasionally collided with one another, sending fragments into orbital resonance bands where they could be affected by the gravity of Jupiter or Saturn.

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

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.

BBC: Asteroid thought to kill dinosaurs may not be guilty