It's only the really big asteroid impacts that are a threat to life on Earth

• 26 October 2002
• Jeff Hecht
• Magazine issue 2366

MOST huge asteroids would not wipe out swathes of life on Earth, if two large impacts are anything to go by.

While the dinosaurs were wiped out by a devastating asteroid impact 65 million years ago, two other massive asteroids which struck the Earth millions of years later had no discernible effect on the mammals living in North America at the time.

The two asteroids struck in the late Eocene, about 35.5 million years ago. While each delivered only about a quarter as much energy as the deadly Chicxulub asteroid that gouged a 180-kilometre crater out of the Yucatan coast off Mexico, they were still huge impacts. One left a 90-kilometre crater at Chesapeake Bay in Maryland and the other a 100-kilometre crater at Popigai in Russia.

That suggests asteroids do not gradually increase their killing power with size. Instead, only the largest asteroids, whose size exceeds a critical threshold, ...