Comet That Killed Mammoths Could Strike Again

We tend to think about cosmic impacts as single events, acts of astronomical providence that can wipe out life on Earth in an instant. Then it's over, and Earth is left to heal.

But what if some asteroids and comets only deposit a portion of their deadly load with each swing by our planet? What if even now, they are circling back around our sun for another pass at us?

That's the implication of a new paper due out in Monthly Notices of the Royal Astronomical Society. Written by Bill Napier of Cardiff University, the study presents a mathematical model which suggests that the impact that killed the mammoths and plunged the planet into a thousand-year ice age isn't done yet.

Caveat time: whether or not the mammoths were killed off by early humans or climate change -- and whether that climate change was caused by an impact 13,000 years ago -- are matters of intense debate. There's interesting evidence arguing against an impact hypothesis, and for it.

But let's say the impact idea is right. Some folks have argued the event was actually a salvo of impacts in quick succession, perhaps thousands of smaller collisions and airbursts like the Tunguska Event, raining down on Earth from a shattered comet.

Comets often break up into swarms of not-so-tiny fragments, so Napier's argument makes sense in that respect. But if he's onto something, where are the other pieces, and what sort of threat do they represent to modern society?

Napier points to a flurry of near-Earth objects (NEOs) called the Taurid Complex, which he suggests came from a comet 50-100 kilometers (31-62 miles) in diameter -- a truly giant hunk of rock and ice -- that entered the inner solar system between 20,000 and 30,000 years ago. The Taurid Complex contains some of the largest NEOs we know about.

If Napier's suppositions are correct -- and he admits that's a big "if" -- we have to rethink how frequently Earth is pelted with enough cometary material to burn down a continent. Our sky surveys suggest an impact of that magnitude would occur only once in several million years. But if objects in the Taurid Complex have our number, they could happen a hundred times more often -- on the scale of tens of thousands of years.