…from asteroids:
To properly explain the crater distribution, Ito and Malhotra say some other factor must have been involved. One possibility is that we simply haven’t seen all the craters yet: the ongoing lunar mapping missions may help on that score.
Another idea is that the Earth’s tidal forces tear Earth-crossing asteroids apart, creating a higher number of impacts than might otherwise be expected.
But the most exciting and potentially worrying possibility is that there exists a previously unseen population of near Earth asteroids that orbit the Sun at approximately the same distance as the Earth. These have gone unnoticed because they are smaller or darker than other asteroids, say Ito and Malhotra.
“More complete observational surveys of the near-Earth asteroids can test our prediction,” they say.
And let’s not waste too much time about it. By some reckonings, asteroid impacts represent the greatest threat to humankind that we are able to calculate.
Even more to the point, it’s not only the greatest one we can calculate, it’s probably the greatest one that we can actually mitigate (short of colonizing the galaxy).