Over untold eons our moon has been pelted by almost every kind of stellar debris. Its surface holds craters exposing a violent history. No erosion exists to smooth the scars, no noxious atmosphere boils to cloud our artificial sight. It's an omnipresent reminder that space is not friendly to warm wet life. That stark truth stands in contrast to the allure it represents as a potential training ground to a species gaining a set of "space legs."

During the exhaustive research for this column I tried to find an estimate of how many craters exist on the moon. There doesn't seem to be a simple answer to such a simple, universally human question. The ready answer is that "they can't be counted, as the number grows by four each day." Fine. (If you want anything done right, you have to do it yourself.)

I can name about two dozen on this side, so there should be at least that many on the far side as well. That would set a lower bound of four dozen. Going the other way, four per day after four and a half million years sets an upper bound at some six and a half billion craters. Assuming no erosion or cover-ups, with a lunar surface of 3.793e13 m2 that means an "average" crater would be around six thousand square meters, if circular about fifty meters across. So we'll call it between 60 and 6 billion. (Hmm, there's a crater for everyone on Earth.)

I checked some more. The internet says there are about half a million craters with diameters greater than 1 km on the moon. Besides upping our lower bound, just for a moment imagine the energy of an impact that leaves a hole a kilometer across. That kind of thinking occurs to me because this week the small (as in, thirty or forty meters across) asteroid 2009 DD45 came within 75 million meters from the Earth. They say it's a Tunguska-class asteroid, which of course do happen, lest we not be able to name them. The unnerving if not cheery part is that 2009 DD45 was found only days ago. (I don't know if the DD stands for "Dooms Day.")

It so happens that after a successful sixteen month mission the Chinese lunar explorer Chang'e lost orbit and impacted the surface of the moon (that means this week the crater count grew by five!). The East continues to prove their ability even in destruction. The West continues to coast, and watch. And wait.