Earth occasinally gets in the way of big nasty rocky things travelling through space. It seems like that daunting reality hasn't been talked about in a while. The interesting question of how likely is it that we'll get hit seems to disappear from people's minds until another close encounter wakes them up. My fascination for the possibility of collision was ignited when 26 years ago I saw, through my backyard telescope, Comet Shoemaker-Levy crash to Jupiter, in July of 1994. What I really observed was the aftermath of the hit: black spots on the face of Jupiter's yellowish gassy striped atomsphere. The spots lasted and could be seen to shift with the planet's slow rotation. Given the incredible stretches of time involved with anything astronomical, it made me wonder: if I could see a comet smashing into the planet Jupiter with my own eyes during my own lifetime, solar system impacts, including collisions with earth, might be far more frequent than I ever thought. It was a sobering thought. I then recalled visiting Meteor Crater in Arizona as a young boy. My father drove me there from our home in southern California. Climbing up the stairs to the rim, the crater view opened up to my sight under a purple-blue sky: a fantastic bowl in the desert. Beautiful, round and still. If anyone needed evidence that things from space can actually hit earth, this was it! It was carved in an instant 50,000 years ago by a massive descending forty-meter rock. Its aftermath was this rimmed basin, twelve hundred meters across. The statistics show that as craters go, it's by no means the biggest traveller that earth has gotten in the way of. But as a small boy standing on its rim, looking into its canyon, I felt completely awed by the knowledge that the falling rock that made it came down from space! I recommend seeing this magnificent crater in person if you can. It may inspire you to visit other impact structures. Or from your desk, journey with Google Earth using the online Earth Impact Database as your travel guide.
The Barringer meteor crater in Arizona
On that point, I found some slides of a presentation I made for a class a few years ago. I've attached it. It shows some of the most dramatic old and gnarled impact structures still visible on the face of this battered planet. It includes statistical charts I made from the data provided by the Earth Impact Database. They show the liklihood of our getting hit. As you may know or have guessed, the likelihood of getting hit is related to the size of the impactor. Little guys hit us a lot; big ones not so much. (It's the middle ones that worry me . . . )
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