As part of my ongoing inquiry into Reasons To Spend Your Life Quivering In Mortal Fear, today's topic is: Death Meteorites.
Astronomers have recently generated some serious buzz on this topic by reporting that the euphoniously-named asteroid 2005YU55 was going to be making a near pass, which it did without incident last night. At its closest approach, the 400-meter-wide block of rock was 324,600 km from Earth, and traveling at 29,000 mph -- which is pretty impressive. And even if 324,600 km seems like a long way away, in astronomical terms it's a close enough shave that there have been several fairly hysterical articles recently describing the havoc that could ensue if one of these things hit the earth.
To be sure, the Earth does get hit regularly. On June 30, 1908, a stony meteorite estimated to be only 50 meters in diameter hit the earth in the Tunguska region of Siberia, creating a tremendous fireball and radially flattening trees for miles around the impact site; it registered on seismographs in London. Artist's renditions of the event, reconstructed from eyewitness accounts, show a brilliant streak across the sky ending in an enormous explosion that for a moment outshone the sun. It was fortunate that it landed in a fairly unpopulated area, and not in a city or even in the ocean, where it would have raised a tsunami that would have dwarfed the December 2007 Indonesian catastrophe.
A 1.2 kilometer wide rock slammed into the earth around 15,000 years ago, leaving a large pockmark in the Arizona desert aptly named Meteor Crater. Given that this is already high desert, it's a little hard to imagine how the area could be any more desolate than it already is, but a collision of this scale must have devastated thousands of square miles.
This, of course, is nowhere near the 10 to 20 kilometer wide meteor that left Chicxulub Crater north of the Yucatan, ending the Cretaceous Era with a (literal) bang and leaving a layer of dust to mark the event in sedimentary rocks worldwide. The devastation that caused is of an unimaginable scale, to me at least, but once again artists have attempted to paint the event as it might have appeared (from a safe distance). This seems to have been the final death knell of the majority of dinosaur clades, with the exception of the one that includes birds. (Yes, birds are dinosaurs. That point is literally beyond question now, since proteins from Tyrannosaurus rex fossils have been successfully sequenced and shown to be unequivocally related to bird proteins. Whether they tasted like chicken remains to be seen, but evidence from bone homology has pointed toward a relationship between birds and deinonychid dinosaurs for years; this is just the final nail. Give that some thought next time you're feeding the chickadees.)
Anyhow, the open question is how soon will another collision will occur, and how big the collision will be. One the size of Tunguska apparently strikes once every century or so. Meteor Crater sized rocks are less frequent, on the order of one every 10,000 years (meaning that we're overdue, not that these events work on any sort of predictable timetable). Era-ending rocks the size of the one that created Chicxulub strike only once every 100 million years.
All of this, however, is only talking about average strike intervals, and you know the problem with averages; if you have one foot in a pot of boiling water and the other encased in ice, on the average you're comfortable. Averages really tell you nothing about actualities, and the reality is that a meteor could strike downtown Detroit tomorrow (undoubtedly doing millions of dollars' worth of improvements), or we might not have one strike for another million years. No way to tell. How's that for a cheery thought?
And to make your day even happier, two questions remain: (1) Will we see a potentially devastating meteor coming? and (2) if we do see it, will we be able to do anything to deflect or destroy it? The answer seems to be no to both. Given that the asteroid that played chicken with the Earth last night is 400 meters wide, and the one that struck Tunguska was only 50 meters wide, you can see that it doesn't take a particularly huge piece of rock to wreak havoc. It's entirely possible that a Tunguska-sized meteor would be missed until it was only days away from striking, and maybe not even then. Given that kind of lead time, there's no way we could send any kind of rocket up to meet it, deflect it, blow it up, whatever. With a bigger rock, we'd see it sooner, and might have more time to react, but the problem is that in that case it's... a bigger rock. Even if we successfully shattered it, the fragments would still pose a hazard, and they would continue on largely the same course as the original rock had (Newton's First Law being strictly enforced in most jurisdictions). Deflecting it using a retrorocket-like device is at least a possibility, but I wonder if we're technologically capable of doing such a thing.
In any case, it's not likely, certainly not soon. Astronomers have most of the near-earth asteroids of any size catalogued, their trajectories predicted for several centuries hence, and they have assured us that no collisions are imminent. There's really no reason to lose any sleep over the fact that there might well be a Cosmic Death Asteroid Hurtling Toward Your Village, and There's Nothing You Or Anyone Else Can Do To Stop It.
Have a nice day.