In late 2004 a near-Earth asteroid was discovered that considerably reduced the giggle-factor usually associated with discussions of asteroid impacts. It was called 2004 MN4. In mid-2005 it received a permanent number, 99942, and a month later it got a name: Apophis, after the Greek name for the Egyptian god of darkness, destruction, and evil; it was an appropriate name.
As preliminary data came in and was analyzed, a chilling picture started to take shape. While this rock was not nearly as big as the Chicxulub asteroid, it would dwarf the Barringer and Tunguska impactors; Apophis is almost three hundred meters in diameter. The calculated yield for its impact was nearly a thousand megatons. By comparison, the Tunguska and Barringer impacts gave about one to three per cent of that yield, and the Krakatoa volcanic explosion of the late 1800s produced only (ha, only) about 200 megatons.
If Apophis hit a populated area the casualties could number in the tens of millions. An impact in the middle of the north Atlantic would produce cataclysmic tsumanis for the east coast of the United States and the west coast of Europe; the Caribbean islands could be wiped clean. The news got worse: the first calculations gave odds of an impact in 2029 at 1 in 233, and it received a 2 rating on the Torino scale for impact hazard. Not bad odds, but the potential consequences as summarized above were horrific. As more data became available over the next several days the odds got much worse, at one point the chances worsened to 1 in 37 for that 2029 meeting. Fortunately, given our pathetic state of readiness to take any remedial action, the observations and subsequent calculations over the following weeks and months revealed much better forecasts for us. Apophis will almost certainly miss the Earth in 2029. During that encounter, Apophis will pass within the orbit of our geosynchronous satellites; that encounter will be on Friday, April 13. In 2036 we will have another close approach with Apophis.
What happens during our 2036 encounter with Apophis will depend on the exact trajectory of Apophis in the 2029 encounter. The orbit of Apophis will change because of its 2029 encounter; the amount of change hinges on how closely it approaches Earth. If Apophis passes through a gravitational keyhole, a tiny region of space near Earth so that our gravity gives it precisely the right (or should that be the wrong?) nudge, Apophis will collide with Earth in 2036. The chances for hitting that keyhole stand at 1 in 45,000. That sounds like very long odds, but it is many times more likely than for any one individual dying in an airplane accident.
We have an identified threat, and we have some time to do something about it. We do not know if there are other threats of the same kind with a shorter time-frame. We should not wait until the last minute to develop the capability to mitigate this kind of threat. Likewise, we should not wait to develop the capability to detect other potential threats of the same nature.
All right, I am through beating you over the head about rocks from space. Next I want to explore some other ways we could be knocked back to the Stone Age.
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