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If you're looking for pragmatic reasons to have a space program, look no further: Preventing the destruction of civilization, due to an asteroid impact. Scientists now seem to agree that, about 65 million years ago, an object about ten kilometers (roughly six miles) in diameter crashed into the earth, and caused the extinction of the dinosaurs. Such an object would have an impact energy of about a hundred million megatons (not a hundred megatons – a hundred million megatons – the explosive equivalent of about two million of the largest nuclear devices ever detonated).
It's possible to use basic physics to calculate how much impact energy you can expect from an asteroid of a given size. An object that's about one kilometer in diameter, or one-tenth the diameter of the dinosaur-killer, would have a thousandth of the volume and a thousandth of the mass. Impact energy is proportional to mass, so its impact energy would be a thousandth of the dinosaur-killer's, or about 100,000 megatons. Similarly, a 100-meter diameter object would have an impact energy a thousandth smaller still, or 100 megatons. Fortunately, objects a lot smaller than 100 meters, unless they're metal, tend to burn up in the atmosphere. |
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Now dinosaur-killer impacts are rare – about once every hundred million years on average. But such an event could occur at any time. And objects much smaller than the dinosaur-killer – one to two kilometers in diameter – could destroy civilization. With an impact energy of one hundred thousand to one million megatons, they could kill a quarter of the world's people, and destroy world agriculture for a year.
Civilization-threatening impacts will occur about once every million years. While this is low-risk, the consequences are so severe they should be considered. An insurance analogy is appropriate. The risk of a house fire or the sudden death of a life insurance policyholder is quite low – which makes insurance against such risks affordable – but the result is so devastating that most people take the insurance. Much smaller impacts, while not enough to destroy civilization, can still cause devastation. In 1908, in Tunguska, Siberia, a meteor exploded over an unpopulated area, with a force of about 10 megatons. (From the calculation above it would have had a diameter of about fifty meters.) Had this occurred over a large city, many thousands would have died. Had it occurred over the ocean, it probably would have generated a destructive tsunami. Tunguska-like impacts probably happen once every century or two, so the risk is quite high that, in a human lifetime, such an impact will occur. Even smaller impacts can have serious effects. On Feb 13, 2013 an asteroid exploded over the Russian city of Chelyabinsk. The asteroid, which my have briefly outshone the sun, detonated with the force of (only) about half a megaton, blew out windows 200 miles away, and caused over a thousand people to seek medical attention. And NASA satellites were able to track the asteroid's plume. Fortunately, the U.S. and Russia have agreements in place to share information about such impacts. So it is unlikely such an event would cause a war, despite the heated rhetoric of some politicians. Now it's possible to track large objects without a space program. In fact, the Spaceguard Survey is using telescopes to do just that. And the probability is good that, should a potentially devastating meteor or asteroid, on a path to strike the earth, be found, there would be a long warning time – probably decades. So there is no reason to start a crash program to develop technologies to divert, or to destroy, or to mine such an impactor. It is, however, prudent to search for them, via telescope, quickly, and determine what the risks are. But when such an object is found (I didn't say if, but when – it's certain one will eventually be found, even if it's thousands of years from now), then what? There are three (actually four) possibilities: |
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| (1) | Diversion (moving the object so it won't hit the Earth); | ||||||||||||||||||
| (2) | Destruction (blowing the object to bits - bits small enough that they'll burn up in the atmosphere); or, if you blow it up far enough in advance, the pieces could miss the Earth altogether; | ||||||||||||||||||
| (3) | Mining (landing on its surface and drilling or digging it up, extracting metals and other valuable materials, until it's all gone); | ||||||||||||||||||
| (4) | Some combination of the first three. For example, one might mine the asteroid unti most of it is gone, then divert what's left. | ||||||||||||||||||