Avoiding an impact
Ken Tapping, May 7th, 2014
Scattered over the Earth are some 188 craters, formed by impacts of large objects from space. Throughout its history the Earth has been hit many more times than that. However most of the evidence has been removed by the continuous recycling of the Earth’s surface by subduction and emergence of new rock. We can get a better idea by looking at the Moon, where there has been little of this recycling. There we see a world smothered with craters. Those great plains we see on the Moon’s surface are lava flows released by particularly big impacts. The lava buried the existing craters, clearing the land for new ones.
Today, with almost no places on Earth uninhabited by people, and with us exploiting our planet’s resources increasingly intensively, we are becoming more vulnerable to ecological disasters, like being hit by large objects from space. Thanks to the discovery of more and more asteroids with orbits crossing the Earth’s path around the Sun, and seeing large objects hit Jupiter, we are now very aware of this threat. What can we do? There are three basic problems. Firstly we need to detect objects that are potential threats. Secondly we need to evaluate that threat. What is the chance of being hit? When? Where? Finally there is the need to do something to avoid the impact.
Most rocky objects out there are lumps of basalt, a dark, volcanic rock. Temperature changes and impacts by micrometeorites over billions of years have chipped away at the surface, producing a coating of fine, dark dust. The result is something that is extremely hard to see against a black sky background. We need telescopes with large mirrors to collect as much light as possible, and to search large areas of sky at one time in order to minimize the risk of missing anything important. However, a 100m diameter asteroid, which is tiny by astronomical standards, would very difficult to detect, but would be devastating if it hit us. Of course we would need to see the object and identify its threat potential with enough notice to have a chance to deal with it. At the moment we don’t have that capability at the level we need, although the instruments are improving rapidly.
In the movies, a bunch of superheroes jump into a spacecraft and plant a big bomb on the incoming asteroid, and the movie ends with a spectacular last-minute explosion that saves humanity. In reality doing this will replace one enormous threat with a huge number of large threats, which could cause an even bigger catastrophe. The best approach is to push the object onto a new path. We can do this by applying a massive shove shortly before the impact or a gentle shove earlier.
However, a lump of rock weighing millions or even billions of tonnes takes a lot of shoving, and a short-sharp shove shortly before impact might actually cause the asteroid to disintegrate, which could be worse. In any case, at the moment we don’t have the means to give suitably large shoves to these objects. The alternative is to give a gentle shove much earlier, and keep that shove going for a long time. For example we could put a small engine on the asteroid that evaporates the surface material in the form of a jet, which over a long time will change the orbit. For that approach we need to identify the threat around a decade before the predicted impact time, to give this method time to work. We are close to having the technological means to do this. However the problem here is to be able to predict the object’s orbit accurately that far ahead. To maximize the chance of saving ourselves, we need to change the orbit by much more than any possible errors in those predictions in order to eliminate the possibility we might have been wiser to leave things well alone.
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