In Tuesday night, a $330 million spacecraft will crash into the asteroid Dimorphos. It is no accident that their nickname “Darts” is reminiscent of the small darts that are thrown at targets. The crash is fully intentional. NASA launched the probe, which has the mass of a small car, into space specifically to crash it into Dimorphos at 23,800 kilometers per hour. A previously deployed miniprobe from Italy will film everything.
What shoud that? Well, one would like to see if such a collision is enough to change Dimorphos’ trajectory. Because the Brocken is 163 meters long. Asteroids of this caliber are the most dangerous in terms of a possible collision with Earth: they are large enough to cause a national disaster at the point of impact. At the same time, they are so small that we know far too few of them.
Almost 30,000 celestial bodies – mostly asteroids – are currently known to come close to Earth’s orbit or even cross it as they orbit the Sun. Two thirds of these are smaller than 140 meters and are therefore comparatively uncritical. When they fall to earth, they burn up in the atmosphere. Only the largest among them reach deeper layers of air. However, they are not always completely harmless. A twenty-meter chunk that fell in Russia in 2013 only disintegrated at such a low level that it triggered a blast wave. It shattered windows across an entire region, injuring more than a thousand people.
We don’t have to be afraid of the really big cars at the moment either. An asteroid with a caliber of ten kilometers, like the one that wiped out the dinosaurs and would at least mean the end of human civilization today, only hits the earth every hundred million years. And such a tumble on earth course is too big to escape the researchers. A catastrophe of this kind is therefore impossible within the next hundred years – however, it is not possible to predict asteroid orbits further into the future with certainty. In the case of boulders with a diameter of one kilometer – such a one would still devastate an entire continent – we are 98 percent sure that none will hit us in the next hundred years.
The problem, however, are asteroids between 140 and 1,000 meters – exactly those like Dimorphos. There are probably many more on a safe collision course to Earth without us knowing about it. Their impact would still have the effect of a very large atomic bomb. So if the worst comes to the worst, he could wipe out a big city.
What to do if we suddenly discover one? If we are not very unlucky, it will still be several years to decades before the impact and one could try to prevent the catastrophe. Because a small push is enough to change its orbit so that it misses the earth. To do this, one could shoot at the asteroid with a laser, for example, or with bundled sunlight from a concave mirror placed in space. Or you detonate an atomic bomb near its surface. However, the simplest thing would be a targeted crash, like the one being tried out on Dimorphos now.
Incidentally, he is only a test object and is not a danger himself. It is also impossible that “Dart” accidentally kicks him towards the ground on Tuesday. Dimorphos is a small moon. Every twelve hours it orbits a slightly larger asteroid, and the crash will shorten this orbit by just 4.2 minutes – if the experiment works and “Dart” can transfer its kinetic energy to the chunk to a sufficient extent.
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