Another near miss
Ken Tapping, May 23, 2018
By the time you read this, asteroid 2010 WC9 will have passed the Earth at around 12.8 kilometres a second (46,000 kilometres an hour), at a distance of roughly 200,000 kilometres, about half the distance between the Earth and Moon. The asteroid is roughly 70 metres in diameter, although it could be a bit larger. If it is made of basalt, the most common rock in the Solar System, it would weigh about half a million tonnes. Being hit by it would have been a disaster, although we as a species would probably survive. Since there are many such objects, some smaller, some larger, in orbits crossing the Earth's path around the Sun, and almost certainly many more we have not yet detected, there is a definite risk of being hit.
The Earth has been hit many times over its 4.5 billion year history. We see a record of this in the oldest rocks, such as those of the Canadian Shield. For example, the mineral riches of Sudbury, Ontario are due to the impact some 1.98 billion years ago of an object 10-15 kilometres in diameter. Another large object hit us some 65 million years ago. It was a major cause of the extinction of about 75% of animal species. If we were hit by a similar object today, it would be a disaster. With people inhabiting most of the Earth's surface, and our lives depending on a world-wide infrastructure of trade and communications, we are now highly vulnerable. What can we do to avoid being hit by an asteroid? Basically, at least for the immediately foreseeable future, not very much. Our problem is reliably predicting the collision in time to prevent it. An asteroid made of basalt, a dark coloured rock, almost certainly covered by a layer of basalt dust, would be extremely hard to see. Unless very lucky, we normally only see the small ones when they are very close, just in time to see them whizz past.
If we want to deflect an asteroid like 2010 WC9, coming towards us at 12.8 kilometres a second, and already quite close, we would have to attach a very large rocket motor to it. This would involve transporting it to the asteroid, matching speeds and then soft-landing it on its surface. To do a last minute change to half a million tonnes of rock will need an extremely powerful rocket. Even if we had such a rocket, it is unlikely we would be able to deliver it to the asteroid in time. In movies we have the heroes blowing up the threatening asteroid with a very large bomb. This would not be the greatest idea. Blasting one object, which would have impacted one part of the world, into many, impacting all over the world, would be far worse.
If we could detect threatening asteroids years before the predicted collision, we could soft-land a low-power rocket motor on them, possibly using the asteroid's own material as fuel. Given enough time, even a low thrust rocket motor could slowly change the asteroid's orbit into a less threatening one. However, it is very hard to predict impacts with any certainty years in advance. The problem is that the asteroids' orbits are being continually tweaked by the gravitational attractions of the other planets, mainly the giant planet Jupiter.
The result is that our predictions contain uncertainties. We can only guarantee that at the forecast encounter time, an asteroid will pass through a volume of space with the Earth lying in it. To avoid disaster, the uncertainties in our prediction have to be much smaller than any change our low-power rocket can make to the asteroid's orbit, so that we can be sure we are making the right correction. This requires better position measurements, over more of the asteroid's orbit. People are working hard on this.
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