June 11, 2012
In 2004, the Earth crossing asteroid Apophis generated much interest when astronomers announced that there was a 2.7 per cent chance that it would hit the Earth in 2029.
The excitement died down when more detailed observations showed that Apophis would actually miss Earth in 2029. And yet, Apophis might still hit in 2036 or 2037--we simply cannot know until nearer the time.
An important question, then, is what to do if astronomers spot Apophis coming our way--what can we do to push this 46 million tonne object away?
Last year, we looked at a Chinese plan to deflect the asteroid by smashing a spacecraft into it using a solar sail.
Today, Massimiliano Vasile and Christie Maddock at the University of Strathclyde in Scotland reveal a plan to blast the asteroid with solar powered lasers, ablating its surface and steering it away from us. "[Our] paper demonstrates how signiﬁcant deﬂections can be obtained with relatively small sized, easy-to-control spacecraft," they say.
Laser ablation is not a new idea. The basic idea is that the material vapourised from the asteroid's surface, pushes it like rocket exhaust, generating thrust. Until now, space scientists had always thought that a job this size required a megawatt class laser, which would need to be powered by a nuclear reactor.
That introduces a host of challenges, not least of which is launching such a device safely and then dealing with the huge amount of heat it produces.
But Vasile and Maddock say that instead of a single large laser, a better option is to use lots of small ones--kilowatt-class lasers, which could each be powered by the Sun.
The advantages are many, they say. First, the problem of dissipating heat in space is a serious one and does not scale linearly with mass. Small spacecraft are easier and cheaper to cool because a smaller percentage of their mass needs to be devoted to radiators and related equipment.
Next, solar powered lasers have the obvious advantage of requiring no fuel and being far simpler and safer to launch than nuclear devices.
And finally, having many small spacecraft ablating the asteroid gives greater scope for redundancy. If one goes wrong, there are several others to plug the gap.
That's not to say that such a mission would be easy to mount. A significant problem for all ablation schemes is that the vaporised rock from the asteroid can end up coating the spacecraft optics and ruining their efficiency.
That's particularly acute for spacecraft that must orbit close the asteroid, such as those that might use mirrors to focus the Sun's rays onto the surface, as some astronomers have suggested.
But laser beams can be collimated and so aimed from much further away. That vastly reduces the risk from ablated material.
Then there is the problem of asteroids with highly eccentric orbits, which are too far from the Sun for much of their orbit for solar power to be much use. In this case, Vasile and Maddock say that solar power spacecraft could still deliver a large enough kick to steer such an asteroid away from us, given enough lead time.
Vasile and Maddock make no attempt to calculate the cost of such a mission or compare it to the cost of other plans. However, there's no question that the price of preventing an Apophis-sized asteroid hitting Earth pales into insignificance compared to the cost of dealing with the consequences of the impact itself.
These ideas might sound like science fiction today but the only question is not whether we will ever need to put such a plan into action but when. If Apophis turns out to be heading our way in 2036, it will turn out to be extremely useful to have sketched out the details already.
Ref: arxiv.org/abs/1206.1336: Design of a Formation of Solar Pumped Lasers for Asteroid Deﬂection
Source: Technology Review