Laser treatment for Earth-bound asteroids

In the cinematic world, our planet’s destruction by the impact of another cosmic body is one of the few science-fiction tropes to bridge the blockbuster (Meteor, Armageddon, Deep Impact) and the arthouse (Last Night, Melancholia). Whatever else that means, it seems to imply that we’re all thinking about it.
But how do you think about it? The impact of a 10-km (6-mile) asteroid would be apocalyptic, but the chances of it happening in the next few generations are all but negligible. The smaller the asteroid, though, the bigger the danger, as witnessed by the recent near miss of the 7-metre (23-feet) 2012 KT42, the sixth closest encounter of any known asteroid. But, Bruce Willis apart, can we even begin to think about averting such an event with today’s technologies?
This is one of those areas in which hard science can degenerate into idle speculation and fantasy. That’s why a paper, as yet unpublished, by two aerospace engineers at the University of Strathclyde in Glasgow, UK, is worth attending to. No one would claim that the plan sketched by Massimiliano Vasile and Christie Maddock to deflect Earth-threatening asteroids with solar-driven lasers is a blueprint for the survival of mankind, or that governments should be rushing to implement the idea. Rather, it’s the sort of ballpark calculation that lets us contemplate the magnitude of the task.
Deflection tactics
But there’s another reason to take note, which the authors don’t mention. Recent announcements of plans to mine asteroids for precious elements and minerals – in particular the launch of Planetary Resources, backed by Larry Page and Eric Schmidt of Google and commercial spaceflight entrepreneur Peter Diamandis – has got people talking about whether some of these cosmic goldmines might be nudged closer to Earth for easier access. Any technology that could alter the course of asteroids might therefore excite more interest from private speculators than from governments wanting to prevent doomsday.
The basic idea behind this approach isn’t new. In fact it goes back to 1994, when planetary scientist Jay Melosh, a specialist on meteorite impacts, and his colleagues proposed that asteroids on a collision course with our planet might be deflected by the type of nuclear blasts favoured in Armageddon.
Another possibility makes use of a huge reflector floating near an asteroid, which could focus sunlight onto the surface and burn off a jet of icy material. According to Newton’s third law of motion, this would gradually change the asteroid’s course: the momentum of the material flung out in one direction would be compensated by a change in the asteroid’s own direction of motion. Some researchers expanded the idea by proposing the use of a whole fleet of mirror-bearing spacecraft around the asteroid. But Vasile and Maddock pointed out that if the reflectors are going to be close enough to the asteroid to achieve strong solar heating, there’s a risk that the mirrors will be covered with the debris coming off the surface.
Laser blast
That’s why the duo now envisage using laser beams instead to heat the surface: they remain tightly focused over large distances, and so can be stationed further away. Lasers too have been considered before for this purpose. But they eat up a lot of energy, and previous proposals have imagined running them from a nuclear power source on a single spacecraft. In contrast, Vasile and Maddock propose using a swarm of craft – which are easier to build – equipped with modest-sized, electrically powered lasers driven by photovoltaic cells, powered in turn by light-collecting mirrors perhaps a few metres in size.
Sounds nice in principle. But can a realistically sized fleet of spacecraft induce enough deflection to head off a potentially hazardous Earth-bound asteroid? The two engineers formulate an answer to this question by looking at an asteroid named 99942 Apophis, which is known to have a trajectory that crosses the Earth’s orbit and has a very small chance of hitting in 2036 or 2037. It’s shaped like a potato, reaching almost 200m (655 feet) along the long axis, and would wreak serious havoc if it hit us.
There are lots of asteroid-deflecting parameters one could vary: the laser power (and consequent solar-cell requirements), the size, number and position of the spacecraft, how they are powered to hold their position, and so on. The craft would have to be actively held in place, not least to counteract the slight push that they will receive from the stuff being blasted off the asteroid. One of the key aims is to find a compromise distance that achieves enough heating without too much clogging of the collector mirrors. The effort any system needs to put in also depends on how much deflection is needed to avoid catastrophe, and how much warning you have: how long you can spend nudging the asteroid, in effect.
All this means that it’s not possible, or indeed meaningful, to say exactly what would be needed in terms of design or cost to get this idea to work. But Vasile and Maddock do manage to establish that if, say, we discovered in ten years time that Apophis really was going to strike, it should be possible to implement a strategy like this based more or less on the technologies already to hand, without any fear of bankrupting the economy. That’s surely a little reassuring, even if it wouldn’t make much of a movie.
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