THANKS to Hollywood, we already know what to do if an asteroid big enough to wipe out life on Earth is spotted coming our way. Haul out our surplus nuclear weapons and get Bruce Willis on the phone.
But there’s a catch. In the past year, asteroid researchers have been warning that it would be a big mistake to rely on nuclear bombs to save the planet. Nuking a killer asteroid as it approaches could be the worst thing our superhero could do, even compared with doing nothing. In February, asteroid researchers submitted a report to NASA headquarters in Washington DC calling for more research into deflection technologies that are a far cry from the “all guns blazing” approach. If they have their way, our toolbox for saving the planet will include ways of gently coaxing the killer into a safe orbit, either by slowing its irregular spin, by giving it a whitewash or even by parcelling it up in wrapping paper.
At first glance, blowing a killer asteroid to smithereens seems like a good idea. Asteroids a kilometre across – the size that threaten the future of our civilisation – should be no match for a 10-megaton nuclear bomb, among the largest in current arsenals. Push the red button with several months to spare and the debris should disperse in time. “You get one hell of a meteor display on the night that was previously going to be Armageddon,” says Alan Harris, an asteroid researcher who retired last year from NASA’s Jet Propulsion Laboratory in Pasadena, California. But as fans of the movie Armageddon know, simply firing a warhead at the asteroid and exploding it on the surface won’t destroy it. Our heroes will need to land on the rock and bury the bomb at least 100 metres deep. This is bound to be a high-risk strategy, however, because getting the depth and digging technique right for an unknown material will be fraught with difficulty. Get it wrong and you risk creating a blast of chunky shrapnel that will spread more devastation across the planet than a localised collision.
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These concerns have led researchers to consider using a nuke to deflect rather than destroy an asteroid. Deflection depends on transferring lots of sideways momentum, rather than explosive energy, to the asteroid. Detonating a bomb on the surface will only vaporise a small portion of the asteroid, but detonating a nuclear bomb well above the surface – about 200 metres for a 1-kilometre asteroid – will vaporise a greater mass. As the mass is blown off the surface, the rest of the asteroid will gain an equal amount of momentum in the opposite direction. A bomb with a kick of between 100 kilotons and 1 megaton, detonated 200 metres above the surface of a solid asteroid a kilometre across, should vaporise the top 20 centimetres. This would be enough to give it a sideways shove that would change its velocity in this direction by about 10 centimetres per second. That would ensure a miss, given seven years’ notice.
Unfortunately, this now also looks tricky. In 2002, a series of optical and radar images taken from Earth and from probes overthrew a key assumption underpinning these calculations. Instead of being solid rocks, most asteroids are porous piles of rubble barely hanging together in space. As many as one in six known asteroids is not even doing that. They are binaries – two rubble piles orbiting each other.
This disturbing discovery led Keith Holsapple of the University of Washington in Seattle to study the effect of nuclear nudges on porous rubble piles. “A very porous material is very effective at absorbing energy,” Holsapple says. Push hard on a rock and it moves; push hard on a porous material and it crushes like polystyrene packing material. Holsapple calculates that a megaton nuclear blast 200 metres away would push a 1-kilometre rubble-pile asteroid only a thousandth as effectively as it would a solid body. So to deflect a 1-kilometre asteroid you’d need a 1-megaton blast up close, says Holsapple. To deflect a 10-kilometre asteroid like the one that saw off the dinosaurs, you’d need a 1-gigaton bomb – a hundred times more powerful than any ever tested.
Holsapple’s work has prompted a change in tactics. At NASA’s Workshop on Asteroid Mitigation last September, no one was talking about nukes. Instead, the motto behind many of the ideas was “softly- softly”.
Several researchers proposed using engines that would thrust gently for a long time, rather than explosively. Former astronaut Rusty Schweickert talked about landing an ion rocket engine on the asteroid and using it to shunt the asteroid gradually to the side. Such engines use solar energy or a small nuclear reactor to heat propellant slowly and eject it continuously, producing a thrust so gentle that the object’s delicate structure should not matter. Schweickert is chairman of the B612 Foundation (named after the home planet of the Little Prince in Antoine de Saint-Exupéry’s book of the same name). The private group, based in Houston, hopes to persuade a consortium of NASA and private funders to let them try changing the orbit of a 100-metre rubbly asteroid by 2015.
As well as designing the propulsion system, the group is planning the delicate space operations needed to pick a landing site, dock there and begin to push the asteroid. “You don’t have to use a lot of fuel, but you have to use a lot of brains,” says Schweickert. Asteroids typically spin on their axis as well as orbiting the Sun. So to push an asteroid in one direction rather than simply increasing or decreasing its spin, an engine on the surface could fire only once per rotation. Instead, Schweickert says the engine will land on the equator, line up with the asteroid’s spin, then fire in the opposite direction. With an asteroid completing one rotation in a few hours, an ion engine should take several months to a year to stop it spinning. The engine can then swivel through 90° and push the asteroid continuously in one particular direction.
But the asteroid’s rotation may not be known precisely, so researcher Jay Melosh of the University of Arizona favours an alternative approach. He suggests using a “solar concentrator” – a giant parabolic mirror – to focus sunlight onto the asteroid’s surface. As the asteroid spins, the light and heat should evaporate material from whatever part of the surface happens to fall in the focal spot. As long as the light is shone at a constant angle to the surface, the gradual momentum gain by evaporating material will move the asteroid gently in the opposite direction. To steady the mirror’s position against the force of the solar wind, which would be significant on a large mirror, a low-thrust rocket engine – similar to those being developed by the B612 Foundation – would be needed. Jim Pawlowski, a specialist in asteroids and orbital debris who has just retired from NASA’s Johnson Space Center in Houston, estimates that a 32-metre mirror would take 10 years to deflect a typical 1-kilometre near-Earth asteroid from its destructive course.
Ideally, researchers would prefer a scheme that, like a nuclear explosion, only required a one-off intervention, but that moved the asteroid very gently over a long time. One idea is to change how much light the asteroid reflects in order to change its orbit.
The orbits of asteroids aren’t just determined by the gravitational forces in the Solar System; other effects also play a part. The sunlit side of a dark asteroid continuously absorbs light energy from the Sun. The momentum of the solar photons is transferred to the asteroid, altering its orbit continuously by an amount that depends on how well it absorbs the light.
There is also a second, less obvious phenomenon. As the asteroid rotates, the “Yarkovsky effect” comes into play. Several hours after having the Sun directly overhead, the surface re-emits absorbed energy in the infrared. The asteroid effectively donates some momentum to the infrared photons, and recoils, altering its orbit.
This sets the stage for a big idea. Most asteroids are so dark they absorb all but a few per cent of the incident light, but if they were coated with something shiny or white, the light would bounce off instead. If the energy is not absorbed, it cannot be emitted later. So a whitewash would all but remove the Yarkovsky effect on the asteroid’s orbit. In the absence of the effect, a typical killer near-Earth asteroid will be deflected by a few millimetres per second from the path it would have taken. Over a couple of centuries, that would be enough for it to miss the Earth.
But how could we possibly hope to change an asteroid’s surface properties? Compared to timing a nuke or training a solar concentrator, it is relatively easy, says Jon Giorgini of the Jet Propulsion Laboratory. Splatting the surface with white paint could do it, but the quantity needed would be heavy and expensive to transport to the asteroid’s orbit. More likely, reflective glass beads or a white powder, such as chalk dust, would be fired into the asteroid’s gravitational field. Because the field is so irregular, the particles would bounce to ground over a variety of different trajectories, eventually covering the entire surface.
But to make the covering really even, Giorgini favours a solar sail. If a giant piece of reflective material were unfolded in the path of the asteroid, it could be made to envelop it, covering the whole thing like a giant birthday present. “It’s technology we could do now,” says Giorgini, although it will take centuries to work.
Not everyone is convinced by the potential for the Yarkovsky effect to save the planet. “Exactly how efficient it is remains to be learned,” says Mike Belton, co-organiser of the NASA workshop and head of Belton Space Exploration Initiatives in Tucson, Arizona. Part of the problem is that the Yarkovsky effect is smaller than the uncertainties in many asteroid’s orbits. This means that even if researchers think a rock is on course for us, they may not know whether changing the Yarkovsky effect on it would be enough.
Even proponents of prospective schemes agree that developing them into a real toolbox of alternatives could take decades. And there’s no time to lose. Although none of the currently known 480 or so near-Earth asteroids over 1 kilometre in size is classed as hazardous, imprecision in measuring their orbits makes it impossible to predict this more than a century into the future.
One exception is asteroid 1950DA, which orbits unusually far from other perturbing objects and in such a way that the gravitational effect of the Earth on it varies in a simple, regular way, a phenomenon known as a resonance. If 1950DA is spinning in one particular direction – and we don’t actually know how it is spinning – there is a 1 in 300 chance it will hit the Earth on 16 March 2880. If it spins the other way, the Yarkovsky effect on it will be different and it is sure to miss.
As surveys continue, they are likely to turn up other relatively high-risk candidates for astronomers to keep an eye on. But if all goes according to plan, by the time we find our first killer asteroid we will be ready with a shiny coat to send it on its way. Saving humanity with baking foil may not seem as daring as blasting the asteroid to kingdom come, but if it works, not even Hollywood will complain.
