WHEN it comes to deflecting an asteroid that is on a collision course with Earth, “most people think of the Hollywood treatment – throw a nuclear weapon at it”, says NASA astronaut Edward Lu. “That’s the blast-and-hope strategy.” It is hard to predict where the shattered pieces would go, and many smaller chunks might still head towards Earth.
Now Lu and fellow astronaut Stanley Love at NASA’s Johnson Space Center in Houston, Texas, have come up with the simplest – and least glamorous – solution yet: park a heavy spacecraft near the asteroid and use gravity as an invisible towline to tug the rock off its deadly course.
Other ideas for dealing with such threats have included detonating nuclear bombs near the asteroid – rather than nuking it directly – to nudge it off track. But this carries the same risks as shattering the asteroid. Some have advocated painting the asteroid white to change the amount of solar energy it reflects, thus altering the forces acting upon it and hopefully changing its course. However, the sheer amount of paint this would require makes it impractical, says Lu.
Advertisement
In another attempt to come up with a practical solution, the researchers envisioned landing a spacecraft on an asteroid and then steering it off course using the craft’s propulsion. But an asteroid’s weak gravity may not hold the spacecraft down, so it would have to be anchored to the surface – a complicated task on a surface that could be loose rubble, says Lu. To make matters worse, asteroids often rotate, and pushing on one might just set it spinning faster rather than alter its trajectory.
It was then that Lu and Love realised that the spacecraft does not need to land. Just getting close and staying there is enough. For instance, their calculations showed that for a 200-metre asteroid, a 20-tonne spacecraft hovering 50 metres above the rock for about a year would change the asteroid’s speed by roughly 2 millimetres per second – enough to knock it off course given time (Nature, vol 438, p 177).
“Most people think of the Hollywood treatment – throw a nuclear weapon at it. That’s the blast-and-hope strategy”
“This is hands down the best idea I have seen,” says Erik Asphaug, an asteroid and comet specialist at the University of California, Santa Cruz. “This will work. But you need to put a large enough spacecraft out there at the right time.”
The spacecraft would need to begin deflecting the asteroid 20 years before a potential impact with Earth, but that is feasible, given that astronomers can predict asteroid paths decades in advance. And such large spacecraft launches are within our grasp, Lu says. NASA’s multi-billion-dollar Prometheus programme, which was set to explore the outer solar system but has now been delayed, included just such a heavy vehicle, propelled by nuclear fission.
