New Scientist reports on a new study by researchers led by Massimiliano Vasile of the University of Glasgow in Scotland have compared nine of the many methods proposed to ward off such objects, including blasting them with nuclear explosions.
The team assessed the methods according to three performance criteria: the amount of change each method would make to the asteroid’s orbit, the amount of warning time needed and the mass of the spacecraft needed for the mission.
The method that came out on top was a swarm of mirror-carrying spacecraft. The spacecraft would be launched from Earth to hover near the asteroid and concentrate sunlight onto a point on the asteroid’s surface.
In this way, they would heat the asteroid’s surface to more than 2100° C, enough to start vaporising it. As the gases spewed from the asteroid, they would create a small thrust in the opposite direction, altering the asteroid’s orbit.
The scientists found that 10 of these spacecraft, each bearing a 20-metre-wide inflatable mirror, could deflect a 150-metre asteroid in about six months. With 100 spacecraft, it would take just a few days, once the spacecraft are in position.
To deflect a 20-kilometre asteroid, about the size of the one that wiped out the dinosaurs, it would take the combined work of 5000 mirror spacecraft focusing sunlight on the asteroid for three or more years.
But Clark Chapman of the Southwest Research Institute in Boulder, Colorado, US, says ranking the options based on what gives the largest nudge and takes the least time is wrongheaded.
The proper way to go about ranking this “is to give weight to adequate means to divert an NEO of the most likely sizes we expect to encounter, and to do so in a controllable and safe manner”, Chapman told New Scientist.
The best approach may be to ram the asteroid with a spacecraft to provide most of the change needed, then follow up with a gravity tractor to make any small adjustments needed, he says.
It is good to have several options for deflection and a survey to detect the specific risks of near earth objects.