Not just any nuclear bomb will do when it comes to deflecting incoming asteroids, a new study reveals – and the differences aren’t just about power. Picking the right bomb is important to getting the asteroid to go in the desired direction, and neutron energy is key.
We now know the Earth is safe from the asteroid Apophis for the rest of the century, removing one short-term danger from space, but sooner or later another space rock will threaten the planet. Many approaches have been suggested to deal with this threat, but nuclear weapons are the most effective proposed so far.
In the event an asteroid or comet is on a collision course with Earth, the appropriate response will vary depending on a number of factors, most importantly time available. Those who have considered the problem have generally agreed that with enough warning, the best solution is to change the asteroid’s orbit. "Over time, with many years prior to impact, even a miniscule velocity change could add up to an Earth-missing distance," Air Force Institute of Technology Masters student Lansing Horan said in a statement.
A dangerously large asteroid will still need a substantial push to achieve this “miniscule” velocity change.
In the popular imagination, a bomb’s shockwave might blow an asteroid off course, but a more important process is explained in Acta Astronautica: “Detonating a nuclear device above an asteroid irradiates a certain surface area. Much of the material near the surface of this region is nearly-instantly melted and/or vaporized by the rapid energy deposition from prompt x-rays, neutrons, and gamma rays. Subsequently, this relatively small amount of superheated material expands away from the asteroid as 'blow-off', inducing a pressure wave in the remaining majority asteroid body.”
Neutrons do most of the work, but bombs release neutrons with varying energies, and the distribution varies by type of nuclear explosion. "If the energy deposition input is incorrect, we should not have much confidence in the asteroid deflection output," Horan noted.
Horan and co-authors have made a start comparing the impact on a spherical silicon dioxide asteroid 300 meters (1,000 feet) across neutrons at the peak energies of specific fusion and fission bombs, but there is a long way to go.
"One ultimate goal would be to determine the optimal neutron energy spectrum, the spread of neutron energy outputs that deposit their energies in the most ideal way to maximize the resulting velocity change or deflection," Horan said. "This paper reveals that the specific neutron energy output can impact the asteroid deflection performance, and why this occurs, serving as a stepping stone toward the larger goal."
It’s just as well we have time to get prepared, but probably also a good idea to get started now.
If we only spot the threat at the last minute, however, blowing it up in the manner beloved of Hollywood disaster films may be required. That makes hundreds of fragments to worry about, rather than just one mighty rock, but Horan said; "Past work found that more than 99.5 percent of the original asteroid's mass would miss the Earth.” That might leave objects large enough to take out a city incoming, but at least spare us a civilization-ending event.
The asteroid Horan modeled has around one ten-thousandth of the mass of the “Dinosaur-killer”. Even intact it would be unlikely to cause a mass extinction, but an ocean strike would still unleash tsunamis whose damage would dwarf any cost for a deflection attempt.