The decision depends on how much time we have and what the asteroid is like, says Jason P. Dworkin, OSIRIS-REx project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. Some asteroids might be easy to crumble into pieces; some could break apart into smaller stones that hit Earth in multiple places. Others might require a really big shove.

“If it was up to me, I would send a rocket with some titanium dioxide on it and paint it white, and have that Yarkovksy effect accelerate, and move it away from the Earth,” Dworkin says. Titanium dioxide is a common ingredient in sunscreen, and it looks chalky white. A spacecraft could fly to a dangerous asteroid, enter orbit around it, and spray-paint it.

“If you have something like 50 years, that could deflect it enough, and it might be a whole lot simpler than sending up nuclear weapons,” Dworkin says.

Radar images of the near-Earth asteroid Didymos and its moonlet from 2003. Didymos will be the target of AIDA’s test of a kinetic impact on a small asteroid. NASA

If that doesn’t work, a nudge might do the trick. NASA and ESA plan to study this with a mission called the Asteroid Impact and Deflection Assessment (AIDA), which will slam a satellite into an asteroid in 2020.

If we don’t have 50 years or a fast enough battering ram, we might need to break up an asteroid instead. Catherine Plesko of Los Alamos National Laboratory uses supercomputers to study how to break up asteroids using nuclear explosions and “kinetic impactors,” essentially giant space cannonballs.

“Cannonball technology is actually very good technology, because you’re intercepting the object at very high speed, so it ends up being more effective than conventional high explosives,” Plesko said at a meeting of the American Geophysical Union in December. “If you really need a lot of energy, though, a nuclear burst is the way to get the largest amount of energy out to the object in the smallest possible container.”

Her simulations include equations that describe how fluids flow; the boiling and melting points of rocks depending on their densities; how light and energy propagates; and other physics. Then she can slip in a model for a given asteroid, and see how it responds to a cannonball.

If Earth decides to nuke it instead, the blast wouldn’t have to blow up the asteroid from within. The plan would entail launching a rocket with the bomb as its payload (a risky prospect, should it blow up on the launchpad) and aiming right for the asteroid. Exploding the bomb at the asteroid’s surface could disrupt its interior enough to break it apart, shifting its course and limiting the threat to Earth.

“For nuclear deflection, it really is the best idea to do an intensive load of these calculations before we even need to contemplate doing anything real,” Plesko says.

If we do need to do something real, it won’t happen overnight. It takes at least five years to design, build and launch a spacecraft that can deflect or destroy an asteroid or comet, according to astronomer Joseph Nuth of Goddard. He says humans would be wise to build a spacecraft now, and ideally two: an observer that could be quickly dispatched to learn more about the asteroid, and an interceptor that could deflect it.

“If we build it now on a normal schedule, and put that interceptor into storage, we can launch it in less than a year,” he explained at the AGU meeting. “This could mitigate the possibility of a sneaky asteroid coming in from a place that’s hard to observe, like toward the sun.” (The Chelyabinsk meteor was one such asteroid.)

No space agency has a deflector mission yet, but scientists know a lot more today than they did even 15 years ago, Bottke says.

“There was a time where we didn’t have a program to look for objects, and it was done privately. You had guys like [astronomer Eugene] Shoemaker driving out every month to Palomar Observatory to look for them,” Bottke says. “Now we have $50 million annually to look for them. Now we’re getting serious science missions to look at these.”

Bottke and others say the increased awareness of asteroid threats has also been a boon for science, and for the people who study asteroids not only as harbingers of doom, but as messengers from Earth’s creation.

“The probability that any large asteroid is going to hit is in the near future is pretty low,” Bottke says. “So I tend to focus on the more uplifting parts of asteroids.”