Deflecting asteroid before impact may take multiple bumps

There’s probably a large space rock out there, somewhere, that has Earth in its cross hairs. Scientists have in fact spotted one candidate — Bennu, which has a small chance of banging into our planet in the year 2182. But whether it’s Bennu or another asteroid, the question will be how to avoid a very unwelcome cosmic rendezvous.

For almost 20 years, a team of researchers has been preparing for such a scenario. Using a specially designed gun, they’ve repeatedly fired projectiles at meteorites and measured how the space rocks recoiled and, in some cases, shattered. These observations shed light on how an asteroid might respond to a high-velocity impact intended to deflect it away from Earth. At the 84th annual meeting of the Meteoritical Society held in Chicago this month, researchers presented findings from all of that high-powered marksmanship. Their results suggest that whether we’re able to knock an asteroid away from our planet could depend on what kind of space rock we’re faced with, and how many times we hit it.

In the 1960s, scientists began seriously considering what to do with an asteroid on a collision course with our planet. The leading idea back then was to launch a projectile that would shatter the space rock into pieces small enough to burn up in Earth’s atmosphere, said George Flynn, a physicist at State University of New York, Plattsburgh. But scientists have since come to realize that achieving such a direct, catastrophic hit is a serious challenge. “It turns out, that’s very hard,” Dr. Flynn said.

The thinking is different today, and it’s not the Hollywood version with a nuclear bomb, either. Rather, the current leading idea is nudging an incoming asteroid aside. The way to do that, scientists generally agree, is deliberately setting up a collision between an asteroid and a much smaller, less massive object. Known as kinetic impact deflection, such a collision alters the trajectory of the asteroid ever so slightly, with the intent that its orbit changes enough to pass harmlessly by Earth.

“It may barely miss, but barely missing is enough,” Dr. Flynn said. Kinetic impact deflection is a promising — and currently feasible — technique, said Dan Durda, a planetary scientist at the Southwest Research Institute in Boulder, Colo. “It doesn’t require science fiction kinds of technologies.”

In 2003, Dr. Flynn, Dr. Durda and colleagues began firing projectiles at meteorites to test the limits of kinetic impact deflection. The goal was to figure out how much momentum could be transferred to a meteorite without shattering it into shrapnel that could continue on a similar orbital path through the solar system.

“If you break it into pieces, some of those pieces may still be on a collision course with Earth,” Dr. Flynn said. Similar laboratory studies in the past have mostly shot projectiles at terrestrial rocks. But meteorites are a much better sample, he said, because they’re fragments of asteroids. The hitch is getting access to them.

Next year, researchers will test kinetic impact deflection on a real asteroid in the solar system for the first time with NASA’s Double Asteroid Redirection Test (DART) mission. The spacecraft’s target asteroid, a roughly 525-foot piece of rock known as Dimorphos, is in no danger of hitting Earth, however. The mission is expected to launch in November.