There is probably a large space rock somewhere with Earth in its sights. In fact, scientists have seen one candidate – Bennu, who has a slim chance of colliding with our planet in the year 2182. But whether it’s Bennu or another asteroid, the question will be how to avoid a highly unwanted cosmic rendezvous.
For nearly 20 years, a team of researchers has been preparing for such a scenario. Using a specially designed gun, they repeatedly fired projectiles at meteorites and measured how the space rocks receded and, in some cases, shattered. These observations shed light on how an asteroid might react to a high-velocity impact intended to deflect it away from Earth.
At the Meteoritical Society’s 84th Annual Meeting held in Chicago this month, researchers presented findings from all that powerful marksmanship. Their results suggest that whether we are able to knock an asteroid away from our planet may depend on the type of space rock we face and how often we hit it.
In the 1960s, scientists began to seriously think about what to do with an asteroid on a collision course with our planet. The guiding idea then was to launch a projectile that would break the space rock into pieces small enough to burn up in Earth’s atmosphere, said George Flynn, a physicist at the State University of New York, Plattsburgh. But scientists have since come to realize that achieving such an immediate, catastrophic hit is a serious challenge.
“It turns out that’s very difficult,” said Dr. flynn.
The thinking is different these days, and it’s not the Hollywood version with an atomic bomb. Instead, the current guiding idea pushes aside an incoming asteroid. The way to do that, scientists generally agree, is to deliberately set up a collision between an asteroid and a much smaller, less massive object. Such a collision, known as kinetic impact deflection, changes the asteroid’s orbit very slightly, intending to change its orbit enough to pass harmlessly by Earth.
“It may barely miss, but barely miss is enough,” said Dr. flynn.
Kinetic impact deflection is a promising — and currently feasible — technique, said Dan Durda, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. “It doesn’t require science fiction-like technologies.”
In 2003, Dr. Flynn, Dr. Durda and colleagues fire projectiles at meteorites to test the limits of kinetic impact deflection. The goal was to find out how much momentum can be transferred to a meteorite without shattering it into shrapnel that can continue on a similar trajectory through the solar system.
“If you break it into pieces, some of those pieces could still be on a collision course with Earth,” said Dr. flynn.
Similar laboratory studies in the past have mainly shot projectiles at terrestrial rocks. But meteorites are a much better sample, he said, because they are fragments of asteroids. The hitch gets access to them.
“It’s hard to persuade museum curators to give you a big chunk of meteorite so you can turn it into dust,” said Dr. flynn.
Over many years, the researchers collected 32 meteorites, most purchased from private dealers. (The largest, about the size of a fist and weighing a pound, cost the team about $900.)
About half of the meteorites belonged to a type known as carbonaceous chondrites, which are relatively rich in carbon and water. The rest were regular chondrites, which typically contain less carbon. Importantly, both types are representative of the near-Earth asteroids that pose the greatest risk to our planet. (Bennu is a carbonaceous chondrite.)
The team turned to an Apollo-era facility to test how the meteorites responded to high-speed impacts. NASA’s Ames Vertical Gun Range in California was built in the 1960s to help scientists better understand how lunar craters form. It can launch projectiles at more than four miles per second, much faster than a rifle.
“It’s one of the few weapons on the planet that can shoot things at the speeds typical of impacts,” said Dr. flynn.
The researchers worked in the facility’s fire chamber, about the size of a walk-in closet, suspending each space rock from a length of nylon rope. They then pumped the chamber to a vacuum — to mimic the conditions of interplanetary space — and fired tiny aluminum spheres at the meteorites. The team launched spheres from one-sixteenth to one-quarter inch in diameter at different speeds. Several sensors, including cameras recording up to 71,000 frames per second, documented the impact.
The goal was to determine the point at which a meteorite is no longer pushed by an impact and instead begins to fragment.
The researchers found a significant difference in the strength of the two types of meteorites they tested. The carbonaceous chondrites tended to fragment much more easily — they could withstand receiving only about one-sixth of the momentum the regular chondrites could before disintegrating.
These results have implications for the deflection of a real asteroid, the team suggests. If an asteroid richer in carbon came our way, it might need to give it a series of gentler nudges to keep it from disintegrating.
“You may need to use multiple effects,” said Dr. flynn.
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. However, the spacecraft’s target asteroid, a roughly 525-foot chunk of rock known as Dimorphos, is in no danger of hitting Earth. The mission is expected to launch in November.
Laboratory studies of kinetic impact deflection shed light on how an asteroid will respond to an impact, said Nancy Chabot, the coordination leader of the DART mission and not involved in the experimental work.
“It’s absolutely important to do these experiments,” says Dr. Chabot, who is also a planetary scientist at the Johns Hopkins University Applied Physics Laboratory.
The DART mission is about being prepared for what is most likely a cosmic inevitability.
“It’s one of these things that we hope we never have to do,” said Dr. chabot. “But Earth has been hit by objects throughout its history and will continue to be hit by objects in the future.”
