NASA smashed an asteroid with a rocket. The debris could hit Mars.

NASA smashed an asteroid with a rocket. The debris could hit Mars.

In the future, if nothing is done to stop it, an asteroid not much larger than a football stadium will crash into the planet. Should it hit a city, it will annihilate it much like a non-radioactive nuclear bomb. There are 25,000 asteroids, roughly 460-feet long, like this zipping about in near-Earth space, and about 15,000 of them are yet to be found.

One way to stop them from hitting Earth is to change their trajectory by crashing into them with a small spacecraft. In September 2022, to test this deflection technique, a van-size spacecraft slammed into a 525-foot-long (harmless) near-Earth asteroid named Dimorphos at 14,000 miles per hour—and in doing so, successfully shifted its orbit around a larger space rock named Didymos.

This was DART—NASA’s Double Asteroid Redirection Test mission—humanity’s first ever planetary defense experiment. It was hailed as a huge success, but it produced some surprising after-effects, including a swarm of boulders that were found enveloping Dimorphos several months post-impact. These relatively small boulders posed no threat to Earth, but scientists did wonder where they might eventually end up.

The Webb telescope records the impact of the DART collision at 22 minutes, 5 hours, and 8 hours after the moment of impact.

Courtesy of STScI/NASA

Now, a recently published study—yet to be peer-reviewed—has offered up some answers. By carefully simulating the myriad ways in which these boulders will orbit the Sun over the next 20,000 years, scientists found that there is no possibility of any of them burning up in Earth’s skies.

“But they’re going to cross the orbit of Mars,” says study author Marco Fenucci, a near-Earth object dynamicist at the European Space Agency’s Near-Earth Objects Coordination Centre. And if both Mars and the mini-asteroids meet at that crossing, some of them will puncture through the thin Martian atmosphere. “And they will arrive to the ground and make a crater,” says Fenucci—creating bowl-shaped scars up to 1,000 feet long.

It’s essentially the first time that humanity has generated its own meteor storm, one that may end up hitting a rocky world. It’s a recreation of a natural process that’s been happening since the dawn of time—an asteroid colliding, and fragmenting, another—liberating space rocks.

It’s a “cascade, from big to small, going on for the entire age of the solar system,” says David Jewitt, an astronomer at the University of California, Los Angeles.

How a swarm of boulders was sent flying 

Because it’s relatively small, telescopic observations of Dimorphos revealed little about the asteroid before DART arrived in spectacular, self-destructive fashion. But with the information they had, astronomers suspected their target was a rubble pile—a weakly bound asteroid barely held together by its own gravity. They were later proven correct when DART’s impact blasted off an excessive amount of debris and even caused the entire object to change shape.

That fleet of errant space rocks was also somewhat unexpected.

The spacecraft’s impact splashed off several fast-moving boulders, and they vanished soon after. But follow-up observations by the Hubble Space Telescope spotted additional debris from the impact: 37 dislodged boulders, some as long as 22 feet, slowly drifting away from Dimorphos.

“We did not expect that many boulders that were that big to be blown off,” says Andy Rivkin, a planetary astronomer at the Applied Physics Laboratory and one of the DART mission’s investigation team leads. “We think that those have to be pre-existing boulders that the shockwave threw off. They were not created during the impact.”

In the short-term, such boulders could theoretically imperil Europe’s Hera spacecraft. Set to launch this October, it’s due to arrive at Dimorphos in 2026 to survey the wreckage that DART made—and flying into one of those vagabond rocks could take it out.

Jewitt, part of the team that used Hubble to spy those 37 boulders, suspects that there are probably more around Dimorphos than telescopes have been able to spy. And if there are some bouldery obstacles present when Hera arrives, they may have to pilot the spacecraft around them.

Fortunately, although studies have found that several space rocks may still be present around Dimorphos in 2026, the odds of a collision are fairly low.

On a path to Mars

To see where these boulders eventually land, Fenucci’s team looked further into the future, taking an average asteroid from the swarm, and plotting out its many possible orbits over the next 20 millennia. There are plenty of uncertainties when looking this far ahead; even small nudges by pressure from sunlight can, over time, push a small asteroid onto a brand-new path.

It seems very likely that, 6,000 years from now—and again 13,000 years from now—the orbits of Mars and these boulders could get extremely close. And at those points, “if [Mars and the boulders] arrive at the crossing point at the same time, there’s a chance that there will be an impact,” says Fenucci.

If these boulders are structurally weak, they could explode in the Martian atmosphere. If not, they’ll leave a hole in the ground.

The only danger these boulders pose to humanity would be to any far-future astronauts that just so happen to be bounding across the surface of Mars at the worst possible time and in the most unfortunate of spots.

This study also offers some insights that may aid planetary defense researchers in their quest.  

Defending Earth from killer asteroids requires knowing where they come from. Scientists think rogue shards mostly come from collisions in the asteroid belt between Mars and Jupiter. But this study shows that “near-Earth asteroids could be the source of meteorites too,” says Federica Spoto, an asteroid dynamics researcher at the Center for Astrophysics, Harvard and Smithsonian.

It’s also plausible that, in the not terribly distant future, asteroids rich in precious materials—from rare metals to rocket fuel-making water—will be mined. “Asteroids that are close to the Earth are natural targets for these missions,” says Fenucci.

But if they are chosen as prime targets for extraction, this sort of research shows why you can’t just split them apart haphazardly. Many small rocks that break away from these rubble piles will harmlessly drift off into deep space. But some of these boulders, perhaps those a little too large, could ultimately head our way.

“If you put into space more stuff that can impact the Earth, then it’s going to be a problem,” says Fenucci.

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