An asteroid called Didymos recently had a close encounter with a spacecraft. Now it has divulged a dizzying secret: the half-mile-wide rock seems to be spinning so rapidly—completing a full rotation every two hours and 16 minutes—that its surface may be ejecting rubble, some carried out into space by solar wind.
Researchers made the discovery soon after NASA’s Double Asteroid Redirection Test (DART) spacecraft brought Didymos into the spotlight. Last September DART slammed into the asteroid’s moonlet Dimorphos nearly head-on, speeding up Didymos’s small companion asteroid enough to change its path and demonstrating that humans could protect Earth from a catastrophic asteroid impact if such a doomsday collision was predicted early enough.
The explosive visit also gave scientists an up-close peek at both asteroids, and DART will be followed by a thorough investigation of the binary asteroid system by the European Space Agency’s Hera mission. Beginning in late 2026, Hera will reconnoiter both Dimorphos and Didymos to take measurements of the rocks and show how DART changed the system.
The new study suggests Hera may also see a stream of rocks and dust flung off the equator of Didymos because of the asteroid’s fast spin, says Adriano Campo Bagatin, a planetary scientist at the University of Alicante in Spain and co-author of the research, which was published online in Icarus on March 11. The mechanism is just one way that scientists are realizing that asteroids can be active, dynamic places rather than quiescent lumps of rock. While it remains difficult to tell just how common such activity may be, the study is a reminder that asteroids across the solar system may be flinging their material out into the cosmos.
And the study nods to a long-standing theory of how DART’s target Dimorphos came to be, says Yun Zhang, a planetary scientist at the University of Maryland, who worked on the DART mission but wasn’t involved in the new research. “It’s like a puzzle to the scientists how the system formed in the first place,” she says.
One theory holds that Didymos may have formed alone, spinning so fast that it couldn’t quite hold together and tossed away material that eventually coalesced to form Dimorphos. “That’s why a lot of researchers want to understand if Didymos has this ability to shed mass from its surface,” Zhang says.
Right now the work is theoretical and based almost entirely on remote observations gathered before DART’s impact. From Earth, Didymos appears only as a bright dot, so scientists can’t gather direct data about material potentially being ejected from the asteroid.
“It’s a good place to start, and that’s all we can do,” says Dante Lauretta, a planetary scientist at the University of Arizona, about the new research, which he was not involved in.
The team members cobbled together all the data they could find on Didymos’s shape, size, mass and composition. Much of this came from past observations from Earth, though the DART mission did help the researchers to firm up the asteroid’s size. Based on years of monitoring since the asteroid was discovered in 1996, the team also knew Didymos’s rapid rotation rate, something that likely resulted in the asteroid’s spinning-top shape: as it turns, material from the poles gets pulled toward the equator to create a bulge.
The researchers fed this information into a computer model, which revealed that Didymos rotates fast enough that material near its equator might accelerate outward with more force than the asteroid’s gravity counteracts, allowing that material to lift off the surface.
From there the material has four potential destinations: it can escape into space, get stuck in orbit around Didymos, fall back to the asteroid’s surface or land on Dimorphos. Campo Bagatin and his colleagues calculated that more than 97 percent of lifted material should fall back to the surface of Didymos within about four hours. The team’s simulation showed that out of the remaining floating bits, most of the larger pieces of rubble—in this case, particles with a diameter at least a few centimeters—ultimately lands on Dimorphos.
This isn’t the first asteroid outed for throwing its own guts into space. Lauretta leads a different NASA asteroid mission called OSIRIS-REx, which will deliver pieces of the space rock Bennu to Earth this September. Soon after the spacecraft arrived at Bennu in December 2018, he and his colleagues were shocked to discover that the asteroid was spitting pebbles into space, albeit through a different mechanism than the one explored in the new Didymos research.
Identifying active asteroids, he says, is a reminder of just how dramatic our cosmic neighborhood can be. “The mindset that the solar system is a static environment is really wrong,” Lauretta says. “It’s a very dynamic place. Things are changing; things are crashing into each other; processes that have been ongoing for four and a half billion years still occur.”
Still, for Didymos specifically, scientists will need to wait for the Hera mission, scheduled to launch in October 2024, to understand just how much the asteroid’s surface is changing. The craft should be able to directly observe this potential rubble with its twin black-and-white Asteroid Framing Cameras, Campo Bagatin says. And it should also notice color differences where material was recently lost on the asteroid’s surface, Zhang notes.
“I am afraid that until the Hera mission will be there, it’s hard to say what really is around Didymos,” Campo Bagatin says.