304 North Cardinal St.
Dorchester Center, MA 02124
304 North Cardinal St.
Dorchester Center, MA 02124
On 26 September, an act of targeted violence will ensue 11 million kilometers from Earth, as a spacecraft about the size of a vending machine smashes into a small asteroid at 6 kilometers per second. Unlike some asteroids that stray worrisomely close to Earth’s orbit, Dimorphos—the 160-meter moon of a larger body—is an innocent bystander, posing no threat to our world. But the looming assault represents humanity’s first-ever field test of a planetary defense mission: NASA’s Double Asteroid Redirection Test, or DART.
The hope is that the collision will nudge Dimorphos into a closer orbit around its 780-meter partner, Didymos, shortening its nearly 12-hour orbital period by a few minutes. A successful strike would support the idea that, in the future, similar efforts could deflect threatening asteroids onto safer courses. But new simulations and lab experiments show the fate of the mission depends heavily on a crucial question: Are such small asteroids solid boulders or—as astronomers increasingly believe—loose heaps of rubble?
The answer, which should be revealed from the crater and ejecta produced by DART’s collision, could determine just how hard to hit an asteroid when the exercise is not a test. “It’s going to be thrilling—and very stressful—but ultimately, I think we’re going to learn a lot,” says Cristina Thomas, a planetary scientist at Northern Arizona University who leads the observation team for the DART mission.
Dimorphos-size asteroids are thousands of times more likely to strike Earth than the larger ones that have triggered previous mass extinction events, and they are still capable of devastating a state or small country, making these smaller bodies the top priority for planetary defense efforts. But they are no more than pinpricks of light to earthbound telescopes, making them hard to detect, let alone study.
When spotted, binary asteroid systems are more revealing, because their light dims whenever one body blocks the other. By monitoring small fluctuations in the light from Dimorphos and Didymos, NASA scientists and others have managed to learn how fast they spin and the frequency of the smaller body’s orbit. This knowledge allowed them to design an autonomous navigation system that, with the help of a new solar-powered ion thruster, will steer DART as it closes in on its prey.
What will happen next is anyone’s guess. “People assume it’s a solid rock, we have a solid spacecraft, and we’re essentially playing a giant game of billiards in space … and you can basically just solve that out as a simple physics equation,” Thomas says. “But there’s so much else that’s happening that makes that not true.”
The biggest uncertainty is the “strength” of Dimorphos, according to DART lead investigator Andy Cheng of the Johns Hopkins University Applied Physics Laboratory. “And that makes a huge difference in terms of the outcome,” he says.
Clues that not all asteroids are solid, monolithic rocks have piled up in recent years. In 2019, Japan’s Hayabusa2 probe shot a 2-kilogram copper projectile into the asteroid Ryugu, blasting a surprisingly large crater, 14 meters across. The experiment indicated that Ryugu’s surface was held together much more weakly than expected. The following year, NASA’s OSIRIS-REx probe landed on the Bennu asteroid and sank right in with little resistance. These missions confirmed the idea of weak asteroids and put constraints on their surface strength—how much force is needed to deform the objects. Scientists estimated these asteroids were held together by about 1 pascal—the pressure from a piece of paper resting on your hand.
“I’m an old-timer, and I find it hard to believe that anything can be so weak,” Cheng says. “It’s reinforced a lesson we’ve learned from planetary geology for decades: You cannot tell whether something is a rock by looking at pictures.”
Unfortunately, forecasting the consequences of an impact is much more difficult when the target is made up of thousands of weakly bound rocks than when it’s a solid boulder. If DART does hit a weak rubble-pile target, the resulting crater would develop over the course of a few hours, a process that would take months or even years to model with traditional computer simulations, says Sabina Raducan, a planetary scientist at the University of Bern.
Recently, she and her colleague optimized a computer code modeling a 3D shock wave to speed up the calculation to a few weeks. Unexpectedly, the new simulations show the DART impact could transfer four to five times more momentum to a weak rubble-pile target than a consolidated one—enough to reshape the entire asteroid rather than simply leaving a small impact crater, they reported in The Planetary Science Journal in June. DART would get more bang for its buck on a weak target because the loose rubble structure would allow more material to spurt from the impact—propelling the asteroid forward like a rocket thruster.
“This [modeling] is cutting-edge,” says planetary scientist Julie Brisset of the Florida Space Institute who is not involved in the DART mission. “You don’t want to stick to the old monolithic [rock] story.”
The push to model rubble-pile asteroids extends beyond computer screens. Scientists often calibrate their impact simulations with lab experiments—shooting high-velocity projectiles at different targets, as James Walker has done at the Southwest Research Institute for more than a decade. Recently, Walker’s and Raducan’s teams have both constructed the first makeshift rubble-pile asteroids for these tests. Walker has been launching masses horizontally at a panel of rocks covered in cement and suspended on a pendulum, whereas Raducan has been firing straight down into a 7-meter-wide sand pit embedded with miniboulders. Both teams have analyses awaiting publication, and one thing is clear: The weaker targets show much more dramatic explosions from the impacts.
A crew of instruments will be watching how well the actual collision matches the simulations. On Sunday, DART deployed a toaster-size CubeSat that will record the collision and its aftermath with two optical cameras. Meanwhile, the James Webb and Hubble space telescopes, along with four ground-based observatories, will take turns monitoring the dot of light. If Dimorphos is a weak rubble pile and its ejecta plume is as large as Raducan predicts, Thomas thinks the observatories should be able to catch it lighting up within hours after the crash.
“That final cloud … is really going to tell us a lot about the actual physical properties of the target,” Thomas says. “It’s not going to take much time for Dimorphos to give us the answers; it’s going to take us more time to figure out what it was telling.”
The full picture won’t come into view for another 4 years, though, when the European Space Agency’s Hera mission arrives to survey Dimorphos’s surface and measure its mass. This will help diagnose the asteroid’s internal structure and aid future planetary defense missions. In the event of a real asteroid threat, the goal is to hit the body just hard enough to divert it but not too hard to vaporize it and send a hailstorm of small rock fragments toward Earth.
Confirming that Dimorphos has rubble-pile structure would also shed light on a bigger question: how the Solar System first took shape. “Understanding what these small bodies went through helps us understand how planetary systems formed,” Brisset says. “They’re the remnants of this process.”
For now, however, the scientists must wait anxiously as DART approaches its bull’s-eye, hoping their preparation work will be the key to unlock its secrets.