DART Did More Than Deter Dimorphos; It Sent It Into a Chaotic Tumble

In 2022, NASA’s DART (Double Asteroid Redirection Test) spacecraft collided with an object named Dimorphos. The objective was to test redirecting hazardous asteroids by deflecting them with an impact. The test was a success, and Dimorphos was measurably affected.

Follow-up research shows that Dimorphos was more than deflected; it was deformed.

In recent decades, we’ve made progress cataloguing the asteroids in the Solar System. Some of them are close enough to Earth to be dangerous. If an object comes within 1.3 astronomical units of the Sun, it’s called a Near Earth Object (NEO.) If it’s more than 140 meters (460 ft) across and crosses Earth’s orbit, it’s called a Potentially Hazardous Object (PHO.) Over 99% of NEOs and PHOs are asteroids, and the remainder are comets.

Earth has suffered many impacts from these objects in the past. The most famous impactor was Chicxulub. When it struck Earth about 65 million years ago, it was responsible for the end of the dinosaurs.

Now that we know the danger these tumbling space rocks pose, NASA and other agencies are preparing to do something about it. DART was a test mission to see how effective a simple kinetic impactor could be at changing the trajectory of an asteroid.

Dimorphos doesn’t pose any threat to Earth. It was chosen as the test target because it’s actually one part of a pair of objects. Dimorphos is a tiny moon of an asteroid named 65803 Didymos. Because Dimorphos is in orbit around Didymos, it’s easy to measure changes in the object’s movement after the impact.

An entire team has been following Dimorphos since DART impacted it to track how the object’s orbit has changed. Their observations show that the test was a success. Dimorphos’ orbit around Didymos was shortened by 32 minutes when the objective was to shorten it by only 73 seconds. Interestingly, it wasn’t the impact that affected Dimorphos’ orbit; it was because of the recoil effect from the ejected debris.

New research published in The Planetary Science Journal shows that Dimorphos was more deeply affected by the impact than thought. The paper is titled “The Dynamical State of the Didymos System before and after the DART Impact.” The lead author is Derek Richardson from the Department of Astronomy at the University of Maryland. Richardson is the team leader for one of the DART investigation teams called the Dynamics Working Group.

“For the most part, our original pre-impact predictions about how DART would change the way Didymos and its moon move in space were correct,” said Richardson. “But there are some unexpected findings that help provide a better picture of how asteroids and other small bodies form and evolve over time.”

Pre-impact observations showed that Dimorphos had an oblate shape. But after the impact, it became prolate or elongated. This went against pre-impact observations, which suggested that Dimorphos was initially elongated.

DART had a mass of 610 kilograms (1,340 lb) and struck Dimorphos at a speed of about 21,000 km/h (13,000 mp/h). The impact had a force equivalent to about three tons of TNT exploding and unexpectedly altered Dimorphos’ shape.

“We were expecting Dimorphos to be prolate pre-impact simply because that’s generally how we believed the central body of a moon would gradually accumulate material that’s been shed off a primary body like Didymos. It would naturally tend to form an elongated body that would always point its long axis toward the main body,” Richardson explained.

“But this result contradicts that idea and indicates that something more complex is at work here. Furthermore, the impact-induced change in Dimorphos’ shape likely changed how it interacts with Didymos,” Richardson said.

Didymos and Dimorphos are connected gravitationally, and after the impact, scattered debris from Dimorphos altered their relationship, reducing Dimorphos’ orbit around Didymos. It isn’t certain yet, but Dimorphos may have entered a tumbling state.

“Originally, Dimorphos was probably in a very relaxed state and had one side pointing toward the main body, Didymos, just like how Earth’s moon always has one face pointing toward our planet,” Richardson explained. “Now, it’s knocked out of alignment, which means it may wobble back and forth in its orientation. Dimorphos might also be ‘tumbling,’ meaning that we may have caused it to rotate chaotically and unpredictably.”

If it’s tumbling, Dimorphos could cause problems—not for Earth but for Hera, the follow-up mission.

The ESA’s Hera mission will be launched in a few weeks. Its mission is to perform a detailed post-impact survey of Dimorphos. To do that, it needs to get close. If Dimorphos is tumbling, its orbit is less predictable, and that will make it difficult for Hera to get close. If that’s the case, the data Hera collects will suffer. It’s possible that, over time, secular damping will calm the tumbling, but there’s a lot of uncertainty at this point.

“While secular damping is possible in the near future, it is unlikely to have major effects on the system when Hera arrives. Thus, Hera may encounter a tumbling Dimorphos, complicating proximity operations,” Richardson and his co-authors write in their research.

DART changed the mutual orbit of Didymos and Dimorphos, and it also changed their orbit around the Sun. The initial impact wasn’t entirely responsible for this change. The ejecta also contributed. “The initial impulse delivered to the system’s barycenter was augmented by the momentum carried by the ejecta that escaped the system,” the authors explain.

Many different researchers have calculated the ejecta, and different observations have arrived at different amounts. However, the researchers say that the impact ejected some tens of millions of kg of material.

The impact could change Didymos’ shape, too. It spins so fast that it’s at greater risk of structural failure when ejecta from Dimorphos strikes it. “Small perturbations, such as ejecta from the impact site on Dimorphos striking Didymos at various speeds, could thus trigger a reshaping process, wherein its equatorial radius increases while its polar radius decreases, resulting in a more oblate shape,” the authors explain.

The impact generated so much ejecta that Dimorphos formed a tail. Some of that debris had to have landed on Didymos, but observations so far show that it hasn’t affected its surface or its dynamics. “This implies Didymos’s surface was strong enough to withstand such impacts,” the authors write. However, in the past, Didymos likely suffered some type of fracturing, possibly due to its fast spin rate, and debris from that event likely formed Dimorphos.

The impact results are uncertain. DART carried a secondary Italian spacecraft named LICIACube (Light Italian CubeSat for Imaging of Asteroids) that separated from DART 15 days prior to impact. It drifted past the asteroid and captured images of the asteroid and the ejecta with its pair of cameras. LICIACube’s observations helped scientists understand what happened, but the small CubeSat executed only a single flyby.

It’s up to the ESA’s Hera spacecraft to answer questions about the impact. It’ll reach Didymos in October 2026, and in December, it will begin about six months of proximity operations. “The primary goal of Hera is to measure the mass of Dimorphos,” the authors write.

The mass is the missing piece that will help us understand how the ejecta contributed to Dimorphos’ altered orbit. Hera also has two CubeSats, Juventas and Milani, and all three will work together to constrain Dimorphos’ mass more precisely. “Once Hera gets closer, its Radio Science Experiment (RSE), involving the main spacecraft and the two CubeSats, Juventas and Milani, should obtain Dimorphos’s mass to higher precision and measure the extended gravity fields and rotational states of both Didymos and Dimorphos,” the paper states.

When you smash an impactor into an asteroid, you can expect some unintended results. But if asteroid redirection is to serve as a tool to protect Earth from dangerous impacts, then we need to know in as much detail as possible what to expect. That’s what DART and Hera are all about. However, they’re also telling us about the relationships between small binary objects.

“The DART mission, together with the Didymos observing campaign, not only represented the first test at a realistic scale of a hazard mitigation technique but also provided unprecedented measurements of dynamical effects in a nonideal small solar system binary for testing theoretical models,” the authors write.

Many of the pre-impact predictions turned out to be true, but other results are surprising, and there are plenty of unanswered questions.

“We look forward to revelations from the Hera mission, which promise to further refine our understanding of small bodies in general and the formation and evolution of binary asteroids in particular,” the researchers conclude.