Not as simple as it seemed: DART mission debris complicates Earth's defense

In September 2022, when NASA's DART spacecraft crashed into Dimorphos , the small moon of the asteroid Didymos, it not only changed its orbit as planned, but also triggered a massive rockfall with momentum more than three times that of the spacecraft itself.

Which means that, while that mission was successful in demonstrating that the kinetic energy of impactors like DART can indeed alter an asteroid's path, the rocks ejected by the impact itself are also capable of creating forces in unexpected directions that could greatly complicate deflection efforts. In other words, asteroid deflection by kinetic impact as a form of planetary defense has proven to be a much more complex task than expected.

The discovery, made by a team of astronomers led by the University of Maryland, has just been published in the Planetary Science Journal.

"We've succeeded in deflecting an asteroid, moving it out of its orbit," says Tony Farnham, lead author of the study. "But our research shows that while the direct impact from the DART spacecraft caused this change, the ejected rocks gave the asteroid an additional 'kick' that was at least as large. A physics-changing factor that needs to be considered when planning these kinds of missions."

On September 26, 2022, NASA's DART (Double Asteroid Redirection Test) spacecraft executed an unprecedented maneuver: it deliberately crashed into Dimorphos, the small satellite orbiting the asteroid Didymos. The mission's primary objective was to demonstrate the feasibility of the "kinetic impactor" technique for deflecting asteroids. The collision, at over 22,000 kilometers per hour, was a resounding success: Dimorphos' orbit around Didymos was shortened by 32 minutes, far exceeding the predefined success threshold of 73 seconds. Humanity had proven, for the first time, that it was capable of altering the course of a celestial body.

However, what Farnham and his team have just discovered is that a significant part of that orbital change came not from the direct impact of the craft, but from the 'blowback' of the materials ejected by the collision, which provided an extra boost, a 'cosmic kick' that was almost as big as the DART impact itself . The ejected material, in effect, acted as a kind of 'extra' propellant, pushing the asteroid with considerable force in the opposite direction to the ejection.

But how were scientists able to unravel this complex phenomenon? The key lies in DART's small but vital traveling companion: LICIACube. This tiny CubeSat, developed by the Italian Space Agency (ASI), separated from DART 15 days before the impact and strategically placed itself in the best position to observe the spectacle. Thus, from a distance of 56.7 kilometers, and just 165 seconds after the collision, LICIACube began sending back to Earth an unprecedented series of images of the immense ejecta plume erupting from the impact site.

Thanks to those images, the team of astronomers was able to track the motion of 104 boulders, with radii ranging from 0.2 to 3.6 meters, as they traveled away from Dimorphos at speeds of up to 52 meters per second (about 187 kilometers per hour). By analyzing these trajectories in three dimensions, Farnham and his colleagues made a surprising discovery: the boulders weren't scattered randomly. Instead, they grouped into two distinct assemblies, with a notable absence of material in other areas. "We saw that the boulders weren't scattered randomly in space," Farnham explains. "Instead, they grouped into two quite distinct groups, with a lack of material elsewhere, which means something unknown was at play there."

The largest debris clump, which comprised about 70% of the measured objects, was ejected southward at high speeds and shallow angles to the asteroid's surface. Scientists hypothesize that these boulders likely came from specific sources, perhaps larger boulders on Dimorphos that were smashed by DART's solar panels just before the main body of the craft impacted the surface. Jessica Sunshine, a co-author on the study, suggests that DART's solar panels could have struck two large boulders on the asteroid, nicknamed Atabaque and Bodhran, and that the southward clump of ejected material would be composed of fragments of Atabaque, a 3.3-meter-radius rock.

Sunshine, who was also deputy principal investigator for NASA's 2005 Deep Impact mission to Comet Tempel 1, compared the results of that mission with those of DART.

"Deep Impact," the researcher explains, "hit a surface composed essentially of very small, uniform particles, so its ejection was relatively smooth and continuous. But here, we see that DART slammed into a stony surface littered with large boulders, resulting in chaotic, filamentary structures in its ejection patterns."

This fundamental difference—a surface made of fine particles versus a rocky, pebbly surface—is crucial to understanding how different types of celestial bodies respond to impacts. It's like comparing a bullet hitting a sandbag versus a bullet hitting a brick wall: the impact and dispersion of the material are completely different. This information, Sunshine says, "is vital to ensuring a future planetary defense mission is successful."

The momentum (the amount of movement) of the boulders ejected by the DART impact was primarily perpendicular to the spacecraft's trajectory. This means that, in addition to altering Dimorphos' orbit, it could have tilted its orbital plane by up to one degree and potentially caused the asteroid to wobble erratically in space. Needless to say, for a mission whose objective is a precise deflection, any unexpected wobble could be a critical factor.

This team's work to understand the impact of debris will be key to the European Space Agency's (ESA) Hera mission, which is scheduled to arrive at the Didymos-Dimorphos system in 2026. Hera, which along with DART is part of the AIDA (Asteroid Impact and Deflection Assessment) collaboration, has the main objectives of studying the Didymos binary system in detail after the 2022 impact, assessing its internal properties, and accurately measuring the outcome of the collision with DART. The Hera mission will deploy its own CubeSats, Milani and Juventas, to collect spectral data of the surface and study the asteroid's subsurface and internal structures.

In short, the University of Maryland study underscores the importance of considering all variables when planning future asteroid deflection missions. It's not enough to predict the main impact; it's essential to understand the physics of the ejection, the size and composition of the ejected material, and how this can influence the asteroid's trajectory and rotation.

"If an asteroid were headed our way," Sunshine concludes, "and we knew we had to move it a specific amount to avoid it hitting Earth, then all these subtleties become very, very important. You can think of it like a game of cosmic billiards. We could miss the mark if we don't consider all the variables."