NASA's DART Impact Reshaped the Asteroid Dimorphos

When the DART spacecraft that NASA launched to test planetary defense whacked the asteroid moon Dimorphos (companion to the larger asteroid 65803 Didymos) on September 26, 2022, the impact was hard enough to redistribute a tenth of its material. That conclusion shakes up our understanding of rubble-pile asteroids.

Researchers had expected the collision to produce a crater, says Sabina Raducan (University of Bern, Switzerland), lead author of a new analysis of the results published on February 26th in Nature Astronomy. Yet, with little holding the rubble pile together, the impact blasted material all over, reshaping Dimorphos and ejecting up to a couple percent of the asteroid moon into space.

NASA built the Double Asteroid Redirection Test (DART) to study what would be necessary to redirect wayward asteroids away from Earth. The plan was to see how much a projectile impact could change the orbital period of a small asteroid in a binary pair. Their target, Dimorphos, is 170 meters wide and orbits the larger Didymos. The spacecraft carried a 580-kilogram (1,280-pound) projectile, navigation equipment, and a CubeSat carrying cameras and sensors, known as LICIACube. The CubeSat, released prior to impact, watched what happened to Dimorphos for a little more than five minutes after the projectile hit.

Later observations showed that the impact reduced Dimorphos’s orbital period by 33 minutes, well above the expected 7 minutes — and well above the goal for planetary defense, researchers reported a year ago in Nature. But more research was needed understand how the impact changed the asteroid’s structure.

The Nature of Rubble Piles

Most asteroids between about 100 meters and tens of kilometers in diameter are thought to be “rubble piles.” The name comes from the jumble of objects, ranging in size from boulders to fine grains, that spacecraft have seen on these asteroids’ surfaces. But we know little about their interiors.

Based on a previous observation — the small crater that Japan’s Hayabusa 2 probe  produced on asteroid 162173 Ryugu in 2019 using a much smaller impactor — Raducan and colleagues expected that the impact of the 580-kilogram DART on Dimorphos would produce a larger crater. Instead, they saw more debris than expected.

“What we saw after the impact was a lot of material being ejected,” she says. “Some formed a tail, like one from a comet.” The tail remained observable for almost a year after the impact, but understanding what happened would take more time.  

Images from LICIACube’s cameras covered only the first few minutes and, initially, Raducan says, the asteroid looked “like a big ball of sand, with very little binding between the grains.”

To trace what happened to the debris over the first hour after impact, researchers used simulations based on LICIACube’s initial observations as well as physical models of single particles. The simulation calculated the outflying rubble’s paths based on the physics of shock propagation. (Laboratory experiments were previously used to validate the model.) 

“The impact was very efficient,” says Raducan. When the 580-kilogram DART hit Dimorphos at 6 kilometers per second (more than 13,000 mph), it blasted 100 million kilograms from the surface. Some 80 million kilograms fell back onto the asteroid while 10 to 20 million kilograms were ejected from the system, forming a comet-like tail.

Other previous models had suggested rubble piles might contain clusters of big boulders at their cores. But the team found instead that the density inside the asteroid was low and the structure loose, with small grains and open space scattered among larger rubble. Although theorists had previously developed models for the insides of rubble piles, data had been lacking.

Kevin Walsh (Southwest Research Institute), who was not involved in the study, likes their model. “They nailed it,” he says. “It's rubble all the way down.”

DART’s efficiency in scattering that rubble is promising for planetary defense. “Rubble piles are everywhere, and with rubble all the way down, that’s very positive,” Walsh adds.

The new data also offer insight into asteroid science, says Ron Ballouz (Johns Hopkins University Applied Physics Laboratory), who also wasn’t involved in the study. Theorists are trying to understand how a binary could form from a single asteroid. With the DART results suggesting inherent differences between Didymos and Dimorphos, he says, “we finally have some real-world data.”

The next step, says Raducan, will be the October launch of the European Space Agency’s Hera mission to the Didymos system. Laden with cameras and other sensors and carrying two CubeSats, Hera will study Didymos and Dimorphos in detail when it arrives in 2026, examining changes in the system since the DART impact.

When asteroid 99942 Apophis passes close to Earth in 2029, it will give us “a real-life experiment and great opportunity for asteroid science”, says Walsh. (Indeed, NASA’s OSIRIS-REX mission, now renamed to OSIRIS-APEX, will visit the asteroid just after its closest approach.) Fortunately, we don't need our planetary defense system then; NASA assures us that the asteroid will miss Earth.