Models of the NASA’s DART impact reshaping the asteroid moon Dimorphos show that this object is “rubble all the way down.”

Close-up view of Didymos and Dimorphos
This image, taken from 570 miles (920 kilometers) away shows the asteroid Didymos (bottom right) and its moonlet, Dimorphos, about 2.5 minutes before the impact of NASA’s DART spacecraft.
NASA / Johns Hopkins APL

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.

nearby view of ejecta streaming out from asteroid
This composite image from the Italian space agency's LICIACube shows the plumes of ejecta streaming from the Dimorphos asteroid after NASA’s Double Asteroid Redirect Test, or DART, mission, made impact with it on Sept. 26, 2022. Each rectangle represents a different level of contrast in order to better see fine structure in the plumes.

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.  

Trailing plume ejected from bright Dimorphos
Astronomers using the NSF’s NOIRLab’s SOAR telescope in Chile captured the vast plume of dust and debris blasted from the surface of the asteroid Dimorphos by NASA’s DART spacecraft when it impacted on 26 September 2022. In this image, the more than 10,000 kilometer long dust trail — the ejecta that has been pushed away by the Sun’s radiation pressure, not unlike the tail of a comet — can be seen stretching from the center to the right-hand edge of the field of view.
CTIO / NOIRLab / SOAR / NSF / AURA / T. Kareta (Lowell Observatory), M. Knight (US Naval Academy); Image processing: T.A. Rector (University of Alaska Anchorage / NSF’s NOIRLab), M. Zamani & D. de Martin (NSF’s NOIRLab)

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.

Dimorphos impact
This simulation incorporates physics of shock propagation to model the effect of DART's impact on Dimorphos. The asteroid moon is noticeably reshaped after impact. (Look "through" your screen at about a foot away in order to see a stereoscopic view.)
S.D. Raducan

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.


Image of mmmackenzie


February 28, 2024 at 5:27 pm

I was interested to read that DART made as big an impact on Dimorphos as it did. A little while ago, I set some A-Level pupils the task of assessing DART's Earth preserving potential. Using mission data and a little A-Level Physics, the pupils found that even if with an optimal collision, DART would have to intercept a Dimorphos-like object 270 years before its arrival.

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February 29, 2024 at 9:35 am

And immortalized in song!

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Tom Kellogg

May 20, 2024 at 10:50 pm

Thanks for the fascinating mission. Nearly 100,000 times more material was wholly ejected than the weight of the space craft. That is amazing. I saw this in the printed July issue which left out the weight of the space craft which led me to look up this on-line article.

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