On Sept. 26, 2022, at precisely 6:14 p.m. ET, a box-shaped spacecraft no bigger than a loveseat smashed directly into an asteroid wider than a football field. The planned impact knocked the space rock off its orbit, showing for the first time that an asteroid can potentially be deflected away from Earth.
The spacecraft was the key part of DART, NASA’s Double Asteroid Redirection Test, which aimed to redirect the paths of Dimorphos and Didymos — two small, nearby asteroids that orbit as a pair. (Neither asteroid has ever posed a threat to Earth). The Johns Hopkins University Applied Physics Laboratory (APL) built and operated the DART spacecraft and manages the DART mission for NASA’s Planetary Defense Coordination Office as a project of the agency’s Planetary Missions Program Office.
As DART closed in on the smaller Dimorphos, the spacecraft’s cameras snapped images of the impending collision, right up to the moment of impact. In the days following, the Hubble Space Telescope zeroed in to track the impact’s aftermath. The DART team has since analyzed the images taken before and after the smash-up. Their findings, published today in Nature, reveal Dimorphos to be a boulder-rich “rubble-pile” that left a trail of debris in its wake after the impact.
DART science investigation team member Saverio Cambioni, the Crosby Distinguished Postdoctoral Fellow in MIT’s Department of Earth, Atmospheric and Planetary Sciences, helped to analyze the collision as part of a larger team led by NASA and APL — including APL’s Andrew Rivkin ’91, who served as the mission’s investigation team co-lead. Cambioni shared with MIT his perspective on the mission’s highlights, and when the Earth might really need a DART-like, asteroid-deflecting defense.
Q: It must have been a nail-biting day for you and the team as DART closed in on its target. What do you remember from that day, personally?
A: It was so exciting! I remember watching the impact event on the NASA TV channel, and I could not wait for the Didymos system to grow from a blurred pixel to a spatially resolved asteroid pair. I joined the DART science investigation team a few years ago, and we discussed in many meetings what the surface geology of Didymos and Dimorphos would look like. We haven’t seen many of these small asteroids, and every time I am always amazed by the diversity of their surfaces. Would the surface be the same as the carbonaceous asteroids Bennu and Ryugu, which were found to be surprisingly rugged with little to no small rock fragments? Or should we instead expect the Didymos system to have terrains rich in pebbles as on the stony asteroid Itokawa?
The most thrilling moment of that day was when the last five-and-a-half minutes of images were streamed to Earth. Didymos, at this point, was well-resolved, and the spacecraft was closing in on Didymos’ moonlet Dimorphos for its intentional collision. At that moment, I started realizing the importance of what the DART mission was accomplishing, not only for the planetary science community, but also for humanity. NASA was on the cusp of demonstrating that a kinetic impact is a viable mitigation technique for protecting the planet from an Earth-bound asteroid or comet, if one were discovered.
After the impact occurred and was successful, perhaps strangely, I thought about the dinosaurs. They did not have the technology to protect themselves and their planet from the impactor that wiped them out, while after DART, humankind is now a step closer to achieving a planetary defense system against hazardous celestial bodies.
Q: Once the team could analyze images from before and after the impact, what were you all able to learn about the asteroid and the effects of the impact?
A: Before DART, little was known about Dimorphos and Didymos. The DART images reveal that Dimorphos’ surface is covered in rocks, with boulders as large as shipping containers near the impact site. Such a boulder-strewn surface suggests that Dimorphos is a rubble-pile asteroid similar to the asteroids Bennu, Ryugu, and Itokawa. However, Dimorphos is shaped like a football, while Ryugu and Bennu are diamond-shaped and Itokawa resembles a peanut. Compared to Dimorphos’ rocky surface, Didymos appears to have both smooth and rocky terrains. Are the smooth terrains made of finer-grained materials? Answering this question will likely have to wait for the rendezvous of the system by the European Space Agency’s Hera mission in late 2026.
The DART’s impact was observed by several telescopes. The telescopes revealed that the impact shortened Dimorphos’ orbit, remarkably, by about 33 minutes — more than 25 times the minimum benchmark for mission success. At the same time, it liberated debris which formed a tail stretching more than the 1,500 kilometers. The team observed the tail with the Hubble Space Telescope for about three weeks and found that its morphology is similar to “active asteroids” that have an asteroid-like orbit and comet-like tail. This similarity indicates that impacts can “activate” asteroids.
Q: What are the chances that we’ll need this technology in the near future? And what do you envision asteroid-defense systems might involve, given what you’ve learned from DART?
A: Neither Dimorphos nor Didymos has ever posed a hazard to Earth, and no known asteroid poses a threat to Earth for at least the next century. However, as we state in one paper, “the catalog of near-Earth asteroids is incomplete for objects whose impacts would produce regional devastation.” To find all the hazardous asteroids before they find us, in 2026 NASA will launch the NEOSurveyor mission, which is an infrared space telescope designed to discover and characterize most of the potentially hazardous asteroids and comets that come within 50 million kilometers of Earth’s orbit.
There are lots of lessons learned from DART that will be useful to design future planetary defense systems. DART showed that it is technologically possible to intercept and impact a subkilometer asteroid, with limited prior knowledge of its shape and surface properties. This means that a future planetary defense mission may not need a precursor probe to characterize the rogue asteroid before another mission is sent to impact it.