DART impact illus.
DART collided with Dimorphos, a moon orbiting the asteroid Didymos, as planned Sept. 26. Credit: NASA/Johns Hopkins APL

Updated after post-impact press conference.

LAUREL, Md. — A NASA spacecraft collided with a moon orbiting a near Earth asteroid Sept. 26 in a demonstration of a technology that could one day be used to protect the Earth from a hazardous object.

NASA’s Double Asteroid Redirection Test (DART) spacecraft hit Dimorphos, an asteroid about 160 meters across orbiting the larger asteroid Didymos, at 7:14 p.m. Eastern. Confirmation of the impact, at a speed of 6.5 kilometers per second, came from a loss of signal from DART at mission control at the Johns Hopkins University Applied Physics Lab (APL) here.

The final approach of DART to Dimorphos appeared to go as planned, with no issues reported by controllers in the last hour. The spacecraft was able to autonomously lock onto Dimorphos and target the asteroid, 160 meters across, with a final reported miss distance of only 17 meters from the center of the asteroid.

“Forty minutes out, you were really getting a good feeling,” Ed Reynolds, DART program manager at APL, said at a post-impact briefing. In the final two minutes, when the spacecraft’s trajectory could no longer be changed and it showed it to be on course for impact, “it was just joy. You got to enjoy the moment.”

A closeup of Dimorphos taken by DART minutes before impact Sept. 26. Credit: NASA/JHUAPL

“We saw that we were going to impact. This asteroid was coming into the field of view for the first time. We really had no idea what to expect,” said Elena Adams, mission systems engineer for DART at APL, said. “All of us were kind of holding our breaths.”

DART launched last November as NASA’s first dedicated planetary defense mission. The purpose of DART is to test the “kinetic impactor” approach to changing the trajectory of asteroids if one was found to be on a collision course with the Earth.

The impact kicks off a campaign of observations to determine the change in the orbit of Dimorphos around Didymos. Nancy Chabot, coordination lead for DART at APL, said more than three dozen observatories around the world plan to observe the asteroids using optical and radar instruments to measure the change. “We want to maximize what we’re able to learn from this first planetary defense mission, so we’ve kept the international, worldwide observation campaign pretty open,” she said.

Cristina Thomas, observation working group lead for the mission at Northern Arizona University, said those observations will continue for about six months, until Didymos is no longer visible from Earth, to get an extremely precise measurement of the orbit change. An initial measurement of that change should be available in a couple weeks.

Telescopes on Earth and in space also observed the impact itself, with some initial reports of a plume of ejecta from the impact visible on ground-based telescopes. Images from LICIACube, an Italian cubesat deployed from DART and which flew by Dimorphos a few minutes after the impact, should be returned over the next several days that may show additional views of the impact.

The size of the orbit change will tell how efficient the impact was in changing the orbit, which will be useful for planning for any future missions. Models of the impact show a wide range of potential outcomes based on the composition, structure and shape of Dimorphos.

“It’s really dependent on what Dimorphos is made of,” said Angela Stickle of APL, who led modeling efforts ahead of the DART impact. “It’s the reason we’re doing the test, because we don’t know a lot about the asteroid.”

While there are wide variations in the size and composition of asteroids, the data from the DART impact can help refine models. “It’s extremely helpful,” said Mallory DeCoster of APL, who also worked on modeling. “Your models need validation with experimental data, even if you have just one data point to extrapolate to.”

Carolyn Ernst, instrument scientist for DART’s camera, said Dimorphos looked like a “rubble pile” asteroid that is an agglomeration of smaller rocks rather than a single rock. “It really looks amazing,” she said. “It looks, in a lot of ways, like a lot of other small asteroids that we’ve seen.”

While it was popular to state that NASA was “smashing” an asteroid, Dimorphos is likely intact other than a crater a few tens of meters across. DART itself is destroyed, although Stickle said the impact speed was not high enough to vaporize the spacecraft. Some debris may be left behind on the surface, she said, although it’s not clear it would be recognizable.

When it comes to the collision between DART and Dimorphos, said Chabot, “the spacecraft is going to lose.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...