A strong recoil effect magnified NASA’s asteroid deflection experiment

Composite image of the Didymos-Dimorphos system taken on November 30 showing its new ejecta tail.

Composite image of the Didymos-Dimorphos system taken on November 30 showing its new ejecta tail.

Scientists continue to ponder the results of NASA’s stunningly successful DART test to deflect a harmless asteroid. As the latest findings show, the recoil caused by Dimorphos’ post-impact debris explosion was significant, further increasing the spacecraft’s impact on the asteroid.

NASA’s refrigerator-sized spacecraft crashed into the 535-foot-long (163-meter) Dimorphos on September 26, shortening its orbit around its larger partner Didymos by a whopping 33 minutes. That equates to several tens of feet, demonstrating the feasibility of using kinetic impactors as a means of deflecting threatening asteroids.

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A stunning side effect of the test was the gigantic and complex plumes that emanated after the asteroid’s impact. The Didymos-Dimorphos system, located 7 million miles (11 million kilometers) from Earth, actually grew a long tail in the experiment’s wake. DART, short for Double Asteroid Redirection Test, had a profound impact on Dimorphos, churning up a surprising amount of debris, or “ejecta” in planetary scientist parlance.

Dimorphos, we’ve learned, is a debris-heap asteroid, as opposed to a dense, tightly-packed body of rock. This undoubtedly contributed to the excessive amount of debris ejected, but scientists weren’t exactly sure how much debris the asteroid shed as a result of the impact. Preliminary results presented Thursday at the American Geophysical Union fall meeting in Chicago shed new light on this and other aspects of the DART mission.

DART not only flung tons of ejected material, but also created a recoil effect that served to push the asteroid farther in the desired direction, Andy Rivkin, leader of the DART investigation team, explained at the meeting. “We got a lot for the money,” he told BBC News.

Had Dimorphos been a more compact body, the same recoil would likely not have occurred. “When you shoot material off the target, you have a recoil force,” explained DART mission scientist Andy Cheng of Johns Hopkins University’s Applied Physics Lab, who also spoke at the meeting. The resulting recoil is analogous to releasing a balloon; As the air rushes out, it pushes the balloon in the opposite direction. In the case of Dimorphos, the ejection stream served as the air exiting the balloon, which also pushed the asteroid in the opposite direction.

Planetary scientists are beginning to get a sense of just how much debris has been displaced. DART, traveling at 14,000 miles per hour (22,500 km/h), hit with enough force to dump over 2 million pounds of material into the void. That’s enough to fill about six or seven train cars, NASA said in a statement. That estimate may actually be on the low side, and the true figure could potentially be 10 times higher, Rivkin said at the meeting.

Scientists assigned DART’s momentum factor, known as “beta,” a value of 3.6, meaning the momentum imparted to Dimorphos was 3.6 times greater than in an impact event that did not produce an ejection plume. “The result of that recoil force is that you put more momentum into the target and end up with greater deflection,” Cheng told reporters. “If you’re trying to save the earth, it makes a big difference.”

That’s a good point, as these values ​​dictate the parameters for an actual mission to deflect a legitimately dangerous asteroid. Cheng and his colleagues will now use these results to infer the beta values ​​of other asteroids, a task that requires a deeper understanding of an object’s density, composition, porosity and other parameters. Scientists also hope to find out how much the DART’s first hit moved the asteroid, and how much of its movement was due to recoil.

The speakers also produced another number – the length of the tail, or ejection plume, that formed after impact. According to Rivkin, Dimorophos grew a tail 30,000 km (18,600 miles) long.

“The asteroid impact was just the beginning,” said Tom Statler, program scientist for DART and moderator at the meeting, in the statement. “Now we’re using the observations to study what these bodies are made of and how they formed — and how we can defend our planet should an asteroid ever come our way.”

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