Scientists continue to ponder the results of NASA’s stunningly successful DART test to deflect a harmless asteroid. As the latest findings suggest, the recoil caused by the blast of debris spewing from Dimorphos after the impact was significant, further increasing the spacecraft’s impact on the asteroid.
NASA’s refrigerator-sized spacecraft collided with the 163-meter Dimorphos on Sept. 26, shortening its orbit around its larger partner, Didymos, by a whopping 33 minutes. That equates to tens of meters, demonstrating the feasibility of using kinetic impactors as a means of deflecting impending asteroids.
A stunning side effect of the test was the gigantic and complex plumes emanating from the asteroid after impact. The Didymos-Dimorphos system, located 7 million miles (11 million kilometers) from Earth, even sprouted a long tail in the aftermath of the experiment. DART, short for Double Asteroid Redirection Test, had a major impact on Dimorphos, kicking up a surprising amount of debris, or “ejecta,” in the parlance of planetary scientists.
Dimorphos, as we have learned, is a rubble asteroid, as opposed to a dense, tightly packed rocky body. This undoubtedly contributed to the excessive amount of debris ejected, but scientists weren’t entirely sure how much debris the asteroid left behind as a result of the impact. Preliminary findings presented Thursday at the fall meeting of the American Geophysical Union in Chicago shed new light on these and other aspects of the DART mission.
Not only did DART create tons of ejecta, it also created a recoil effect that pushed the asteroid further in the desired direction, as Andy Rivkin, leader of the DART research team, explained at the meeting. “We got great value for money,” he told BBC News.
Indeed, if Dimorphos had been a more compact body, the same level of recoil probably wouldn’t have happened. “When you shoot material off the target, you have a recoil force,” explained DART mission scientist Andy Cheng of the Johns Hopkins University Applied Physics Lab, who also spoke at the meeting. The resulting recoil is analogous to the release of a balloon; as the air flows out, it pushes the balloon in the opposite direction. In the case of Dimorphos, the stream of ejecta served as the air coming out of the balloon, which also pushed the asteroid in the opposite direction.
Planetary scientists are starting to get a sense of how much debris has been moved. DART, traveling at 14,000 miles per hour (22,500 kph), slammed with enough force to spill more than 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 could actually be on the low side, and the actual figure could potentially be 10 times higher, Rivkin said at the meeting.
The scientists assigned DART’s momentum factor, known as “beta,” a value of 3.6, meaning that the momentum transferred to Dimorphos was 3.6 times greater than an impact event that did not create an ejection plume. “The result of that recoil force is that you put more momentum into the target and eventually get a bigger deflection,” Cheng told reporters. “If you’re trying to save the Earth, it makes a big difference.”
That’s a good point, since those values will dictate the parameters for an actual mission to deflect a legitimately dangerous asteroid. Cheng and his colleagues will now use these results to derive the beta values of other asteroids, a task that requires a better understanding of an object’s density, composition, porosity and other parameters. The scientists also hope to find out how much DART’s initial hit moved the asteroid and how much of its movement occurred as a result of the recoil.
The speakers also produced another figure: the length of the tail, or ejection plume, that formed after impact. According to Rivkin, Dimorophos sprouted a tail 30,000 km long.
“The asteroid impact was just the beginning,” Tom Statler, the program scientist for DART and a presenter at the meeting, said in the statement. “Now we’re using the observations to study what these bodies are made of and how they formed — as well as how to defend our planet if an asteroid ever heads our way.”
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