Asteroid Struck by NASA Spacecraft Might Have Healed Itself

The asteroid that NASA flung a spacecraft into was deformed by the collision, but it appears to have reshaped itself and healed.

The asteroid, named Dimorphos, was slammed into by NASA's Double Asteroid Redirection Test (DART) in September 2022, in an attempt to test how much a collision would deflect space rocks.

The collision with the asteroid knocked a huge chunk of its mass into space, but much of this was redistributed around the asteroid, reshaping and resurfacing the space rock, according to a paper in the journal Nature Astronomy.

The DART mission's purpose was to see if an asteroid heading toward Earth could be deflected by kinetic means. That is, by slamming another body of mass into it and altering its path.

Dimorphos is part of a binary asteroid system. Dimorphos, which is 525 feet in diameter, orbits a larger, 2,560-foot space rock known as Didymos.

The collision of the spacecraft—which weighed about 1,300 pounds—had a noticeable impact on the asteroid and its movements.

"The simulation results based on observations of the asteroid Dimorphos have shown that the asteroid now orbits around its larger companion [Didymos] 33 minutes slower than before. Its orbit has gone from 11 hours, 55 minutes to 11 hours, 22 minutes," Ian Whittaker, a senior lecturer in physics at Nottingham Trent University, wrote about the asteroid in The Conversation.

The new paper reveals that rather than just creating a large crater on Dimorphos' surface, the collision has reshaped the asteroid entirely in a process called global deformation.

"Observations from the first few hours after impact imply that more than 1.3–2.2 × 107 kg of ejecta were released from the DART impact, equivalent to 0.3–0.5% of Dimorphos's mass," the authors write in the paper.

"Moreover, up to 8% of Dimorphos's mass may have been displaced or ejected at below the escape velocity of Dimorphos," they continue. "In all impact scenarios simulated here, the DART impact does not produce a conventional impact crater but instead causes global deformation of the target."

The asteroid appears to have a loose, rubble-pile structure, weakly held together by gravity, meaning that it is easily reshaped and re-formed after collisions like the DART mission.

The authors write: "Since the DART spacecraft probably caused global deformation of Dimorphos, we can infer that similarly formed asteroid moons are easily reshaped and their surfaces are relatively young. Overall, the findings of this study provide valuable information for understanding the formation and characteristics of binary asteroids and will inform future exploration and asteroid deflection efforts."

Additionally, the findings regarding the asteroid's changed orbit, as well as its reshaping, indicate that asteroids like Dimorphos could be deflected using collisions.

Not only did the lost mass ejected from Dimorphos' surface slow the asteroid down due to recoil but the collision also altered the asteroid's momentum. Additionally, the asteroid's change in shape may have modified its gravitational field, further changing its path and speed orbiting around its companion asteroid.

"A global deformation of Dimorphos would have modified the gravitational field between Didymos and Dimorphos with important implications for its orbit," the authors write. "The shape change would cause an additional perturbation to Dimorphos's orbit, on top of those caused by the spacecraft momentum and ejecta recoil, and this effect can account for a few seconds to several minutes of the observed orbit period change (~33 min), depending on the magnitude of the deformation."

This implies that smaller, rubble-pile asteroids like Dimorphos can be efficiently deflected using techniques like DART. However, not every asteroid heading toward the Earth will be similar in composition to Dimorphos.

Additionally, if the asteroid is noticed only when it's close to Earth, that might not be enough time to deflect it entirely away from our planet.

"The big variable here is how much warning we will have, because a change in orbit of 30 minutes—as was observed when DART struck Dimorphos—will make little difference if the asteroid is already very close to Earth," Whittaker wrote.

"However, if we can predict the object path from further out—preferably outside the Solar System—and make small changes, this could be enough to divert the path of an asteroid away from our planet," he said.

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