NASA’s asteroid deflection test had unexpected and puzzling outcome

After NASA’s DART mission successfully collided a spacecraft with an asteroid, the orbit of Dimorphos began to change over the following month in ways that have baffled astronomers.

In September 2022, the Double Asteroid Redirection Test (DART) impacted the near 600-kilogram satellite Dimorphos, which revolves around a larger asteroid known as Didymos.

Initially, Dimorphos completed an orbit every 11 hours and 55 minutes. Observations after the collision indicated that the impact had shortened that orbital period by approximately 30 minutes. However, in the weeks and months following, researchers noted a further contraction of around an additional 30 seconds in its orbital period.

Some astronomers speculated that debris from the impact might have been gradually ejected from Dimorphos, causing a loss of energy and thus a reduction in the orbital size. But new calculations by researchers including Harrison Agrusa and Camille Chatenet from Côte d’Azur University, France, suggest this is not feasible, given the mass and structure of Dimorphos.

Agrusa explained, “A boulder may fly past Dimorphos, but eventually, that momentum will return to Dimorphos.” This indicates that any ejected materials would likely end up coming back and exerting their effects on the asteroid’s motion.

The researchers propose instead that the complex spin dynamics induced by the impact might be at play. This spinning motion could cause surface materials, such as rocks and boulders, to shift, leading to friction and heat production from their collisions. Agrusa notes that this energy loss in the form of heat might sufficiently account for the gradual change in orbital dynamics over the month following the impact, even if not directly modeled in their study.

“Disturbing materials on Dimorphos’ surface could adjust its gravitational potential energy,” Agrusa said. “This might explain the gradual decrease in orbital period as a result of a long-lived process.”

Although these findings emphasize the unpredictability of how asteroid orbits can shift post-impact, Agrusa asserts this knowledge is mostly irrelevant concerning potential asteroid deflection efforts aimed at protecting Earth. This is because binary systems like Didymos and Dimorphos are relatively uncommon, and such effects would be less likely for solitary asteroids whose paths are of concern for potential Earth impacts.

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