When NASA crashed its DART spacecraft into the asteroid Dimorphos, the goal was to shorten the space rock's orbit around its parent asteroid. The mission succeeded - but Dimorphos' orbit has continued to shrink and it isn't clear why.
A high school teacher and his students have discovered that an asteroid hit by a NASA spacecraft, in a test run for saving Earth from a collision, is behaving unexpectedly. The find could have implications for future planetary defence missions.
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| Dimorphos as seen by the Hubble Space Telescope after colliding with NASA’s DART spacecraft NASA/ESA/STScI/Hubble |
On 27 September 2022, NASA intentionally crashed its Double Asteroid Redirection Test (DART) spacecraft into the asteroid Dimorphos. The goal was to slow the space rock’s nearly 12-hour orbit around a parent asteroid, called Didymos, and see if the momentum of the spacecraft could alter the asteroid’s trajectory. The mission was successful, with Dimorphos’ orbit shortening by 33 minutes in the weeks after the impact.
Video Courtesy: NewScientist
Teacher Jonathan Swift at Thacher School in California and his students used the observatory there to follow-up the findings. Although relatively modest, the school’s 0.7-metre telescope was big enough to observe the asteroids, and they found the orbit of Dimorphos continued to shrink by another minute more than a month after the collision.
“The number we got was slightly larger, a change of 34 minutes,” says Swift. “That was inconsistent at an uncomfortable level.”
His team presented their findings at a meeting of the American Astronomical Society in New Mexico in June, receiving positive responses. “We tried our best to find the crack in what we had done, but we couldn’t find anything,” says Swift.
The observations suggest something has caused the orbit of Dimorphos to continuously slow since the impact. One possibility is that the asteroid is now tumbling, having previously been tidally locked to Didymos, affecting its orbit. “That could cause tidal forces to change the orbital period,” says Swift.
Harrison Agrusa, a DART team member at the Côte d’Azur Observatory in France, says there is some evidence for tumbling. “After the impact, it’s significantly librating,” he says, meaning that Dimorphos is wobbling in its position relative to Didymos, like the moon does with Earth. “Then it’s possible, depending on inertia, this would evolve into more chaotic tumbling where it can flip around,” he says.
However, Agrusa says this tumbling is unlikely to cause a shortening of Dimorphos’ orbit, as the orbit would instead vary randomly. More likely is that material kicked up by the impact, including boulders up to a few metres across, stayed in orbit near Dimorphos and fell back onto the surface, slowing its orbit more. “I think that’s the most likely explanation,” he says.
Nancy Chabot, the coordination lead on DART at Johns Hopkins Applied Physics Laboratory in Maryland, says the DART team has continued to observe Dimorphos and will publish its own results in the coming weeks.
Those observations also show the orbital period continued to decrease, albeit by a lesser amount, some 15 seconds, before plateauing, perhaps supporting the idea of some ejected material continuing to hit the surface before dispersing. “We don’t see any change after roughly the first month,” she says, referring to observations up to the end of February when the asteroid was no longer in view from Earth.
An upcoming European Space Agency mission called Hera, set to arrive at Dimorphos in 2026, could tell us for certain what happened after the impact. There may be implications for future attempts to redirect an asteroid, as it is important to know exactly how impacted space rocks behave.
“It’s really good to know what we did to Dimorphos,” says Chabot. “Those specific details are really key to applying this technique in future if you needed to.”
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