Last year, Nasa successfully crashed a spacecraft into an asteroid as part of a planetary protection test mission.

Now, a high school teacher and his students have discovered that the asteroid is behaving weirdly.

Dimorphos was hit by a Nasa spacecraft called Double Asteroid Redirection Test (DART) in September 2022. The goal of the collision was to change the asteroid’s orbit around its parent asteroid, Didymos and alter its trajectory.

The mission was successful, and Dimorphos’ orbit was shortened by 33 minutes in the weeks after the impact.

However, a team of high school students led by teacher Jonathan Swift at Thacher School in California have discovered that Dimorphos’ orbit 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,’ said Mr Swift. ‘That was inconsistent at an uncomfortable level.’

This unexpected behaviour could have implications for future planetary defence missions.

The team used their school’s 0.7-metre telescope was to observe the asteroids and presented its findings at a meeting of the American Astronomical Society in New Mexico in June.

One possibility is that the asteroid is now tumbling, having previously been tidally locked to Didymos.

Another possibility is that material kicked up by the impact, including boulders up to a few meters across, stayed in orbit near Dimorphos and fell back onto the surface, slowing its orbit more.

Harrison Agrusa, a DART team member at the Côte d’Azur Observatory in France, thinks the second theory is more likely.

The DART team is continuing to observe Dimorphos and will publish their own results in the coming weeks.

An upcoming European Space Agency mission called Hera, set to arrive at Dimorphos in 2026 which could tell us for certain what happened after the impact.

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