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An asteroid that Nasa crashed a special spacecraft into to knock it off course, is now behaving in a weird way.

The spacecraft named Dart successfully smashed into the Dimorphos asteroid in September last year - it was part of an experiment to change the space rock's direction and test Earth's defences against asteroids in the future.

However, a teacher and his class studying the rock have now discovered that since the collision, it has moved in a strange and unexpected way.

Dimorphos is in orbit around a much larger asteroid called Didymos.

By crashing into the smaller asteroid, the Dart mission successfully altered Dimorphos' orbit by "tens of metres". Dart, which stands for Double Asteroid Redirection Test, used a spacecraft around the size of a fridge.

By successfully crashing directly into Dimorphos, Nasa was testing if it could use similar methods to knock an asteroid off course, if one is in danger of hitting the Earth.

However, using their school telescope, a team of children and their teacher Jonathan Swift at Thacher School in California have found that more than a month after the collision, Dimorphos' orbit continuously slowed after impact... which is unusual and unexpected.

As reported in the New Scientist, the team presented their findings at a meeting of the American Astronomical Society.

After discovering the unusual behaviour of Dimorphos, it's likely that Nasa will have to factor in the high school's findings, if they ever launch another asteroid redirection mission in the future.

Following the collision last year, a huge trail of debris was seen floating off from the asteroid after impact.

One explanation for the asteroid's orbit continuing to change so long after the Dart collision is that material thrown up by the impact, including rocks several metres across, eventually fell back onto the surface of the asteroid, changing its orbit even more.

The European Space Agency is launching a mission called Hera, which will arrive at Dimorphos in 2026 and could reveal more details as to what happened to the asteroid following the impact.