Didymos and Dimorphos

A binary near-Earth asteroid

Didymos is an asteroid 780 metres across that spins once every 2.26 hours, and Dimorphos, a moonlet of 151 metres, circles it 1.19 km away. Neither has ever threatened Earth, which made the pair the ideal place to test whether an asteroid could be pushed off its course.

On 26 September 2022, NASA's DART spacecraft struck Dimorphos at 6.1 km/s. Its orbit, measured from Earth afterwards, had shortened by 33 minutes, against the 73 seconds that would have counted as a success: the first time humanity deliberately changed the motion of a celestial body. The impact site is marked here, and ESA's Hera is on its way to survey it.