The target of ESA's upcoming Hera mission is asteroid Didymos, an Apollo-type rocky near-Earth object.
Didymos is a binary asteroid; the primary body has a diameter of around 780 m and a rotation period of 2.26 hours, whereas the Dimorphos secondary body – or moon – has a diameter of around 160 m and rotates around the primary at a distance of around 1.2 km from the primary surface in around 12 hours.
Dimorphos will be the subject of a kinetic impact by the NASA DART spacecraft in October 2022. Its orbital velocity of about 17 cm/s is expected to be shifted by around half a millimetre per second, changing its rotation period around the primary body by about 200 seconds – a fraction of one per cent, but enough to be measured roughly with Earth-based telescopes.