Hera approaches Dimorphos to investigate effects of DART impact

Dimorphos is the smaller member of an asteroid pair: according to ESA, the celestial body is about 151 metres across and orbits the 780-metre-wide Didymos. Hera is on its way to this pair and is due to arrive there in November. [1]

NASA’s DART struck Dimorphos on 26 September 2022. Just before the collision, the probe saw a surface covered in large boulders, many ranging in size from cars to houses. But those images are not a reliable description of its current state. The impact hurled material into space and, according to ESA, Dimorphos has probably been substantially deformed. Establishing how much the celestial body has changed is precisely what Hera is meant to do. [1]

Hardly any gravity

Conditions at the asteroid pair differ greatly from those on Earth. ESA estimates gravity on Didymos at about one forty-thousandth of Earth’s gravity. On Dimorphos, it is only about one two-hundred-thousandth.

To help readers imagine this weak gravitational pull, ESA describes a hypothetical astronaut descending towards Dimorphos. From a distance of two kilometres, the fall would take about two Earth days, provided the astronaut adjusts their course correctly. Their arrival speed would be only about ten centimetres per second. Even so, bouncing back too strongly could be enough to send them back into space. This is an illustrative scenario, not an announced crewed mission. [1]

Questions about the surface also remain unanswered. How firmly the boulders hold together is uncertain. There is no direct evidence yet of a possible layer of dust beneath them: DART’s camera may have captured too little detail to show it, but the layer may also be absent.