A hitchhiker’s guide to Hera’s target asteroids

Hera is en route to the Didymos–Dimorphos binary asteroid system, arriving in November after a journey through deep space. Dimorphos, roughly 151 m across, orbits the 780-m diameter Didymos at an average distance of 1.15 km, completing a circuit every 18 cm/s. Both asteroids consist of dark silicon-rich rock with surfaces comparable in brightness to the lunar highlands, and Hera will meet them at 1.5 times Earth’s distance from the Sun, where sunlight is dimmer but still intense.

The asteroids present stark contrasts: Didymos, shaped like a spinning top, rotates every 2.2 hours and features smooth dusty plains, while Dimorphos resembles a rubble pile dominated by house-sized boulders. NASA’s DART spacecraft observed these differences just before its 26 September 2022 impact on Dimorphos, which ejected material into space and reshaped its surface. Hera’s mission includes deploying two CubeSats to study the system in detail.

A hypothetical astronaut on Hera could reach Dimorphos by stepping off the spacecraft and drifting toward its surface, though Didymos’ stronger gravity would require careful navigation. Falling from 2 km away would take about two days, with a landing speed of just 10 cm/s—akin to a gentle docking. However, the low gravity risks multiple hours-long bounces, similar to the Rosetta mission’s Philae lander experience on comet 67P in 2014.

On Dimorphos, movement would resemble swimming over a reef due to negligible gravity, with loose dust potentially forming persistent clouds. Visiting Didymos would require precise sideways jumps to avoid orbiting, and its equatorial spin would create a steady downward tug except at the poles. Hera’s team has used such imaginative scenarios to refine mission operations, including the deployment of CubeSats to study the asteroid pair.