A hitchhiker’s guide to Hera’s target asteroids

ESA's Hera spacecraft is currently speeding through deep space at 12 km/s relative to Earth and is due to reach the Dimorphos asteroid this November. The article uses the thought experiment of an astronaut teleported onto the small-car-sized Hera at arrival to describe what they would see and what it might feel like to walk on the asteroid surface.

Hera is headed for two asteroids rather than one. Dimorphos is 151 m across—about the size of the Great Pyramid of Giza—and orbits the mountain-sized, 780-m-diameter Didymos at an average distance of 1.15 km, with a rate of just 18 cm/s. Both bodies are made of relatively dark, silicon-rich rock, and their rough surfaces have brightness comparable to the lunar highlands, the brighter regions of Earth's Moon. Hera will rendezvous at nearly 1.5 times the Earth-Sun distance, so sunlight will be less intense than on Earth but still extremely bright; because this is about the same distance as Mars's orbit from the Sun, lighting conditions would be similar to those at the Red Planet. The pair would loom like iron-grey hills bathed in sunlight.

NASA's DART spacecraft observed the two asteroids just before it impacted the smaller one on 26 September 2022. The spinning-top-shaped Didymos has relatively smooth dusty plains and rotates every 2.2 hours—not fast enough to make anyone dizzy, but fast enough that it is close to coming apart, suggesting Dimorphos might have been created from material previously sloughed off the larger body. The astronaut might even witness loose dust rise free of Didymos because of this fast spin; something similar has been observed on the Bennu asteroid by NASA's OSIRIS-REx spacecraft, although the exact reason remains uncertain.

Little is known about what the hitchhiker would see on Dimorphos specifically, since it has likely been extensively reshaped by the DART impact, which ejected a plume of material into space. But as DART saw it, Dimorphos resembled a classic rubble-pile asteroid, with a surface dominated by surprisingly large boulders, many between the size of cars and houses. To venture down to Dimorphos, the easiest route—even from many kilometres away—would be to gently step off the top of Hera and let the gravity of the two asteroids do the work. The challenge is that Didymos's gravitational pull is much stronger than Dimorphos's: the excerpt notes that Didymos's gravity is about 40,000 times weaker than...