A hitchhiker’s guide to Hera’s target asteroids

To start with, Hera is actually headed for two asteroids rather than one. Dimorphos – at 151 m across about the size of the Great Pyramid of Giza – is actually orbiting around the mountain-sized 780-m diameter Didymos asteroid. Dimorphos orbits an average 1.15 km from its parent body, at a rate of just 18 cm/s.

Both asteroids are made from relatively dark silicon-rich rock. Their rough surfaces possess comparable brightness to the lunar highlands, the brighter regions of Earth’s Moon.

Hera is rendezvousing with the asteroid pair at nearly 1.5 times the distance of Earth from the Sun, so the intensity of sunlight will be lessened compared to what our astronaut would be used to – but still extremely bright (because this is around the same distance as Mars orbit from the Sun, lighting conditions are similar to what they would experiece at the Red Planet). The pair would loom like iron grey hills bathed in sunlight.

The two asteroids are extremely different, as observed by NASA’s DART spacecraft just before it impacted the smaller of the two on 26 September 2022.

The spinning-top shaped Didymos is distinguished by relatively smooth dusty plains, and rotates every 2.2 hours – not so rapid to make anyone on it dizzy but fast enough that it is close to coming apart, suggesting that Dimorphos might well have been created out of material previously sloughed off from this larger body.

It might even be possible that our hitchhiking astronaut could witness loose dust rise free of Didymos due to this fast spin – Something similar has been observed on the Bennu asteroid by NASA’s OSIRIS-REx spacecraft (although the exact reason why remains uncertain).

Rocky terrain

Strictly speaking we know little of what the hitchhiker would see on the smaller Dimorphos, since it has likely been extensively reshaped by the DART impact, which ejected a plume of material into space. But Dimorphos as DART saw it resembled a classic ‘rubble pile’ asteroid, with its surface dominated by surprisingly large boulders, many between the size of cars to houses.

Sooner or later our astronaut is going to want to venture down to Dimorphos. The easiest way to do that, even from many kilometres away, would be to gently step off the top of Hera and let the gravity of the two asteroids do their work.

The challenge is that the gravitational pull of the larger Didymos body is much stronger than Dimorphos’s: the former has gravity about 40 000 times weaker than Earth’s, while Dimorphos has about 200 000 times weaker gravity than Earth’s. So all other things being equal, our astronaut would have to use retro-rockets to steer themselves specifically towards the smaller of the two.

Two-day fall to the surface

Assuming they manage to fine-tune their direction, it would take around two terrestrial days to fall to the surface of Dimorphos from a distance of 2 km away. Don’t worry though; they’d hit the surface at a vanishingly low speed of 10 cm/s – more like a gentle docking than an actual fall.

The astronaut would need to land as limply as possible, because if they bounce too strongly they might inadvertently achieve escape velocity and be shot all the way up again back into space.

Even if not, it is highly likely our astronaut would experience multiple hours-long bounces before finally coming to rest – resembling the experience of the Rosetta mission’s Philae lander as it touched down on comet 67P/Churyumov-Gerasimenko in 2014.

By now we are well into speculation: one of the main reasons Hera is going to Dimorphos is to find out how much the DART impact transformed it. But assuming it still resembles the asteroid as it was, the hitchhiker would probably find themselves atop one of the large boulders dominating the surface.

Not walking but gliding above the asteroid

‘Walking’ in such low gravity is next to impossible. It might be more practical to push your way along just off the surface, more like a Scuba diver over a coral reef. Besides, it remains uncertain how cohesive these boulders are. It might be that any force applied might cause them to crumble apart – and any dust loosened through surface interaction might surround the vicinity in a cloud that would persist for hours.

In theory there could be a layer of dust beneath the plentiful boulders littered across Dimorphos, although direct evidence has yet to be spotted – either because DART’s camera lacked the imaging resolution to do so, or it simply isn’t there.

If present, such dust would likely be highly abrasive – because it has never undergone weathering within a planetary atmosphere – and prone to electrostatic cling.

Jumping from Didymos to Dimorphos

If our hitchhiker grows tired of the lumpy, potentially dusty surface of Dimorphos, then they could always try and visit Didymos instead. All they’d need to do is take a jump, but they would have to be oriented just right in the sideways as well as upwards direction, to avoid going into orbit instead.

Assuming success, it would take up to half an hour to reach Didymos, but the hitchhiker would hardly feel more stable there.

While its relatively stronger gravity might attempt to hold them in place, the asteroid’s equatorial spin would cancel out most of it, producing a slight but steady downward-feeling tug towards the equator. The only places where this spin effect would be cancelled out would be at the asteroid’s north and south poles.

While on Didymos, the hitchhiker might want to hunt out debris cast across its surface from when Dimorphos impact plumes fell onto it, helping to recreating the details of the DART impact. Otherwise it might be a good time to dematerialise back to Earth again!

In real life, no human visitors to Dimorphos and Didymos look likely in the conceivable future, but Hera team members have made such imaginative flights of fancy many times, to help optimise spacecraft instruments and fine-tune the orbits and eventual landings of the two CubeSats that Hera will deploy after arrival.

Meet the Hera Space Companion

Do you have questions about Hera’s journey? Ask the Hera Space Companion: not just an AI chatbot but a dynamic mission-linked communication platform, designed to make the European Space Agency’s first planetary defence mission accessible to everyone!