25/03/2025
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As ESA’s Hera planetary defence mission flew past planet Mars it autonomously locked onto dozens of impact craters and other prominent surface features to track them over time, in a full-scale test of the self-driving technology that the spacecraft will employ to navigate around its target asteroids.
“There was no time to test this autonomous surface feature tracking as thoroughly as the rest of Hera’s autonomous functions before we left Earth,” explains ESA’s Jesus Gil Fernandez, Hera’s guidance, navigation and control engineer.
“But during Hera’s Mars flyby we were able to operate it for 20 minutes. Despite the spacecraft being in motion several orders of magnitudes faster and further from the red planet than it will be around its eventual destination, our system immediately managed to acquire features that were entirely new to it, all across Mars. Then – with one new image acquired every 48 seconds using Hera’s Asteroid Framing Camera – it was able to continue tracking them throughout the test.
“Landmark tracking has been demonstrated before with previously charted features but tracking unmapped markings in this way is really unprecedented. This technology experiment was regarded in advance as a little bit risky – because if it locked up the flight computer in any way then Hera might have failed to acquire the rest of its Mars science data – but thankfully the system performed really well, giving us high confidence in the phase of Hera’s mission when it will use this technique to autonomously navigate around its asteroids and acquire close-up images of the crater produced by NASA’s DART spacecraft impacting the Dimorphos asteroid.”
On 12 March Hera came to within 5700 km of the red planet, using its gravity field to shift the spacecraft’s trajectory towards its final destination: Dimorphos and the larger Didymos asteroid it orbits around. This manoeuvre shortened Hera’s journey time by many months and saved a substantial amount of fuel.
This flyby also gave the Hera team their first chance to employ the spacecraft’s instruments beyond Earth and the Moon, to image the surface of Mars, the planet’s enigmatic moon Deimos and glimpse its other moon Phobos. It also offered the opportunity to perform the first test of the autonomous surface feature tracking system developed for Hera by a team from GMV in Spain and Romania.
During Hera’s proximity operations starting at around 30 km away it will navigate close to the asteroids in various ways. To start with the spacecraft will keep the larger Didymos body framed in its cameras as an overall reference point, observing the contrast between the asteroid’s edges and the deep space around it. When coming closer, Hera’s guidance, navigation and control (GNC) system shifts to ‘centroid tracking’, focused on bright Sun-illuminated areas towards the centre of the asteroid.
“We were actually able to test out this centroid tracking technique using the binary system Earth and Moon as Hera headed into deep space, the pair taking the place of Didymos and Dimorphos,” explains Andrea Pellacani, technical manager for Hera GNC at GMV. “It turned out to perform well, but that still left Hera’s experimental feature tracking GNC technique untested – until our recent Mars flyby.”
Towards the end of its six-month investigation phase, Hera will approach closer than 2 km from the smaller Dimorphos asteroid, at which point its field of view will be entirely filled by the asteroid surface. At this point autonomous surface tracking becomes essential: by imaging the same features – such as boulders and craters – in successive pictures Hera will be able to derive its own altitude and trajectory with respect to Dimorphos.
“Our team of Spain and Romania has been working on this technology for almost 15 years. The same was originally proposed for allowing soft precise lunar landing,” adds Andrea. “The system needs to have a rough shape model and the ‘rotational ephemerides’ of the target body – how much it is rotating, and in which direction – but is overwise quite robust. The problem was that before launch it had only been tested on GMV’s GNC robotic testbed platform-art in Madrid, but there had been no time to do the same on the full-scale Hera Avionics Test Bench at prime contractor OHB in Bremen.
“So we were very grateful for the chance to try it out for real as Hera flew past Mars. We were confident it would work well, because we simulated Mars in detail using ESA’s Planetary and Asteroid Natural scene Generation Utility, PANGU, and ran it through our GNC testbed. The actual flyby results broadly matched our simulation, but the system impressed us with its robustness: we didn’t lose track of any targets across the planet during the activation.”
The autonomous surface tracking system acquires up to 100 features, distributed equally across four quarters of the target surface, but employs only the top six for calculating its relative position and direction, so as limit its computational load.
“We’re really delighted with the success of this technology experiment,” adds Jesus. “This isn’t something that any space agency has done before. NASA’s OSIRIS-Rex asteroid mission performed autonomous optical navigation in support of acquiring surface samples, but only after performing detailed mapping in advance which allowed known landmark matching. Our system doesn’t need any previous surface knowledge to start navigating, giving it a lot of potential.”
Hera mission manager Ian Carnelli notes: “This technology can be reliably used for close proximity autonomous operations, lunar and planetary landings, paving the way for a variety of ambitious space missions.”
Hera’s performs vision-based processing of surface features using a separated core of the spacecraft’s main flight computer, akin to the way a gaming laptop might include a dedicated graphics card. But a GMV-built Image Processing Unit has also been placed on Hera, which runs on two customised Field Programmable Gate Array microprocessors. The specific software to operate this IPU from Hera’s main computer is still being finalised, ahead of the mission’s arrival at Didymos at the end of 2026, but it will be able to perform the same vision-based processing in a fraction of a time, holding plenty of follow-on mission potential.
About Hera
Launched on 7 October 2024, Hera is on its way to visit the first asteroid to have had its orbit altered by human action.
By gathering close-up data about the Dimorphos asteroid, which was impacted by NASA’s DART spacecraft in 2022, Hera will help turn asteroid deflection into a well understood and potentially repeatable technique.