"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," said Jesus Gil Fernandez, ESA'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 March 12, Hera flew within 5700 km of Mars, using the planet's gravity to steer itself more efficiently toward its final destination: the binary asteroid system composed of Dimorphos and the larger Didymos. This gravity-assist maneuver shaved months off the journey and preserved fuel.
This close encounter also provided the first opportunity to activate Hera's full suite of instruments beyond Earth's vicinity, capturing detailed images of Mars, its small moon Deimos, and a distant view of Phobos. It also served as the debut test of Hera's autonomous surface feature tracking system, designed by teams at GMV in Spain and Romania.
During later mission phases, Hera will maneuver around the asteroids starting from about 30 km away. Initially, it will rely on the silhouette of the larger Didymos against space as a visual guide. As it closes in, the navigation system will transition to centroid tracking, focusing on the sunlit center 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," explained 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."
In the final stages of its mission, Hera will descend to within 2 km of Dimorphos. At that range, the asteroid will dominate the spacecraft's view, necessitating autonomous navigation based on visual landmarks. By comparing successive images of the same boulders and craters, Hera will estimate its position and movement relative to the asteroid.
"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," added 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 otherwise 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 system is capable of identifying up to 100 unique surface features, spread evenly across the target area. However, only the top six are used for calculating position and orientation to minimize computing demands.
"We're really delighted with the success of this technology experiment," added 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 remarked: "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 image processing is handled separately from its main flight systems, similar to how gaming laptops use discrete graphics processors. In addition, the spacecraft carries a GMV-developed Image Processing Unit powered by two custom FPGA microprocessors. Though the software that will interface this unit with Hera's main computer is still in development, it promises faster processing capabilities with future mission applications in mind.
Launched on October 7, 2024, Hera is en route to the first asteroid ever deflected by human intervention. By gathering in-depth data on Dimorphos, struck by NASA's DART mission in 2022, Hera will provide key insights into making asteroid deflection a repeatable and dependable planetary defense method.
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