Hera asteroid mission spies Mars’s Deimos moon

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.

Hera’s 12 March flyby of Mars was an integral part of its cruise phase through deep space, carefully designed by ESA’s Flight Dynamics team. By coming as close as 5000 km away from Mars, the planet’s gravity shifted 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.

Moving at 9 km/s relative to Mars, Hera was able to image Deimos from as close as 1000 km away, surveying the less-seen opposite side of the tidally locked moon from the red planet. Measuring 12.4 km across, dust-covered Deimos might actually be a leftover of a giant impact on Mars or else a captured asteroid.

“Our Mission Analysis and Flight Dynamics team at ESOC in Germany did a great job of planning the gravity assist,” comments ESA’s Hera Spacecraft Operations Manager Caglayan Guerbuez. “Especially as they were asked to fine-tune the manoeuvre to take Hera close to Deimos –which created quite some extra work for them!”

Three Hera instruments were used during the flyby:

- Hera’s black and white 1020x1020 pixel Asteroid Framing Camera, used for both navigation and scientific investigation, acquires images in visible light.

- Hera’s Hyperscout H hyperspectral imager observes in a range of colours beyond the limits of the human eye, in 25 visible and near-infrared spectral bands, to help characterise mineral makeup.

- Hera’s Thermal Infrared Imager, supplied by the Japan Aerospace Exploration Agency (JAXA), images at mid-infrared wavelengths to chart surface temperature, in the process revealing physical properties such as roughness, particle size distribution and porosity.

ESA’s Hera mission scientist Michael Kueppers explains: “These instruments have been tried out before, during Hera’s departure from Earth, but this is the first time that we have employed them on a small distant moon for which we still lack knowledge – demonstrating their excellent performance in the process!”

Hera Principal Investigator Patrick Michel, Director of Research at CNRS / Observatoire de la Côte d'Azur, adds: “Other Hera instruments we will utilise once we reach the Deimos and Dimorphos asteroids were not activated, either because they are not usable at such long range and rapid speed from a target – such as our PALT laser altimeter, possessing a maximum range of 20 km – or because they are hosted aboard Hera’s pair of CubeSats which will only be deployed at the asteroids.”

Hera also performed some joint observations of Deimos with ESA’s own Mars Express, which has been in orbit around the red planet for more than two decades.

Results from the Deimos close encounter should help guide operational planning for next year’s Martian Moons eXploration Mission, MMX, being led by JAXA in collaboration with NASA, the French space agency CNES, the German Aerospace Center (DLR), and ESA. MMX will not only collect detailed measurements of both martian moons but also land on Phobos to collect a sample and return it to Earth for analysis.

With Didymos being 780 m across and Dimorphos just 151 m across, Hera’s twin destinations are many times smaller than the city-sized Deimos moon, but Hera is now headed on course towards them. A follow-up manoeuvre next February, followed by a series of ‘impulsive rendezvous’ thruster firings starting in October 2026 will fine-tune its heading to reach the Didymos system that December.

ESA Hera mission manager Ian Carnelli comments: “This has been the Hera team’s first exciting experience of exploration, but not our last. In 21 months the spacecraft will reach our target asteroids, and start our crash site investigation of the only object in our Solar System to have had its orbit measurably altered by human action.”