Hera asteroid mission’s side-trip to Mars

ESA’s Hera asteroid mission for planetary defense will make a swingby of Mars next March, borrowing speed to help reach its target Didymos binary asteroid system.

In the process the spacecraft will venture as near as 6,000 km from the surface of the Red Planet, closer than the orbits of the two martian moons. Its trajectory will be tweaked so that it can train its science instruments onto Mars’s smaller moon Deimos from within 1,000 km away, while also observing Mars itself.

Details of the swingby are being presented at this week’s Hera Science Community Workshop at ESA’s ESTEC technical center in the Netherlands.

“This swingby is part of the scheduled maneuvers to get Hera to Didymos by the end of its two-year cruise phase,” explains Michael Kueppers, ESA’s Hera project scientist.

“By swinging through the gravitational field of Mars in its direction of movement the spacecraft gains added velocity for its onward journey. This close encounter is not part of Hera’s core mission, but we will have several of our science instruments activated anyway. It gives us another chance to calibrate our instruments and potentially to make some scientific discoveries.”

Flight Dynamics engineer Pablo Muñoz, part of the Mission Analysis team at ESA’s European Space Operations Center in Germany, said, “It´s truly fortunate that Mars happens to be at the right location and at the right time to give Hera a hand. This enabled us to design a trajectory that uses the gravity of Mars to push Hera towards its rendezvous with Didymos, resulting in great fuel savings for the mission. Part of the excess propellant can then be spent in advancing the arrival at the binary asteroid by a few months, thus maximizing the mission’s planetary defense and science return.”

Hera is due for launch in October this year, headed for the mountain-sized Didymos asteroid and the Great-Pyramid-sized Dimorphos moonlet that orbits around it. On 26 September 2022 NASA’s van-sized DART spacecraft impacted the Dimorphos asteroid at around 6.1 km/s. This first test of the ‘kinetic impact’ method of planetary defense succeeded in modifying the orbit of the target asteroid around its larger parent.

Next Hera will perform a close-up survey of Dimorphos, to gather crucial missing information on the asteroid’s mass, makeup and structure that can turn DART’s grand-scale experiment into a well-understood and potentially repeatable planetary defense technique.

“Hera’s instruments have been designed to observe Dimorphos of course, but the potential is there to turn up interesting insights about the distinctively asteroid-like Deimos as well,” notes Patrick Michel Director of Research at CNRS at Observatoire de la Côte d’Azur in Nice and Hera’s Principal Investigator.

Orbiting 23,460 km from Mars, Deimos—its name deriving from the Greek for “Fear”—is the further and smaller of the two martian moons. The lumpy body has a diameter of 12.4 km across and has a dark surface reminiscent of C-type asteroids. One theory is that both Deimos and its fellow martian moon Phobos are in fact captured asteroids from the main Asteroid belt. Their surface characteristics have features in common with the planet below them however, conversely suggesting an impact-based origin.

“Deimos has not been observed before with Hera’s combination of science instruments, so hope to make some discoveries,” adds Patrick Michel. “We will also be observing in synergy with the Emirates Mars Mission ‘Hope Probe,’ which launched in July 2020 and entered orbit around Mars in February 2021. Co-observations with ESA’s own Mars Express and ExoMars Trace Gas Orbiter missions are also under consideration.

“Additionally, the imagery and data we gather will help with planning the Japanese-led Martian Moons eXploration mission, MMX, which is due to launch in 2026. MMX will survey both moons while also landing a small French-German rover on Phobos and acquiring samples to return to Earth.”

Hera will employ three of its instruments during its swingby of Mars and Deimos. Its main Asteroid Framing Camera will gather visual imagery while its HyperScout-H instrument will observe in a range of colors beyond the limits of the human eye, gathering mineralogical data in a total of 25 visible and near-infrared spectral bands.

Finally its Thermal Infrared Imager is a heat mapper, able to make out features through local night-time and measure how surface temperatures change over time to help constrain surface properties.