Hera asteroid mission tracks Mars’s Deimos moon in a fly-past

En route to Dimorphos asteroid

The European Space Agency’s Hera mission has made an extraordinary leap in its planetary defence efforts, marking its first scientific use of advanced instruments beyond Earth and the Moon.

On its way to explore an asteroid whose orbit was altered by human intervention, the spacecraft captured stunning images of Mars and its moon, Deimos, during a flyby on March 12, 2025. This flyby is crucial in the mission’s journey and will provide key data for future asteroid deflection techniques.

Hera, launched on October 7, 2024, is currently en route to visit Dimorphos, the first asteroid ever impacted by a human spacecraft when NASA’s Dart spacecraft struck it in 2022. By examining the aftermath of this collision, Hera will provide invaluable information to help refine asteroid deflection as a potential planetary defence strategy.

The flyby of Mars on March 12 was a well-planned manoeuvre during the mission’s cruise phase through deep space. The spacecraft came as close as 5,000km from the Red Planet, using Mars’s gravity to alter its trajectory toward the Didymos system, including Dimorphos and its larger companion, Didymos. This gravity assist saved the mission valuable fuel and significantly shortened the time for Hera to reach its target asteroids, reducing the overall journey by several months.

Moving at a velocity of 9km/s relative to Mars, Hera could observe Deimos from just 1,000km away, capturing images of the moon’s surface, including its far side—an area rarely seen by previous space missions.

Deimos, which spans approximately 12.4km in diameter, is covered in a thick layer of dust and might be a remnant from a massive impact on Mars or a captured asteroid from the outer solar system.

The Hera spacecraft used three scientific instruments to gather detailed data during the flyby. The first was Hera’s Asteroid Framing Camera, a black-and-white imaging device with a resolution of 1020×1020 pixels that helps with navigation and scientific analysis.

The second instrument, the Hyperscout H hyperspectral imager, observed in 25 spectral bands beyond the visible range, allowing scientists to characterise the mineral composition of the surfaces it encountered.

The third tool, Hera’s Thermal Infrared Imager, was developed with collaboration from the Japan Aerospace Exploration Agency (JAXA). It captures mid-infrared wavelengths to assess surface temperature and physical properties such as roughness, particle size distribution, and porosity.

According to Michael Kueppers, Hera’s mission scientist, these instruments had been tested earlier during the spacecraft’s departure from Earth. Still, the flyby of Mars marked their first full scientific deployment on a distant moon like Deimos. “This is the first time we’ve used them to investigate an object like Deimos, which is still relatively unknown,” Kueppers said, adding that the performance of the instruments exceeded expectations.

Caglayan Guerbuez, ESA’s Hera Spacecraft Operations Manager, praised the efforts of the Mission Analysis and Flight Dynamics team at ESA’s Space Operations Centre (ESOC) in Germany. “They did a fantastic job planning the gravity assist,” Guerbuez commented. “Fine-tuning the manoeuvre to bring Hera so close to Deimos was quite challenging and required a lot of extra effort from the team.”

In addition to Hera’s observations, the spacecraft conducted joint observations of Deimos in tandem with ESA’s Mars Express, which has been orbiting Mars for over two decades. This cooperation provided a more comprehensive view of Deimos and will help guide the planning for future missions aimed at further exploring Mars’s moons.

The results from Hera’s close encounter with Deimos will contribute to the preparation for the Martian Moons eXploration Mission (MMX), which is led by JAXA and involves collaboration with NASA, the French space agency CNES, the German Aerospace Centre (DLR), and ESA. MMX aims to conduct detailed studies of both Martian moons and land on Phobos to collect a sample, which will be returned to Earth for analysis.

Though Deimos is much larger than Hera’s final destinations, Didymos and Dimorphos, which measure 780 and 151 metres across, respectively, the spacecraft is now on course to reach these asteroids. A follow-up manoeuvre in February 2026, followed by a series of thruster firings starting in October of that year, will help fine-tune Hera’s trajectory to reach the Didymos system by December 2026.

Ian Carnelli, Hera’s mission manager, reflected on the significance of the mission’s progress. “This flyby has been an exciting first experience for the Hera team in planetary exploration,” he said. “But it is only the beginning. In just 21 months, we will reach our asteroid targets and begin investigating the only object in the Solar System whose orbit has been altered by human intervention. This is truly groundbreaking work that could lay the groundwork for planetary defence in the future.”

Hero image: The Hera spacecraft used three scientific instruments to gather detailed data during the flyby of Mars on March 12, 2025. Credit: ESA