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ESA's Hera Mission Swings Past Mars, Sees Deimos

The Hera mission sent back unprecedented views of the Martian moon Deimos as a prelude to the mission’s final destination: the asteroid Didymos.

Deimos is dark uneven rock with bright Mars landscape in the background

It has been a busy week for the Red Planet. First, NASA’s Europa Clipper flew 884 kilometers (550 miles) over the surface of the planet on March 1st, as it passed en route to Jupiter by 2030. Then yesterday, the European Space Agency’s Hera mission passed Mars on its way to asteroid 65803 Didymos.

Hera will arrive at the Didymos system on December 28, 2026, and will explore the asteroid and its moonlet Dimorphos for six months. The mission is a follow-on to study the long term effects of the impact of NASA’s DART mission, which impacted moon Dimorphos in 2022. Hera will be the first mission to complete a rendezvous with a binary asteroid pair.

To get there, Hera had to pass by Mars for a gravity assist, a maneuver that changes the spacecraft's momentum. The flyby provided the opportunity for unprecedented close-ups of the Martian moon Deimos against the dramatic backdrop of Mars.

applauding people near TV screen showing flyby image

The misshapen moon is 12.4 kilometers across and orbits Mars once every 30 hours. First spotted by American astronomer Asaph Hall using the U.S. Naval Observatory’s 26-inch refractor in August 1877, the moon was imaged more closely in 1972 by the Mariner 9 mission. It’s still an open question as to whether the moon is a captured asteroid, or a chunk of Mars blown into orbit early in the planet's formation.

Hera's closest approach to the moon occurred at 8:07 a.m. EDT / 12:07 UT on March 12th, at a range of about 1,000 kilometers. Then, 44 minutes later, the spacecraft passed within about 5,000 kilometers of Mars, moving at a speed of 5.6 meters per second (13 mph) relative to the planet.

“Our Mission Analysis and Flight Dynamics team at ESOC in Germany did at great job of planning the gravity assist,” said Caglayan Guerbuez (ESA) in a recent press release. “Especially as they were asked to fine-tune the maneuver to take Hera close to Deimos — which created some extra work for them!”

Sped-up simulation of Hera's close approach

ESA

This was the first time the Hera team had a chance to use some of the instruments on the spacecraft since its departure from Earth. It was also the only planetary encounter before Hera reaches its final destination. So the team made the most of this dress rehearsal, turning on three instruments during closest approach.

The first instrument on deck was the spacecraft’s Asteroid Framing Camera, which captures images in visible light for scientific and navigation purposes. The camera grabbed black-and-white, 1020 × 1020-pixel images of Deimos during the flyby.

Deimos is dark uneven rock with bright Mars landscape in the background

Next, the Hyperscout H hyperspectral imager observed Deimos and Mars in 25 bands, from visible light into the infrared. The data Hyperscout H provides is key to understanding mineral composition on the target asteroid.

Mars appears blue in the background, while small Deimos in the foreground appears dark gray

Finally, Hera’s Thermal Infrared Imager was on hand to study Deimos and Mars at mid-infrared wavelengths. Built and provided by the Japan Aerospace Exploration Agency (JAXA), this imager will provide information on surface temperature, texture, and porosity. The flyby serves as a teaser for JAXA, as the agency's Mars Moons Exploration mission should head to Mars next year. The probe will scout out both moons and even attempt to land on Phobos, returning a sample of that moon to Earth.

Mars appears in a gradient from deep purple to orange, while Deimos appears a much brighter orange-white

Other instruments on Hera were turned off during the flyby. For example, the Planetary Altimeter measures the distance from the spacecraft to the surface but only when within about 20 kilometers (12 miles). Hera also has a pair of CubeSats aboard (named Milani and Juventas) that won’t be deployed until the mission reaches Didymos.

Hera’s observations of Deimos were carried out jointly with ESA’s Mars Express mission, which orbits the Red Planet and periodically sees both moons.

Now, with the flyby complete, it's onward to the Didymos system. When the spacecraft arrives in late 2026, we can look forward to seeing the long-term effects of DART's impact on the asteroid pair.