ESA’s Planetary Defense Mission: Hera Asteroid Spacecraft Is Complete

ESA’s Hera mission has been fully assembled in Germany and is set to study the Dimorphos and Didymos asteroids. It will soon undergo rigorous testing in the Netherlands to ensure space readiness. Credit: ESA/Science Office

ESA’s asteroid defense mission, Hera, has been successfully assembled at OHB Bremen in Germany.

Hera is complete. ESA’s asteroid mission for planetary defense was built and prepared in two halves, but now, through a painstaking operation, they have been mated together to make a single spacecraft, ready for full-scale testing of its readiness for space.

The mating took place at OHB Bremen in Germany, with Hera’s Core Module raised more than 3 m above its Propulsion Module and then gradually and carefully slotted into place, over a three-hour period. The modules had been placed in cages to ensure their correct alignment relative to each other down to a few tenths of a millimeter.

“The mission keeps on hitting milestones right now, but this is a big one, and a very emotional moment for the team,” explains Paolo Martino, Hera system engineer. “Previously we had these two modules, now you can say the spacecraft has been born.”

This Hubble Space Telescope image of the asteroid Dimorphos was taken on December 19, 2022, nearly four months after the asteroid was impacted by NASA’s DART mission (Double Asteroid Redirection Test). Hubble’s sensitivity reveals a few dozen boulders knocked off the asteroid by the force of the collision. The free-flung boulders range in size from three feet to 22 feet across, based on Hubble photometry. They are drifting away from the asteroid at a little more than a half-mile per hour. TCredit: NASA, ESA, David Jewitt (UCLA), Alyssa Pagan (STScI)

Hera is Europe’s contribution to an international planetary defense experiment. Following the DART mission’s impact with the Dimorphos asteroid last year – modifying its orbit and sending a plume of debris thousands of kilometers out into space – Hera will return to Dimorphos to perform a close-up survey of the crater left by DART. The mission will also measure Dimorphos’ mass and make-up, along with that of the larger Didymos asteroid that Dimorphos orbits around.

Hera’s Propulsion Module incorporates its propellant tanks – housed within a central titanium cylinder, the ‘backbone’ of the spacecraft – along with piping and thrusters, which will have the job of hauling the mission across deep space for more than two years, then to maneuver around Dimorphos and Didymos. This module was mated with Hera’s Core Module at OHB Bremen to complete the spacecraft structure. Credit: OHB

To make its rendezvous with Dimorphos Hera has to lift off in October 2024. So to maximize working time the mission was constructed by prime contractor OHB as two separate modules, which could be worked on in parallel.

Hera’s Propulsion Module incorporates its propellant tanks – housed within a central titanium cylinder, the ‘backbone’ of the spacecraft – along with piping and thrusters, which will have the job of hauling the mission across deep space for more than two years, then to maneuver around Dimorphos and Didymos.

Meanwhile, Hera’s Core Module can be thought of as the brains of the mission, hosting its onboard computer, mission systems, and instruments.

Hera’s Core Module can be thought of as the brains of the mission, hosting its onboard computer, mission systems and instruments. Enclosed in a cage, it is seen here being lowered onto the spacecraft’s Propulsion Module at OHB Bremen to complete the spacecraft. Credit: OHB

Manufactured together, the Core Module remained at OHB while the Propulsion Module traveled to Avio near Rome in Italy for the addition of its propulsion system. The pair were then reunited in Bremen to prepare for the mating operation.

“A similar double-module process is often used for telecom missions, but those are usually standardized designs,” adds Paolo. “This is the first time it has been applied to a deep space mission, on a much more ad hoc basis.”

Hera is complete. ESA’s asteroid mission for planetary defense was built and prepared in two halves, but now, through a painstaking operation, they have been mated together to make a single spacecraft, ready for full-scale testing of its readiness for space. The mating took place at OHB Bremen in Germany, with Hera’s Core Module raised more than 3 m above its Propulsion Module and then gradually and carefully slotted into place, over a three-hour period. The modules had been placed in cages to ensure their correct alignment relative to each other down to a few tenths of a millimeter. Credit: OHB

The mating had been exhaustively simulated in advance using CAD software, but OHB’s assembly, integration, and testing team were still checking alignment as the crane lowered the Core Module every step of the way. The cleanroom door was kept sealed during the mating to prevent any distractions.

“We studied a lot together with our designers on which were the most critical parts of the process, so most of them were already taken into account,” explains Matteo Grimaldi, Senior Assembly, Integration and Testing technician at OHB.

Hera, her CubeSats, and their rocky target destination. Credit: ESA/Science Office

Once the tip of the Propulsion Module cylinder met the top deck of the Core Module the mating was complete. Then an initial test bolt was inserted to check the alignment was entirely correct in advance of the two modules being fully bolted together.

“The two modules are now together forever, as they will be in space, barring any major unexpected problem,” explains Paolo.

 “If we need to, we can still access internal units through side panels. Next, we will be adding some payload units to the spacecraft’s top deck which we are receiving directly from the manufacturers once Hera moves to its next stop.

“That is at the end of this month, when Hera is being transported to the ESTEC Test Centre in the Netherlands, where it will go through a full-scale environmental test campaign to check its flight readiness.”

Meet Hera, our very own asteroid detective. Together with two CubeSats – Milani the rock decoder and Juventas the radar visionary – Hera is off on an adventure to explore Didymos, a double asteroid system that is typical of the thousands that pose an impact risk to planet Earth.