The GNC system is being validated through virtual maneuvers conducted in Spain and Germany. At GMV's headquarters in Madrid, Hera's On-Board Computer is tested using a model asteroid for realistic proximity operations. In Bremen, OHB tests the spacecraft using a full-scale hardware replica called the Hera Avionics Test Bench.

"The system for Hera's interplanetary cruise phase - which of course is the most critical to be ready for launch - is now fully tested using the actual spacecraft flight model," explains ESA GNC engineer Jesus Gil Fernandez. "This phase will end at asteroid arrival when camera images will be used to distinguish the asteroid from background stars by spotting its gradual motion across successive images. GNC for the follow-on proximity operations phase is what we are concentrating on now, involving the spacecraft initially coming as close as 30 km from the asteroid pair, then much closer later on, down to 1 km."

After its October launch, Hera will cruise for two years, including a Mars flyby for science observations of Deimos, before reaching the Didymos binary asteroid system. The system's gravity is tens of thousands of times weaker than Earth's.

To operate safely, Hera's GNC system will fuse data from various sources, similar to self-driving cars. "Its main data source will be its main Asteroid Framing Camera, whose images are being used both for science and navigation," adds Jesus. "These images will be combined with other inputs to make a robust estimate of its position, notably the mission's PALT-H laser altimeter, which bounces down laser pulses to the asteroid's surface, as well as inertial sensors. This GNC system is designed to be operated manually from the ground initially, but once Hera's CubeSats are deployed, autonomous navigation will be needed to fulfil core mission objectives."

Hera will maintain its position around Didymos using hyperbolic arcs, a technique borrowed from ESA's Rosetta mission. These arcs involve regular thruster firings to reverse flybys. "Hera's hyperbolic arcs are designed so that if a thruster firing has a small error then the spacecraft would keep at a safe distance from the asteroids anyway," adds Jesus. The low velocities mean that maneuvers need to be executed accurately to avoid collision risks.

As Hera nears the asteroids, it will rely on autonomous surface feature tracking for navigation. "Once we come closer than 2 km then Dimorphos will fill the camera's field of view. Then comes the most ambitious navigation mode of all, based on autonomous surface feature tracking with no absolute reference," Jesus explains.

GNC testing will continue after launch to prepare Hera for its October 2026 arrival at Didymos.

Related Links
Hera at ESA
Asteroid and Comet Mission News, Science and Technology