Brake! Time for ESA’s Hera asteroid mission to go slow
Hera launched from Cape Canaveral Space Force Station in Florida on 7 October 2024, just ahead of Hurricane Milton. Over the two years since, gravity has done most of its driving, with two deep-space manoeuvres and a spring 2025 Mars flyby that shifted its course and boosted its speed. Hera is now moving relative to Earth at more than 12 km/s, has crossed Mars orbit, and is closing in on the orbit of the Didymos binary asteroids, so it must brake to match its targets' velocity.
Early next Tuesday, 15 October, Hera will rotate so its ring of Orbit Control Thrusters faces the Didymos system. Three hydrazine-based thrusters will ignite at once and burn continuously for 93 minutes to reduce Hera's velocity relative to the asteroids by a few hundred metres per second. This is tricky flying: the Reaction Control Thrusters, mounted in pairs on the tips of the cube-shaped spacecraft, must also help maintain attitude, with supportive fine-tuning of the Orbit Control Thrusters. To prepare for this initial Braking Manoeuvre, BRM-1, ESA performed a brief test burn on 2 October and then began generating the full telecommand sequence, according to Flight Dynamics engineer Francesco Castellini. His team determines where ESA missions are in space and calculates how to move them where they need to go.
As soon as possible after the burn ends, ESA will acquire a signal from Hera via one of its two 35-metre deep-space antennas in New Norcia, Australia. The hard work then begins: using Hera's GNC sensor outputs in telemetry plus ranging and Doppler data in the signal, the team will assess whether the braking's direction and magnitude match the anticipated delta-v.
After several busy days of checks and calculations, the telecommand sequence for a second, shorter braking manoeuvre, BRM-2, will be uplinked to Hera for implementation early on Thursday, 22 October. That approximately 31-minute thruster burn should complete most of the delta-v needed to match the velocity of the Didymos system.