Cruise control: the team guiding Hera to its asteroid
First the European Space Agency's Hera asteroid mission separated from its launcher on 7 October 2024, next its lengthy ‘cruise phase' began – the car-sized spacecraft's journey through millions of kilometres of nothingness to converge on its rocky destination.
Major moments in the life of Hera – starting with that initial launcher separation – are supervised by the mission's flight control team from the Main Control Room of ESA's mission control centre ESOC, in Darmstadt, Germany, a well known location from TV and newspaper coverage of dramatic spacecraft operations.
But for everyday oversight of the spacecraft during its cruise phase, the action shifts to the separate Building D and a much smaller glass-walled chamber, resembling a standard open plan office.
This Interplanetary Control Room is employed for all ESA missions beyond Earth orbit, including the BepiColombo Mercury mission and Solar Orbiter probe to the Sun, along with Hera.
Linking to deep space
An approximately seven-strong flight control team communicate with Hera from here during the cruise phase, at a typical frequency of two ‘passes' per week.
These are the approximately eight-hour periods when one of ESA's 35-diameter deep space antennas, located across the globe, points itself to the precise point in the sky where Hera is located.
Meanwhile Hera itself lines up its own antenna with Earth, enabling two-way communications to take place: new commands are uplinked to the spacecraft at the same time as telemetry is downlinked, giving insight into the current state of Hera and its subsystems.
Caglayan Guerbuez, Hera Spacecraft Operations Manager, explains: "During the cruise phase we perform periodic platform maintenance, ensuring that our various subsystems remain healthy, plus instrument checks for the scientists. But the intensity of these activities is kept low as much as possible, allowing us to focus on preparing for the future exploratory ‘asteroid phase'."
Keep calm and carry on
Visitors might be surprised by the team's relaxed manner, Caglayan adds: "It's quite an extensive and challenging workload but we aim for a calm atmosphere. We don't need to be in the control room all the time. We're not in continuous contact with Hera after all, and we're certainly not steering with a joystick!
"Instead we have all our well-validated processes in place, automated as much as possible, and our coming month of instruction to the spacecraft is already planned out in detail – being what we call our short-term planning cycle.
"We're always living in the future, basically, continuously planning ahead!"
After a pass, if something is found to be operating non-nominally – known as a ‘contingency' – then corrective action might be required. Such actions are decided on a group basis however, in plenty of time for the next pass later in the week, with instructions that are always rigorously checked before being uplinked.
Preparing for Hera's asteroid phase
Short-term activities are supplemented by medium- and longer-term planning. This includes liaising with the scientists to schedule observations within the flight plan and preparing all other aspects of the asteroid phase – the six-month period when Hera will perform its close-up asteroid survey.
Sylvain Lodiot, heading ESA's Outer Solar System and Planetary Defence Operations, notes: "The frequency of passes will go up to one per day or even more during key events such as when we performed our two deep space manoeuvres, our March 2025 flyby of Mars, and our recent software update, then later during the asteroid phase to come.
"But that's not to say the rest of the time is quiet! In fact, this has been among the most demanding cruise phases of any ESA mission. This is because it was absolutely essential for Hera to launch on schedule: a lift-off off two years later – the next time Mars would be lined up for the flyby that contributed an essential velocity shift for the mission – would have led to a much longer overall travel time. With this in mind, plenty of tasks had to be left until after launch to be taken care of."
Safety first
This included the mission's flight software, which initially lacked full ‘fault detection, isolation and recovery' (FDIR) capability. What this meant was that the mission might not be able to recover autonomously from any onboard error.
This was equivalent in space terms to be walking a highwire without a safety line, so the team had to watch over the mission carefully until the Hera consortium achieved FDIR capability. This critical activity was led by ESA, with support from prime contractor OHB, central software provider Spacebel and GMV for onboard guidance systems.
Preparing Hera and its CubeSats to fly by themselves
The next major item on the task list was one of the most complex test campaigns ever performed at ESOC: rehearsing all Hera's autonomous functionality around its two target asteroids and interactions with the pair of CubeSats it will deploy.
The real challenge is the inherent complexity of a trio of spacecraft self-navigating themselves around a pair of asteroids – in the absence of any possibility of real-time human oversight. Validating that the necessary autonomy will work as planned took more than a year and a half to conclude and required 50 ground test days in all.
Sylvain Lodiot adds: "The purpose of this testing was to give us rock solid certainty that the updated flight software for Hera's asteroid phase was ready to be uploaded, which duly took place last month. Now we have this software aboard we are able to commission all of Hera's instruments and test out how it will fly, and communicate with its CubeSats, all by itself."
The software allowing autonomous navigation around the asteroid system is entirely new for Hera explains Ritchie Kay, a flight control team engineer and veteran of ESA's Rosetta mission: "With this functionality then spacecraft can maintain good pointing to either of the asteroids at all times no matter how the trajectory of the spacecraft evolves. This was not possible on Rosetta but it allows Hera to fly closer to the asteroids safely – the spacecraft is able to react and fly away if it detects a collision risk, for instance."
Along with ground simulations, the controllers – working with ESOC's Flight Dynamics team – also took advantage of practical opportunities arising to try out Hera's autonomy functionality for real. This included performing surface feature tracking during Hera's Mars flyby and a post-launch attempt to turn the spacecraft towards Earth and the Moon, as a stand in for its target binary asteroids.
Ritchie recalls: "It was late at night and the whole team was tired after our Launch and Early Orbit Phase operations, but everyone came in to watch and enjoy the moment as Hera downlinked pictures of Earth and the Moon – having actively tracked them with its autonomous software!"
Testing and validating Hera's updated software was a gruelling team effort, but the final commanding of the software update was performed by Jens Kolbenschlag, an ESA Graduate Trainee since September last year: "For the preparation of the software update alone we sent several hundred commands, where each has an impact on the spacecraft's configuration. We had to make sure that at any point in this long procedure, the spacecraft was safe even if another problem were to occur in this vulnerable state.
"We conducted the update at a time where our commands took about eight minutes to reach the spacecraft, and another eight minutes to receive a response. You can imagine the suspense in the control room after sending a critical command, waiting to receive the confirmation that everything executed successfully!"
Concluding Hera's cruise phase
Hera's next milestone is its braking manoeuvre to rendezvous with Didymos and Dimorphos this October, which will formally conclude its cruise phase after more than two years. The resulting thruster firings – involving a velocity change of about 700 m/s – will be overseen from the Interplanetary Control Room.
A still more demanding activity comes early next year, when Hera will deploy its two CubeSats, one after the other within a couple of weeks.
The unprecedented nature of this operation means it will be overseen from ESOC's Main Control Room, with initial training simulations due to occur this autumn.
Then, early in the new year, the team will begin a whole new operational mode: overseeing three spacecraft rather than one, which self-steer and communicate with each other via inter-satellite links, just a few kilometres away from a binary asteroid system in deep space.
Thank you for reading the article!
