Imagine being tasked to install new software onto a multi-million-euro computer, sight unseen. You don’t get to touch the hardware in question because it is already busy overseeing a spacecraft in deep space, in onward motion at more than 12 km per second.

Instead you send instructions by text message, relayed via 35-m diameter communication antennas directed at precisely the right patch of sky, working around a nearly eight-minute one-way signal delay due to the inherent distance involved.

Having uploaded the software successfully, then comes the crucial phase when the entire spacecraft needs to be rebooted – twice!

Hera’s onboard computer, overseeing all its instruments and subsystems, runs on parallel processor streams for redundancy. A pair of reboots allowed each to be evaluated in turn. The seven-strong core Hera team stood ready in their control room in case the spacecraft did not signal back as scheduled, but thankfully it rebooted nominally on each occasion, right on time.

“The success of this more that two week operation leaves Hera finally ready for what we call its ‘asteroid phase’,” says Anna Schiavo, Hera Spacecraft Operations Engineer. “The update will allow us to commission all of the spacecraft’s remaining instruments and to use  the autonomous functionality that Hera will rely on to navigate around its target asteroids – along with the inter-satellite links Hera will employ to communicate with the two CubeSats it will deploy early next year.”

Before being applied in flight the software underwent one of the most complicated test campaigns ever undertaken at ESA’s ESOC mission control centre in Darmstadt, Germany. Occurring over a year and a half and requiring 50 ground test days in all, this involved rehearsing all Hera’s autonomous functionality around its two target asteroids and also interactions with its CubeSats.

Sylvain Lodiot, heading ESA’s Outer Solar System and Planetary Defence Operations, explains: “The testing involved running the software on a functional replica of Hera located at the mission’s prime contractor OHB in Bremen, called the ‘Bench’, flying around simulated models of the asteroids and communicating with actual CubeSat replicas though the inter-satellite links.”

Caglayan Guerbuez, Hera Spacecraft Operations Manager.: “It’s not so unusual that a deep space mission is launched without its final software aboard, but Hera was really a mission in a hurry: it had to launch when it did in October 2024 in order to benefit from a flyby past Mars the following spring, otherwise the mission would have taken years longer to reach Dimorphos.

“So we have been kept busy during the cruise phase to catch up, with the valued support of software provider Spacebel as well as OHB.”

Hera is ESA’s first planetary defence mission. The van-sized spacecraft is headed to the Dimorphos asteroid, which orbits in turn around the larger Didymos asteroid. Dimorphos is already historic as the first object in the Solar System to have had its orbit changed by human action – when NASA’s DART spacecraft impacted it in September 2022.

DART’s impact caused a brightening visible from distant Earth telescopes, but nobody knows what it did to Dimorphos itself. Hera is going there to perform a close-up crash site survey, to help turn DART’s kinetic impact experiment into a well understood planetary defence deflection technique that might be tailored against incoming objects if required.