The European Space Agency (ESA) is working on a specially radiation-hardened computer to control its Hera deep-space probe on its task to observe the effects of the Asteroid Impact and Deflection Assessment (AIDA) mission. Scheduled to launch in October 2023, Hera will be controlled by an autonomous computer that can operate in the radioactive environment of interplanetary space 490 million km (304 million mi) away from Earth for a period of four years.
Spacecraft computers have come a long way from the ones that guided the Apollo missions. However, the very advances in ever-smaller microelectronics that have made sophisticated robotic deep-space probes possible have also made their computers increasingly vulnerable to both the hazards of radiation and a poorly-written line of code.
Being developed under the oversight of QinetiQ Space in Belgium, the Hera spacecraft's computer is designed to operate hundreds of millions of miles from Earth with a minimum of supervision from mission control, in a hostile radiation environment. To achieve this requires a hardened architecture, a careful choice of components, and software that can work around the damage that is certain to occur due to charged particles cascading through the shielding – these could corrupt the computer's memory, or cause a series of short circuits in the microcircuitry.
"Our computers use flash memory – the same as in your own laptop or smartphone – but we perform rigorous radiation testing to ensure the batches we use meet the necessary performance standards," says Peter Holsters of QinetiQ Space. "The next level of managing the problem is on the software side, with speedy error detection and checking in the memory management, including the ability to identify and work around 'bad blocks' in memory."
Another aspect of the Hera computer's design is that it must be both autonomous and highly reliable while using as little power as possible. Since Hera will travel beyond the orbit of Mars, its solar panels won't get as much sunlight as in the vicinity of Earth, so it must be economical. It also has to work with a minimum risk of going into safe mode or rebooting at critical moments.
"In Earth orbit a mission's computer going into safe mode is no big deal – the satellite itself is not going anywhere, there's time to reconfigure it," says Holsters. "But in deep space, with big asteroids whirling around, any recovery from failure will have to be done autonomously, and as quickly as possible.
"That implies maximum redundancy and fast switch-over times from the failing element to its backup. We actually have good experience of such hot redundancy from another company project: developing a safety-critical docking mechanism according to the International Berthing and Docking Mechanism Standard, which is used for making the connection between crewed and uncrewed spacecraft on one end and the International Space Station or in future the Lunar Gateway station, on the other.
"Our benchmark for Hera is that reconfiguration from any computer failure should be extremely fast, a matter of 10 to 20 seconds. Another design strategy is to deliberately not have all the functionality in the central onboard computer. On Hera the image processing – which can potentially be used for autonomous spacecraft navigation – will be performed by a dedicated unit, being developed by GMV in Romania."
The Hera computer uses a dual-core LEON-3 processor that's based on the Advanced Data and Power Management System (ADPMS) that was developed for the Proba-2, Proba-V, and Proba-3 mini-satellites. This has already had 15 years of successful flight time and will also be used on the Altius ozone-monitoring mission. The one destined for Hera is currently in the engineering model stage of testing.
"This testing – supported through ESA's General Support Technology Programme – is taking place under our ProbaNEXT project, which is developing our next-generation Proba platform for a wide variety of uses and users," says an ESA spokesman. "Currently, we are qualifying the redundancy and fast switch-over time element of the design. This testing is allowing us to demonstrate all relevant functioning that Hera needs, so once the decision is made to fly the mission then we will be ready."
The video below shows Hera's part in the AIDA mission.
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