NASA’s Curiosity rover has already long outlived its minimum projected lifespan with more than six Earth years — more than 2,000 Martian days or “Sols.” However, NASA has announced that it recently had to make use of the rover’s computer redundancy to keep the mission going. Curiosity has flipped over to its backup computer system after the main system started experiencing errors last month.
Many NASA spacecraft and surface missions have redundant systems built-in. Once they’ve launched from Earth, there’s no way to repair damage to critical systems, so it makes sense to double-up on the vital components. That includes Curiosity’s computers, which were designed specifically for the harsh environment on Mars.
The rover has a pair of identical brains running a 5-watt RAD750 CPU. This chip is part of the PowerPC 750 family, but it has been custom designed to survive high-radiation environments as you’d find on Mars or in deep space. These radiation-hardened CPUs cost $200,000 each, and NASA equipped the rover with two of them. Each computer also has 256 kB of EEPROM, 256 MB of DRAM, and 2 GB of flash memory. They run identical VxWorks real-time operating systems.
When Curiosity landed on Mars in 2012, it used the “Side-A” computer. However, just a year later in 2013 (Sol 200), the computer failed due to corrupted memory. The rover got stuck in a bootloop, which prevented it from processing commands and drained the batteries. NASA executed a swap to Side-B so engineers could perform remote diagnostics on Side-A. In the following months, NASA confirmed that part of Side-A’s memory was unusable and quarantined it. They kept Curiosity on Side-B, though.
Several weeks ago, Side-B began experiencing problems that prevented the rover from storing key science and engineering data. The team spent a week evaluating Side-A and preparing it for the swap. NASA has now switched Curiosity back to Side-A while it investigates the problem in greater detail, which it can only do when the other computer is active. Side-A still has damaged memory from 2013, but the computer won’t use those blocks.
NASA hasn’t said how much of Side-A’s RAM is bad, and it only had 256MB to start, but the team does intend to move Curiosity operations back to Side-B if possible. For now, the mission is functioning normally on Side-A. There is no immediate need to switch back, but NASA would prefer to utilize the larger memory capacity on Side-B.
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