200 million miles away, on the surface of the Red Planet, humankind’s most advanced interplanetary endeavor — Mars Science Laboratory Curiosity — is attempting one of the most complex and dangerous maneuvers possible: Switching out its primary on-board computer for the identical, redundant fail-safe computer. It is hoped that the swap will restore Curiosity to full operational capability.

On Wednesday, Curiosity failed to “sleep” — a mode that it enters daily to recharge its batteries for the next day’s activities. NASA performed diagnostic work here on Earth using the Rover Simulation and Visualization Program (essentially a Curiosity simulator), and decided that the failure to enter sleep mode was due to some corrupted flash memory.

Most spacecraft, including Curiosity, are outfitted with two identical computers that can be swapped out if one develops a fault. Famously, back in November 2012, the Mars Odyssey orbiter — which beams data back to Earth from Curiosity — switched to a redundant computer that had been sitting idle since before the orbiter’s launch in 2001. In Curiosity’s case, there are two Rover Compute Elements (RCEs), A and B. Curiosity started with RCE A at launch, switched to B for some of its journey to Mars, and has been using A since just before its landing in August 2012. Curiosity has now switched back to RCE B, in the hope that B’s flash memory isn’t corrupted. We won’t know if the switch-over was a success until NASA has carried out more diagnostics, which will occur over the next few days.

Each of Curiosity’s RCEs is a single-board RAD750 computer — a radiation-hardened computer (pictured above) made by BAE that has a PowerPC 750 (G3) CPU clocked at around 200MHz, 256MB of RAM, 2GB of flash, and 256KB of EEPROM. To switch computers, Curiosity enters “safe mode” — a “minimal-activity precautionary status” — and then begins the switch-over from one RCE to the other. It isn’t entirely clear how the swap occurs — is there a plug that is literally switched over? — but there is presumably a third, smaller computer that maintains communications with NASA, and is capable of gracefully switching between the two RCEs. After the swap occurs, NASA will work hard to fix RCE A’s memory corruption, so that Curiosity has a fail-safe that it can fall back to in the future. (See: Inside NASA’s Curiosity: It’s an Apple Airport Extreme… with wheels.)

In other news, here are the latest beautiful photos to be beamed back from the surface of Mars.