The Dawn spacecraft also is using two identical processors -- one for main use and one for backup. It also has two identical radio transmitters, two receivers and two star trackers, which are used to orient the craft.
The spacecraft doesn't use artificial intelligence but it does have self-monitoring software. The software, built by NASA, monitors the system's software and hardware, along with conditions like temperatures, currents and pressures, and will take active steps to protect the spacecraft in the event of a problem.
"If the radio transmitter doesn't seem to be working correctly, it will power that transmitter off and power the other one on," explained Rayman. "If it determines it's been too long since it's received a command from Earth, it will then turn its receiver off and turn the other one on. It also may swap to another computer in case that's the problem. It will take an action, point to Earth sending a signal and requesting a signal and then, if it still doesn't receive a signal back, it will take another action."
Engineers coded into the system a series of possible explanations for various problems so the system itself can work through them on its own.
Dawn has had trouble with its reaction wheels, which control the spacecraft's orientation, and is expected to stop working early next year.
NASA engineers have figured out a way to keep the spacecraft going even without the requisite number of reaction wheels, but that fix has been quickly using up the craft's supply of conventional propellant, which is used to rotate it in one direction or another.
The spacecraft will run out of its rotation propellant by early next year and will remain in permanent orbit around Ceres.
"It will just become an inert monument to human ingenuity orbiting around the first dwarf planet to ever be orbited," Rayman said. "What more fitting tribute to the spacecraft itself than to leave it around a body that no other spacecraft has had the capability of reaching?"
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