"The one thing we really like about this computer is that it doesn't get destroyed by radiation," said Lemke. "It can be upset, but it won't fail. We've done a lot of testing on the different parts in the computer. When it sees radiation, it might have to reset but it will come back up and work again."
It takes just 20 seconds for the computer to reset, but for a system that runs everything on a craft hurtling through space at thousands of miles an hour, even 20 seconds of down time is too much. That's why there are two flight computers onboard, giving the spacecraft a redundant system.
For Orion's first test flight, which will have the spacecraft flying through an area of high radiation known as the Van Allan belt, the vehicle will carry a third computer as an extra precaution.
"Since we'll be going through a lot of radiation for quite a while, we've added another computer -- a third -- so if the two main computers go down because of radiation, this one will know the state of the vehicle if those two are lost," said Lemke. "When the first two reset, they'll go to the third and get the current data."
When it comes to computer redundancy, it's all about probabilities.
There's a chance of a single onboard computer going down in one out of every 3.7 missions, according to Lemke. And there's a one-in-8,500 chance of the second computer going down within 20 seconds of the first.
"There is a lot of uncertainty in just how bad the radiation is in the Van Allen belt, so we knew the probability could really be worse than what was calculated," said Lemke. "We don't want to even take a one-in-8,500 risk, so that's where the third computer came in."
The chance of losing all three computers at the same time is one in 1,870,000 missions.
"What does all of this mean?" asked Lemke. "It means that we won't be at all surprised if we experience a flight computer reset during the mission but we are very confident that radiation won't cause a problem with this test flight."
The computers are running IBM's PowerPC 750FX single-core processors, which were first launched in 2002.
NASA fit two of the processors into each flight computer, setting them up to run identical software and monitor each other. If the processors don't do the exact same thing, the system will stop giving commands and reset itself.
"The processors are obsolete already but they have the property of just getting upset by radiation, instead of being permanently damaged," said Lemke, noting that NASA has been using the processors for more than 10 years. "You could do it with something newer, but all the engineering that would go into making it work right would make it a lot more expensive for us to build it."
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