Although the GPCs run the spacecraft during a mission, astronauts take a number of relatively modern computers with them into orbit in the form of laptops. Crews carry modified IBM ThinkPad A31p computers into space with them and use them for rendezvous assistance, entry and landing simulations and e-mailing Earth.
The laptops also are much faster than the GPCs and connect with devices not available to the GPCs. The Thinkpads use one of these connections to relay photos of the external tank falling away after launch to mission control at NASA's Johnson Space Center in Houston.
But that modernity has a trade-off: the laptops are not nearly as reliable as the GPCs due to radiation effects and use of less critical commercial off-the-shelf software, Klausman said.
The laptops, however, don't work on life-support or high-criticality systems that require the reliability found in the GPCs.
"For critical operations, I can't come anywhere close to that reliability with the laptops," Klausman said. "They are wonderful items, but they are susceptible to radiation particles, they are susceptible to badly written software. I could put five laptops on board and all five would suffer radiation upsets within the first day."
With a ThinkPad 760XD laptop, two to three memory changes due to radiation occur during a shuttle flight to the Station, Klausman said. That number balloons up to 30 for a mission to NASA's Hubble Space Telescope. The reason is that Hubble orbits about 150 miles higher than the station, where the radiation protection from Earth's magnetic field is not as strong.
Designers also found out that laptops would crash when the shuttle passes through the "South Atlantic Anomaly," which is an area where the magnetic field draws in to Earth, again offering less radiation filtering for spacecraft flying through it.
The GPCs don't crash for radiation concerns because the GPC hardware includes a memory scrubber that prevents the system from reading radiation-changed memory.