Magellan Spacecraft Has Computer Memory Problem En Route to Venus
PASADENA, Calif. (AP) _ NASA said Wednesday a computer chip has failed on the Magellan spacecraft, but the defect won’t endanger the $550 million mission to map the surface of cloud-shrouded Venus starting in August.
Until engineers locate the faulty computer memory chip, they must send Magellan new commands every other day to make sure it is pointing in the proper direction, said Edwin Sherry, a technical assistant at the space agency’s Jet Propulsion Laboratory.
″There’s no threat to the mission,″ he said.
Magellan was launched from space shuttle Atlantis on May 4. It will go into a polar orbit around Venus on Aug. 10. About 18 days later, it will start using its radar to peer through the planet’s thick clouds, making maps that eventually will reveal the landscape of at least 70 percent of the planet.
As of Wednesday, Magellan was nearly 112 million miles from Earth and 107 million miles from Venus. But since its launch, it has traveled a curving path of 424 million miles, and still has 369 million miles to go before reaching the second planet from the sun, Sherry said.
He said the problem developed Sunday when an error was detected in a tiny portion of Magellan’s computer memory as the spacecraft got ready to take a fix on two distant stars to make sure it was pointing the right way.
The error prompted Magellan to shift to a backup computer, point its solar panels toward the sun to increase the power supply, and make sure it stayed in contact with Earth by slowing the rates at which it receives commands and sends information.
The failure was apparently the result of electrical corrosion at a junction between two types of material on a single memory chip, leaving the chip unable to retain data, Sherry said.
However, he said engineers haven’t yet ruled out the possibility that the chip was damaged by an electrically charged particle spewed out by the sun, which is near the peak of its 11-year cycle of activity.
Magellan uses gyroscopes to sense when pressure from solar wind makes the spacecraft drift slightly, or point in the wrong direction. The gyroscopes normally issue automatic commands to three spinning wheels, which correct the spacecraft’s alignment.
Magellan’s main computer is programmed to take a fix on the two stars each day to determine the spacecraft’s actual alignment. If this ″star calibration″ shows the gyroscopes failed to align Magellan correctly, they again command the wheels to adjust the craft’s position.
Magellan is still operating on its backup computer. Once the faulty chip is located - scientists hope within a few days - a new computer program will be sent to the spacecraft so the main computer can be used while the faulty chip is bypassed, Sherry said.
He said a similar computer chip failure happened before Magellan was launched, and that such a failure is expected about once annually.
″You’d hope for zero faults like this,″ Sherry said. ″But they’re typical of working with state-of-the-art equipment. It’s remarkable we have so few.″
Engineers working on the problem are located at Jet Propulsion Laboratory in Pasadena and at Martin Marietta in Denver. The company built Magellan and is responsible for its daily operations under National Aeronautics and Space Administration supervision.