Engineers have determined why NASA's Voyager 1 probe has been transmitting incomprehensible information for nearly five months, raising hopes of recovering humanity's most distant spacecraft.
Voyager 1, which was traveling outward about 15 billion miles (24 billion kilometers) from Earth, began sending unreadable data to ground controllers on November 14. For about four months, NASA knew Voyager 1 was still alive, and it continued to broadcast a steady signal. But he couldn't decipher anything he was saying.
To confirm their hypothesis, engineers at NASA's Jet Propulsion Laboratory (JPL) in California confirmed that it was a small portion of corrupted memory that caused the problem. The faulty memory bank is located in Voyager 1's Flight Data System (FDS), one of three computers on the spacecraft. The FDS system works in conjunction with a central command and control computer and another device that oversees attitude and signal control.
FDS duties include packaging Voyager 1's science and engineering data for transmission to Earth through the vehicle's telemetry modulation unit and radio transmitter. According to NASA, about 3 percent of the FDS memory has been corrupted, preventing the computer from performing normal operations.
Optimism grows
Susan Dodd, NASA's project manager for the twin Voyager probes, told Ars in February that this was one of the most serious problems the mission has ever faced. And that's saying something because Voyager 1 and 2 are NASA's longest-lived spacecraft. They were launched 16 days apart in 1977, and after flybys of Jupiter and Saturn, Voyager 1 flies farther from Earth than any spacecraft in history. Voyager 2 lags behind Voyager 1 by about 2.5 billion miles, even though the two probes are heading out of the solar system in different directions.
Typically, engineers try to diagnose a spacecraft's malfunction by analyzing the data it sends back to Earth. They couldn't do that in this case because Voyager 1 was sending data packets that showed a recurring pattern of ones and zeros. However, the Voyager 1 ground team identified FDS as the likely source of the problem.
The Aviation Data Subsystem was an innovation in computing when it was developed five decades ago. It was the first computer on a spacecraft to use volatile memory. Most NASA missions operate repetitively, so each Voyager spacecraft is launched with two FDS computers. But the FDS backup on Voyager 1 failed in 1982.
Due to the age of the Voyagers, engineers had to consult paper documents, memos and diagrams to help understand the details of the spacecraft's design. After months of brainstorming and planning, the JPL teams sent a command in early March to prompt the spacecraft to send a readout of the FDS memory.
It worked, and Voyager.1 responded with a different signal than the code the spacecraft had been sending since November. After several weeks of careful examination of the new code, engineers identified the bad memory locations.
“The team suspects that one chip responsible for storing part of the affected portion of FDS memory is not working.” NASA said in an update Published Thursday. “Engineers cannot pinpoint the cause of the problem with certainty. Two possibilities are that the chip was hit by an energetic particle from space or that it has simply worn out after 46 years.”
Voyager 1's distance from Earth complicates troubleshooting efforts. The one-way travel time for a radio signal to reach Voyager 1 from Earth is about 22.5 hours, meaning it takes engineers on Earth about 45 hours to figure out how the spacecraft responds to their commands.
NASA must also use its largest communications antennas to communicate with Voyager 1. These 230-foot (70 m) diameter antennas are in high demand by many other NASA spacecraft, so the Voyager team must compete with other missions for troubleshooting time. This means it will take some time to return Voyager 1 to normal operations.
“Although it may take weeks or months, engineers are optimistic that they can find a way to operate FDS normally without unusable memory devices, which will enable Voyager 1 to start returning science and engineering data again,” NASA said.