JPL announced today that engineering data was successfully received from the Voyager 1 spacecraft on Saturday April 20, the first time since Nov 2023.
A new set of commands were sent on Thursday to relocate some code around a failed memory chip in the Flight Data System (FDS) computer and as expected that fixed the problem that had caused Voyager’s transmission to be stuck with sending a 0101... pattern since Nov 14, 2023.
Only engineering data (spacecraft health data) is being received. Successful reception of science data (data from its instruments) will require additional code relocation in the tight sparse free memory areas in the FDS. www.jpl.nasa.gov/…
Voyager 1, launched more than 46 years ago, is 24.3 billion km away from earth; it takes light and comm. signals 22.5 hours to travel this distance.
The Problem
The problem began around Nov 14, 2023, when ground control noticed that data received from Voyager 1 was stuck at a simple repeating 0101.. bit pattern.
Schematic of Voyager 1 computers and telecom unit
However the Voyager team was able to verify that the spacecraft was able to receive and execute commands.
The Diagnosis
NASA engineers had a good hunch that the problem lay in the FDS computer which formats data sent over the comm. link.
NASA also suspected that problem lay with some corrupted memory in the FDS, which occurs occasionally due to cosmic rays and particles.
A similar problem had affected Voyager 2 in 2010. Received science data (but not engg data) was garbled. The problem was traced to a flipped bit in the program stored in the FDS. A command was sent to flip the bit, which fixed the problem.
During December and January, NASA sent commands to read back memory from the FDS but all it got was 0101. So, finding the locations of the corrupted memory was no easy task.
Many science writers were preparing obituaries for Voyager 1.
But the tenacious folks at JPL never gave up. After months of experimentation, the commands to read back memory sent on March 1 resulted in a successful reception of the FDS memory contents. That breakthrough led to the identification of the corrupted memory locations.
On March 20 at a meeting of the National Academies’ Committee on Solar and Space Physics, Joseph Westlake, director of NASA’s heliophysics division stated that the diagnosis was a failed memory chip which affected about 200 16-bit words of code.
The Fix
NASA engineers figured out the sections of code that needed relocation and relinking with the rest of the code. Some new commands and code fragments were sent last Thursday April 18.
And lo behold, the reception on Saturday (45 hours later) contained engineering data, not 0101 anymore!
More work needs to be done to enable science data to be received since that is sent at a higher data rate and uses different sections of code.
Let’s keep in mind that this code surgery was done remotely on a system that was running the code; there is no option to switchover to a backup unit (the backup FDS failed in 1981).
About Voyager’s Hardware
Voyager's computer systems were custom-built using 1960s technology, with clock speeds measured in KHz and RAM in kbytes, running hand-crafted software, crammed into 4 Kwords of CMOS memory per computer.
And yes, it uses a digital 8-track tape for storage of science data!
The custom-designed hardware, (upgraded) software and most instruments are still functioning after 46 years in space.
Some technical info on the Voyager FDS computer –
- There was a backup FDS unit but it failed in 1981.
- Custom CMOS CPU - 36 instructions. 80 KIPS, 115 kbps data rate.
- 128 registers, kept in memory.
- CMOS memory, a first in space, 8KB.
- No separate memory for program storage vs execution. The CMOS memory is non-volatile kept powered on by the RTG.
- DMA access to memory by hardware. Instead of “cycle-stealing”, the instructions indicated cycles where DMA can occur.
- KIPS = Kilo instructions per second
DMA = Direct memory Access
RTG = Radioisotope Thermoelectric generator
About the Voyager Spacecraft
The two Voyager spacecraft, Voyager 1 and 2, were launched on Sept 5, 1977 and Aug 20, 1977 resp, and have been traveling in space for over 46 years.
Voyager 1 is farther away from earth at 24.3 billion km (22.5 light hours), while V2 is 20.3 billion km away, located below the ecliptic. Both spacecraft are in interstellar space.
Here are the locations and some vital stats on the two Voyager spacecraft.
The Future of Voyager
In spite of the heroic efforts over the past few decades to keep Voyager operational, their days are numbered. The power levels generated by the Radioisotope thermoelectric generators (RTG) have steadily declined by about 50% over the past 46 years. Hence, various instruments have been shut off over the years. E.g., V1 cameras were shut off after the iconic images of the solar system and the pale blue dot were taken in 1990.
Here is the current instrument status from voyager.jpl.nasa.gov/...
The Voyager probes will be unable to keep instruments powered on much beyond the early 2030s, although NASA engineers may find clever ways to keep comms up for longer. The probes will keep traveling in space. In ~40K years, Voyager 1 will pass within 1.6 light-years of star Gliese 445; Voyager 2 will pass 1.7 light-years from star Ross 248. Perhaps, some distant civilization in the distant future will find and decipher the Golden records carried by the two probes.
Epilogue
It took a great deal of ingenuity and perseverance to fix this 1970s era computer, traveling for 46+ years, with no repairs or maintenance, 24.3 billion km away (it takes almost 2 days to get a response to a command from Voyager 1), with software written in a language no one uses anymore, by engineers and scientists who are no longer present or available and crammed into less than 4 kwords of memory. It is a testament to human creativity and resolution that this could be done. Hats off to everyone that designed this magnificent machine and to those who keep it going and going.
Smiles and applause among members of the Voyager flight team in a conference room at NASA’s Jet Propulsion Laboratory on April 20
www.jpl.nasa.gov/...
Further Reading:
- voyager.jpl.nasa.gov/…
- NASA’s Voyager 1 Resumes Sending Engineering Updates to Earth — www.jpl.nasa.gov/...
- The Brains of the Voyager Spacecraft: Command, Data, and Attitude Control Computers — www.allaboutcircuits.com/…
- Computers in Spaceflight: The NASA Experience — ntrs.nasa.gov/...
