Voyager 1’s 24-Hour Signal Delay Is a Deep-Space Engineering Test
The symbolic nature of Voyager 1 crossing 1 light-day does not have much significance for operations. NASA expects Voyager 1 to travel to a distance of one light-day from Earth on 15 November 2026, which means that the spacecraft will cover approximately 16.1 billion miles or 25.9 billion kilometers of space. This translates to 24 hours for the radio command sent from Earth to reach the probe and another 24 hours before Voyager 1 would respond. For the engineers at JPL, it means a turnaround of 48 hours on every command sent to and from the vehicle.
Suzy Dodd, the Voyager project manager, put this in a rather colorful way by noting that “If I send a command and say, ‘good morning, Voyager 1,’ at 8 a.m. on a Monday morning, I’m going to get Voyager 1’s response back to me on Wednesday morning at approximately 8 a.m.” As you can guess from Dodd’s statement, there is a lot of planning and pre-programming required when communicating with Voyager 1 at extreme ranges in deep space.
But precisely for this reason, Voyager 1 becomes a useful engineering example of spacecraft operations at such great distances and limited resources. Launched from the Cape Canaveral Air Force Station on a Titan IIIE-Centaur rocket on 5 September 1977, the vehicle completed its assigned Jupiter and Saturn missions after passing by Saturn in November 1980. However, despite being 30 years past its designated mission, Voyager 1 continues to function using the same core components and architecture it started out with while working around multiple age-related issues.
Firstly, there is a limit on the power available. Voyager 1 carries three plutonium-238-powered radioisotope thermoelectric generators (RTGs). At the moment of launch, they generated 470 watts and currently deliver 250 watts, losing approximately 4 watts per year. In a vacuum, RTGs remain the best solution because they convert the heat released from radioactive decay directly into electricity without moving parts, allowing for prolonged power supply without maintenance. But there are no constant-power solutions in the real world – every year, the probe gets less electricity to work with.
Secondly, a 24-hour one-way communication latency impacts operations in a significant way. If engineers cannot diagnose and solve an anomaly almost instantly, they are not dealing with Earth-orbiting satellites. Every command sequence has to be carefully thought out in advance because the troubleshooting process is twice as long and takes a lot more planning. Thus, the importance of onboard autonomy and safe-state behaviors grows significantly, since it allows the spacecraft to protect itself until engineers figure out what happened.
This was proven by the fact that Voyager 1 experienced several issues recently. In May 2022, it suffered a problem with attitude control, which was resolved by November of last year, when it suddenly stopped returning readable science data. After five months of work, engineers established that one onboard computer had a faulty memory chip and rewired everything. Although the problem was solved, the process is quite illustrative of working at great distances.
It also tells us something about the capabilities of 1970s designs. Even though Voyager 1 wasn’t designed to operate indefinitely, its builders gave it a strong framework and enough resistance to sustain a long-term cruise in interstellar space. Such a combination is exactly why engineers are able to recover functionality from aging equipment by making difficult decisions and using their experience.
In addition, there is a practical aspect to the issue. Voyager 1 travels at 38,000 miles per hour and covers 3.5 astronomical units annually, which means that its link budget will only deteriorate with time. So the farther it is, the weaker its signal will be when received on Earth. For this reason, JPL has switched to using light-time for describing distances between the spacecraft and Earth.
Voyager 1 will continue traveling farther from Earth even after it stops transmitting meaningful scientific data back. What is more interesting now, however, is what it teaches about managing spacecraft operations with limited power supplies, aging hardware, and 24-hour latencies. For that matter, the spacecraft serves as an ongoing example of U.S. deep-space operations engineering.
Its lessons are fairly straightforward: manage power and keep the system stable, isolate failures accurately yet slowly, and extract the maximum science data out of aging hardware. These principles are exactly what will determine Voyager 1’s further progress – one light-hour at a time.
By David Whitaker — Associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into accessible technical stories.
