‘Dead’ thrusters revived to help Voyager 1 through a communications pause

Engineers at NASA say they have successfully revived thrusters aboard Voyager 1, the farthest spacecraft from our planet, in the nick of time before a planned communications blackout. A side effect of upgrades to an Earth-based antenna that sends commands to Voyager 1 and its twin, Voyager 2, the communications pause could have occurred when the probe faced a critical issue — thruster failure — leaving the space agency without a way to save the historic mission. The new fix to the vehicle’s original roll thrusters, out of action since 2004, could help keep the veteran spacecraft operating until it’s able to contact home again next year. Voyager 1, launched in September 1977, uses more than one set of thrusters to function properly. Primary thrusters carefully orient the spacecraft so it can keep its antenna pointed at Earth. This ensures that the probe can send back data it collects from its unique perspective 15.5 billion miles (25 billion kilometers) away in interstellar space, as well as receive commands sent by the Voyager team. Within the primary set are additional thrusters that control the spacecraft’s roll, which enables Voyager 1 to remain pointed at a guide star so it can remain oriented in space. If Voyager can’t control its roll motion, the mission could be threatened. But as the thrusters fire, tiny amounts of propellant residue have built up over time. So far, engineers have managed to avoid clogging by commanding Voyager 1 to cycle between its original and backup thrusters for orientation, as well as a set of thrusters that were used to change the spacecraft’s trajectory during planetary flybys in the 1980s. The trajectory thrusters, however, do nothing to contribute to the spacecraft’s roll. Voyager 1’s original roll thrusters stopped working more than two decades ago after power was lost in two internal heaters, which means the spacecraft has been relying on the backup roll thrusters to remain pointed at a guide star ever since. “I think at that time, the team was OK with accepting that the primary roll thrusters didn’t work, because they had a perfectly good backup,” said Kareem Badaruddin, Voyager mission manager at NASA’s Jet Propulsion Laboratory in Pasadena, California, in a statement. “And, frankly, they probably didn’t think the Voyagers were going to keep going for another 20 years.” Now, Voyager 1 engineers are concerned that clogging from the residue could cause the spacecraft’s backup roll thrusters to stop working as soon as this fall — and they had to get creative, as well as take risks, to revive the long-defunct primary roll thrusters. Fixing broken equipment in space When the heaters on the primary roll thrusters failed in 2004, engineers thought they couldn’t be fixed. But with the threat posed by clogging looming, the team returned to the drawing board to see what had gone wrong. Engineers considered the possibility that a disturbance in the circuits controlling the power supply to the heaters flipped a switch to the wrong position — and flipping it to the original position might restart the heaters, and in turn, the primary roll thrusters. But it wasn’t a straightforward solution for a probe that’s operating so far away. The spacecraft is currently beyond the heliosphere, the sun’s bubble of magnetic fields and particles that extends well beyond the orbit of Pluto. The mission team had to take a risk by switching Voyager 1 to its primary roll thrusters and turning them on before attempting to fix and restart the heaters. The heaters could only function if the thrusters are also switched on. If Voyager 1 drifted too far from its guide star, the spacecraft’s programming would trigger the roll thrusters to fire — but if the heaters weren’t turned on yet at that moment, the automatic sequence could have triggered a small explosion. A nail-biting test In addition to the risk, the team, which began its work earlier this year, was facing a time constraint. A giant Earth-based antenna in Canberra, Australia, went offline May 4 for upgrades that will be ongoing until February 2026. NASA’s Deep Space Network enables the agency to communicate with all of its spacecraft — but its Canberra antenna is the only one with enough signal strength to send commands to the Voyager probes. “These antenna upgrades are important for future crewed lunar landings, and they also increase communications capacity for our science missions in deep space, some of which are building on the discoveries Voyager made,” said Suzanne Dodd, Voyager project manager and director of the Interplanetary Network at JPL, which manages the Deep Space Network for NASA, in a statement. “We’ve been through downtime like this before, so we’re just preparing as much as we can.” While the antenna will briefly operate in August and December, the mission team members wanted to command Voyager 1 to test its long-dormant thrusters before they could no longer communicate with the spacecraft. This way, if they need to turn on the thrusters in August, the team would know whether that was a viable option. On March 20, the team waited to see the results return from Voyager 1 after sending a command to the probe the day before to activate the thrusters and heaters. It takes more than 23 hours for data to travel back from Voyager 1 to Earth due to the sheer distance between the two. Had the test failed, Voyager 1 may have already been at risk. But the team watched the data stream in, showing the temperature of the thruster heaters rising dramatically, and knew it had worked. “It was such a glorious moment. Team morale was very high that day,” said Todd Barber, the mission’s propulsion lead at JPL, in a statement. “These thrusters were considered dead. And that was a legitimate conclusion. It’s just that one of our engineers had this insight that maybe there was this other possible cause and it was fixable. It was yet another miracle save for Voyager.”