Engineers working on NASA’s Voyager 1 probe have successfully mitigated a problem with the spacecraft’s engines, which allow the distant probe to point toward Earth so it can receive commands, send engineering data and provide the unique science data it collects.

After 47 years, a fuel tube inside the engines became clogged with silicon dioxide, a byproduct of the aging of the rubber diaphragm in the spacecraft’s fuel tank. The clogging reduces the efficiency of the engines, which can generate thrust. After weeks of careful planning, the team switched the spacecraft to a different set of engines.

The engines are powered by liquid hydrazine, which turns into a gas and is released in breaths lasting tens of milliseconds to slowly tilt the spacecraft’s antenna toward Earth. If the flooded engine were working, it would need to make about 40 short pulses a day.

Both Voyager probes were equipped with three sets of engines, or arms: two sets of guidance engines and one set of orbit-correction maneuvering engines. During the mission’s flybys, each type of engine was used for different purposes. However, since Voyager 1 was traveling on a fixed path outside the solar system, its thrust requirements were simpler and both arms of the engine could be used to guide the spacecraft toward Earth.

In 2002, the mission’s engineering team, based at NASA’s Jet Propulsion Laboratory in Southern California, noticed a blockage in some fuel lines in the thrust branch used for guidance, so the team switched to a second branch. When that branch showed signs of blockage in 2018, the team switched to the trajectory correction maneuver engines and have used that branch ever since.

Now these orbital correction engine pipes are much more clogged than the original branches that the team replaced in 2018.

The clogged tubes are located inside the engines and direct fuel to the catalyst layers, where it turns into gases. (These are different from the fuel tubes that send hydrazine to the engines.) While the diameter of the tube hole was originally only 0.01 inch (0.25 millimeters), the blockage reduced it to 0.0015 inches (0.035 mm), or about twice the width of a human hair. As a result, the team had to turn back to one of the propulsion branches.

Engine warming up

Switching to other propellants would have been relatively straightforward for a mission in 1980 or even 2002. But the spacecraft’s age introduced new problems, primarily related to energy supply and temperature. The mission shut down all non-essential onboard systems, including some heaters on both spacecraft, to conserve power supplies that were diminishing due to the decay of plutonium.

While these steps worked to reduce power, they also cooled the spacecraft, which was exacerbated by the loss of other non-essential heat-generating systems. As a result, the steering thruster branches cooled, and opening them in this situation could damage them, rendering the thrusters unusable.

The team decided that the best option was to turn on a non-essential heater before starting the engines. However, as with so many problems faced by the Voyager crew, this became a dilemma: The spacecraft’s power supply was so low that turning on the optional heaters would have required the mission to turn off something else to provide enough power for the heaters, and all of these things are now working, which is considered essential.

While investigating the problem, they ruled out shutting down one of the science instruments that was still operating because of the risk of non-recovery. After further investigation and planning, the engineering team determined that they could safely shut down one of the spacecraft’s main heaters for an hour, freeing up enough energy to turn on the engine heaters.

It worked. On August 27, they confirmed that the engine arm needed to help direct Voyager 1 toward Earth was back online.

“All the decisions we have to make in the future are going to require much more analysis and careful consideration than they did before,” said Suzanne Dodd, Voyager project manager at the Jet Propulsion Laboratory, which manages Voyager for NASA.

The spacecraft explores interstellar space, the region outside the sun’s bubble of particles and magnetic fields that no other spacecraft can visit for long. The mission’s science team is working to keep the Voyagers operating for as long as possible so they can continue to explore what the interstellar medium is like.