NASA's Voyager 1 team overcame a complex challenge by successfully switching to backup thrusters after the main thrusters became clogged with silica. The probe uses thrusters to keep it pointed at Earth, but after 47 years in space, some of its fuel tubes have become clogged. This adaptation is complicated by the fact that the ship is now in interstellar space with decreasing power.

Artist's concept shows the Voyager spacecraft traveling through the stars. Image source: NASA/JPL-Caltech

Engineers on NASA's Voyager 1 probe have successfully mitigated issues with the spacecraft's thrusters, which keep the distant probe pointed at Earth so it can receive commands, send engineering data and provide the unique science data it is collecting.

After 47 years, a fuel line inside the thruster has become clogged with silica, a byproduct of the rubber diaphragms in the spacecraft's fuel tanks that develop over time. This blockage reduces the efficiency of the propeller in producing power. After weeks of careful planning, the team replaced the spacecraft with a different set of thrusters.

The thruster's fuel is liquid hydrazine, which is converted into a gas and released in pulses tens of milliseconds long, gently tilting the spacecraft's antenna toward Earth. If the clogged thruster is healthy, it needs about 40 of these short pulses per day.

NASA's Voyager spacecraft model. The twin Voyager spacecraft have been flying since 1977, exploring the outer regions of the solar system. Image source: NASA/JPL-Caltech

Both Voyager probes have three sets of thrusters, or branches: two sets of attitude propulsion thrusters and one set of trajectory correction maneuvering thrusters. Both sets of thrusters are used for different purposes during planetary flybys on missions. However, since Voyager 1's flight path outside the solar system remains unchanged, its thruster requirements are relatively simple, and any thruster branch can be used to point the spacecraft toward the Earth.

In 2002, the mission engineering team at NASA's Jet Propulsion Laboratory in Southern California discovered that some fuel lines in the branch of the attitude thruster used for pointing were clogged, so the team switched it to a second branch. When the branch showed signs of congestion in 2017, the team switched to trajectory correction maneuvering thrusters and has been using the branch ever since.

The pipes are now more clogged than the original branches when the team replaced them in 2018. The clogged tubes are located inside the propeller and direct the fuel to a catalyst bed where it is converted into gas. (These are not the same fuel lines that feed hydrazine into the thrusters). The diameter of the pipe opening, which was originally only 0.01 inches (0.25 millimeters), was reduced to 0.0015 inches (0.035 millimeters) after the blockage, which is about half the width of a human hair. Therefore, the team needed to switch back to one of the attitude propulsion thruster branches.

For missions from 1980 or even 2002, switching to a different thruster was a relatively simple operation. But the aging of spacecraft brings new challenges, mainly related to power supply and temperature. The mission shut down all non-essential onboard systems on both spacecraft, including some heaters, to conserve dwindling power supplies generated by decaying plutonium.

Each of NASA's Voyager probes was equipped with three radioisotope thermoelectric generators (RTGs), including the one pictured here. RTGs power spacecraft by converting heat generated by the decay of plutonium-238 into electrical energy. Source: NASA/JPL-Caltech

While these measures had the effect of reducing power, they also caused the spacecraft to get colder, an effect exacerbated by the loss of other non-essential systems that generate heat. As a result, the branches of the attitude thrust thrusters have become cold, and turning them on in this state could damage them, rendering the thrusters unusable.

The team decided that the best approach would be to warm the thrusters before the switch by turning on heaters that were deemed non-essential. However, like many of the challenges faced by the Voyager team, this also presented a conundrum: The spacecraft's power supply was so low that turning on non-essential heaters would require shutting down other equipment to provide sufficient power to the heaters, and all equipment currently operating was considered essential.

While studying the problem, they ruled out the possibility of shutting down a still-running scientific instrument for a limited period of time because of the risk that the instrument would not be able to be restarted. After further study and planning, the engineering team determined they could safely turn off one of the spacecraft's main heaters for up to an hour, freeing up enough power to turn on the thruster heaters.

They succeeded. On August 27, they confirmed that the required thruster branch was working again to help Voyager 1 point toward Earth.

"All the decisions we have to make going forward will require more analysis and caution than ever before," said Suzanne Dodd, Voyager program manager at Jet Propulsion Laboratory, which manages Voyager for NASA.

These spacecraft are exploring interstellar space, a region outside the bubble of particles and magnetic fields produced by the Sun, where no other spacecraft will be able to visit for a long time. The mission science team is working to keep Voyager operating as long as possible to continue to reveal what the interstellar environment is like.

Compiled from /ScitechDaily