Voyager 1 probe fires long-dormant thrusters in interstellar space

Artist’s concept of the Voyager 1 spacecraft. Credit: NASA/JPL-Caltech

NASA’s Voyager 1 spacecraft has fired a set of four control thrusters for the first time in 37 years, giving the long-lived probe a new way to point itself on its cruise into interstellar space 13 billion miles from Earth.

The backup rocket jets were originally designed to help the Voyager 1 spacecraft aim its instruments at planets and moons on its journey through the solar system.

Launched in September 1977, the plutonium-powered probe encountered Jupiter and its moons in March 1979, then flew by Saturn in November 1980, before heading northward out of the ecliptic plane, the disk in which the solar system’s planets reside. The identical Voyager 2 spacecraft visited Jupiter and Saturn, then went on to explore Uranus and Neptune.

Voyager 1’s primary thrusters have been degrading in the last few years, NASA said in a statement. The tiny rockets generate around 0.2 pounds of thrust, roughly equivalent to the force from the weight of a deck of playing cards.

The MR-103 thrusters, provided by Aerojet Rocketdyne, are designed to fire in pulses to rotate the spacecraft and keep its 12-foot (3.7-meter) antenna pointed at Earth, but engineers have noticed more firings were needed recently, indicating the jets were losing some of their performance.

Ground controllers looked for a solution, and they decided to try out a separate rocket pack with four identical MR-103 “trajectory correction maneuver,” or TCM, thrusters on the back side of the spacecraft that were used to nudge Voyager 1 and keep it on course during flybys earlier in the mission.

The TCM thrusters were last fired on Nov. 8, 1980, on approach to Saturn, and Voyager 1 used the miniature rocket engines in a more continuous firing mode, not in individual pulses as needed now, NASA officials said.

“On Tuesday, Nov. 28, 2017, Voyager engineers fired up the four TCM thrusters for the first time in 37 years and tested their ability to orient the spacecraft using 10-millisecond pulses,” NASA said in a statement. “The team waited eagerly as the test results traveled through space, taking 19 hours and 35 minutes to reach an antenna in Goldstone, California, that is part of NASA’s Deep Space Network.

“Lo and behold, on Wednesday, Nov. 29, they learned the TCM thrusters worked perfectly — and just as well as the attitude control thrusters,” the statement said.

Engineers had to do some detective work to make sure the thrusters could be safely tested.

“The Voyager flight team dug up decades-old data and examined the software that was coded in an outdated assembler language, to make sure we could safely test the thrusters,” said Chris Jones, chief engineer at NASA’s Jet Propulsion Laboratory.

Controllers at JPL plan to switch to the TCM thrusters full-time in January, giving Voyager 1’s other rocket jets a rest. The spacecraft will have to turn on one heater on each of the four TCM thrusters, putting a further drain on Voyager 1’s decaying plutonium power source.

Voyager 1 will switch back to its other thrusters once the probe no longer has enough power for the heaters, officials said.

“With these thrusters that are still functional after 37 years without use, we will be able to extend the life of the Voyager 1 spacecraft by two to three years,” said Suzanne Dodd, project manager for Voyager at JPL.

Each Voyager spacecraft carries four TCM thrusters and 12 small attitude control rocket jets, divided into two redundant chains of six thrusters. Voyager 1 was already operating on its backup branch of attitude control thrusters.

NASA will likely do a similar test of Voyager 2’s TCM thrusters, which could be used if that spacecraft’s attitude control thrusters further degrade in the future.

Voyager 1 is the most distant human-made object, and scientists announced in 2013 the craft’s crossing of the boundary between the sun’s influence and interstellar space, making it the first mission to explore the void between the stars. The spacecraft’s plutonium generator should produce enough power to keep it operating until around 2025.

Scientists expect Voyager 2 to make the crossing into interstellar space within a few years, heading in a different direction from the sun.

NASA’s Pioneer 10 and 11 spacecraft are also in interstellar space, but no longer have enough power to contact Earth, and New Horizons spacecraft is on a trajectory to leave the solar system after its encounter with Pluto.

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