NASA Powers Down Experiment on Voyager 2

NASA’s oldest operational mission is slowly drawing down. This week, controllers announced the decision to power down Voyager 2’s plasma science instrument in an effort to conserve the spacecraft’s power.

The determination to shut down the experiment is part of a decade-long goal of extending the mission’s conclusion into the 2030s. Currently 20.5 billion kilometers (almost 138 times the distance from Earth to the Sun) away, the mission is now probing the realm outside of the heliosphere, the area around the Sun where its protective magnetic field shields us from intergalactic particles.

Launched in 1977, Voyagers 1 and 2 took advantage of a rare alignment of the outer planets to carry out a series of historic flybys. Both Voyagers went past Jupiter and Saturn, and Voyager 2 is to date the only mission to carry out a close reconnaissance of the ice-giant worlds, Uranus and Neptune; a close pass near Titan ejected Voyager 1 from the plane of the solar system. On a trajectory headed out of the solar system, both carry Golden Records containing messages and greetings from Earth for any future alien salvagers.

Controllers turned off Voyager 1’s plasma science instrument in 2007, but the same science package on Voyager 2 was crucial to determining the flow and direction of electrically charged atoms, or plasma, in the spacecraft's vicinity. Understanding those dynamics was pivotal in determining in 2018 that the spacecraft has crossed over the boundary of the heliosphere and into interstellar space.

The instrument consists of four "cups," each of which collects plasma for measurement. Three of the cups point sunward, while a fourth scoops up material at right angles to the others. Researchers saw a dramatic drop in solar wind particles in 2018, a key sign that the spacecraft had in fact departed the realm dominated by the Sun.

The fourth cup measures the particles in the space between the stars, called the interstellar medium, once every three months — every time the spacecraft completes a rotation around its sunward axis. But this maneuver uses precious hydrazine fuel, which Voyager 2 needs to stabilize and continue communications with NASA’s Deep Space Network. The need to conserve fuel played a role in the decision to power down the plasma science instrument rather than the others still operating on the spacecraft.

The decision was planned-for and unrelated to issues that Voyager 2 had relaying science and engineering data with Earth late last year. Voyager 1 had its own share of technical issues earlier this year, but both are now in good health.

Both spacecraft are powered by a radioisotope thermoelectric generator (RTG), which uses decaying plutonium-238, a radioactive element with a half-life of 87.7 years. For Voyager 2, the the RTG provided 470 watts of power at launch; Voyager 2 loses about 4 watts of power output each year.

NASA sent the command to power down the instrument on September 26th. Now 19 light-hours away, mission controllers waited for the return call to ensure the shutdown didn’t affect other systems. The remaining instruments will focus on studying the interstellar medium. In particular, the plasma wave subsystem still monitoring plasma density in the spacecraft’s vicinity when waves from solar eruptions pass by.

The mission is far from over, with another decade of juice: “As one of two interstellar explorers, Voyager 2 will continue to observe the evolution of particles and fields in the interstellar medium,” says Linda Spilker (NASA-JPL). “It collects scientific data about the region, including the composition and behavior of cosmic rays and energetic particles, the characteristics of plasma oscillations, and the structure and detection of the magnetic field.”

It’s strange to think: Voyager 2 will be a full light-day away from us in the coming years. “The Voyager mission has transformed our knowledge about interstellar space, providing the first direct measurements of this interesting place,” says Spilker.

Now drifting off into interstellar space in the direction of the southern constellation Pavo, the Peacock, Voyager 2 will continue to chronicle its journey into interstellar space until the day comes for the mission to end.