NASA switches off Voyager 2 plasma instrument to stretch out juice

Veteran probe set to score a half-century while still doing science

Engineers have turned off Voyager 2's plasma science instrument in an effort to eke out the veteran probe's dwindling power supply.

The instrument has become less useful in recent years due to its orientation relative to the direction that plasma is flowing in interstellar space. It consists of four "cups," three of which point toward the Sun to observe the solar wind inside the heliosphere. A fourth points at a right angle to the other three to observe the plasma in planetary magnetospheres, and now interstellar space.

It proved critical when Voyager 2 exited the heliosphere in 2018, following its sibling, Voyager 1, into interstellar space.

Scientists took a year to confirm that Voyager 1 had entered interstellar space since it lacked a working plasma science instrument – the device stopped working in 1980 and was turned off in 2007. However, the instrument aboard Voyager 2 continued to function, and scientists noted a dramatic fall in the flow of plasma into the three cups facing the Sun as it exited the heliosphere.

The heliosphere is the bubble of particles and magnetic fields created by the Sun that surrounds the solar system.

According to NASA: "The most useful data from the fourth cup comes only once every three months, when the spacecraft does a 360-degree turn on the axis pointed toward the Sun. This factored into the mission's decision to turn this instrument off before others."

The switch-off command, which was transmitted to the probe at the end of September, now means that only four instruments remain active, the same as Voyager 1. According to engineers, the spacecraft has enough power to continue studying the region outside the heliosphere with at least one operational science instrument into the 2030s.

The Voyagers are powered by radioisotope thermoelectric generators (RTGs), which convert the heat generated by decaying plutonium into electricity. The decay process has meant the generator produces slightly less power year after year. This wasn't so much of an issue early on in the mission, but as the probes approach the half-century mark, dwindling power means difficult decisions must be made.

All non-essential systems, including some heaters, have been turned off. To postpone deactivating another science instrument, engineers adjusted Voyager 2's voltage regulator last year.

It took 19 hours for the command to turn off the instrument to reach Voyager 2 and another 19 hours for the return signal to reach Earth. The team reported that the command was executed as expected and that the probe was operating normally. ®