NASA shuts down Voyager 2 plasma instrument to extend mission

NASA mission engineers turned off the plasma science instrument aboard the Voyager 2 spacecraft due to the probe’s power supply gradually dwindling.

Traveling more than 20.5 billion miles from Earth, the spacecraft continues to use four scientific instruments to study the region outside our heliosphere, the protective bubble of particles and magnetic fields created by the sun. The probe has enough power to continue exploring this region with at least one operational scientific instrument until the 2030s.

Mission engineers took steps to avoid turning off a science instrument for as long as possible because the science data collected by the twin Voyager probes is unique. No other man-made spacecraft has operated in interstellar space, the region outside the heliosphere.

The plasma science instrument measures the amount of plasma (electrically charged atoms) and the direction in which it flows. It has collected limited data in recent years due to its orientation relative to the direction in which plasma flows in interstellar space.

Both spacecraft are powered by decaying plutonium and lose about 4 watts of power each year. After the Voyager twins completed their exploration of the giant planets in the 1980s, the mission team turned off several scientific instruments that would not be used in the study of interstellar space. This gave the spacecraft a lot of extra power until a few years ago.

Since then, the team has deactivated all onboard systems not essential to the proper functioning of the probes, including some heaters. To delay the shutdown of another scientific instrument, they also adjusted how Voyager 2’s voltage is monitored.

Monitoring results

On September 26, engineers gave the order to turn off the scientific plasma instrument. Sent by NASA’s Deep Space Network, it took 19 hours to reach Voyager 2, and the return signal took another 19 hours to reach Earth.

Mission engineers are still carefully monitoring changes to the 47-year-old spacecraft’s operations to ensure they don’t generate any unwanted side effects. The team confirmed that the shutdown command was executed without incident and that the probe is operating normally.

In 2018, the plasma science instrument proved key to determining whether Voyager 2 had left the heliosphere. The boundary between the heliosphere and interstellar space is demarcated by changes in atoms, particles and magnetic fields that Voyagers’ instruments can detect.

Inside the heliosphere, particles from the sun flow outward, away from our nearest star. The heliosphere is moving in interstellar space, so at Voyager 2’s position near the front of the solar bubble, the plasma is flowing almost in the opposite direction as the solar particles.

The plasma science instrument consists of four “cups”. Three sections point in the direction of the sun and observe the solar wind inside the heliosphere. A fourth points at right angles to the other three and observes plasma in planetary magnetospheres, the heliosphere and now interstellar space.

When Voyager 2 left the heliosphere, the plasma flow in the three sun-facing cups decreased significantly. The most useful data from the fourth cut is provided only once every three months, when the spacecraft makes a 360-degree turn on the axis pointed toward the sun. This factored into the mission’s decision to turn off this instrument before the others.

Voyager 1’s plasma science instrument stopped working in 1980 and was turned off in 2007 to save power. Another instrument aboard Voyager 2, called the plasma wave subsystem, can estimate plasma density when solar flares shock through the interstellar medium, producing plasma waves.

The Voyager team continues to monitor the health of the spacecraft and its available resources to make technical decisions that maximize the mission’s science outcomes.