NASA turns off heating of Voyager probes to keep them operational longer

NASA has a plan to keep the two Voyager probes launched more than 40 years ago operational for longer. The generators supply less power, so choices have to be made. Turning off heating elements is one of those choices.

NASA recently decided to disable a Voyager 2 heating element. Voyager 2 was chosen because this probe has an extra scientific instrument on board compared to its sister and therefore has a slightly higher energy requirement. It concerns the cosmic ray subsystem instrument. This can measure cosmic rays and played a crucial role in determining last November that Voyager 2 had left the heliosphere behind.

Despite turning off the heating for this instrument and the fact that the temperature has dropped to -59 degrees Celsius as a result, it still sends data back to Earth. The current temperature is lower than the -45 degrees Celsius at which the instrument was tested 42 years ago. In 2017, NASA also turned off a heater on a Voyager 1 spectrometer, and this instrument continued to function for years after the temperature dropped below the tested values.

Turning off instrument heating is part of a new plan to best manage the power needs of both probes. This is necessary because the thermoelectric radioisotope generators, of which both Voyagers have three on board, gradually generate less and less power. This has to do with the decay of the isotope plutonium-238, whereby the released heat is converted into electrical current. Partly due to this decay, each probe produces approximately 4W less power per year. That means that the generators produce about forty percent less than during the launches 42 years ago. Since Voyager 2 is currently more than 18 billion kilometers and Voyager 1 21.9 billion kilometers from the sun, solar panels would not help.

NASA previously indicated that steps such as turning off heating should ensure that the Voyager probes can collect and send data until 2025. More options will be explored in the coming years to save energy, including the option to switch off more heaters. In the cold of interstellar space, it is important to closely monitor the temperature of both probes. For example, sufficient power must be available to heat the fuel supply of the thrusters. If it freezes, the probes will no longer be able to rotate and Voyagers may not be able to point their antennas toward Earth. Technicians can then no longer send commands to the probes or receive data back.

These thrusters are also starting to perform less well. That’s why NASA deployed a separate set of backup thrusters on both probes. Voyager 1 did that in December 2017; the thrusters switched on at the time had not been used since 1980. With Voyager 2, current thrusters are also underperforming, requiring more short pulses for probe orientation. This month, NASA engineers decided to switch on a separate set of thrusters for Voyager 2 as well; these have not been used since the probe flew past Neptune in 1989.