Engineers shut down the cosmic ray subsystem instrument on Voyager 1 on February 25.
The space agency plans to turn off the low-energy charged particle experiment on Voyager 2 on March 24.
Once it goes dark, just three scientific instruments will remain operational on each craft.
Voyager 2 crossed into interstellar territory in 2018.
To date, they are the only two spacecraft to have achieved this milestone.
Both probes leverage aradioisotope power systemwhich generates electricity from decaying plutonium.
After more than 47 years, however, the fuel source is starting to dry up.
According to NASA, the systems lose about four watts of power each year.
Others were shut off in succession in order toconserve energyand extend the missions.
Each probe launched with a nearly identical set of 10 science instruments.
On Voyager 1, only the low-energy charged particles, magnetometer, and plasma wave subsystem remain operational.
