In brief: Mission control specialists at NASA's Jet Propulsion Laboratory are taking additional energy-conserving measures to ensure their Voyager probes can continue operating in interstellar space for as long as possible. Engineers shut down the cosmic ray subsystem instrument on Voyager 1 on February 25. This tool, which consists of three telescopes that study cosmic rays, helped NASA determine when Voyager 1 left the heliosphere.The space agency plans to turn off the low-energy charged particle experiment on Voyager 2 on March 24. The LECP instrument features two subsystems and is used to measure electrons, ions, and cosmic rays in space. Once it goes dark, just three scientific instruments will remain operational on each craft.NASA launched its twin Voyager probes in 1977 to study planets in the outskirts of our solar system including gas giants Jupiter and Saturn. Voyager 1 completed its planetary objectives and entered into an extended mission phase in 1980, eventually becoming the first human-made object to reach interstellar space in 2012. Voyager 2 crossed into interstellar territory in 2018. To date, they are the only two spacecraft to have achieved this milestone.Both probes leverage a radioisotope power system which 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.Instruments that specialized in collecting data during the early-day planetary flybys were turned off shortly after those missions wrapped up. Others were shut off in succession in order to conserve energy and extend the missions. Each probe launched with a nearly identical set of 10 science instruments. // Related StoriesOn Voyager 1, only the low-energy charged particles, magnetometer, and plasma wave subsystem remain operational. Voyager 2 also still has its magnetometer and plasma wave subsystem, as well as its cosmic ray subsystem; the low-energy charged particles instrument will be shut off later this month.NASA believes the latest shutdowns should buy them another year or so before they have to turn another instrument off.