Voyager 2 Has Another Instrument Turned Off

Space.com reports that NASA engineers have turned off one of the few remaining scientific instruments that was running on Voyager 2, as the power available on the space probe continues to fall with age.  

Voyager 2 launched back in August of 1977 on a trip to Jupiter and a "Grand Tour" of the outer solar system. That mission was completed decades ago, and the probe left the solar system on Nov. 5, 2018.  Yeah, you can say the two Voyagers are in the 47th year of a 4 year mission. 

...It is currently 12.8 billion miles (20.5 billion kilometers) from Earth and is using four science instruments to study space beyond the heliosphere, the sun's bubble of influence around the solar system. NASA thinks that Voyager 2 has enough power to keep running one science instrument into the 2030s, but doing that requires selecting which of its other instruments need to be turned off. 

Mission specialists have tried to delay the instrument shutdown until now because Voyager 2 and Voyager 1 are the only two active probes humanity has in interstellar space, making any data they gather unique. Thus far, six of the spacecraft's initial 10 instruments have been deactivated. Now, losing the seventh has become unavoidable, and the spacecraft's plasma science instrument drew the short straw. On Sept. 26, engineers gave the command to turn off the instrument.

The plasma science instrument is an interesting concept.  It consists of four "cups" - detectors - to sense the amount of plasma flowing around the spacecraft.  Three of those are pointed toward the sun to measure the charged particles in the solar wind while the fourth is pointed forward, to look for charged particles coming from interstellar space.  To be honest, the instrument really hasn't done much since the probe left the heliosphere and entered interstellar space. Ironically, though, it was that instrument that verified it had left the heliosphere.  

This NASA graphic shows the locations of NASA's Voyager spacecraft in interstellar space. NASA announced the arrival of Voyager 2 in interstellar space on Dec. 10, 2018. Voyager 1 reached the milestone in 2012. (Image credit: NASA/JPL-Caltech)

The situation was that the three detectors pointed back where it came from didn't detect anything because there's no solar wind after it crossed the heliopause, and the other detector only provided useful data once every three months when the spacecraft made a 360-degree turn on its axis. It was a very practical choice of an instrument to shut down.

Both Voyagers are in the same condition, losing about 4 Watts every year.  In the 1980s, after their primary mission to the outer planets was complete, several instruments were shut down; things that were never expected to be used again. Voyager 1's plasma experiment equivalent to this one failed back in 2007 and has been off since then.   

It's a common thing among human beings to look for "big round numbers" and the race that both of the Voyagers are in is to see if they make their 50th anniversaries in space. They've both recently crossed into their 47th year, having launched in August and September of 1977.  

Reality, though, is that the Voyagers don't have much time left.  Both probes are powered by Radioisotope Thermal Generators (RTGs) and those are expected to keep the few instruments that need to run alive until 2025, but that could change with the random failure of any one of thousands of components.  The RTGs might operate longer than 2025 although probably not much longer.  Either way, eventually the RTGs will no long be able to power enough of the instruments to get data and transmit it back.  Eventually, first one Voyager then the sister spacecraft will go silent.  Even though they won't generate enough heat to power the instruments, the RTGs might keep the Voyagers a little warmer, but eventually they'll cool to almost absolute zero.  

As I said over a decade ago, if we're lucky some day a ship from Earth may find one and bring her back to whatever serves as the equivalent of the Smithsonian in those days.  In all probability, they will simply follow the Newtonian laws of motion, cool to a couple of degrees Kelvin and glide away forever, all alone in the night.