After 47 years of exploring the universe and uncovering countless secrets, the Voyager 2 plasma science experiment has been shut down for the final time. Although this was inevitable, since Voyager 2 needed to conserve power, this was a sad end to one of humanity’s most successful space experiments.
The probe is currently 12.8 billion miles (20.5 billion kilometers) away from Earth. It needs to conserve as much energy as possible to continue sending us at least some data from its current location.
This has forced the team behind Voyager 2’s ongoing maintenance to shut down this venerable scientific device. By closing PSE, we are closing a chapter in our exploration of plasma dynamics in the solar system, which is key to understanding space weather and planetary magnetospheres.
Join us as we look at these now-retired and more distant human science experiments.
What is a plasma science experiment?
Developed at MIT, the PSE, also known as the Plasma Spectrometer (PLS), serves as the eyes and ears of Voyager 2, measuring charged particles impacting our solar system and beyond. The instrument is designed to measure charged particles in planetary magnetospheres, the solar wind, and the interstellar medium – the matter found between stars.
As NASA explains, PLS uses two Faraday cup detectors. One is oriented along the Earth-spacecraft line, and the other is positioned at right angles to this line.
When switched on, the Earth-directed detector measures the macroscopic properties of plasma ions, allowing precise determination of their velocity, density and pressure. Three sequential energy scans are performed during these measurements with a relative energy variation (ΔE/E) of 20%, 7.2% and 1.8%.
A diagram showing one of Voyager’s probe pairs with its instruments. Source: NASA
These scans covered a range of subsonic to extremely supersonic flows – with a profiled Faraday cup measuring electrons within the energy range of 5 eV to 1 keV.
Since its launch aboard the Voyager 2 spacecraft in 1977, PLS has revealed new phenomena near exoplanets and within the solar wind throughout the solar system. This experiment confirmed the exact moment Voyager 2 crossed the heliosphere, leaving the influence of the Sun and entering interstellar space.
This is the first in a series of scientific instruments that will gradually stop working over the coming years. On September 26, Voyager 2 PLS sent its final communication from a distance of approximately 13 billion miles before receiving the order to shut down for good.
PLS has made some amazing discoveries
Throughout its life, PLS (and Voyager 2) has made amazing contributions to our understanding of the universe. One of the most important was the discovery of the ring around Jupiter’s moon Io.
This is a donut-shaped bubble of plasma surrounding the gas giant’s moon. The instrument also determined that the plasma cake is composed primarily of sulfur and oxygen.
Since its discovery, scientists have become confident that the donut was likely formed by Io’s chain of volcanoes. These were also discovered by other instruments on board the Voyager probe.
But this is only one of many discoveries made by the probe and the PLS probe. It has also made important discoveries about Saturn, Neptune and Uranus, said John Richardson, principal research scientist at MIT’s Kavli Institute for Astrophysics and Space Research.
“On Saturn, PLS found a magnetosphere filled with water and oxygen removed from Saturn’s icy moons. On Uranus and Neptune, the tilt of the magnetic fields led to PLS seeing lower density features, with Uranus’ plasma disappearing near the planet.”
Another breakthrough was the discovery of the end of heliotropy in our solar system, which was confirmed by a marked reduction in the outward flux of plasma from the Sun.
“This marks the end of the solar wind and the beginning of the local interstellar medium (LISM). Although PLS was not designed to measure LISM, it has been consistently measuring interstellar plasma currents outside the heliosphere,” Richardson told MIT.
Voyager 2’s power is supposed to continue until 2030
Both Voyager probes were equipped with three several-hundred-watt radioisotope thermoelectric generators (MHW RTGs). Each RTG contains 24 pressurized plutonium oxide spheres, meaning that upon launch, each RTG generated approximately 158 watts of electrical power.
These power supplies have no moving parts and are widely believed to be a very reliable option for spacecraft. The long legacy of the Voyager probes is testimony to this.
With a half-life of about 87.7 years, this should give the probe more than enough energy until at least 2020. Since then, the probe has been borrowing the additional power it needs from its backup battery, designated for emergency use.
However, this is not sustainable if the probe continues to perform some scientific experiments in the future. For this reason, PLS was chosen as the first of its instruments to be retired.
Voyager 2 undergoes solar thermal testing before launch. Source: NASA/Wikimedia Commons
According to Richardson, Voyager 2’s onboard power is declining by about 4 watts per year. The probe itself is nuclear powered by plutonium. With PLS down indefinitely, there should be enough power on board to keep other lower-power devices running well into the 2030s.
After that, the RTGs will likely degrade to the point where the probe’s electronics will no longer have enough power to operate or send signals back to Earth. Beyond that, the probe could survive for billions of years with most or all of its instruments turned off!
Voyager 2 will outlive humanity
Voyager 2 is expected to reach the closest stars to Earth in about 30,000 years and then continue in deep space indefinitely.
At least until the Milky Way collides with the Andromeda Galaxy in about 4.5 billion years. Depending on what happens during this event, the probe could be ejected into the intergalactic medium, where it could survive for trillions of years.
At that time, the Earth, the Sun, the Milky Way and many other parts of the galaxy will be a distant memory. It’s an alarming thought that, ultimately, a tiny spacecraft launched in the 1970s could be the last thing that exists as evidence of our species’ existence, or indeed any sentient life, at all.
But, in some way, Voyager 1 and 2 and the future deep space probes we are likely to send to the stars will be humanity’s lasting legacy. So, please don’t feel too sad for Voyager 2 and its twin.
