In an effort to conserve Voyager 2's dwindling energy and extend the spacecraft's mission, NASA has shut down another of its instruments. They did it with the Plasma Spectrometer in October 2024, and it won't be the last. In March, Voyager 2's Low-Energy Charged Particle instrument will be powered down.

What does this mean for the durable spacecraft?

"If we don’t turn off an instrument on each Voyager now, they would probably have only a few more months of power before we would need to declare end of mission." - Suzanne Dodd, Voyager Project Manager, JPL

Things have changed a lot since the pair of Voyager spacecraft were launched in 1977. Our planet is hotter, the human population has ballooned, and Battlestar Galactica came and went—twice.

Voyager 1 and 2 have surprised us all with their longevity. When they were launched, their planned mission length was a mere five years. Now, almost 50 years after their launch date, they've both reached interstellar space, a remarkable achievement.

This image shows Voyager 2 blasting off on a Titan-Centaur rocket from Cape Canaveral on August 20th, 1977.

Image Credit: NASA

Though both spacecraft have proven to be durable, nothing lasts forever, not even plutonium. When they were launched, they both carried about 13.5 kg of plutonium-238 in their Radioisotope Thermoelectric Generators (RTGs). RTGs generate electricity by running the heat from the decaying plutonium through a thermocouple. However, as the plutonium decays, its power output is reduced. That necessitates lowering the spacecraft's power demands.

That's where NASA is at with both Voyagers. They've had to sequentially shut down systems that are no longer providing much scientific benefit. Fortunately, some of the spacecraft's instruments were aimed at planetary science and are less critical in interstellar space.

"The Voyagers have been deep space rock stars since launch, and we want to keep it that way as long as possible," said Suzanne Dodd, Voyager project manager at JPL. "But electrical power is running low. If we don’t turn off an instrument on each Voyager now, they would probably have only a few more months of power before we would need to declare end of mission."

Each Voyager spacecraft carries the same payload of 10 science instruments. NASA has shut down different instruments on each one at different times to achieve the best science outcomes.

In October 2024, NASA turned off Voyager 2's Plasma Spectrometer. On March 24th, NASA will shut down Voyager 2's Low-Energy Charged Particle Instrument (LECP), leaving it with only three active instruments: the Triaxial Fluxgate Magnetometer (MAG), the Cosmic Ray Subsystem (CRS), and the Plasma Wave Subsystem (PWS).

Those three instruments still allow Voyager 2 to gather valuable scientific data.

Voyager 2 captured this image of Jupiter and Io when it was 24 million km away.

Image Credit: NASA/JPL

Voyager 2's MAG instrument measured the magnetic fields of Uranus and Neptune and how the solar wind interacted with their magnetospheres. It also played a vital role in determining exactly when Voyager 2 crossed the heliopause into interstellar space. Now that the spacecraft is in interstellar space, MAG is measuring the strength of interstellar magnetic fields and how they interact with the Sun's magnetic fields.

The CRS instrument helped scientists measure energetic particles inside the magnetospheres of the outer planets. It also provided irreplaceable data on the composition, energy, and distribution of cosmic rays. By measuring cosmic ray nuclei, it helped scientists understand how these rays are accelerated and propagated. By measuring cosmic ray flux in interstellar space, the CRS revealed some of the details about the ISM.

The PWS measured the density of electrons near the Solar System's planets. Early in the Voyager missions, the instrument detected lightning storms on Jupiter and other giant planets, a significant development in understanding these planets. In interstellar space, it's measuring the density of the interstellar plasma. Its measurements are critical to understanding the interstellar medium (ISM).

Throughout its mission, the LECP instrument has told scientists about the energy of charged particles and the dynamics of the Sun's solar wind. It has also shown how some particles can leak out of the heliosphere into interstellar space. As Voyager 2 continues its journey into interstellar space, the LECP will tell us more about the heliopause and how particles behave differently in the heliosphere and interstellar space.

"Every minute of every day, the Voyagers explore a region where no spacecraft has gone before." - Linda Spilker, Voyager project scientist at JPL

The LECP instrument will be shut down later this month, reducing Voyager 2 to only three instruments. Nothing illustrates Voyager's longevity and robustness more than the LECP. It's only being shut down because of energy constraints, not because of degraded performance.

Voyager 2 uses a stepped motor to rotate the instrument 360 degrees and provides a 15.7-watt pulse every 192 seconds. During development and testing, the motor was tested to 500,000 steps. That was enough to see it through until the spacecraft encountered Saturn in August 1980. However, the motor will have completed more than 8.5 million steps by the time it's deactivated later this month.

Like other facets of the Voyager program, the LECP has lasted so long that its principal investigator, Stamatios Krimigis, is now 86 years old and has retired into an honorary position. He's now Emeritus Head of the Space Exploration Sector of the Johns Hopkins Applied Physics Laboratory (APL). Maybe both the man and the instrument will fully retire at the same time.

Voyager 1 and 2 are our first interstellar probes, though they were never intended to be. Everything they're showing us about interstellar space is bonus knowledge. Many of the people behind the program are gone now, but both spacecraft live on. There's a poignancy to that that goes beyond science, charged particles, and the details of the interstellar medium. They're humanity's first unintentional envoys into interstellar space and are starting to outlast their creators.

"The Voyager spacecraft have far surpassed their original mission to study the outer planets." - Patrick Koehn, Voyager Program Scientist

This graphic from 2019 shows the locations of both Voyage probes in relationship with the heliosphere.

Image Credit: NASA/JPL-Caltech/Johns Hopkins APL

However, the Voyagers are scientific missions, and they're still stubbornly fulfilling those missions.

"The Voyager spacecraft have far surpassed their original mission to study the outer planets," said Patrick Koehn, Voyager program scientist at NASA Headquarters in Washington. "Every bit of additional data we have gathered since then is not only valuable bonus science for heliophysics but also a testament to the exemplary engineering that has gone into the Voyagers — starting nearly 50 years ago and continuing to this day."

NASA is determined to milk the Voyager spacecraft for as much data as possible. Once Voyager 2's LECP is turned off later this month, both Voyagers should be able to operate for another year before another instrument will need to go dark. For Voyager 1, this means it will lose its LECP. Voyager 2's CRS will be shut off in 2026.

NASA engineers say that their power conservation program should let both spacecraft operate into the 2030s, albeit with a single instrument each. However, they have been operating in deep space for almost 50 years, and it's not a benign environment. It's only rational to expect some other problems to crop up.

It's easy to gloss over the success of the Voyager program now that space missions launch every month, powerful rovers explore Mars, and high-resolution cameras deliver a steady stream of yummy images to our hungry browsers. It's also easy to forget that they've both travelled more than 20 billion km. In fact, when Voyager 2 sends us a signal, it takes 19.5 hours to reach us. For Voyager 1, the signal travel time is even greater: 23.5 hours. Those signal travel times will only grow as the spacecraft continue their journeys. And every kilometre of their journeys is a new frontier for humanity.

"Every minute of every day, the Voyagers explore a region where no spacecraft has gone before," said Linda Spilker, Voyager project scientist at JPL. "That also means every day could be our last. But that day could also bring another interstellar revelation. So, we’re pulling out all the stops, doing what we can to make sure Voyagers 1 and 2 continue their trailblazing for the maximum time possible."

• Press Release: NASA Turns Off 2 Voyager Science Instruments to Extend Mission

• Press Release: 40 Years Ago: Voyager 2 Explores Jupiter

• Published Research: Whistlers observed by Voyager 1: Detection of lightning on Jupiter