NASA engineers have come up with an energy-saving strategy to get more time—and more science—out of the Voyager probes, humanity’s longest traveling spacecraftas they continue to venture into unexplored regions of interstellar space.
And time is of the essence: Voyager 1 and 2 have been flying since 1977, and their power sources have gradually faded, putting their instruments at risk. Out in the great abyss of space, unfathomably far from our sun, solar energy is not profitable. That’s why engineers equipped each Voyager with a trio of radioisotope thermoelectric generators, or RTGs. These work by converting the heat from the decay of radioisotope fuel, plutonium-238, into electricity. They’re basically nuclear batteries – and they’re finally running out of juice, losing a predictable 4 watts per year. While the Voyagers don’t need that power for propulsion, it’s essential to their ability to collect scientific readouts of distant charged particles and magnetic fields—so far, humanity’s only opportunity to sample this data in interstellar space.
A couple of years ago, NASA began researching ways to keep Voyager’s instruments running as long as possible. The first step, in 2019, was to start turning off the heaters for the science instruments. It worked; the devices continued to function despite temperatures dropping to about 50 degrees Celsius, much colder than the conditions they had been tested in. But it still wasn’t enough, so in late March a NASA team initiated a power-saving strategy on Voyager 2 that dips into reserve power that is intended to protect systems from voltage spikes.
Although this strategy makes the craft more vulnerable, the risk of such spikes appears to be very low, said Suzanne Dodd, the Voyager project manager at NASA’s Jet Propulsion Laboratory in Southern California. Assuming all goes well, they will start similar voltage management on Voyager 1 this fall. Overall, Dodd believes this could buy the probes’ science missions a few extra years. Voyager is still a mission of discovery, she says, and every bit of data the spacecraft gets in interstellar space is valuable. “I continue to be amazed by these spacecraft and by the engineers who come up with clever ways to use them,” Dodd says.
Now 45 years old, the Voyagers spent their first two decades flying through the solar system, snapping photos of Jupiter, Saturn, Uranus and Neptune as they zoomed by. Voyager 1 also captured the iconic “pale blue dot” photograph of a small, distant earth. As they continued, they continued to capture data. They have long outlived their predecessors, Pioneer 10 and 11, which were the first probes to fly by the gas giants but were shut down more than 20 years ago. Both Voyagers have flown well beyond the Kuiper Belt, a region that hosts Pluto and other small, icy bodies. In 2012, Voyager 1 left the heliosphere, the protective bubble of particles and magnetic fields generated by the Sun, beyond which lies the interstellar medium. Its twin followed six years later, when both were officially in interstellar territory, cruising at 35,000 miles per hour into the unknown.
Today, Voyager 1 is 159 AU from home, and Voyager 2 is at 133 AU and traveling in a different direction. (1 AU is the distance between Earth and the Sun, or about 93 million miles.) The spacecraft is certainly showing signs of age—the team dealt with telemetry issues on Voyager 1 last year—but the cosmic workhorses continue.
It is not uncommon for NASA missions to far outlive their life expectancy and be granted extensions after achieving their primary objectives. Opportunity Mars rover rolled on for almost 15 yearsrather than three months. The Saturn-focused Cassini orbiter, which NASA operated in collaboration with the European Space Agency, endured for 20 years instead of four. But the Voyagers certainly take the cosmic cake. If the energy-saving venture of Dodd’s team works, the two could reach the unprecedented age of 50 – with a “stretch goal” of reaching 200 AU around the year 2035.
But this will require sacrificing the scientific instruments one by one.
Voyager 2 still has five instruments humming along: a magnetometer, a plasma wavemeter, a plasma science experiment, a cosmic ray detector, and a low-energy charged particle detector. The first two only take about 2W to run, and their electronics are in the body of the probe, so they’ll probably be the last to shut down. The others are housed on the boom of the boat, where it’s cold, and they use between 3 and 5 watts each, so turning each one off would buy another year of life.
Interstellar space may seem completely empty, but it’s not: There are still solar particles and magnetic phenomena to study. “The farther we get from the Sun, the more interesting it gets because we really don’t know what we might find. And having two Voyager spacecraft is like looking through binoculars,” said Linda Spilker, Voyager Project Scientist at JPL .For example, astrophysicists expected that outside the heliosphere, the sun’s magnetic field would slowly rotate in the direction of the interstellar medium, and the Voyagers would be able to track it. But they haven’t seen any such rotation yet, Spilker says, suggesting that models of the magnetic fields need to be updated.
Missions based on newer probes will take advantage of Voyager’s ongoing solar science. As early as 2025, NASA plans to launch Interstellar mapping and acceleration probe (IMAP) to investigate the heliosphere. The Voyagers are already well outside the heliosphere, so the measurements from the distant probes can be compared with those from the much closer new one. “Having Voyagers out there under IMAP will be really wonderful. Because we’re imaging with IMAP, Voyagers will also make valuable measurements locally,” said David McComas, a Princeton physicist who leads the IMAP collaboration. He likens it to doctors takes a CAT scan of a person’s brain for the big picture, plus a biopsy for detailed information.
The Voyagers aren’t done yet, but they already have an impressive legacy. That includes NASA’s New Horizons probe, which slipped by Pluto in 2015. Now 55 AU from Earth, that spacecraft is probing the edge of the heliosphere with newer, better sensors than the Voyagers are equipped with, and it has already captured images of objects that hadn’t even been discovered when the Voyagers were launched, such as Pluto’s moons and a Kuiper belt object called Arrokoth. “For all of us on New Horizons, the Voyager team, they are our heroes,” said Alan Stern, the collaboration’s principal investigator and a planetary scientist at the Southwest Research Institute. New Horizons is the only other remote human-made probe still in operation, and it could last until 2050, Stern said. The team is now looking for a new target for a flyby.
Inspired by Voyager’s tremendous success, engineers are already designing next-generation spacecraft concepts, such as those that could be powered by lasers and light sails and could one day whiz into our interstellar surroundings faster and farther than the probes of the 1970s could. What advice should they take from Voyager’s long and healthy life? First, Dodd says, it’s useful to have lots of fuel and redundant systems, because even robust instruments eventually fail. And it is important to pass knowledge on, she says, if the craft survives the generation of engineers who designed it.
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