NASA’s Voyager 2 spacecraft is recovering from a scary moment earlier this week, when its onboard computer automatically shut down the spacecraft’s science instruments to keep the spacecraft from draining its power supply. Engineers at NASA’s Jet Propulsion Laboratory say the science instruments are back on, and Voyager 2 should get back to science as usual soon.
The trouble started on January 25, when Voyager 2 was supposed to make a 360⁰ roll to help calibrate its magnetometer. But when the spacecraft’s computer tried to execute the commands it needed to make the roll, it got the timing wrong. As a result, two systems that don’t normally run at the same time ended up running at the same time – and both systems draw a lot of power. Since Voyager 2 is running on a limited power supply courtesy of its onboard generator, this was exactly the sort of problem that could have put the mission in mortal peril.
Fortunately, Voyager 2 (like its twin sibling Voyager 1) was programmed to deal with problems like this. When the spacecraft’s computer realized how much power it was drawing, it shut down the science instruments to save power for really critical things, like heating fragile subsystems and communicating with engineers back home on Earth, who could help find and fix the problem.
It took a solid three days to get one of the overlapping, power-hungry systems shut down and the science instruments turned back on – but engineers spent a lot of that time waiting. At the moment, Voyager 2 is 18.5 billion km (11.5 billion miles) from Earth (and counting), and even for radio communications at the speed of light, it takes 17 hours for a message to get from Earth to Voyager, and another 17 hours for a message to get from Voyager back to Earth. So every time engineers at NASA sent Voyager a command, like “turn off that extra system now,” it took at least 34 hours to find out if the command worked and whether it fixed the problem.
That’s the challenge of maintaining an aging spacecraft that’s now well beyond the outer boundary of our solar system and flying further into interstellar space with every passing moment.
The radioisotopic thermoelectric generator that turns heat from radioactive decay into electricity for Voyager 2 is producing 40% less power now than when the spacecraft launched; that’s because the same radioactive decay that produces the energy also decreases the amount of material available to produce more energy. Voyager 2’s power budget decreases by about 4 watts every year, and engineers on Earth have had to make some tough decisions about what to keep running and what to turn off.
In 2019, they switched off the main heater for Voyager’s cosmic ray instrument, which detects fast-moving, high-energy particles emitted by our Sun or by sources outside the solar system. The data that instrument sent home helped determine when Voyager 2 finally crossed outside the range of the Sun’s “wind” of ionized particles and into interstellar space. So far, the cosmic ray instrument is still running, despite being chilled to -59⁰C (-74⁰F), much colder than it was tested for.
So far, NASA engineers have kept Voyager 2 flying for 42 years (to put that into perspective, Voyager 2 launched just 3 months after Star Wars Episode IV: A New Hope first appeared in theaters). And the space agency says both Voyager 2 and Voyager 1 have several more years of useful science ahead of them. Voyager 2’s plasma instruments and magnetometer are still sending home data on the thin, scattered clouds of gas and dust in interstellar space. And in Nov. 2019, five papers published new descriptions of the region just outside the reach of our Sun’s stellar wind.
Voyager 2 launched on August 20, 1977 – two weeks before Voyager 1 – and took the long way out of the solar system, passing Jupiter, Saturn, Neptune, and Uranus; in the process, the mision gave humanity its only close look (so far!) at the often-overlooked ice giants, Neptune and Uranus. Meanwhile, Voyager 1 took a shorter, quicker route past Jupiter and Saturn, skipping the ice giants. Together, the intriguing data the Voyagers sent home paved the way for the 1999-2003 Galileo mission to Jupiter and the 1997-2017 Cassini mission to Saturn. Without the Voyagers, we wouldn’t know about Titan’s methane lakes, Enceladus’ watery plumes, Europa’s hidden icy ocean, or Io’s volcanoes.
NASA’s Juno mission is now studying Jupiter and its moons, and the European Space Agency’s JUICE (JUpiter ICy moons Explorer, because you can play fast and loose with capitalization when you’re a multinational space agency) is set to join it in 2022, followed by NASA’s Europa Clipper, which is set to launch in 2025. That’s two or three generations of space missions (depending on how you count them), all built on the foundation of the Voyagers’ first encounters with the outer planets 42 years ago.
But all that is behind the Voyager probes now. They’re moving forward (at about 16 km/second). As they push further into interstellar space, they’ll keep sending new information home to Earth, even if every message takes longer to send, and even if the power supply is slowly dwindling. After all, we named them Voyager.
