After more than 44 years of traveling farther from Earth than any man-made object before, the Voyager spacecraft is entering its very last phase.
Both Voyagers were launched from Cape Canaveral in 1977 – with Voyager 2 actually the first to lift off – taking advantage of a rare (once in 176 years) alignment of Jupiter, Saturn, Uranus and Neptune to shoot into space interstellar.
They were designed to last five years and study Jupiter and Saturn, but remarkably both spacecraft are still functioning despite having escaped beyond the bubble of hot plasma known as the heliopause that defines the start from the edge of our solar system.
Speaking to the magazine American Scientist of the probes powering down, NASA physicist Ralph McNutt said, “We’re at 44½, so we’ve done 10 times the warranty on the damn things.”
Both spacecraft are powered by radioisotope thermoelectric generators (RTGs) – fueled by heat from decaying plutonium spheres – although the power of these RTGs decreases by about four watts each year.
This means that the instruments are switched off one by one.
To date, Voyager 1 has only four functioning instruments and Voyager 2 has five.
It is certain that at some point the plutonium powering the spacecraft will decay beyond what is capable of keeping the probes functional. Some estimate it could be as early as 2025, while others hope it will be later.
But they have so far surprised NASA engineers, who expected to start turning off Voyager 2’s instruments first, one by one, from 2020. Instead, nothing was turned off since 2008.
“If all goes very well, we may be able to extend the missions into the 2030s. It just depends on the power. That’s the endpoint,” said Linda Spilker, who started work on the missions. Voyager before their launch, speaking to Scientific American. .
Get out of the solar system
It took about 36 years for Voyager 1 to break the heliopauseand the data it has since returned suggests some fascinating qualities about the role of magnetic fields in the universe.
Voyager 2 then passed in interstellar space in 2018 – 41 years after launch – crossing the outer boundary of the heliopause where the warm solar wind meets the cold space known as the interstellar medium.
But space is very, very big, and none of the probes are currently considered to be outside the solar system. The final boundary is believed to be the Oort cloud, a collection of small objects still under the influence of the Sun’s gravity.
NASA says it will take Voyager 2 about 300 years to reach the inner edge of the Oort cloud, and possibly 30,000 years to fly beyond it.
Voyager 1 is currently 14.5 billion miles (23.3 billion km) from Earth and it takes 20 light hours and 33 minutes to travel that distance, meaning it takes two days to send a message to the spaceship and get a response.
Voyager 2 is not that far away, just 20 billion kilometers from Earth, just under 18 light hours from us.
Both spacecraft carry a gold-plated disc containing multicultural greetings, songs and photographs, in case they ever encounter intelligent life – although some astronomers have warned humanity may regret making first contact .
Carl Sagan dismissed this concern: “The fact is that, for better or for worse, we have already announced our presence and our location in the universe, and continue to do so every day.
“There’s a sphere of radio transmission about thirty light-years thick that stretches outward at the speed of light, announcing to every star it envelops that the earth is full of people.
“Our television programs flood space with signals detectable at enormous distances by instruments not much larger than our own. It is a sobering thought that the first news of us may be the result of the Super Bowl “, he wrote.
Just last month NASA said its engineers were working to solve a mystery affecting Voyager 1’s telemetry data, although Voyager 2 continues to operate normally – although some instruments are now shut down for longevity .
The problem probe has an Attitude and Articulation Control System (AACS) that is in charge of orienting the spacecraft, including keeping its antenna pointed precisely at Earth so it can send data at home.
This data is still coming in, suggesting AACS continues to work as intended, but the telemetry data itself is not valid according to NASA. It appears to be randomly generated or does not reflect any possible state the AACS might be in, the space agency explained.
“A mystery like this is kind of normal at this point in the Voyager mission,” said Suzanne Dodd, project manager for Voyager 1 and 2 at NASA’s Jet Propulsion Laboratory in Southern California.
“The spacecraft are both nearly 45 years old, which is far beyond what mission planners had anticipated. We are also in interstellar space – a high-radiation environment in which no spacecraft has flown before.
“So there are big challenges for the engineering team. But I think if there is a way to solve this problem with AACS, our team will find it,” Ms Dodd added.
On February 14, 1990, as Voyager 1 passed Uranus, it turned back to Earth to take a picture of our planet as a tiny dot.
Four years later, astronomer Carl Sagan reflected on the importance of photography to a Cornell University audience, coining his name as the “Pale Blue Dot” and delivering one of the most widely published speeches in all the time.
“Consider this point again. This is here. This is our home. This is us. On this one, everyone you love, everyone you know, everyone you’ve heard of, everyone human beings who have ever existed, have lived their lives.
“All of our joys and sorrows, thousands of religions, ideologies and confident economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young loving couple, every mother and father, every hopeful child, inventor and explorer, every moral teacher, every corrupt politician, every ‘superstar’, every ‘overlord’, every saint and sinner in the history of our species y have lived – on a speck of dust suspended in a ray of sunshine.”
We probably won’t see his like again any time soon.
NASA said that while it’s possible to turn the cameras back on, it’s not a priority for the interstellar mission.
The agency added that the image probably wouldn’t be as good as the one taken in 1990: “It’s very dark where the Voyagers are now. Although you can still see some brighter stars and some of the planets with the cameras, you can see these stars and planets better with amateur telescopes on Earth.”
For those still holding out hope, NASA warns the attempt could be a waste of the probes’ dwindling resources: “The computers on the ground that understand the software and analyze the images no longer exist.
“The cameras and their heaters have also been exposed for years to very cold conditions deep within our solar system.
“Even if mission officials recreated the computers on the ground, reloaded the software onto the spacecraft, and were able to turn the cameras back on, it’s not clear they would work.”