In 1977, NASA loaded two spacecraft, each the size of a Honda Civic, into a rocket and sent them off on a five-year mission to explore our solar system’s outer planets.
Named Voyager 1 and Voyager 2, these twin craft were designed to fly past Jupiter, Saturn, Neptune and Uranus on their way out of our solar system. Along their journeys, they would send data and images to Earth. And just in case these craft ran into another intelligent species, each also carried a gold-plated record that contained information and sounds representing humanity.
Thanks to their sturdy technology and smart engineering, these spacecraft are now nearing their 50th birthdays. And they more than met their goals. For more than five years now, both craft have been sailing beyond our solar system. Still at work, these most distant explorers are now bringing back to Earth details of previously uncharted interstellar space.
What they’ve seen along the way has transformed what we know about our solar system. Tending to these aged spacecraft and watching what they discover has also helped us understand our place in the universe.
“They’ve turned what started out as a very exciting planetary mission of discovery into an expedition of humanity,” says Bill Kurth. He’s a space physicist at the University of Iowa in Iowa City. He started working on the Voyager mission in 1974 as a graduate student. Today, he still works on Voyager instruments that study space weather, or how conditions in space change due to activity on the sun’s surface.
To celebrate Voyager’s unexpected legacy, let’s take a look at the mission’s biggest science moments to date.
1. First close-up photos of the outer planets
Until Voyager, scientists knew little about our solar system’s outer planets.
In 1610, Galileo identified the first four of what would turn out to be Jupiter’s 95 moons. But it wasn’t until Voyager 1 and 2 flew past Jupiter in 1979 — then Saturn, Neptune, and Uranus in later years — that we finally saw these planets in all their glory.
Pictures from Voyager revealed our solar system in technicolor. “It really opened our eyes to the outer solar system,” Kurth says. This also prompted specific questions about how these giant-planet systems work, he says. It planted the seeds for follow-up missions to understand Jupiter’s composition and to better study Saturn’s 146 moons (more than any other planet).
2. A pale blue dot
Most of Voyager’s discoveries taught us about the solar system beyond Earth. But they also hinted at what it means to be human. In some of the last photographs that Voyager 1 took before leaving our planetary environment, it turned its eye toward home one last time.
Voyager 1 snapped a photo of a tiny, pale blue dot — Earth — almost obscured by space dust. “Look again at that dot. That’s here. That’s home. That’s us,” astronomer Carl Sagan wrote about the image in his 1994 book, Pale Blue Dot: A Vision of the Human Future in Space. “On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives … on a mote of dust suspended in a sunbeam.”
3. Volcanoes on Io
Before the Voyager mission, scientists had assumed that moons of all planets would resemble ours. Our moon is beautiful, but there’s nothing happening on its surface.
So scientists were surprised when Voyager’s flyby of Jupiter revealed active volcanoes on the surface of its moon Io. This confirmed suspicions from radio scientists in the 1950s about Io’s role in Jupiter’s system, Kurth says. Researchers have continued studying Io and found it to be a major player in Jupiter’s environment.
“It deposits approximately one ton per second of material from these volcanoes into the magnetic bubble around Jupiter. That drives the workings of Jupiter’s entire system,” he notes. For instance, because of its interaction with Jupiter’s magnetic field, Io creates a huge electric current that sparks lightning in Jupiter’s atmosphere.
This discovery showed scientists that a moon could be “just about anything,” adds Linda Spilker. She’s also been part of Voyager’s journey since she was a graduate student 1977. Today she’s the mission’s Project Scientist. A planetary scientist, she works at the Jet Propulsion Laboratory in Pasadena, Calif.
4. Rings around the outer planets
The Voyager mission was the first to send home breathtaking images of ring systems surrounding Saturn, Jupiter, Uranus and Neptune.
Saturn’s enormous rings were discovered in the 1600s by Galileo using a telescope. But Jupiter’s narrow rings were unknown until Voyager 1 flew by them in 1979.
And while Saturn’s icy rings were well known, Voyager 1 and 2’s flybys captured them in detail for the first time. Voyager 2 flew so close to this gas giant’s rings that the mission team detected hundreds of tiny impacts per second. These were tiny dust grains from the rings bombarding the craft. But don’t worry, Kurth says. The spacecraft escaped unharmed.
As the Voyagers flew by Uranus, Spilker remembers receiving photos of the rings from a side hidden from Earth. In this photo, the rings were backlit by the sun. “It was spectacular,” Spilker says. “It showed us all the dust lanes.”
And though researchers had suspected Neptune, too, might host rings, confirmation of that didn’t emerge until late August 1989. That’s when Voyager 2 — then nearly 2.9 billion miles from Earth — sent home the first images of this planet’s two main rings. Another two also orbit this planet, an ice giant.
5. Lightning on Jupiter
Voyager’s discovery of lightning on Jupiter was the first time lightning had ever been observed beyond Earth.
Voyager 1 initially detected a radio-wave emission called a whistler. This is caused by atmospheric lightning, Kurth says. The spacecraft picked up these radio waves when it looked back at the night side of Jupiter. The craft then “saw splotches of light,” Kurth said. This is “where lightning below the clouds had lit the clouds above the lightning stroke.”
6. Understanding the Great Spots of Jupiter and Neptune
Voyager also sent home images of huge storms on Jupiter and Neptune. They appeared as big spots.
Astronomers first observed the Great Red Spot on Jupiter hundreds of years ago. Its Red Spot is the largest known storm in the solar system and is almost twice the size of Earth! When Voyager 1 flew by Jupiter, it saw that the storm was still raging.
Jupiter’s storm may be the oldest and largest, but a storm on Neptune has the strongest winds. This storm was called the Great Dark Spot. Voyager 2 was the first to photograph it in August 1989.
In its flyby, Voyager 2 measured wind speeds from Neptune’s storm to be up to 2,400 kilometers per hour (1,500 miles per hour). On Earth, uneven heat from the sun tends to spur winds. But on Neptune, Spilker says, heat from the planet itself might be driving these winds.
In fact, Neptune lacks a surface. “So there’s no mountains or valleys to slow the wind down,” she explains. Exactly how the winds work remains unknown. “We’re really not sure exactly why Neptune has the fastest winds in the solar system.”
7. Lunar oceans
Voyager discovered that two moons in our outer solar system could host oceans on their surfaces — Jupiter’s moon Europa and Saturn’s moon Enceladus. The spacecraft picked up on the icy surfaces of the two moons, Spilker says. “It set the stage for the discovery that these worlds actually have liquid-water oceans under their surfaces.”
These first finds paved the way for future missions to Enceladus, including Cassini, which Spilker also worked on. In 2017, Cassini discovered hydrothermal vents on its surface. “Because thermal vents on Earth can have colonies of life, we wonder perhaps [if there’s also] life in the oceans of these worlds,” she says. A mission looking to better understand Europa’s oceans launched October 14, 2024.
8. Geysers on Triton
One of Spilker’s fondest memories from working on Voyager was receiving images of Neptune’s moon, Triton. The Triton data arrived late at night, she recalls. “During the two or three days around closest approach, I put a sleeping bag under my desk on the floor. I would just sleep there because I didn’t want to miss it.”
During its 1989 flyby of Neptune, Voyager 2 was the first to observe geysers on the moon’s surface shooting 8 kilometers (5 miles) high. On Earth, the Old Faithful geyser at Yellowstone National Park spurts less than 60 meters (200 feet) high.
The flyby images would appear on small TVs line by line, Spilker says. “I remember watching those first pictures come back,” she says. “It was just so exciting to know I’m seeing data from this world that we really don’t know much about.”
9. Entering interstellar space
Voyager 1 and 2 sailed past their final planet, Neptune, in 1989. But their mission was far from over.
Over the next several decades, the spacecraft made their way to the edge of our solar system. Both have now entered interstellar space — Voyager 1 in 2012 and Voyager 2 in 2018 — which is a place they were never designed to visit. In fact, this was the first time that any spacecraft had explored so far from Earth.
As its name suggests, interstellar space is the area between stars. People tend to think of it as empty. But it’s actually full of charged particles, atoms, molecules and plenty of dust.
So far, the Voyager spacecraft have had smooth sailing through interstellar space. But Voyager now faces new risks far, far from home. This includes radiation, notes Jamie Rankin. She’s a space physicist at Princeton University in New Jersey. Rankin joined the team as a graduate student in 2012. That was just days after Voyager 1 entered interstellar space. She’s now working alongside Spilker as one of Voyager’s project scientists.
Cosmic rays are high-energy particles that whizz through space at nearly the speed of light, and they’re much more abundant in interstellar space than in our solar neighborhood. “On Earth, galactic cosmic rays are heavily shielded by the sun,” Rankin says. But “there are a lot more of those out there where the Voyagers are. Some cosmic ray might hit a [computer] chip and fry it.”
Luckily, Rankin notes, the Voyagers’ age actually provides an advantage in dealing with these rays. Because their computer technology is so old — and therefore large — it can survive more hits than newer technology can.
Alan Cummings, another veteran team member, works at Caltech in Pasadena, Calif. He joined the Voyager mission in 1974 and can boast being the last person to touch both Voyagers before launch. For this cosmic-ray physicist, some of Voyager’s most exciting science is happening now, some 15 billion miles from home.
Cummings didn’t expect all that much from the Voyagers in interstellar space. “I thought it would just be the same every day. But that didn’t turn out to be right.” The curious incoming data included measurements of magnetic fields and the density of plasma in space. Plasma is a state of matter containing charged particles.
In the last five years, the spacecrafts’ measurements of magnetic field strength and plasma density have spiked. That suggests that Voyager is exploring an unknown region of space, Kurth says. The unexpected measurements may be related to the sun’s activity.
And although it’s widely accepted that both Voyagers have entered interstellar space, work is ongoing to understand where exactly that is. The definition of what boundary the craft passed is still being written. The transition between solar wind and interstellar wind is called the heliopause. Research published in early 2024 suggests it may be shaped differently than once believed.
“This paper offers a mechanism to explain the mystery of why the direction of the magnetic field did not change at the Voyager crossings of the purported heliopause,” explains Cummings. Instead of a thin boundary line as once thought, this new work suggests a thick boundary layer.
Such a layer may form where magnetic fields from the heliosphere and interstellar space interact.
“Voyager will not be in the ‘pristine’ interstellar medium until that boundary is crossed. And it is likely 100 or more AU beyond Voyager 1’s present location,” says Kurth. (One AU, or astronomical unit, is 150 million kilometers or 93 million miles.) “There are aspects of this region yet to be understood.”
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10. Voyager phones home
Keeping the aging spacecraft running smoothly has required feats of engineering. This was the case in November 2023 when Voyager 1 stopped transmitting readable data back to Earth.
Bob Rasmussen, a software engineer at NASA’s Jet Propulsion Laboratory, is part of the team responsible for keeping the Voyager spacecraft up and running. Rasmussen worked on the Voyager mission when it first launched. He came out of retirement in 2022 to help keep the mission going.
A broken memory chip onboard Voyager 1 caused a communications shutdown in 2023. The spacecraft could receive signals but no longer send any back. Despite the urgency of fixing Voyager’s data transmission, the team took its time to work carefully.
“All hard problems need to be solved step by step, starting with what you know, and building from there,” Rasmussen says. “Walking our way through this maze took a few months. But we finally figured out what had happened.”
It took sending different signals to Voyager to test which parts of the system were working. This task was made especially difficult because the decades-old papers explaining how these systems worked were written by hand or on typewriters. They couldn’t be easily searched.
Also, owing to the craft being 15 billion miles from home, each message took nearly a full Earth day to make a one-way trip. Thanks to the team’s persistence and smart engineering, Voyager 1 started sending usable data again in April 2024.
The journey continues
It’s not clear how much longer we’ll be able to communicate with the Voyager spacecraft. Experts are optimistic that the Voyager mission will make it until the spacecraft celebrate their 50th birthdays in 2027. But their end will eventually come. Sooner or later, their instruments will fail as they reach the end of their working life or the spacecraft no longer have enough power to run them.
In 2024, for instance, Voyager 2 turned off its plasma-science instrument to conserve power. Voyager 1 is doing a little bit better in terms of available power, Spilker says. Four of the 11 instruments each craft carries are still working.
The end of the Voyager mission will be the end of an era. Many scientists dedicated their careers to these spacecraft and found the mission an opportunity of a lifetime.
“This has been my entire career, [so] I’ll be a little depressed,” Cummings says. “I actually plan to retire [with Voyager].”
Whether the mission continues a few more years or a few more decades, it has left a lasting impact on both the scientific community and the public. “Voyager has been a true pathfinder and explorer,” Spilker says. “It has really changed our view of not only our own solar system but also of interstellar space.”
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