NASA’s Voyager 1 spacecraft has been breaking distance records for decades. Launched almost 50 years ago, it became the farthest human-built object from Earth when it overtook the pathfinding Pioneer 10 probe in 1998. It was the first spacecraft to leave the solar system’s protective bubble, and it will be humankind’s first emissary to the stars.
The cameras on Voyager 1, along with those on its twin spacecraft, Voyager 2, captured powerful megastorms in the outer solar system, the first images of dozens of moons and never-before-seen details of the gossamer rings around Jupiter, Saturn, Neptune and Uranus. The images and data sent home have shaped theories of how the solar system formed and evolved, inspired subsequent missions and posed enough new questions to keep space scientists busy for decades.
And this month, Voyager 1 will set another big record. On November 18 at 5:16:07 a.m. Eastern Standard Time, it will become the first spacecraft to reach one light-day from Earth: the distance light travels in 24 hours. That’s a journey of 25.9 billion kilometers, or more than four times the distance from the sun to Pluto. At that point, signals from Earth will take a full day to reach Voyager 1; it will take another day to hear back.
“I just never imagined [the mission] would last that long,” says Linda Spilker, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who began working on Voyager shortly before the probes launched in 1977. Spilker was in Cape Canaveral, Fla., for the first launch. She even planned the births of her children, now in their 40s, around mission milestones. After stepping away to lead another mission, she is now back as Voyager head scientist.
Two other scientists have been with Voyager since the beginning. Others weren’t yet born when the spacecraft launched. “Voyager, to this day, is a multi-generational mission,” Spilker says. But it isn’t a mission of nostalgia. There’s still a world of discovery out beyond the solar system’s edge.
“Every piece of data, every day that goes by that we get data from the Voyagers, tells us something new,” says physicist Jamie Rankin of Princeton University, who started her Ph.D. in August 2012, the same month that Voyager 1 left the solar system for the cold, calm seas of interstellar space. “Every day, it’s in places nobody’s ever explored before.”
Exploring our neighborhood
The original mission of the Voyager probes was to explore the outer parts of our solar system, which until then had remained largely mysterious. A rare planetary alignment meant that a spacecraft launched in the late 1970s could swoop past Jupiter, Saturn, Uranus and Neptune, getting a gravitational boost from each planet that it passed. With these gravitational assists, the spacecraft could complete the tour in less than 15 years. Such an alignment won’t happen again until the 2150s.
The project was a bold step for its time. When work began in the early 1970s, no spacecraft had gone farther than Mars. NASA set a conservative goal for the mission: flybys of Jupiter and Saturn. In 1972 and 1973, NASA launched two Pioneer probes, which made sure the trek was possible and gathered data along the way. As with Voyager, there were two spacecraft in case one failed.
After nail-biting launches and a wait of nearly two years, the Voyagers began uncovering many of Jupiter’s and Saturn’s long-held secrets, including the volcanoes on Jupiter’s moon Io, details of Jupiter’s Great Red Spot and hints of geologic activity on Saturn’s moon Enceladus.
Mission accomplished, Voyager 2 went on to pass Uranus and Neptune. It took the first images of the planets’ rings and discovered more than a dozen moons along the way. The spacecraft also spotted a perpetually churning storm on Neptune and erupting geysers on Neptune’s moon Triton.
“The highlights of Voyager were clearly the planetary encounters,” said Ed Stone, the mission’s original head scientist, in a NASA interview before his death in 2024. “We had a lifetime of discovery packed into each one.”
The uncontested stars of these encounters were the images shared in news reports and TV broadcasts as the planets and their moons came into view. Jupiter’s moon Io turned out to be a shocking splotchy orange color that looked like a pizza.
A humble navigation image of that moon led to one of the most surprising discoveries of the entire mission. Four days past Voyager 1’s closest approach to Jupiter, after the excitement had died down, project astronomer Linda Morabito of the Jet Propulsion Laboratory started processing additional images of Io. These images had been taken after the flyby, when Voyager was looking back over its shoulder.
As Morabito enhanced one image’s brightness, she spotted an odd bulbous formation protruding from Io’s edge. “What’s that?” she asked herself. Her supervisor had the same reaction: “Jesus, what’s that?” she later recalled him saying.
The bulb turned out to be a volcanic plume — an erupting fire fountain extending 250 kilometers above Io’s surface. It was the first discovery of active volcanism anywhere other than Earth, and it transformed our view of the outer solar system. Instead of a cold, inert icebox, this was a bubbling and dynamic region.
The findings inspired many future missions. While visiting Jupiter from 1995 to 2003, the Galileo probe found that Europa may have a subsurface ocean that could be hospitable to life. When the Cassini probe orbited Saturn from 2004 to 2017, it discovered that the moon Enceladus also has a potentially habitable ocean that spews icy geysers into space.
In 1990, with Voyager 1 about 6 billion kilometers from the sun, astronomer Carl Sagan convinced mission leaders to turn the spacecraft backward for one last look at the planets. Voyager 1 took a family portrait in which Earth appeared faintly as a “pale blue dot,” Sagan said. “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,” he wrote in the 1994 book Pale Blue Dot.
To save power, Voyager 2’s cameras were shut off in 1989. After the family portrait, Voyager 1’s cameras were also turned off. Where the spacecraft were headed, there wouldn’t be much to see.
The edge of nowhere
The probes were now on their way out of the solar system along their own separate paths, both headed toward a mysterious boundary that no human emissary had ever crossed. In this next phase of their voyage, no one knew what to expect. How long would the spacecraft keep communicating? How far would the powerful forces that drive the sun reach? And what would it be like to taste interstellar space?
For its first 35 years in space, Voyager 1 was swaddled in the solar wind. This stream of hot, charged particles flows constantly from the sun, forming a vast bubble called the heliosphere. The bubble surrounds the solar system, protecting it from cosmic radiation; it extends billions of kilometers past the planets up to a boundary called the heliopause. Heliophysicists consider this boundary, where the solar wind bumps into interstellar space, the edge of the solar system, although the sun’s gravitational influence extends much farther.
Scientists expected the Voyagers to cross the heliopause, but they didn’t know when — or what the spacecraft would find when they did. Rankin had just started her Ph.D. at Caltech when in August 2012 Voyager 1 detected an abrupt jump in the number of high-energy cosmic rays alongside a drop in solar particles, hinting that the spacecraft had crossed the boundary. But magnetic field measurements suggested otherwise. Instead, those measurements indicated that the sun still exerted some influence. The interstellar magnetic field diverted around the heliosphere like a river around a rock.
A Voyager instrument that could have settled the question of whether the probe had passed the heliopause was no longer working. But scientists got a lucky break in April 2013 when an eruption on the sun sent a pressure wave rippling outward. It set the electrons around Voyager 1 vibrating, which allowed scientists to estimate the density of charged particles nearby.
The results showed that Voyager 1 had in fact crossed the heliopause on August 25, 2012, when it was about 18 billion kilometers away, or 120 times Earth’s distance from the sun. Data from Voyager 2 later backed up that finding. “This is the beginning of a new era of exploration for us,” Stone told Science News in 2013.
The space beyond
But the line between our solar system and interstellar space isn’t solid. The sun’s reach extends farther than anyone expected, and “this boundary is not very sharp or very thin,” says physicist Jeewoo Park of NASA’s Goddard Space Flight Center in Greenbelt, Md.
With Voyager on the other side of the heliopause, particle speeds and gas temperatures have dropped. The magnetic field is weaker. Even dust particles are about half as abundant compared with inside the heliosphere. “For the most part, it’s been very quiet and very consistent,” Rankin says.
But it’s not entirely silent. In four instances, scientists saw spikes in the magnetic field strength and particle density measurements around the probe. After the fourth, in mid-2020, those numbers have remained high. No one knows why. Perhaps Voyager 1 continues to ride the aftershocks of solar eruptions, says Park, who reported on the events in June in the Astrophysical Journal.
Where pure interstellar space begins is still an open question. So is the heliosphere’s shape. Most scientists think it is elongated, like a comet, but others think it is a sphere or even a croissant.
Space telescopes launched in 2008 and 2025, the Interstellar Boundary Explorer and Interstellar Mapping and Acceleration Probe, could help answer that question.
Meanwhile, the Voyager probes are silently sailing onward. Scientists estimate that the onboard power sources that keep the crafts’ instruments running will dry up in 10 to 15 years, preventing them from sending data back to Earth. Our conversation will come to an end. But the journey goes on.
Voyager 2 will approach a red dwarf star known as Ross 248 in about 42,000 years, coming within 700 light-days. It will be 44,000 years before Voyager 1 next encounters another star, Gliese 445, at a similar distance. “The Voyagers become our silent ambassadors, a little bit of us that continues into space,” Spilker says.
They carry a message, should anyone find it, contained on the Golden Record. A copy of the phonograph record, which is plated in gold, rides on each of the probes. The long-distance dispatches feature sights and sounds of Earth, including a map of the solar system; images of Earth’s monuments; greetings in dozens of languages; and the sounds of whale song, laughter, classical composers and Navajo chants.
That time capsule captures the optimism and promise of the mission, Spilker says. “It may well outlast humanity.”