Voyager 2, despite its name, was the first of two twin probes NASA launched in 1977 to explore the outer planets of our solar system. While Voyager 1 focused on Jupiter and Saturn, Voyager 2 took close-up pictures of those planets as well as Uranus and Neptune. Voyager 2's mission to those last two planets would be humanity's only visit there in the 20th century.
As of early November 2018, NASA announced that the space probe had crossed the outer edge of our solar system and is now more than 11 billion miles (18 billion kilometers) from Earth. (Voyager 1 crossed the boundary into interstellar space 2012.)
Voyager 2 launched on Aug. 20, 1977, and Voyager 1 launched about two weeks later, on Sept. 5. NASA planned for the Voyager spacecraft to take advantage of an alignment of the outer planets that takes place only every 176 years. The alignment would allow both probes to swing from one planet to the next, with a gravity boost to help them along the way.
Backup for Voyager 1
Although there was not enough money in Voyager 2's budget to guarantee it would still work when flying past Uranus and Neptune, its trajectory was designed to go past those planets anyway. If the spacecraft were still working after Saturn, NASA could try to take pictures of the other planets.
Voyager 2 was ready as a backup for Voyager 1. If Voyager 1 failed when taking pictures of Jupiter and Saturn, NASA was prepared to alter Voyager 2's path to follow Voyager 1's trajectory. It would cut off the Uranus and Neptune option, but still preserve the possibility of capturing images.
The backup plan was never executed, though, because Voyager 1 went on to make many discoveries at Jupiter and Saturn, working well enough for NASA to carry out its original plans for Voyager 2.
Jovian and Saturnian discoveries
Voyager 2 reached Jupiter in 1979, two years after launching from Cape Canaveral. Since Voyager 1 had just gone through the system four months earlier, Voyager 2's arrival allowed NASA to take valuable comparison shots of Jupiter and its moons. It captured changes in the Great Red Spot and also resolved some of the moon's surfaces in greater detail.
Voyager 2 took pictures of many of Jupiter's satellites. Among its most spectacular findings were pictures from the icy moon Europa. From 128,000 miles (205,996 km) away, Voyager 2 snapped detailed photos of the icy moon's cracks and revealed that there's no change in elevation anywhere on the moon's surface.
Proving that moons are abundant around the outer planets, Voyager 2 happened to image Adrastea, a small moon of Jupiter, only months after Voyager 1 found two other Jupiter moons, Thebe and Metis. Adrastea is exceptionally small, only about 19 miles (30.5 kilometers) in diameter at the smallest estimate.
Next in line was Saturn. Voyager 2 arrived at its closest point to the ringed planet on Aug. 26, 1981, and took hundreds of pictures of the planet, its moons and its rings. Suspecting that Saturn might be circled by many ringlets, scientists did an experiment. They watched the star Delta Scorpii for nearly two and a half hours as it passed through the plane of the rings. As expected, the star's flickering light revealed ringlets as small as 330 feet (100 meters) in diameter.
Uranus, Neptune and beyond
Voyager 2 provided the only close-up glimpses of Uranus and Neptune that we've ever seen. Its closest approach to Uranus was on Jan. 24, 1986. It made observations of the planet, noting that the south pole was facing the sun and that its atmosphere is about 85 percent hydrogen and 15 percent helium.
Additionally, Voyager 2 discovered rings around Uranus, 10 new moons and a magnetic field that, oddly, was 55 degrees off the planet's axis. Astronomers are still puzzling over Uranus' orientation today.
Voyager 2's pictures of the moon Miranda revealed it to be perhaps the strangest moon in the solar system. Its jumbled-up surface appears as though it was pushed together and broken apart several times.
The spacecraft then made it to Neptune, reaching the closest point on Aug. 25, 1989. It skimmed about 3,000 miles from the top of the planet's atmosphere, and spotted five new moons as well as four rings around the planet. [Photos of Neptune, The Mysterious Blue Planet]
Voyager 2's legacy
Voyager 2's observations paved the way for later missions. The Cassini spacecraft, which was at Saturn between 2004 and 2017, tracked down evidence of liquid water at the planet's icy moons several decades after the Voyagers initially revealed the possible presence of water. Cassini also mapped the moon Titan after the Voyagers took pictures of its thick atmosphere.
Voyager 2's images of Uranus and Neptune also serve as a baseline for current observations of those giant planets. In 2014, astronomers were surprised to see giant storms on Uranus — a big change from when Voyager 2 flew by the planet in 1986. Storms have also been brewing on Neptune, under the watchful eye of the Hubble Space Telescope. Scientists have been comparing these observations to what Voyager 2 saw up close in 1989, and as of June 2017, NASA was considering a future mission that would explore Uranus or Neptune.
The Voyager spacecraft both celebrated 40 years in space in 2017, prompting accolades from several people in NASA.
"It's amazing that the two spacecraft are still working after 40 years," Ed Stone, a Voyager project scientist who's been with the mission since the its inception in 1972, told Space.com at the time. "When we launched, the Space Age itself was only 20 years old, so this is an unparalleled journey, and we're still in the process of seeing what's out there."
"I believe that few missions can ever match the achievements of the Voyager spacecraft during their four decades of exploration," Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA headquarters in Washington, D.C., said in a statement in August 2017. "They have educated us to the unknown wonders of the universe and truly inspired humanity to continue to explore our solar system and beyond."
Voyager 2 is only the second spacecraft to cross the heliopause, a bubble created by solar wind flowing from our sun that also marks where interstellar space begins. And although it's 11 billion miles away, Voyager 2 still has a long scientific lifetime ahead of it. NASA expects the spacecraft will still have enough power for its instruments at least through 2025.
- Real-Time Odometer: Where are the Voyagers?
- Read more about Voyager 2's mission, from NASA
- Latest Voyager news from NASA's Jet Propulsion Laboratory
This article was updated on Dec. 10, 2018 by Space.com Reference Editor, Kimberly Hickok.