Voyager 2, despite its name, was the first of two twin probes launched in 1977 to explore the outer planets. 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.
Even today, the spacecraft is still beaming back information from the very edge of the solar system. Carrying a golden record bearing sounds from Earth, Voyager 2 is poised to enter interstellar space and rake in new scientific discoveries, 40 years after its launch.
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. It would let each probe swing from one planet to the next, using a gravity boost to help it along the way.
The Voyager spacecraft tour of the solar system was a scaled-back version of NASA's original plans. The agency originally planned a series of four probes, with two of them flying past Jupiter, Saturn and Pluto and the other two going by Jupiter, Uranus and Neptune. The two Voyager spacecraft, however, ended up exceeding engineering expectations. They remain in good working health more than four decades after launch. Voyager 1 crossed the boundary into interstellar space in 2012, while Voyager 2 is expected to follow it there in the coming years.
Backup for Voyager 1
Both Voyagers were supposed to take pictures of Jupiter and Saturn. But NASA wanted to try something a bit more ambitious. 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. If the spacecraft were still working after Saturn, NASA would try to take pictures of the other planets.
Voyager 2 was supposed to act as a backup to Voyager 1, too. 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 some pictures.
Of course, 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. [Infographic: How the Voyager Space Probes Work]
Jovian and Saturnian discoveries
Voyager 2 arrived at Jupiter in 1979, two years after launching from Cape Canaveral. Since Voyager 1 had just gone through the system four months previously, Voyager 2's arrival allowed NASA to take valuable comparison shots of Jupiter and its moons. It saw 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 Europa. From 128,000 miles away, Voyager 2 took more detailed images of the moon's cracks. Scientists detected no change in elevation on the planet; one famously commented that the moon looked as smooth as a billiard ball.
Proving that moons are abundant around the outer planets, Voyager 2 happened to image Adrastea only months after Voyager 1 found Thebe and Metis. Adrastea is a tiny moon, 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, taking 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 humans have ever had. 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.
Voyager 2's legacy
NASA has said Voyager 2's observations have been crucial to later observations. 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 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, which are done by telescope. 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; these observations are also compared to what Voyager 2 saw up close in 1989. 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," said Ed Stone, who has been a Voyager project scientist since the mission's inception in 1972, in an interview with Space.com in September 2017. "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."
In December 2017, NASA announced it had successfully used a set of backup thrusters on Voyager 1 to help the spacecraft orient itself toward Earth for communications. (The original thruster set was losing in performance, prompting NASA to try a set of thrusters on Voyager 1 that were last used for a Saturn flyby in 1980.) The agency said it planned to run a similar backup-thruster test on Voyager 2, although its primary thrusters are in better shape than Voyager 1's.
As of February 2018, Voyager 2 is roughly 117 astronomical units (sun-Earth distances) from Earth. That is roughly 10.9 billion miles, or 17.5 billion kilometers.
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.