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, 35 years after its launch. It is NASA's longest-running mission
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 only takes place 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.
Backup for Voyager 1
Both probes 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 was 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 planets. 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.
Today, the spacecraft is approaching the spot in the solar system where the solar wind gives way to interstellar space. Voyager 2 is almost 100 astronomical units (or Earth orbit equivalents) away from Earth, so far away that it takes signals 27 hours to reach there and back.
More than three decades after its launch, Voyager 2 still has a long scientific lifetime ahead of it. NASA expects the spacecraft will still have enough power for its instruments through 2025.
— Elizabeth Howell, SPACE.com Contributor