In 1977, NASA sent the Voyager spacecraft to visit the gas giants Jupiter and Saturn. And though they’re twins, the two spacecraft followed very different trajectories: Voyager 1 left the plane of the solar system after flying by Saturn on a path towards the edge of the heliosphere while Voyager 2 stayed in line with the planets to visit the outer ice giants Uranus and Neptune.
More than 30 years later, it’s Voyager 1 that’s in the news all the time since it is well on its way to becoming our first true interstellar spacecraft. But today, on the 32nd anniversary of Voyager 2’s launch (Aug. 20, 1977), it’s time to look back at some of the amazing sights it saw during its planetary flybys.
We might not think about it much, but as the only spacecraft to see the two furthest planets up close, the breathtaking images we have of Uranus and Neptune are all thanks to Voyager 2.
Though the second of the pair by numerical designation, Voyager 2 was actually the first spacecraft to launch. It left Cape Canaveral on a Titan IIIE rocket on Aug. 20, 1977; Voyager 1 launched on Sept. 5, 1977. It was the first to launch, but Voyager 2 followed a slower trajectory that would see it encounter Jupiter months after Voyager 1.
Voyager 1 started imaging Jupiter in the January of 1979, and after taking some 19,000 pictures in four months, it passed the baton to Voyager 2 in April. The second interplanetary spacecraft added another 14,000 images to the Jupiter image file, making its closest approach to the planet on July 9.
Jupiter wasn’t an unknown entity when the Voyager spacecraft arrived; astronomers had been studying it from the Earth for centuries. But what the two spacecraft found surprised scientists -- the physical, geologic, and atmospheric processes that have shaped the planet and its system of satellites. Most notably, the active volcanoes on the Galilean moon Io. It was totally unexpected to find volcanic activity on a moon, and the more scientists studied of Io’s volcanism the more interesting it became. The moon’s volcanic eruptions seem to be the primary source of material in Jupiter’s magnetosphere.
Before leaving the Jovian system, Voyager 2 took a series of pictures of the Galilean moon Ganymede, which mission scientists have stitched into this mosaic. Ganymede is the largest satellite in our solar system, larger even than Mercury, meaning if it were to orbit the sun and not Jupiter it would easily be classified as a planet. And it would be an ice world. On top of the moon’s metallic iron core (that generated a magnetic field) and the layer of rock is a thick shell of ice that might contain some amount of rocky material. In 1996, astronomers found evidence of a thin oxygen atmosphere on Ganymede using the Hubble Space Telescope.
Voyager 2 reached Saturn two years after its close encounter with Jupiter, and it imaged the ringed planet with more sensitive cameras than Voyager 1. It saw elongated ovals in Saturn’s atmosphere, tilted features in the east-west shear zones, and other features that were on the whole similar to those seen on Jupiter but smaller and more subdued. Some of Saturn’s atmospheric variations can be seen in this image that combines the view through ultraviolet, violet, and green filters.
As it approached the ringed planet, Voyager 2 saw Saturn and its moons Tethys, Dione, and Rhea together (though this picture was stitched together from individual frames). The spacecraft, at this point, was 13 million miles from Saturn, and the black dot in planet’s southern hemisphere is the shadow cast by Tethys.
Voyager 2 also caught this view of Saturn’s moon Titan. It might not look like much, but the white glow is Titan's thick atmosphere. It’s clearly visible around the whole of the moon, particularly in the upper left. At the time, the spacecraft was a little over 620,000 miles away on Aug. 25, 1981.
On Jan. 24, 1986, Voyager 2 became the first spacecraft in history to visit Uranus. At its closest pass, the spacecraft came within 50,600 miles of the planet’s cloudy upper layer. While flying by the planet, Voyager 2 managed to image ten previously unseen moons and Uranus’ ring system in fine detail. Data gathered during the flyby revealed the planet’s rate of rotation is about 17 hours and 14 minutes, that it has a significant magnetic field, and that the global temperature is generally fairly consistent even though Uranus’ equatorial region gets less sunlight than its poles.
In studying Uranus’ moons, Voyager 2 found complex surfaces indicative of a varied geologic past. This detailed view of the Miranda shows at least three types of terrain of different ages and geologic styles. To the left is an ancient hilly region that has been cratered over time. Towards the center of the image is a deeply grooved terrain with linear valleys and ridges. On the right along the terminator is a complex terrain with irregular ridges and few craters.
As Voyager 2 left Uranus, it caught this stunning shot of the planet as a slim crescent against the black backdrop of space.
Perhaps the most recognizable picture of Neptune, this picture of the planet’s full disk was stitched together from images taken on Aug. 16 and 17, 1989. Within two weeks, the spacecraft would make its closest flyby not only of Neptune but of any planet it visited. Voyager 2 passed within 3,000 miles of Neptune’s north pole on Aug. 25, 1989 before beginning its journey to the edge of our solar system.
Voyager 2 caught this stunning view of Neptune and its moon Triton three and a half days after making its close flyby of the planet. Triton is Neptune’s largest moon and the only moon in our solar system that orbits in the opposite direction that its host planet rotates. This uncommon orbit combined with its icy composition has led some scientists to suspect that Triton was an independent body that was captured by Neptune’s much stronger gravity. When it took this picture, Voyager 2 was about 3 million miles from Neptune.
Three hours before its closest approach, Voyager 2 caught this close view of linear clouds near Neptune’s eastern terminator. The light and shadow shows that these clouds are raised above Neptune’s already thick cloudy surface. Voyager 2 took this picture from about 98,000 miles away. Scientists figured that the cloud streaks are 31 to 124 miles long, 18 to 31 miles wide, and as tall as 31 miles.