1989

Voyager 2 at Neptune

The launch of the Voyager 1 and 2 missions in 1977 fortuitously occurred at a time when a rare alignment of Jupiter, Saturn, Uranus, Neptune, and Pluto could allow a space probe to gravitationally slingshot from one to the next and enable flybys of all five planets with little extra propulsion needed. Advocates for this “Grand Tour” of the outer solar system realized that such an opportunity would not occur again for 176 years.

Though the Pluto flyby option was given up in favor of a closer flyby of Saturn’s moon Titan by Voyager 1, the Voyager 2 probe was able to complete the initial reconnaissance of all four of the giant planets. Its last stop on the Grand Tour was in August 1989 at Neptune, where the spacecraft bent its path downward to a 24,000-mile (38,000-kilometer) flyby of the large icy moon Triton.

The Voyager 2 Neptune encounter revealed much beauty and mystery in the Neptune system. The planet’s hydrogen-, helium-, and methane-bearing atmosphere was much more dynamic than Uranus’s had been in 1986, with dark- and light-blue belts and white clouds circling a giant cyclonic storm called the Great Dark Spot (analogous to Jupiter’s Great Red Spot). Planetary scientists discovered that Triton’s relatively youthful surface contains active geysers of icy nitrogen, water, and carbon dioxide, erupting nitrogen into the moon’s thin atmosphere of nitrogen, carbon monoxide, and methane. A new large moon, 250 miles (400 kilometers) across, was discovered close to the planet and was named Proteus, after the shape-shifting Greek sea god. Five other small moons were discovered and characterized during the flyby. Subsequent missions have gone on to orbit Jupiter (Galileo) and Saturn (Cassini), and plans are slowly being formulated to send orbiters to Uranus and Neptune to expand on Voyager’s initial discoveries.

Neptune’s second-largest moon, Proteus, was discovered in images taken during the Voyager 2 Neptune flyby in August 1989.

SEE ALSO Uranus (c. 4.5 Billion BCE), Neptune (c. 4.5 Billion BCE), Great Red Spot (1665), Discovery of Neptune (1846), Triton (1846), Voyager “Grand Tour” Begins (1977), Jovian Rings (1979), An Ocean on Europa? (1979), Voyager Saturn Encounters (1980, 1981), Rings Around Neptune (1982), Voyager 2 at Uranus (1986), Kuiper Belt Objects (1992), Galileo Orbits Jupiter (1995), Cassini Explores Saturn (2004–2017).

A composite view assembled from Voyager 2 images simulating a view of Neptune from the surface of the large icy moon Triton.