Voyager: The Grand Tour

In the mid-1960s, aerospace engineer Gary Flandro at NASA's Jet Propulsion Laboratory calculated that a rare geometric alignment of Jupiter, Saturn, Uranus, and Neptune—occurring only once every 176 years—would allow a single spacecraft to swing from planet to planet using gravitational slingshots. Launched within sixteen days of each other in late summer 1977, Voyager 1 and Voyager 2 set out on this historic 'Grand Tour,' cutting the travel time to Neptune from thirty years down to just twelve.

During their encounters with Jupiter (1979) and Saturn (1980–1981), the Voyagers discovered active sulfur volcanoes on Jupiter's moon Io, intricate braided structures and shepherd moons within Saturn's rings, and a thick nitrogen-rich atmosphere on Titan. While Voyager 1's trajectory bent northward out of the ecliptic plane after Titan, Voyager 2 continued onward to perform humanity's only close flybys of Uranus (1986) and Neptune (1989), revealing tilted magnetospheres, ten new Uranian moons, and active nitrogen geysers erupting from Neptune's icy moon Triton.

In August 2012, Voyager 1 crossed the heliopause at 121 AU to become the first human-made object to enter interstellar space, followed by Voyager 2 in November 2018. Each spacecraft carries a gold-plated phonograph record—the Voyager Golden Record—curated by a committee chaired by Carl Sagan, preserving the sounds, music, scientific diagrams, and greetings of Earth for any spacefaring civilization that may encounter them across cosmic time.