In early August 1989, Voyager 2 approached Neptune after a 12-year journey across the solar system, sending back images that revealed the planet's Great Dark Spot and its unexpectedly dynamic, storm-streaked atmosphere. The spacecraft would make its closest approach on August 25, becoming the only probe ever to visit Neptune and completing the Grand Tour of the outer planets begun in 1977.
By the time Voyager 2 reached the outskirts of the Neptune system in August 1989, it had already outlived its original mission design by more than a decade. Launched in 1977 to take advantage of a rare planetary alignment, the probe had already visited Jupiter, Saturn, and Uranus, using each planet's gravity to slingshot itself onward in a maneuver mission planners called the Grand Tour.
As Neptune grew from a distant point of light into a resolvable disk in the probe's cameras, scientists on Earth were stunned by what emerged: a planet far more turbulent than the placid blue orb they'd expected. Winds howled at over 1,200 miles per hour, the fastest recorded anywhere in the solar system, and a massive rotating storm system, the Great Dark Spot, churned in its atmosphere.
Communication with the spacecraft, by then over 2.7 billion miles from Earth, took more than four hours one way, meaning every image and course correction operated on a profound time delay. Voyager 2's radio signal was so faint that NASA's Deep Space Network had to link multiple giant dish antennas together just to detect it. The Neptune encounter would mark the last planetary flyby of the Voyager program before both probes headed toward interstellar space.