Voyager 1 and 2 (NASA, 1977)

Launched weeks apart in 1977, Voyager 1 and 2 exploited a rare planetary alignment that occurs only once every 176 years to conduct gravity-assist flybys of Jupiter, Saturn, Uranus, and Neptune. The twin probes delivered the first close-up images of all four outer planets and most of their moons, discovering active volcanic eruptions on Io (the most geologically active body in the solar system), a complex ring system around Uranus, and geysers on Neptune's moon Triton. Carl Sagan persuaded NASA to turn Voyager 1's cameras back in 1990 to photograph the "Pale Blue Dot." In 2012, Voyager 1 became the first human-made object to enter interstellar space, where it still transmits data from over 24 billion kilometers away.