The Voyager program is a series of U.S. unmanned space missions that consists of two unmanned scientific probes, Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a rare planetary alignment of the late 1970's. Although they were officially designated to study just Jupiter and Saturn, the two probes were able to continue their mission, well beyond what was intended, into the outer solar system. Voyager 1 is currently the farthest human made object from Earth.
Both missions have gathered large amounts of data about the gas giants of the solar system, of which little was known at the time.