The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s. Although officially designated to study just Jupiter and Saturn, the probes were able to continue their mission into the outer solar system, and are, as of May 2012^[update], on course to push through the Heliosheath. These probes were built at JPL and were funded by NASA. Voyager 1 is 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 previously known. In addition, the spacecraft trajectories have been used to place limits on the existence of a hypothetical trans-Neptunian planet.

The Voyager probes were originally conceived as part of the Mariner program, and designated Mariner 11 and Mariner 12, respectively. They were then moved into a separate program named Mariner Jupiter-Saturn, later retitled Voyager because it was felt that the probes' designs had moved sufficiently far from the Mariner family that they merited a separate name.Voyager is essentially a scaled-back version of the Grand Tour program of the late 1960s and early 1970s. Gary Flandro, a Jet Propulsion Laboratory employee on the study team, discovered that the alignment of the outer planets would make it possible to use a gravity assist from Jupiter to go to Saturn and on to Uranus and Neptune. The Grand Tour's plan was to send a pair of probes to fly by all the outer planets; it was scaled back because of budget cuts. In the end, Voyager fulfilled all the Grand Tour flyby objectives except for Pluto, which at the time was considered a planet by the IAU.