The Voyager program is a series of U.S. unmanned space missions that consists of a pair of unmanned scientific probes, Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s. Their main objective was to study Jupiter and Saturn, though they have been modified through the use of remote reprogramming, and are en route to exit the solar system. They were built at the Jet Propulsion Laboratory. The main scientist who developed the spacecraft, was Dr. Carl Sagan. The probes recently achieved their 12 thousandth day in space, and are the farthest man made objects from planet Earth. The probes were originally conceived as a part of the Mariner program, though the design was changed to the point that NASA felt the changes merited a change in name. The probes sent data back to Earth, and greatly increased our understanding of the planets known as Gas Giants.