A Voyager spacecraft flies through the vast reaches of space in this artist's conception.
Launched a few weeks apart in 1977, each probe carries six data-gathering instruments, including television cameras, ultraviolet and infrared imagers, and radio and plasma wave detectors.
Perhaps the Voyager probes' most famous cargo is twin copies of the Golden Record, a version of a time capsule that contains sounds and images meant to represent Earth to any intercepting aliens. The record's contents were selected by a committee chaired by the late astronomer Carl Sagan.
"The launching of this bottle into the cosmic ocean says something very hopeful about life on this planet," Sagan said at the time.