On August 20, 1977, NASA's Voyager 2 spacecraft launched from Cape Canaveral, Florida.
Voyager 2 is now 12.1 billion miles (19.5 billion kilometres) away and continues to transmit data about the distant and unknown heliopause.
Scientists are beginning to wonder how long the iconic space probe will be able to operate.
Voyager 1 is now the most distant artificial object from Earth, at over 14.5 billion miles (23.3 billion kilometres).
Because of its incredible multidecade tour of the giant planets, Voyager 2 is arguably more iconic.
Though it was launched second, Voyager 1 was so named because it was the first to reach Jupiter and Saturn — in March 1979 and November 1980, respectively.
"Both Voyager 1 and Voyager 2 left tremendous legacies for planetary exploration," said NASA. Lunine, Jonathan