'Knocking on eternity's door' — NASA's Voyager mission turns 40

The Voyager probes have sent messages about our solar system back to Earth for 40 years. They also double as messengers: Each carries a copy of the iconic Golden Record, a phonograph packed with data and messages about human beings. As their power runs down and they eventually cease returning data to Earth, their only functional purpose will be to send that Golden Record out into space.