Are we alone?
Forty years ago, NASA rocket scientists sought to answer this question by launching the Voyager spacecraft, twin unmanned spaceships that would travel further than any human-made object in history.
They are still traveling.
When Voyager 1 and 2 launched about two weeks apart in 1977 from Cape Canaveral, Florida, scientists knew little about the outer planets in our solar system, and could hardly imagine the scope of their upcoming space odyssey.
"None of us knew, when we launched 40 years ago, that anything would still be working, and continuing on this pioneering journey," said Voyager project scientist Ed Stone.
Voyager's main mission was to explore other planets including Jupiter, Uranus and Neptune, but it also carried the story of humanity into deep space.
On board each Voyager is a golden record -- and record player -- that is built to last one billion years or more and contains key information about humanity and life on planet Earth, in case of an alien encounter.
The sounds include the calls of humpback whales, the Chuck Berry song "Johnny B. Goode," Beethoven's Fifth Symphony, a Japanese shakuhachi (a type of flute), a Pygmy girls' initiation song, and greetings in 55 languages.
Late American astronomer Carl Sagan, one of the lead scientists involved with the project, also asked his son, Nick, who is now 46, to record his voice on it.
"Hello, from the children of planet Earth," says the young American boy.
A total of 115 images are encoded in analog form, including the Great Wall of China, telescopes, sunsets, elephants, Jane Goodall with chimps, an airport, a train, a breastfeeding mother, dolphins and images of the human sex organs.
- Exploring other worlds -
Voyager 2 launched first, on August 20, 1977, followed by Voyager 1 on September 5 on a faster, shorter trajectory that would eventually take it into the lead.
The timing of the mission took advantage of a rare planetary alignment that only happens every 175 years, along with "gravity-assist" boosts from passing by planets that minimized the fuel needed.
This handout photograph from Sept. 5, 1977 shows the launch of NASA's Voyager 1 spacecraft from NASA's Kennedy Space Center at Cape Canaveral, Florida
But Voyager faced many challenges from the start, from tight budgets to the limits of modern technology in the 1970s.
One project scientist famously recalled wrapping Voyager's cables in aluminum foil -- the store-bought kind for kitchen use -- to protect the spacecraft from frying in heavy radiation.
Then-president Richard Nixon wanted to contain the mission to a two-planet flyby and a five-year life, but scientists building Voyager planned for a longer mission just in case.
The csmic duo went on to unveil planetary features unlike anything ever seen, including details about the surface of Jupiter with its raging storm the size of two Earths, known as the Great Red Spot.
"These were, at that point, better pictures than any ground-based telescopes could provide," said Alan Cummings, a senior research scientist at Caltech who worked on the project beginning in 1973.
The Voyager mission also discovered the first active volcanoes beyond Earth on Jupiter's moon Io, and signs of a subsurface ocean on Jupiter's moon Europa.
Saturn's moon, Titan, turned out to have the most Earth-like atmosphere in the solar system, while Neptune's moon Triton was revealed to be spewing icy geysers.
- 'Cherish that blue dot' -
Astronomy textbooks were rewritten on a wide scale, thanks to Voyager, which zoomed past Jupiter, Saturn, Neptune and Uranus and "revolutionized the science of planetary astronomy," NASA said in an online remembrance of the famed mission.