Launched in 1977, NASA's twin Voyager spacecraft are now poised at the very edge of interstellar space.
Powered by plutonium 238 batteries, both probes remain in fine fettle after 34 years in deep space. And they are beaming back a repeat message that, according to one NASA scientist, is "both unsettling and thrilling." The message, programmed for the fun of it by project scientists back in the 1970s, is-"Expect the unexpected!"
Ed Stone, Voyager Project scientist since 1972, said what made the twin mission so historic was the fact that they took advantage of a rare alignment in the solar system of all the outer planets, except Pluto.
"Voyager 1 visited Jupiter and Saturn, while Voyager 2 flew past Jupiter, Saturn, Uranus and Neptune. Voyager 2 is still the only probe to visit Uranus and Neptune," Stone said during a space agency news conference held last week.
Stone listed all the amazing discoveries of the Voyager robot team:
•Volcanoes erupting on Jupiter's moon Io.
•An ocean far below the ice surface of Jupiter's moon Europa.
•The magnetic poles of Uranus and Neptune are mysteriously tipped.
•Geysers erupt regularly on Neptune's moon Triton.
•Methane rain falls on Saturn's moon Titan.
•Evidence was found showing that planetary winds are faster as you move away from the Sun.
Stone said the Voyagers are now passing through the solar system's heliosheath.
"The heliosheath is a very strange place, filled with a magnetic froth no spacecraft has ever encountered before, echoing with low-frequency radio bursts heard only in the outer reaches of the solar system, so far from home that the sun is a mere pinprick of light," he said. "In many ways, the heliosheath is not like our models predicted."
According to Stone, "No one knows exactly how many more miles the Voyagers must travel before they pop free into interstellar space. The heliosheath is 3 to 4 billion miles in thickness. That means we'll be out within five years or so."
Heat from Voyagers' nuclear batteries should keep things running smoothly onboard until the year 2020, possibly longer.
"The Voyagers will become our silent ambassador to the stars," Stone said. "Each probe is famously equipped with a Golden Record, literally, a gold-coated copper phonograph record... A team led by Carl Sagan assembled the record as a message to possible extraterrestrial civilizations that might encounter the spacecraft."
Stone said the chance of aliens finding the Voyagers is "fantastically remote."
In 1979, the first "Star Trek" movie was about a Voyager spacecraft that merged with an alien spaceprobe. The two shared their circuitry and became a fantastic, super entity bent on destruction.
"Well, that's science fiction-make believe," said Stone. "But with our real Voyagers, 'expecting the unexpected' is even more fantastic."
Lou Varricchio, M.Sc., was a science writer at the NASA Ames Research Center.