"I think it was the science adviser at the time, and the NASA administrator, who went to visit [then-President Richard] Nixon," Krimigis said, noting that Nixon was lukewarm on the mission. "They told him that the opportunity only arose once every 175 years -- 'and Jefferson missed it.' " Nixon signed on.
The twin probes were launched less than a month apart in the summer of 1977. Each weighed 1,800 pounds and carried 11 cameras and instruments. Computer memory was 80 kilobytes (an entry-level PC today has nearly 10,000 times as much). Electricity came from nuclear-powered generators fueled by the heat of decaying plutonium. Krimigis said they knew from the start that the probes could get a lot farther than Jupiter and Saturn.
Two Voyager spacecraft are still transmitting as they near the outer limits of the sun's influence.
And they did. A "Grand Tour" of the four planets ended in 1989. Both spacecraft visited Jupiter and Saturn, after which Voyager 2 went on to the first and, thus far, only flybys of Uranus and Neptune. Voyager 1 headed toward the frontier of interstellar space, overtaking the earlier Pioneer 11 spacecraft, launched in 1973, whose power supply is dead.
Today Voyager 1, about 9 billion miles from Earth and traveling at 46,000 mph, and Voyager 2, about 7 billion miles away doing 63,000 mph, are flirting with the edge of the solar system, where the sun's magnetic field and the solar wind give way to interstellar wind.
Virtually nothing is known about this boundary. Data from the spacecraft show periodic jumps in radiation levels -- expected when the solar wind is no longer able to block incoming cosmic rays -- followed by smaller declines.
"By 2006, the spacecraft may have crossed into the outermost layer of solar atmosphere, where the supersonic wind has slowed and heated to a million degrees as it interacts with the interstellar wind," said California Institute of Technology physicist Edward C. Stone, Voyager's chief scientist from the outset. "If Voyager is terminated, we will lose the opportunity to observe [this] interaction."
For Stone, 69, and Krimigis, 66, Voyager has been what Stone described in an e-mail as "the defining event that has shaped my career for the last 30 years," and the Voyagers have amassed accomplishments unsurpassed by any spacecraft.
The two probes have discovered 22 moons at four planets. Voyager 1 has traveled farther than any other spacecraft, and took the first portrait of the solar system from the outside looking in.
The probes found exploding volcanoes on Jupiter's moon Io, huge fault canyons on Uranus's moon Miranda, geysers on Neptune's moon Triton, and flew by Saturn's methane-enshrouded moon Titan almost 14 years before the European Space Agency's Huygens probe landed there this January.
The Voyager mission today has a full-time staff of 10 people, down from 300 at the height of the Grand Tour. The probes' software has no storage capability, so they must transmit their instruments' readings in real time.
The signal takes 12 hours to reach far-away Earth.