A quarter-century after NASA’s twin Voyager spacecraft
departed Earth to visit outer planets, the historic mission is
flying a race against time.

During the first 12 years after launch in 1977, the Voyagers
chalked up a wealth of discoveries about four planets and 48
moons, including fast winds on Neptune, kinks in Saturn’s rings
and volcanoes on Jupiter’s moon Io. As scientists and engineers
mark the mission’s silver anniversary, they hope at least one
Voyager will pass beyond the boundary of the Sun’s influence
before the onboard nuclear power supply wanes too low to tell us
what’s out there. Voyager 1 is now the most distant human-made
object, about 85 times as far from the Sun as Earth is. Voyager 2
is now about 68 times the Sun-Earth distance.

“After 25 years, the spacecraft are still going strong,” said Dr.
Edward Stone, Voyager project scientist since 1972 and former
director of NASA’s Jet Propulsion Laboratory (JPL), Pasadena,
Calif. “Back in 1977, we had no way to know they would last so
long. We were initially just on a four-year journey to Jupiter
and Saturn.”

The Voyager team at JPL still receives information almost daily
from the durable spacecraft traveling beyond all the planets. The
Voyagers are examining the far reaches of the solar wind, a gusty
flow of particles hurled outward by the Sun. The eventual goal is
to become the first spacecraft to taste interstellar space.
Voyager 1, which launched on Sept. 5, 1977, flew past Jupiter and
Saturn, then angled northward out of the plane of the planets’
orbits. After Voyager 2 launched on Aug. 20, 1977, and completed
its tour of Jupiter and Saturn, NASA extended the spacecraft’s
adventure with flybys of Uranus in 1986 and Neptune in 1989.

“A radio signal traveling at the speed of light takes nearly 12
hours to travel between Voyager 1 and Earth. That raises
operational concerns,” said Ed Massey, Voyager’s project manager
at JPL. ” If something went wrong on board, at least a full day
would lapse before a signal revealing the problem could reach
Earth and commands to fix it could be returned.

It could be too late.” So the project team tries to anticipate
any emergencies and program the spacecraft’s computers with
advance instructions on how to react to them, he said.

Both spacecraft are studying the vast bubble the Sun inflates
around itself by outward pressure of the solar wind. The bubble
has a boundary, called the heliopause, where this outward
pressure is counterbalanced by inward pressure of the
interstellar wind in our neck of the galaxy. The interstellar
wind outside that boundary is a flow of atoms and other particles
blasted from explosions of dying stars. The location of the
heliopause varies with the level of solar activity during the
Sun’s 22-year sunspot cycle and with changes in the interstellar
wind, Stone said. Some scientists suggest that, on a much longer
time scale, the interstellar wind may occasionally press the
boundary far enough inward to sway Earth’s climate.

Voyager 1 is rushing toward the heliopause at about one million
miles (1.6 million kilometers) a day. Whether it gets there
before about 2020, while it still has adequate electrical power,
depends on how far away the heliopause is. Recent estimates are
that, depending on that distance, it would take Voyager 1 between
seven and 21 years to reach the heliopause.

Voyager 1 has already discovered that the outbound solar wind
around it is slowing from effects of inbound interstellar
particles leaking through the boundary. A much better prediction
of the boundary’s location will come when the spacecraft
encounters the termination shock, the zone where the solar wind
begins piling up against the heliopause. That encounter may come
within the next three years, Stone estimates.

Whatever their future holds, Voyager 1 and Voyager 2 have already
earned a prominent place in the history of exploration. Among
their big surprises: Jupiter’s moon Io has active volcanoes.
Jupiter’s atmosphere has dozens of huge storms. Saturn’s rings
have kinks and spoke-like features. The hazy atmosphere of
Saturn’s moon Titan extends far above the surface. Miranda, a
small moon of Uranus, has a jumble of old and new surfacing.
Neptune has the fastest winds of any planet. Neptune’s moon
Triton has active geysers.

Long after they fall silent, the Voyager twins will keep speeding
away from our solar system, each carrying an “interstellar
outreach program” of recorded sounds and images from Earth,
Massey said.

More information about Voyager is available at:


JPL, a division of the California Institute of Technology in
Pasadena, manages Voyager for NASA’s Office of Space Science,