Astronauts can make service visits to the Earth-orbiting
Hubble Space Telescope, but what do you do if the spacecraft
needing a replacement part is the farthest human-made object
from Earth, more than twice as distant as Pluto?

The answer, as the flight team for the Voyager
Interstellar Mission recently demonstrated, is to plan ahead
and keep top-notch engineers available.

Last month, the team cautiously activated a backup
position-sensing system, including a Sun sensor and star
tracker, on Voyager 1. The spacecraft had been carrying those
components and other spare parts since it was launched in 1977
on what was then slated as a four-year mission.

“After sitting on the shelf for 25 years, it’s like new
equipment,” said Ed Massey, Voyager project manager at NASA’s
Jet Propulsion Laboratory, Pasadena, Calif. Voyager 1 and its
twin, Voyager 2, completed their history-making tour of outer
planets in 1989 and are now headed toward the boundary zone —
called the heliopause — where the Sun’s influence cedes way
to interstellar space. Both spacecraft have adequate power and
communication capabilities to explore that frontier for about
20 more years, if other onboard systems hold up.

The original designers’ foresight in building backup
systems into the Voyagers helps, but making changeovers aboard
a spacecraft more than 12.5 billion kilometers (7.8 billion
miles) away presents enormous challenges. Anything that could
go wrong needs to be foreseen, because reaction time is
unforgivingly slow. Communication signals take nearly 12 hours
each way traveling to or from Voyager 1 at the speed of light.
And the task now falls to a Voyager flight team of just 14
people, compared with a Voyager team of more than 300 in the
1980s.

The success of the recent modifications gives the team
confidence in switching to other backup systems on both
spacecraft when concerns arise about original systems, Massey
said.

Voyager 1’s original attitude-control system showed
slowly increasing signs of trouble in the past two years, said
Tim Hogle, a flight-team engineer at JPL. Diagnostics pointed
to an electronic component that takes analog signals from
position-sensing devices and converts them into digital values
for an onboard computer. Because of the system’s design,
switching to that component’s backup also meant activating the
backup Sun sensor and star tracker, which provide the
reference points for the spacecraft’s orientation in space.

“We had to plan this switch very carefully,” Hogle said.
This backup equipment hadn’t been tested since Voyager 1 was
approaching Saturn in 1980. If it didn’t work right, switching
to it could confuse the onboard computer about the
spacecraft’s orientation, which could lead to faulty pointing
of the antenna and loss of communications with Earth.

Among other precautions, the team programmed a temporary
changeover with an automatic reversion to the original system,
allowing just enough time to evaluate the backup. The planning
effort identified potential trouble points in advance and made
provisions to be able to correct them, if necessary, before
the final changeover, nine days after the temporary one. The
cautious approach paid off. The system made an unexpected lock
onto the Sun during the temporary switch, so the spacecraft
was instructed to keep itself steady with gyroscopes during
the final switch.

Calibration of the backup system was completed April 1.

“By switching to the backup before the original system
failed, we now have the original as a backup if we need it,”
said flight-team member Steve Howard of JPL.

The changeover plan benefited from soliciting experienced
advice from several Voyager veterans now working on other JPL
projects. “We have resources here you just could not find
anywhere else,” Howard said.

Massey said, “The switchover went relatively smoothly. It
is certainly a testament to the people who designed and built
the spacecraft, and to the expertise and dedication of the
flight team.”

Information about Voyager’s grand tour of the outer
planets and the current Voyager Interstellar Mission is
available online at http://voyager.jpl.nasa.gov . JPL, a
division of the California Institute of Technology in
Pasadena, manages Voyager for NASA’s Office of Space Science,
Washington, D.C.