Distant spaceprobe in repair drama

Voyager: Way beyond the most distant planet

By Dr David Whitehouse BBC News Online science editor

Scientists have carried out an instrument swap on a spacecraft that is twice as distant as Pluto, the farthest planet.

A back-up computer chip and optical sensors on the spaceprobe had to be activated after the primary ones showed signs of failing.

The task itself is not very unusual, except that the spacecraft is Voyager 1, which is more than 12.5 billion kilometres (7.8 billion miles) from Earth.

A signal takes 12 hours to reach the probe.

'Like new equipment'

After their launch in 1977, Voyager 1 and its twin, Voyager 2, completed their tour of outer planets in 1989.

Ed Massey led the Voyager team

The spacecraft are now heading towards the heliopause, where the Sun's influence gives way to interstellar space.

Both have adequate power and communication capabilities to explore that frontier for about 20 more years, if other onboard systems hold up.

Last month, Voyager's controllers cautiously activated a back-up position-sensing system, including a Sun sensor and star tracker.

"After sitting on the shelf for 25 years, it's like new equipment," said Ed Massey, Voyager project manager at the American space agency's (Nasa) Jet Propulsion Laboratory (JPL), California, US.

Tim Hoggle and Voyager's faint signal

Voyager 1's original attitude-control system had showed 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 analogue 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 back-up also meant activating the back-up Sun sensor and star tracker, which provide the reference points for the spacecraft's orientation in space.

'Relatively smoothly'

"We had to plan this switch very carefully," Dr Hogle said.

The back-up equipment had not been tested since Voyager 1 was approaching Saturn in 1980.

Among other precautions, the team programmed a temporary changeover with an automatic reversion to the original system, allowing just enough time to evaluate the back-up.

"By switching to the back-up before the original system failed, we now have the original as a back-up if we need it," said flight-team member Steve Howard.

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."