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 anticipated, 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.