After almost four decades of dormancy, NASA has successfully ordered the Voyager 1 deep space probe to fire a set of thrusters that have been inactive for 37 years. The November 28 test was carried out by mission control at the Jet Propulsion Laboratory (JPL) in Pasadena, California, as part of an effort to keep the unmanned explorer functional for up to an additional three years as it speeds into interstellar space.

Launched 40 years ago, Voyager 1 is the most distant and fastest manmade object ever to leave the Earth, and at a distance of 13 billion mi (22 billion km) away a signal from the nuclear-powered spacecraft takes 19 hours and 35 minutes to reach one of NASA's Deep Space Network antennae at Goldstone, California. In 2013, it became the first probe to enter interstellar space and the space agency hopes that its failing radioactive batteries will continue to function until 2025.

However, how long Voyager's nuclear power source operates will be irrelevant if it's unable to keep its main antenna pointed at Earth. If its attitude starts to drift, there is the danger of contact being lost forever and the spacecraft shutting itself down automatically. To prevent this from happening, the probe is equipped with a set of gyroscopes and 16 hydrazine MR-103 thrusters (eight primary and eight backup) built by Aerojet Rocketdyne.

These thrusters were of vital importance during the complex maneuvers as Voyager 1 flew by Jupiter and Saturn. They not only ensured that the spacecraft kept on its proper slingshot trajectory to boost its velocity high enough to reach its next target and, eventually, escape the Solar System, but were also essential for properly aligning the spacecraft's antenna and pointing its instruments in the right directions.

Once Voyager left Saturn three years into its mission, most of the thrusters became redundant and NASA ordered the probe to shut them down and stop heating them to conserve electrical power. Currently, it relies on only four primary attitude control thrusters to keep it pointing at Earth using tiny puffs of gas lasing for a few milliseconds. The problem is that the propellant in that thruster system is limited and the engines themselves started degrading three years ago, so they are generating less and less force.

To keep Voyager 1 alive, a group of JPL propulsion experts recommended to mission control to switch over to four of the dormant Trajectory Correction Maneuver (TCM) backup thrusters that haven't fired since 1980. This meant retrieving data from the earliest years of the project that included software written in an assembler language from the days when BASIC and Fortran were cutting edge. It also meant programming the TCM thrusters to fire in short bursts, which they weren't designed to do.

According to NASA, the test last Wednesday saw the probe fire the TCM thrusters for 10 milliseconds in a series of pulses that demonstrated that they were capable of taking over the task of attitude control. The space agency plans to heat up the four thrusters one at a time in January to bring them fully online. They will then be used for attitude control until there is no longer adequate power for the heaters, after which the job will revert to the original system.

In addition, the agency plans to do a similar switch over on Voyager 2, which is scheduled to enter interstellar space in a few years.

Source: NASA