The planet-hunting space observatory Kepler will run out of fuel within months, ending almost a decade of service, NASA has revealed.

The spacecraft, currently about 140 million kilometres from Earth, is down to its last reserves and scientists expect it to cease functioning by Christmas. Given that the craft has already surprised its operators several times, however, no one is prepared to make firm predictions.

“Without a gas gauge, we have been monitoring the spacecraft for warning signs of low fuel— such as a drop in the fuel tank’s pressure and changes in the performance of the thrusters,” says system engineer Charlie Sobeck.

“But in the end, we only have an estimate – not precise knowledge.”

Kepler was launched in 2009 with a mission plan that assumed it would stop working after three and a half years. The craft, however, proved considerably more robust and continued surveying the Milky Way for exoplanets until 2013, when an onboard component known as a reaction wheel broke.

This meant it could no longer accurately hold focus. NASA techs, however, reconfigured the remaining instrumentation and worked out a way in which the pressure of sunlight could be used to hold Kepler’s lens steady.

Rebranded, thus, as “K2”, the observatory entered a second phase of life – shifting its focus to a new bit of the galaxy every three months. Initially, NASA estimated the observatory had enough fuel remaining to do so 10 times.

Once again, however, Kepler confounded expectations. It has now shifted focus 17 times and may well have energy for a couple more shifts left in its tank.

Because of its remote location, Kepler is exempt from the safety concerns that govern end-of-life operations for other NASA missions. Unlike craft in near-Earth orbits, there is no requirement to conserve enough fuel to conduct a controlled re-entry. And unlike Cassini, there is no risk of it crashing into a potentially life-supporting moon.

The observatory, thus, can be left to run until its fuel is completely exhausted. NASA says its primary concern is judging the point at which Kepler has just enough fuel remaining to be able orient itself so that it can transmit its last ever tranche of data.

After that, Kepler, the observatory that refused to die, will go silent for ever.

Quelle: COSMOS