Rosetta operations strategy modified following a RCS test
23 September 2010
In 2011 the Rosetta spacecraft will go into a 2.5 year hibernation until its reactivation to prepare for its encounter with Comet 67P/Churyumov-Gerasimenko in 2014.
In September 2006, during a period without contact with Earth, a pressure reading in the Reaction Control System (RCS) had dropped unexpectedly to zero. A leak had not seemed the most likely explanation at the time. Nonetheless, all eventualities had been evaluated before any spacecraft tests were carried out this year. During a test performed on the RCS on 9th September 2010, the data on the pressure showed there was a leak in an area which cannot be isolated.
The leak implies a modification in planned spacecraft operations and thus the RCS will not be re-pressurised as planned in January 2011. The system will be operated using existing pressure in so-called "blow-down" mode. This increases operational uncertainties, but the margins remain comfortable.
Ground tests have been done to characterise fully the behaviour of the Rosetta thrusters at low pressure. This has allowed identification of operational measures giving an optimised rendezvous manoeuvre strategy and an optimised thermal control of the fuel tanks during rendezvous manoeuvres. This was also aided by the fact that fuel allocated for uncertainties on manoeuvres in the first part of the mission has not been used and can now be utilised for this optimisation.
Thus, despite the unwelcome news, an operation strategy has already been defined that conserves the mission objectives without re-pressurisation of the RCS system.
The strategy achieves the required mission by delaying the start of the post-hibernation rendezvous manoeuvre by about one week. This small delay is safely within the uncertainties still existing in the overall comet operations timeline.
The rest of the mission manoeuvres will be performed at low pressure, with lower but acceptable efficiency. No impact on the comet science operations is expected and in particular the date for lander delivery is kept within the originally planned window.