Search

The Attitude and Orbit Control System of the 3 axis stabilised spacecraft (S/C) XMMNewton was designed to use three reaction wheels (RW) with a fourth wheel in cold redundancy. The 3 wheel configuration has been used for more than 13 years for the momentum management, to re-orientate the S/C with slews in order to achieve the required scientific pointing. With 3 wheels, frequent reaction wheel unloading is used both to compensate the external torque and to re-orientate the angular momentum to allow the slew execution, which results in fuel consumption of around 6 Kg per year. A fourth wheel in control gives the advantage of a higher momentum envelope and introduces the null space in the wheels matrix, allowing a wheel speed variation without changing the total angular momentum. With four wheels the reaction wheel bias will compensate only for the accumulation of angular momentum due to the external torques, resulting in saving a significant amount of fuel and extending the mission lifetime. The use of four reaction wheels implied the development of new attitude control algorithms and a software change on board. A design phase of this change was successfully completed and the project went through the implementation

Extending the Lifetime of ESA’s X-Ray Observatory XMM-Newton Marcus G. F. Kirsch, Jim Martin and Andreas Rudolph ESA ESOC, Darmstadt, Germany Alastair McDonald and Rainer Kresken CGI, Germany Anders Elfving ESA ESTEC, Noordwijk, The Netherlands Mauro Pantaleoni and Thomas Godard Rhea Systems S.A., Wavre, Belgium Norbert Pfeil Terma, Darmstadt, Germany Timothy Finn, Frederic Schmidt, Detlef Webert and Uwe Weissmann Telespazio Vega, Darmstadt, Germany Andre Vasconcelos SCISYS Deutschland GmbH, Darmstadt, Germany

Mauro Pantaleoni 1 Rhea Systems S.A. working at ESOC, Darmstadt, Germany Patrick Chapman 2 Airbus DS (former Astrium Ltd.), Stevenage, UK Thomas Godard 3 Rhea Systems S.A. working at ESOC, Darmstadt, Germany Rob Harris 4 Rhea Systems S.A. working at Airbus DS (former Astrium GmbH), Friedrichshafen, Germany Marcus Kirsch 5 ESA ESOC, Robert-Bosch Strasse 5, Darmstadt, Germany Rainer Kresken 6 CGI, Darmstadt, Germany Jim Martin 7 ESA ESOC, Robert Bosch Strasse 5, Darmstadt, Germany Alastair McDonald 8 CGI, Darmstadt, Germany Paul McMahon 9 Airbus DS (former Astrium Ltd.), Stevenage, UK Frederic Schmidt 10 Vega Space GmbH, Darmstadt, Germany Tommy Strandberg 11 Airbus DS (former Astrium GmbH), Friedrichshafen, Germany Uwe Weissmann 12 Vega Space GmbH, Darmstadt, Germany Detlef Webert 13

ESA’s XMM-Newton space observatory, the flagship of European X-ray astronomy, is after it’s launch in 1999 the most powerful X-ray telescope ever placed in orbit. The mission originally designed for a 10 years lifetime is planned to be operated long into this decade since spacecraft and instruments are operating admirably without major degradation. Therefore recently a system called XMM Early Warning System (XEWS) is developed to perform near-real-time trend analysis of spacecraft parameters in order to detect early degradation of components. This will enable the mission to perform early counter measures in case degradation is detected. During the development of XEWS it has been spotted that one of XMM-Newton’s reaction wheels shows since 2008 non-periodic friction increase during stable pointing. We present an analysis of all four reactions wheels since the start of the mission identifying the periods of increased friction and giving some possible causes and cures for this effect that is as well know as “cage instability” and describe the impact on operations. Additionally a comparison with the wheels of ESA’s INTEGRAL spacecraft which is using effectively the same Attitude and Orbit Control System will be presented.

The XMM-Newton X-ray observatory mission has achieved ten years of successful scientific observations and spacecraft operations. Due to this outstanding success, the mission has been extended until 2012. This long term objective can only be achieved by maintaining the instruments in a healthy state with the platform able to operate correctly. The spacecraft operates in a strong radiation environment that requires monitoring, taking action according to the strength of radiation to protect detectors and electronics on board. Long term exposure to radiation may degrade the CCD detector of the Star Tracker (STR) modifying the behaviour of the pixels and their response to optical photons. Periodically a survey of the status of the CCD sensor is performed, with a dedicated operational procedure, in order to spot pixels exhibiting anomalous behaviour, here called blemish pixels. During the last survey operation, 6 new blemishes were spotted, increasing the total number to 12. The blemishes result in a number of Single Event Upsets (SEU), loss of guide star and inaccuracy of the pointing. In order to cope with these effects the regions around the blemishes are excluded at mission planning level, rejecting candidate guide stars which would be imaged close to the problematic pixels. This method results in the reduction of the effective area of the CCD, proportional to the number of blemishes. A further method can be used to cope with the blemishes: subtracting the energy of the blemish pixel in the STR software on board, corrects the signal of the pixel. With this method the full effective area of the STR CCD could be used again. The method has not been used so far. This paper will report on a possible implementation and its aspects in S/C operations. It requires testing and validation using the S/C simulator as well as specific tests on the satellite. Special care needs to be taken to avoid saturation of the pixel signal.

The XMM-Newton mission was conceived and designed in the early 90's, and the technology and programming techniques used on its instruments are therefore more than 15 years old, being this also applicable to the SW validation and development facilities kept on ground. The XMM-Newton instrument SW modification and maintenance was originally the responsibility of the institutions that provided the instruments, although a parallel development and validation capability was maintained under ESA control. Along the six years of mission, this situation has evolved such that the SW maintenance is today mainly performed by ESA, with the institutions acting under support contracts, related to the maintenance of the instrument spare units and general consultancy. More than twenty SW modifications have been performed to the instruments while in flight and almost every on- board memory has been patched since launch. Modifications include corrections to SW anomalies, adaptation to the in flight conditions, provision of new designs to improve performance, enhancement to the autonomous instrument safety and coping with instrument degradation. This paper describes the SW maintenance activities performed so far in the XMM-Newton instruments, focusing deeper into the different requirements that led to modifications of the SW, the complexity of dealing with different testing and validation levels and scenarios including operational interfaces between the MOC at Darmstadt and the SOC at Madrid, and facing at the same time the need of reducing to a minimum the impacts on science activities. Furthermore, it addresses the challenges and constraints that missions with a relatively long estimated lifetime have to face to keep expertise and tools for SW modifications, and looks into possible scenarios to ensure many more successful years for XMM-Newton..

Advanced solar electric power systems and electric propulsion technology constitute viable elements for conducting human Mars transfer missions that are roughly comparable in performance to similar missions utilizing alternative high thrust systems, with the one exception being their inability to achieve short Earth-Mars trip times. A modest solar electric propulsion human Mars scenario is presented that features the use of conjunction class trajectories in concert with pre-emplacement of surface assets that can be used in a series of visits to Mars. Major elements of the Mars solar electric transfer vehicle can be direct derivatives of present state-of-the-art Solar array and electric thruster systems. During the study, several elements affecting system performance were evaluated, including varying Earth orbit altitude for departure, recapturing the transfer stage at Earth for reuse, varying power system mass-to-power ratio, and assessing solar array degradation on performance induced by Van Allen belt passage. Comparisons are made to chemical propulsion and nuclear thermal propulsion Mars vehicles carrying similar payloads.