The ESA spacecraft Mars Express is the first European mission to the red planet and has contributed many scientific discoveries in the search for water and traces of life on Mars since its arrival at Mars in late 2003. Launched in June 2003 it has exceeded its planned lifetime by more than eleven years. After more than 14 years in orbit, aging components and diminishing on-board resources present a substantial challenge: continuing to probe Mars and to generate invaluable scientific data becomes a matter of innovative resource management strategies. Mars Express uses three lithium-ion batteries which were dimensioned for a 4 year mission duration. The batteries have now been in use for much longer than their intended lifetime and the effect that ageing and degradation has had in reducing their capacity is becoming significant. This limits the maximum eclipse length the batteries can support and increases their depth of discharge during every eclipse: this, in turn, further accelerates the ageing process. An additional obstacle is that Mars Express is the oldest scientific spacecraft using lithium-ion batteries and therefore there exists no comparable data set one could rely on to predict the remaining battery lifetime and degradation behaviour. These aspects, increasing eclipse lengths, decreasing battery capacity and limited predictability, are major life limiting factors which need to be addressed by the Flight Control Team. Eclipse lengths could only be reduced through orbit change manoeuvres which would consume more fuel than the very tight budget would allow, so the only option to enable the spacecraft to continue to survive long eclipses as the batteries continue to degrade and also to slow the rate of their degradation is to reduce the spacecraft's power consumption during eclipses. To that end the Flight Control Team conducted a review of the spacecraft power balance. The eclipse power budget comprises power supplied by batteries and power required by the platform and the thermal subsystem. Л power saving regime was devised around the largest single consumer during eclipses - the thermal subsystem. It accounts for almost 50% of the spacecraft's power demand. This paper introduces the new thermal power saving regime for Mars Express. It is based on a combination of three individual strategies: warm-up pointings, boost heating, and spacecraft cooling. Warm-up pointings and boost heating are used to increase the thermal energy of the spacecraft prior to eclipse entry. This is achieved by changing the spacecraft attitude to expose a certain side to the Sun absorbing its thermal radiation and by using spare power to run additional heater cycles, respectively. After eclipse entry the thermally 'charged' spacecraft will dissipate the accumulated energy, however, due to its thermal inertia this process will sufficiently delay the need for heater activity until well into the eclipse. At this point the spacecraft cooling strategy comes into effect. It temporarily lowers the temperatures allowed by the Thermal Control System, whilst still respecting the myriad hard limits on platform and payloads further delaying the heater activity. The combined strategy was operationally applied during the 2017 eclipse season and yielded a mean DoD reduction of 10 percentage points. This remarkable result changes the outlook of the Mars Express mission substantially, since it conceivably enables the spacecraft to survive the harsher eclipse seasons post 2020 from a battery perspective. Furthermore, the smaller depth of discharges will reduce the rate of battery degradation and extend the lifetime of batteries and spacecraft.
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