Extending Mars Express gyros for a scientific lifeline

Having reached the milestone of 20 years in space, Mars Express remains one of the oldest operational spacecraft orbiting Mars. The spacecraft subsystems, scientific payload and ground segment are in overall good health, with ongoing smooth science operations. Of the 3 lifetime-limiting elements of the spacecraft (fuel, gyros, and batteries) the ring laser gyro situation may be the one to watch more closely (see also Martin et al., “Getting the most out of Mars Express”, EPSC, 2022). Gyro 3 failed in March 2024 during a routine wheel off-loading. This gyro was the most degraded of the remaining gyros and was the next expected to fail but was not predicted to reach end of life until July 2024. This does not affect the prediction for the overall lifetime of the gyros but does reduce confidence in the predictions. Considered that the spacecraft needs at least three functioning gyros to compute its attitude based on inertial information, the failure of Gyro 5 (the next projected one to fail) will drive the end of this capability. This is not expected before 2029-2030 with the current low (~5%) average duty cycle of the Inertial Measurement Unit (IMU).

 

For the purpose of ensuring that the mission can fulfill its mission extension goals over the coming years, it was decided to examine if additional measures could be taken to further reduce the duty cycle of the gyros, with the caveat that the fuel consumption should not be increased and if possible even decreased. Investigations have been started between Flight Dynamics, the Flight Control Team and the Science Ground Segment, with the aim to increase Mars Express’ operational lifetime. Among the main points is the option of not turning the IMU on during wheel off loadings. Gyros are switched on (and lifetime consumed) during maintenance blocks (wheel off-loadings), Orbit Control Manoeuvres (OCM), a very small number of science operations that require the spacecraft to slew faster than can be supported by the gyroless estimator, and other operation incidents, e.g., the 2023 Safe Mode. Maintenance blocks account for the vast majority (~85%) of gyro usage, with observations in gyrostellar mode and operational incidents making up the remaining ~15%. It therefore makes the most sense to focus on the maintenance blocks when looking for reductions in gyro usage.

 

Maintenance blocks are performed roughly every 4 orbits and last about 100 minutes, divided into 3 roughly equal phases: slew to the maintenance attitude, wheel off-loading in a fixed attitude and slew back from the maintenance attitude. Regarding gyro operations, there are 2 classes of maintenance blocks: (i) “Gyrostellar” where the maintenance is performed with the gyros on for just under 40 minutes and the spacecraft in gyrostellar mode. (ii) “Gyroless” where the spacecraft attitude is controlled by the gyroless estimator throughout the maintenance block. However, the gyrostellar estimator is switched on, but not put in the loop during the rate reduction phase. Based on this there are several possible options to reduce gyro usage in maintenance blocks: Decrease the frequency of wheel off-loadings, perform fewer maintenance blocks in Gyrostellar, or stop activating the gyros in hot standby in gyroless maintenance blocks. The latter would be easy to implement by Flight Dynamics, but the concern is that it could increase the risk of Safe Mode.

 

Implementing one or several of these options will increase Mars Express’ lifetime potentially by a significant number of months, which will give it an extended scientific lifeline. This will allow the mission not only to achieve new science opportunities and take longer benefit from recently added observation modes (e..g., MARSIS Phobos mode, MEX-TGO radio occultations), but also to ensure its capabilities remain robust to support joint science with JAXA’s Martian Moon eXplorer (MMX) mission to Phobos from 2027 onward.