A Practical Application of Relativity to Observation and Control of MESSENGER Timekeeping

he MESSENGER mission to Mercury, designed, implemented, and managed by The Applied Physics Laboratory, uses a partially automated timekeeping system to ensure that the knowledge of time is properly maintained onboard the spacecraft and on the ground. Since 2007, accurate time has been maintained when required through the use of a very stable oscillator. The behavior of that oscillator, and hence of the timekeeping system itself, has varied in flight in a manner that was not observed on the ground but which conforms to the special and general theories of relativity. Through accounting for the effects of relativity, we have been able to determine the rate of aging of the oscillator and are therefore able to predict the future behavior of the timekeeping system. This knowledge is now routinely applied by the MESSENGER Mission Operations Center to predict when new time parameters will need to be uploaded to the spacecraft as well as the values that need to be uploaded.