Planning and operations for superior solar conjunctions during the MESSENGER mission

Solar conjunctions pose many challenges to the planning and operation of the MESSENGER mission to Mercury. A solar conjunction occurs when the spacecraft and the Sun appear near to each other in the celestial sky relative to Earth-based antennas, within 3° of each other from the point of view of project planning. The conjunction can be a superior solar conjunction, during which the spacecraft is on the far side of the Sun relative to Earth, or an inferior solar conjunction, during which the spacecraft is between Earth and the Sun. Experience has shown that special planning and operations are not required for the inferior solar conjunctions. Superior solar conjunction durations have ranged from 48 days when MESSENGER was still far from the Sun to only three days, depending on the phasing relative to Earth. During superior conjunction periods, both uplink and downlink communication with the spacecraft can be degraded and communications are often deemed unreliable because of interference from solar plasma and scintillation. Superior conjunctions are therefore actively managed during both the cruise and orbital phases of the mission. The MESSENGER operations team has developed a flight-proven suite of activities to mitigate the impact of superior solar conjunctions. These include prudent command-load boundary placement, temporary extension of the command-loss timers, active momentum management techniques, shutting off unnecessary power loads including instruments, special management of the solid-state recorder and command volume, and modification of Deep Space Network track coverage.