Getting the Message to MESSENGER: Overview of the Weekly Planning and Sequencing of MESSENGER Orbital Activities

摘要

For over three years, the MESSENGER spacecraft has been in orbit about the planet Mercury. Because of the thermal and radiation constraints imposed by MESSENGER'S proximity to the Sun, assembling weekly command loads for the payload requires tight coordination across all operational subsystems and engineering teams. In addition to close team interaction, science planning requires a thorough understanding of mission constraints, accomplished with detailed models of recorder usage, communication uplink and downlink, orbit ephemeris and spacecraft attitude. Because of the many complexities in the orbital phase of the MESSENGER mission, such as the demanding thermal environment and changes in lighting conditions, the overlapping process of these weekly command-load builds must start approximately three weeks prior to onboard execution. The mission's planning and scheduling process is mature, having been originally designed and successfully implemented to assemble the command-load sequences for the Near Earth Asteroid Rendezvous (NEAR) orbital mission. This planning system architecture is also used on the New Horizons mission to Pluto. Refinements were periodically made during the interplanetary cruise and early orbital phases of the MESSENGER mission, and the process and code now constitute a resilient operational system as demonstrated by three very successful and productive years of Mercury orbital science operations (12+ Mercury years), with over 200,000 images captured and over 2,800 orbits completed to date. MESSENGER'S command loads are assembled through a structured process that includes guidelines for load content and data recorder management. Weekly instrument command sequences are generated to fulfill the science objectives without violating instrument and spacecraft operational constraints. The SciBox science planning software serves as a coordination tool that allows the instrument teams to plan their observations, and it uses their plans to produce a consolidated payload command load that is packaged by the mission operations team for upload to the spacecraft. Given the complex set of constraints required to safely operate MESSENGER in orbit about the planet closest to the Sun, this integrated approach ensures that potential command conflicts are resolved in a timely manner, and that the chance is minimized that an erroneous or anomalous command sequence that jeopardizes health and safety or results in data loss could ever make it through all the gates of the three-week process and then to the spacecraft.