CHAPTER 8 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT 150 advanced warning available, the more time flight controllers have to work the intricate details of this power-down plan. When necessary, however, the ISS can execute a burn with as little as 3 hours’ notice (about 5 hours and 20 minutes before the conjunction if the burn is done at the standard 2 hours and 20 minutes). An emergency unplanned power down is normally required in order to perform a burn this quickly. This power down is a preplanned power down, which is always available but unrefined, and may result in noncritical systems being powered down rapidly, thus potentially impacting research on board. For this reason, the emergency power down is an option of last resort. Time Event Comments Pre-burn Request additional Tracking and Data Relay Satellite (TDRS) time from space network If necessary, MCC-H will request to fill gaps in TDRS satellite coverage of the ISS to provide command and telemetry links during critical periods leading up to the PDAM burn as well as the burn itself. TDRS satellite coverage is usually quite good, but there are often 10- to 30-minute gaps in coverage while the satellite network is shared with other users. These gaps can often be negotiated with other users and filled to provide more time for MCC-H and MCC-M to command configurations in preparation for the reboost. Pre-burn Safe payload racks and other vehicle systems Certain payload and vehicle systems are sensitive to thruster firings (for example, the treadmill needs to be fixed to the ISS structure and not in use during reboost burns). TIG – 2:40 ISS power down Power down the ISS systems to support the feathering of solar arrays. TIG – 2:40 Generate burn options If not already done, TOPO generates burn options and supply to JSpOC for debris screening and provides options to MCC-M. TIG – 2:25 Power up redundant navigation equipment MCC-H powers up redundant rate gyro assemblies and enables US Segment accelerometers to support burn. Redundant gyros are brought up to protect against failures during the burn (in some cases, the burn may stop without the redundant equipment). The accelerometers in the US Segment are used by the Russian Segment to calculate the end of the burn. TIG – 2:25 Park and lock solar array joints Position solar arrays for propulsive support and reboost. Solar arrays are usually positioned in a way that minimizes the structural effects caused by thruster firings. TIG – 1:30 Select final burn option If not already done, TOPO selects and approves the final burn time based on JSpOC screening and provides the final burn option to MCC-M. TIG -1:30 Final go/no go for PDAM The MCC-H and MCC-M flight directors authorize execution of the debris avoidance burn. TIG – 1:20 MCC-H commanded handover of attitude control to the Russian Segment MCC-H commands attitude control to handover from United States On- orbit Segment (USOS) non-propulsive CMG attitude control to Russian Segment thrusters attitude control. TIG – 1:00 MCC-M commands burn execution sequence initialization MCC-M will issue a command to begin the automatic burn sequence. This sequence will configure systems on the Russian Segment (including the propulsion and attitude control systems), maneuver the stack to burn attitude, and then execute the burn exactly 1 hour after the command is received. TIG Burn execution Under Russian Segment Service Module automatic control, several minutes in duration depending on configuration of propulsion system and engines used. The ISS is maneuvered back to attitude post burn. TIG + 00:40 MCC-H commanded handover back to the USOS MCC-H commands attitude control back to USOS non-propulsive CMG attitude control. Post-burn Clean up MCC-H places solar arrays back in solar auto track to maximize power generation and repowers systems to restore normal operations.