149 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT CHAPTER 8 Compartment-1 on the bottom of the ISS (which requires an attitude maneuver to point the Progress engines in the correct direction). A standard DAM also allows MCC-M and MCC-H to custom pick the burn duration to change the orbit velocity (also referred to as the delta-V). This can be helpful when shaping the orbit to account for where the ISS needs to rendezvous, and to undock cargo and crew delivery vehicles that are planned over the next few months. A PDAM is a “canned” burn plan with limited delta V options that is always ready and loaded in the Russian Segment. The PDAM is a capability that was first made available in 2012. Prior to that time, all DAMs were standard DAMs. The PDAM was developed in response to the increasing number of conjunctions that were occurring (due to the increase of orbital debris over the past decade) as well as the development time required for the standard DAM. Prior to the development of the PDAM capability, if a conjunction was detected between the ISS and a piece of debris within approximately 24 hours, there was insufficient time to develop a cyclogram and maneuver the ISS out of the way. The crew and flight control team were left with simply isolating the crew members in their Soyuz vehicles as protection from the effects of an impact. PDAMs are pre-developed plans that permanently reside in the Service Module software and have burns available of 0.3, 0.5, 0.7, or 1.0 m/s (1.0, 1.6, 2.3 or 3.3 ft/s). MCC-H and MCC-M can select a burn magnitude that will best change the orbit to avoid a conjunction as well as keep the ISS in the proper position for later operations, such as rendezvous of crew and cargo vehicles that are weeks or months away. It is also possible that one burn magnitude may clear the ISS out of the path of the original debris, only to find it in the path of some other object. Having four options ensures that one solution can be found that will clear the path of all debris. Because a PDAM is already built and on board the space station computers, a PDAM can be planned and executed with as few as 5.5 hours remaining until a conjunction will occur. The 5.5-hour minimum is driven by the time required to configure the solar arrays (3 hours) prior to the avoidance burn, which is typically done 2 hours and 20 minutes prior to the closest approach. The solar arrays need to be positioned in specific orientations to avoid being struck by the exhaust of the thrusters that are used for both the reboost and the attitude control. The 2-hour-and-20-minute burn point is driven by the need to travel for a short period of time in the new, slightly adjusted orbit to move away from the conjunction. Because the PDAM can be executed rapidly and cancelled late, and because most conjunctions become green as the time before closest approach decreases, the preferred strategy for debris avoidance is usually to wait out the conjunction and plan to do a PDAM at the latest possible time, if it is still required. The only downside to this strategy is that the burn magnitudes are limited to those in the canned burns. In some cases, MCC-H and MCC-M may elect to plan for a standard burn if a more tailored reboost would be beneficial from a trajectory point of view to preserve, for example, a particular rendezvous opportunity several weeks away. Predetermined Debris Avoidance Maneuver Execution When a space debris threat continues to be yellow or red with less than 24 hours remaining to TCA, or when JSpOC notifies MCC-H of a late- notice conjunction, PDAM planning goes into high gear to prepare the ISS for an escape maneuver. As previously discussed, the TOPO will select a candidate burn time (usually 2 hours and 20 minutes prior to TCA), and the MCC-H flight director will approve the time and direct teams to plan for a burn. This planning will attempt to configure the space station systems (especially power and payload) as gracefully as possible based on how much time is available before the burn executes. Most of this planning centers around power availability, since (as in many thruster firings) the solar arrays are repositioned to best protect them from thruster firings rather than optimized for power generation. Generally, this means less power is available than had originally been planned and certain systems need to be turned off. With careful planning, certain allowances can be made—for example, an experiment that requires a few more hours to finish will be allowed to complete before being turned off. The more