CHAPTER 1 SYSTEMS: INTERNATIONAL SPACE STATION PLANNING—A ROADMAP TO GETTING IT ALL DONE 14 Planners also contend with sequences of activities where each activity requires the preceding one to be completed, sometimes with additional time between end and start. For example, the repair of a piece of equipment might require the ground control team to power the device off for a few hours before the astronaut performs the repair so that the device will cool down enough to be handled. Even gathering the tools for the repair has to be taken into account since, as with everything in microgravity, activities take longer than they do on Earth. The astronaut also must ensure that another crew member is not using the one item needed for the repair. Also, since every bit of available space is used to stow equipment or supplies, the equipment that needs to be repaired could be situated behind another object, which would need to be temporarily moved to another area. A simple repair can be tough to schedule with the addition of more temporal constraints such as the crew members’ desire to eat their meals together, which can be critical for psychological support when away from home and their usual routine for so long. Crew time is another limited resource that his highly constrained. Ground rules and constraints limit overall scheduled crew time per day to 6.5 hours, with the remaining days’ time comprised of exercise (see above), sleep, morning and evening preparation (i.e., time to review the current or next day’s plan, review procedures to be used, etc.), and midday meal time. Further, the 6.5-hour scheduled time is bounded by being allowed only after the morning crew/ground planning conference and needing to conclude by the start of the evening crew/ ground planning conference. The goal of managing crew time is to provide as much time as is practical for science scheduling. Science planners also contend with other limited resources such as batteries for small handheld devices, consumable gases (e.g., argon, nitrogen), water, test tubes, sample bags, test strips, etc., thereby further complicating the scheduling problem. Finally, planners must manage constraints imposed or required by the external environment such as day/night cycle requirements, attitude constraints, microgravity requirements, and satellite communications availability. Planners at the various ISS operations control centers manage many of their own resources as well as the use of common resources such as crew time, power, air, other gases (previously mentioned), tools, etc. To accomplish this, the planning teams use complex scheduling software to define and manage all these constraints and resources, and to generate valid effective timelines of crew and ground operations to support each day of ISS operations. Primary to its other tasks, the Houston flight control team, led by the ISS flight director, is responsible for integrating all these plans into a single, integrated plan that is presented in the OPTIMIS. Lessons Learned As previously mentioned, on-orbit crew time is at a premium. All efforts are made to minimize unnecessary use of this limited resource. One problem that all planners face is accurate prediction of task durations. Underestimating task duration leads to the replanning of uncompleted activities and, in many cases, requires crews to work longer hours to avoid getting too far behind the general plan. Planners usually arrive at task duration estimates through ground procedure verification and simulation, as well as through previous related experience, as described above. The time is usually increased to the predicted duration for new tasks or for astronauts executing a task for the first time on-orbit. In many cases, additional time is scheduled for crews to review procedures. A good example of this occurred during Expedition 1. The crew was asked to connect a newly flown control box to an on-board laptop to allow manual control of the Control Moment Gyros heaters (see Chapter 7) that had been experiencing extreme thermal fluctuations. Ground controllers, at the time, had no means to control the heaters. Upon reviewing the procedures on the ground, planners initially determined that 2.5 hours would be required for the astronauts to review the procedure and execute the task. The initial performance by the crew took 2.5 hours. However, subsequent performance only required 1.5 hours because of the familiarity gained with the apparatus and procedures. Early planners quickly learned the importance of accounting for the overhead involved in worksite preparation, equipment gathering, worksite cleanup, and equipment stow. A good example of this occurred during Expedition 4. The crew was asked to take samples of the US Laboratory Low and Moderate Temperature Loops (see Chapter 11) to check for microbial growth or particulate contamination. Ground task duration estimates predicted about 1 hour of crew time for the activity, including gathering the equipment. However, upon review of the procedure, the crew members pointed out that to access the loops, they would need to remove two