CHAPTER 8 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT 148 Debris Avoidance Maneuvers DAMs are planned as a small orbital- raising (or reboost) maneuver. A reboost maneuver uses the small rocket thrusters on the aft of the ISS to push it slightly higher in orbit. The ISS is designed such that all propulsion is done by the Russian Segment. The core of the propulsion system is the Service Module, which is operationally controlled by Mission Control Center-Moscow (MCC-M). Small thrusters on the Progress resupply vehicles can be used for both reboost and attitude control, depending on the specific docking ports, whereas the propulsion system is under the control of MCC-M, the Service Module, and the Service Module computers. Although MCC-M actually controls and fires the rockets, MCC-H provides threat assessments based on JSpOC data. The control centers work together to keep the ISS in the correct orbit. Reboost maneuvers are already periodically scheduled to raise the orbit of the ISS, which decays naturally due to atmospheric drag. A DAM is the same thing, but is planned with flexibility in mind so the maneuvers can be executed in only a few hours, if necessary. DAM reboosts are relatively small. A DAM performed in 2012 fired the Service Module engines for 54 seconds, which raised the altitude of the ISS by 1.5 km (0.9 miles). This “nudge” in the orbit is all that is required to clear a conjunction successfully and carry the ISS away from the future potential impact. These nudges are usually targeted to occur 2 hours and 20 minutes prior to a potential impact, which provides sufficient time in the new orbit to travel away from the point of collision before the close approach occurs. When the need to at least plan for a debris avoidance burn becomes apparent (typically ~30 hours before TCA for a conjunction that is not improving), the TOPO along with his or her counterparts at MCC-M will plan a DAM burn. The burn will be optimized to minimize the impact on the ISS operations (for example, if possible, to avoid periods where feathering the solar arrays and a power down would present a power challenge), to minimize impact to downstream planning of vehicles that will be docking to the space station or returning to Earth from the ISS in the coming months, and finally to place the ISS on a new trajectory that has been preemptively evaluated by JSpOC as clear and free of debris (at least for the next several days). Detailed execution planning is handed to MCC-M as soon as the TOPO has designed a DAM consisting of a specific burn magnitude, duration, and time. Engineers at MCC-M will build a software script that allows the computers in the Service Module to physically execute the burn. This script is referred to by its Russian moniker: cyclogram. The Russian software executes a series of cyclogram-defined commands on the Russian Segment automatically. Russian flight controllers at MCC-M will build the cyclogram on the ground, verify by running it on a software test bed with the same computers used on the space station, and uploading it for use on the actual reboost. It takes a day to complete this process if the cyclogram is being built from scratch. The cyclogram contains detailed commands that begin execution approximately 90 minutes prior to a debris avoidance burn, immediately after attitude control is handed over from the US Segment to the Russian Segment. The cyclogram will then configure the propulsion system to fire thrusters, maneuver the ISS to the reboost attitude (i.e., aim the main engines so that the thrust will increase the orbital altitude), and fire the engines at the appropriate time and for the appropriate duration. After the burn ends, flight controllers at MCC-M and MCC-H will work together to hand attitude control back over to its normal long-term configuration using the US Segment Control Moment Gyroscopes (CMGs). There are two kinds of DAMs— a standard DAM and a predetermined DAM (PDAM)—each with its own cyclogram. A standard DAM is essentially identical to a planned reboost except that MCC-H builds the software load on an expedited schedule. Normal reboosts to counter atmospheric drag or to set up phasing for a visiting vehicle are calculated months in advance. A standard DAM requires approximately 24 hours to build and verify, but it has greater flexibility for selecting burn duration and choosing the vehicle that will conduct the reboost. For example, the Service Module itself can perform a reboost if a Progress is not docked to the aft of the Service Module or Docking