CHAPTER 8 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT 146 performed during the short 4-orbit rendezvous of Soyuz and Progress, the last 30 hours of the longer Soyuz 34-orbit rendezvous, or the final proximity operations of the Progress 34-orbit rendezvous. This is to protect for cases where the DAM would potentially prevent the rendezvous of the visiting Soyuz/ Progress to the ISS. n Black: Pc greater than 10-2 (Pc greater than 1 in 100). A DAM must be performed prior to TCA unless the burn is in the final minutes of a Soyuz or Progress docking operation. This is a brief window of exposure. The action levels provide a good example of risk management as it appears in flight rules. On a green conjunction, the risk is not zero. Rather, it is reasonably small— 1 in 100,000 chance of impact. Although the risk of collision could be driven closer to zero by moving the green line to, for example, a 1 in 1,000,000 chance, it would also mean the ISS would need to perform far more debris avoidance burns, perhaps even weekly. Burns of this frequency would be unsupportable. Research on the ISS would be impacted, propellant would be depleted, and the orbit would change so often that some cargo missions would need to be delayed. The threshold chosen represents a balanced and accepted risk where it is as low as possible while still allowing the ISS to be useful as an orbiting laboratory. The yellow—and especially the red and black—thresholds, however, represent unacceptable risk to the crew and ISS, and are where the space station will actively move out of the way of debris. In the best circumstances, the conjunction is with a well-tracked object and the TOPO will have several days prior to the TCA to calculate multiple updates to the object, refine the Pc, and trend the conjunction. Sometimes, however, an object may be more difficult to predict. For example, a large flat piece of metal from an expended rocket stage in a low orbit has high atmospheric drag in one orientation, and low drag in another. Since it may be tumbling, the drag slowing it down and changing its orbit could vary and makes the TOPO’s job more difficult. This is the sort of situation in which additional tracking can help. As the conjunction nears, the Pc typically gets greener (as described in the next section) with additional tracking and less uncertainty because the orbit of the object becomes better understood and less time will elapse (and introduce prediction error) before the conjunction. For this reason, and because the maneuver can be disruptive to ongoing ISS operations, the general philosophy is to delay a DAM as long as possible to provide the TOPO the opportunity and data to ensure the necessity of that maneuver. In the end, it often comes down to the flight director making the best decision possible by balancing the impacts and risks per the flight rule, based on the available data and expertise of the TOPO. Tracking and Evaluating Figure 7 shows the trend of a piece of debris (in this case, part of an exploded rocket upper stage) in 2013. The format of the table is exactly the same as that used by the TOPO as he or she works with the SSN to refine the Pc over time. In the Figure 7 example, data were first provided to MCC-H as an Orbital Conjunction Message (OCM) 68 hours prior to the TCA. Additional OCMs were provided every 4 to 6 hours as JSpOC, the TOPO, and the flight director continued to evaluate the risk posed by the object to the ISS. The first four OCMs do not include a Pc since several computations over time are required to generate an official Pc to collect data and provide an accurate assessment of the collision risk. As can be seen by examining the miss distances in early OCMs (1-4), the miss distances are changing relatively significantly due to uncertainty in the position of the object. Collision Avoidance History NASA first implemented conjunction assessment on STS-26 in 1988 by using a simple 4 x 10 x 4 km (2.5 x 6.2 x 2.5 mile) football-shaped volume as a “keep out” zone around the Space Shuttle based on simple miss distance, which worked well for the relatively maneuverable shuttle. Prior to the ISS first element launch in 1998, NASA and the DoD implemented the higher-fidelity risk-based assessment to better understand when a burn was required and to avoid the operational and science impact of performing an unnecessary DAM.