147 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT CHAPTER 8 OCM # TCA (hours) U (Radial) (km/miles) V (Downtrack) (km/miles) W (Cross track) (km/miles) R (Spacing) (km/miles) Pc 1 68.9 1.06/0.66 45.90/24.46 -11.41/-7.07 47.31/29.33 2 62.6 1.02/0.63 43.70/27.09 -10.86/-6.73 45.04/27.92 3 58.3 -0.57/-0.35 -12.60/-7.81 3.10/1.92 12.99/8.05 4 54.8 -0.22/-0.14 -1.05/-0.65 0.24/0.15 1.10/0.68 5 49.8 0.93/0.58 44.48/27.58 -11.07/-6.86 45.85/28.43 2.1x10-5 6 46.4 1.47/0.91 70.57/43.75 -17.55/-10.88 72.74/45.10 1.6x10-5 7 42.0 2.73/1.70 203.32/126.06 -50.56/-31.34 209.53/129.91 0.0x100 8 38.6 2.73/1.70 210.20/130.32 -52.25/-32.40 216.52/134.24 0.0x100 9 34.6 2.25/1.40 272.34/168.85 -67.78/-42.02 280.66/174.01 0.0x100 Figure 7. Tracking of a sample conjunction. The information is arranged as follows: Column 1: Orbital Conjunction Message (OCM) number. The OCM is the official message from JSpOC to Mission Control Center-Houston (MCC-H) on an impending potential collision. Multiple messages are generally received on a particular object over time, and are represented by individual rows. In this case, nine messages were received before the object was cleared. Column 2: Time remaining until TCA: The number of hours until the TCA between a tracked object and the ISS. Columns 3-5: Predicted miss distance between the object and the ISS, broken out by axes described in Figure 6. Column 6: Predicted overall miss distance between the object and the ISS. Column 7: Official Pc at the time the message was generated (color coded based on thresholds set in flight rules), as computed by the TOPO in MCC-H. During this time period, the TOPO and JSpOC often work together to devote more observation time and assets of the network to help refine the understanding of the location and velocity of the object as it orbits the Earth, which is reflected in the later OCMs. When an official Pc was determined, it was for a yellow conjunction at 49.5 hours before TCA on OCM 5. Yellow conjunctions can eventually result in a DAM if an object stays yellow closer to TCA however, in general, additional tracking and reduction in uncertainty often clears an object. In this case, the Pc was calculated as zero and the object went green 42 hours prior to TCA. Unfortunately, some conjunctions stay yellow or become red as more tracking becomes available. Debris that has high atmospheric drag or is in an unusual orbit (such as a highly elliptical orbit) is less predictable, and some collision threats are not identified until late (less than a day before TCA). Figure 8 shows a conjunction that began as green and eventually developed into enough of a threat to require a DAM. OCM # TCA (hours) U (Radial) (km/miles) V (Downtrack) (km/miles) W (Cross track) (km/miles) R (Spacing) (km/miles) Pc 1 69.3 -0.19/-0.12 -7.26/-4.51 -9.93/-6.17 12.30/7.64 2.610-9 2 60.9 -0.18/-0.11 -11.28/-7.10 -15.41/-9.58 19.09/11.86 7.510-7 3 53.1 -0.18/-0.11 -2.55/-1.58 -3.48/-2.16 4.32/2.68 1.410-6 4 45.3 -0.22/-0.14 -1.33/-0.83 -1.82/-1.13 2.26/1.40 5.510-13 5 37.5 -0.32/-0.20 6.68/4.15 9.13/5.78 11.32/7.03 7.0x10-17 6 29.0 -0.14/-0.09 2.95/1.83 4.04/2.51 5.00/3.11 6.3x10-5 7 21.4 -0.07/-0.04 -0.72/-0.45 -0.98/-0.61 1.22/0.76 1.1x10-3 8 12.8 -0.16/-0.10 2.30/1.43 3.15/1.96 3.90/2.42 7.6x10-5 9 11.4 -0.09/-0.06 -0.88/-0.55 -1.19/-0.74 1.48/0.92 2.5x10-3 10 9.8 -0.04/-0.03 -0.04/-0.03 -0.05/-0.03 0.08/0.05 1.310-2 11 5.1 -0.01/-0.01 -0.66/-0.41 -0.91/-0.66 1.12/0.70 1.710-2 Figure 8. Development of a conjunction, which requires a maneuver. See Figure 7 for data definitions. In these cases, a DAM is planned and, if necessary, executed.