145 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT CHAPTER 8 Debris Screening The DoD Joint Space Operations Center (JSpOC) at Vandenberg Air Force Base, California, performs an assessment of the orbit of the ISS against this catalog of debris every 8 hours. In this assessment, the orbit of the ISS is projected out several days, along with tracked space debris that is orbiting such that it may come close to the ISS. When this assessment predicts a potential close approach between the ISS and a piece of space debris (usually within the following 72 hours), JSpOC will provide data on the close approach, including predicted miss distance and time of closest approach (TCA) to the Trajectory Operations Officer (TOPO) flight controller in Houston. The TOPO screens an imaginary box of space around the ISS (sometimes referred to as the “pizza box”) that is ± 25 km (±15.5 miles) in the direction of motion, ± 25 km (±15.5 miles) perpendicular to the direction of motion, and ± 0.5 km (±0.3 miles) radially from the ISS as it flies in orbit. This is shown in Figure 6. U (Radial) V (Velocity Vector) W (Out-of-Plane) +25 km (+16 miles) +25 km (+16 miles) +0.5 km (+0.3 miles) -.5 km (-0.3 miles) -25 km (-16 miles) -25 km (-16 miles) U V W Figure 6. The imaginary box (aka the pizza box) around the ISS. If the predicted miss distance is within this box, the TOPO will notify the flight control teams in Houston and Moscow of a potential collision hazard. TOPO will use tracking data on the object and the position of the ISS (see Chapter 7) to calculate a probability of collision (Pc) that is a mathematical representation of the likelihood of a collision between the ISS and an object during the close approach. The computation takes into account variables that impact the known orbits of the ISS and the target object, such as uncertainties in atmospheric drag and quality of radar tracks on the target. Just because an object can be detected by radar does not always mean the precise location can be pinpointed. For example, depending on the size or composition of an object, it might be barely detectable, meaning the radar may get glimpses of an object but not a clear view. The TOPO continuously refines the Pc of the object as the TCA approaches, iterating up to several times per shift, depending on the level of concern regarding the potential collision. JSpOC will also work with TOPO and the SSN on increasing coverage on a problem space object to better understand its orbit as required, especially if it becomes a threat to the ISS. Coverage can be increased by using more radars and/or telescopes to gather more data on the object. Coverage can be increased by tracking an object multiple times per day as it passes over various tracking sites when ordinarily it may only be tracked once every few days. Evaluating the Risk of a Potential Collision Flight rules define four levels of concern and actions for orbital debris that will have a close approach and has a calculated Pc: n Green: Pc less than 10-5 (less than 1 in 100,000 chance of collision)— no action required. n Yellow: Pc between 10-5 and 10-4 (Pc greater than 1 in 100,000 but less than 1 in 10,000)—a debris avoidance maneuver (DAM) should be attempted prior to TCA unless there is a major impact to the ISS operations (such as loss of a rendezvous opportunity with a cargo vehicle). n Red: Pc between than 10-4 (Pc greater than 1 in 10,000) and 10-2 (1 in 100)—a DAM should be performed prior to TCA unless the burn itself will place the crew at greater risk. DAMs may also not be