determines whether any cataloged objects are predicted to enter an area around the orbiter (called an alert box) with dimensions approximately 5 km radially, 25 km along the track of the orbiter (either leading or trailing), and 5 km out of the orbital plane during the first two hours of the mission (Flight Rule A4.1.1–3). The Eastern Range is responsible for notifying NASA if the orbiter will enter a 50 km×200 km×50 km region around another crewed vehicle during the first orbit after a launch (Flight Rule A2.1.1–1.).

If either of these warnings indicates a possible collision, the launch is usually delayed until the next even minute. Additional analyses are requested using the new launch time, and further holds may be ordered. To date, two shuttle launches have been delayed to avoid potential collisions with orbiting objects (Reeves, 1997).

On-Orbit Operations

When the shuttle orbiter is in orbit, the SCC screens the entire satellite catalog for objects that could approach the orbiter within a 5 km×25 km×5 km alert box at any time during the mission. The SSN is tasked with providing more intensive tracking of the approximately one to two objects per day that penetrate this box. The objects are then reassessed using a more accurate and computationally intense “special perturbations” algorithm to determine if any will come within a “maneuver box” of 2 km radially, 5 km along the orbiter’s track, and 2 km out of plane. (The alert and maneuver boxes are shown in Figure 5–1.) The large box size relative to the size of the orbiter is necessary because the current and future positions of tracked objects are not known precisely. The accuracy of the special perturbations algorithm is obviously dependent on the availability of accurate sensor data.

Information about potential close conjunctions is passed to NASA flight controllers who apply Flight Rule A4.1.3–6 (see Box 5-1), which stipulates that a maneuver be performed “if the maneuver does not compromise either primary payload or mission objectives.” Like all flight rules, this one can be superseded by real-time decisions.

FIGURE 5–1 The space shuttle alert and maneuver boxes.



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