ON-ORBIT COLLISION AVOIDANCE

¶The Space Control Center, operated by the 21st Space Wingʼs

¶1st Space Control Squadron (a unit of Air Force Space Command), maintains an orbital data catalog on some 9,000

¶Earth-orbiting objects, from active satellites to space debris, some of which may be as small as four inches. The Space

¶Control Center ensures that no known orbiting objects will transit an Orbiter "safety zone" measuring 6 miles deep by

¶25 miles wide and long (Figure A) during a Shuttle mission by projecting the Orbiterʼs flight path for the next 72 hours

¶(Figure B) and comparing it to the flight paths of all known orbiting or re-entering objects, which generally travel at

¶17,500 miles per hour. Whenever possible, the Orbiter moves tail-first while on orbit to minimize the chances of orbital debris or micrometeoroids impacting the cabin windscreen or the Orbiterʼs wing leading edge.

If an object is determined to be — 25

¶within 36-72 hours of collid- mi s ing with the Orbiter, the Space le

¶Control Center notifies NASA, and the agency then determines a maneuver to avoid a collision.

¶There were no close approaches to Columbia detected during

¶STS-107.

¶Figure A. Orbiter Safety Zone

¶Figure B. Protecting the Orbiterʼs flight path

¶a maneuver back to the bay-to-Earth, tail-forward attitude from 10:17 to 10:21 a.m. It is possible that this maneuver imparted the initial departure velocity to the object.

¶Although various Space Surveillance Network radars tracked the object, the only reliable physical information includes the objectʼs ballistic coefficient in kilograms per square meter and its radar cross-section in decibels per square meter. An objectʼs radar cross-section relates how much radar energy the object scatters. Since radar cross-section depends on the objectʼs material properties, shape, and orientation relative to the radar, the Space Surveillance Network could not independently estimate the objectʼs size or shape. By radar observation, the objectʼs Ultra-High Frequency (UHF) radar cross-section varied between 0.0 and minus 18.0 decibels per square meter (plus or minus 1.3 decibels), and its ballistic coefficient was known to be 0.1 kilogram per meter squared (plus or minus 15 percent). These two quantities were used to test and ultimately eliminate various objects.

¶~12" ~ 9.0"

¶~ 8"

~10.5" ~4"

¶RCC Panel Fragment 37736

¶RCC Panel Fragment 2018

¶(From STS-107 Right Wing (From STS-107 Right Wing panel #10) panel #10)

¶Figure 3.5-1. These representative RCC acreage pieces matched the radar cross-section of the Flight Day 2 object.

¶In the Advanced Compact Range at the Air Force Research Laboratory in Dayton, Ohio, analysts tested 31 materials from the Orbiterʼs exterior and payload bay. Additional supercomputer radar cross-section predictions were made for Reinforced Carbon-Carbon T-seals. After exhaustive radar cross-section analysis and testing, coupled with ballistic analysis of the objectʼs orbital decay, only a fragment of RCC panel would match the UHF radar cross-section and ballistic coefficients observed by the Space Surveillance network. Such an RCC panel fragment must be approximately 140 square inches or greater in area to meet the observed radar cross-section characteristics. Figure 3.5-1 shows RCC panel fragments from Columbiaʼs right wing that represent those meeting the observed characteristics of object 2003-003B.10

¶Note that the Southwest Research Institute foam impact test on panel 8 (see Section 3.8) created RCC fragments that fell into the wing cavity. These pieces are consistent in size with the RCC panel fragments that exhibited the required physical characteristics consistent with the Flight Day 2 object.

¶Findings: CAIB/NAIT Working Scenario (Appendix D.7) and the Re-

¶F3.5-1 The object seen on orbit with Columbia on Flight

Day 2 through 4 matches the radar cross-section and area-to-mass measurements of an RCC panel fragment.

¶F3.5-2 Though the Board could not positively identify

the Flight Day 2 object, the U.S. Air Force ex- clusionary test and analysis processes reduced the potential Flight Day 2 candidates to an RCC panel fragment.

¶Recommendations:

None