The shuttle orbiter has been struck many times by small meteoroids and orbital debris, but it has not yet been damaged severely. Because it was not designed with the meteoroid and orbital debris hazard in mind, however, some orbiter components are at risk of being damaged by meteoroids or debris. This damage can range from damage that does not affect a mission but increases refurbishment costs (such as pitting of window surfaces) to damage that could force the crew to abort a mission (such as penetration of a radiator pipe) to damage that would prevent the orbiter from successfully returning to Earth (such as a large hole in the leading edge of a wing) to damage that would rapidly result in the loss of life or the vehicle (such as a collision with a large fragment from the breakup of a spacecraft). Astronauts conducting extravehicular activities are also at risk from meteoroids and orbital debris.
RISK MANAGEMENT STRATEGY
A National Aeronautics and Space Administration (NASA) guideline states that the risk of critical failure (i.e., penetration of the orbiter that results in the loss of vehicle or loss of life) from the impact of a meteoroid or orbital debris should not exceed 1/200 for a particular mission. This compares to the median calculated risk of critical failure of 1/248 for the shuttle’s launch and ascent to orbit. Compared to the efforts NASA has made to reduce other risks to the shuttle, the efforts made to understand and reduce the risk from meteoroids and debris has been small. NASA should consider changing the 1/200 guideline to reduce the maximum allowable risk from meteoroids and orbital debris.
NASA has not conducted a systematic assessment of the survivability of the shuttle with respect to the meteoroid and orbital debris hazard. Similar analyses, however, have been conducted by the U.S. Department of Defense (DOD) to assess aircraft survivability. NASA should improve its approach to calculating the risk to the shuttle from meteoroids and orbital debris by establishing a survivability assessment process and conducting an end-to-end survivability assessment of the entire shuttle orbiter—including all subsystems and components—against the hazard. The assessment should be integrated with assessments of other hazards, such as the risk during ascent and reentry, to create a complete, integrated, peer-reviewed probabilistic risk assessment for the shuttle.
NASA should also continue its efforts to assess in detail the vulnerability of areas of the shuttle orbiter that they predict to be most likely to experience critical damage, mission-limiting damage, or damage requiring costly repairs. This information should be used to refine assessments of the overall risk to the shuttle, to determine which areas require more protection, and to determine whether operational and procedural modifications can decrease the risk.
TOOLS FOR RISK ASSESSMENT
NASA uses computer models to assess risks and guide its efforts to protect the shuttle from meteoroids and orbital debris. A model of the meteoroid and orbital debris environment (ORDEM96) is used as input for a threat assessment model (BUMPER) to predict the magnitude of the risk to the shuttle from meteoroids and orbital debris.
ORDEM96 is arguably the best available model of the debris environment, and BUMPER is probably the best available model for orbital debris risk assessment. However, both models incorporate a number of simplifying assumptions, and the magnitude of uncertainty in their predictions has not been well characterized. NASA should strive to refine BUMPER and ORDEM96 so that their results include appropriate error bars and associated confidence levels. To begin this process, NASA should analyze the sensitivity of the output of both models to changes in the various input parameters.
Because the data are limited and the population of debris smaller than about 5 mm in diameter varies widely, ORDEM96’s predictions of debris fluxes for individual shuttle missions may be highly inaccurate. To predict the short-term hazard to the orbiter from orbital debris more accurately, NASA should expand its data gathering and modeling efforts to better understand the sources (e.g., solid rocket motors and debris wakes) of the sub-5 mm debris population in the shuttle’s orbital regime.
The DOD Space Surveillance Network (SSN) warns the space shuttle program of possible close conjunctions with cataloged orbiting objects. But probably
more than 95 percent of the objects that could cause critical damage to the orbiter are not cataloged because they are too small to be reliably detected by SSN sensors.
The capabilities of the SSN to support NASA’s efforts for collision avoidance are eroding, and until recently, NASA had issued no requirements that might have helped to halt this erosion. NASA and the DOD should work together to satisfy these requirements, to identify impending changes to the SSN that will affect debris tracking, and to identify changes that would improve the SSN’s ability to track smaller objects that pose a hazard to crewed spacecraft.
Once NASA has received a warning of an upcoming close conjunction, it must decide whether to maneuver the shuttle to avoid a collision. Two flight rules (A4.1.3–6 and C4.3.2–1) that are relevant to this decision appear to place mission success ahead of flight safety. NASA should re-examine these rules and consider restating them to establish when a maneuver is mandatory for safety reasons. NASA plans to use a new probability-based approach to determine when a collision avoidance maneuver is necessary, but the collision avoidance data currently provided by the SSN is not accurate enough for this new approach to be effective.
The space shuttle program has developed operational procedures, and is about to implement hardware modifications, to improve the survivability of the shuttle orbiter and crew in the face of the meteoroid and orbital debris hazard. In the future, however, when the orbiter is supporting the International Space Station, many of the operational techniques developed to improve the orbiter’s survivability will not be employed because the shuttle’s freedom to maneuver and control its attitude will be constrained to satisfy requirements for space station power, thermal conditions, and attitude control. The effect of these restrictions on the shuttle’s survivability should be reassessed.
NASA plans to modify the orbiter’s radiators and wing insulation to reduce the risk of early mission termination and critical failure. These modifications appear to be positive steps that will have a minimal negative impact on the program. NASA should continue to investigate potential modifications to the orbiter to improve its survivability against meteoroids and orbital debris. NASA should also reconsider conducting on-orbit surveys to detect exterior impact damage and repair it as necessary.