National Academy of Sciences

December 16, 1997

Orbital Debris May Pose Significant Risk to the Space Shuttle

WASHINGTON -- NASA should perform a thorough risk assessment to determine the likelihood that the space shuttle could be severely damaged by meteoroids and orbital debris, says a new report* from a committee of the National Research Council. In addition, the spacecraft should be re-examined to identify components that may require additional protection.

"Although NASA is taking steps to protect the shuttle from orbital debris such as spent rocket bodies, satellite fragments, and paint chips, there still is a real risk that a collision could cripple the shuttle or threaten the safety of the crew," said committee chair Frederick Hauck, president and chief executive officer, AXA Space, Bethesda, Md. "NASA needs to obtain a more precise picture of the potential dangers and assess additional methods for reducing these threats."

The most recent examination of overall risks posed to the shuttle, conducted in 1995, did not assess the hazards from orbital debris. For some missions, the committee said, adding the debris threat to estimates of potential risks -- such as the risks during launch or re-entry to the Earth's atmosphere -- almost doubles the likelihood that the crew will be harmed or that the spacecraft will incur major damage. NASA should consider reducing its allowable level of risk from orbital debris.

Moreover, the space agency should expand efforts to model the sources of debris that could be in the shuttle's orbit. Predictions of risk for individual missions may be highly inaccurate -- especially for estimates of the number of very small objects, the committee said. Because debris can collide with the shuttle at speeds of six miles per second, even tiny debris like paint chips can cause considerable damage.

Providing Protection

A complete evaluation of the shuttle's components and subsystems and their vulnerability to damage should be performed, the committee said. NASA should explore further modifications to hardware that would protect critical systems. For example, the agency plans to add shielding to the shuttle's radiator system. In addition, NASA should examine whether new operational procedures could minimize damage caused by orbital debris. In-flight inspections and repairs of the shuttle's exterior should be considered.

NASA's four operating space shuttles, designed in the 1970s, were not built to repel bombardment by orbital debris because such objects were not recognized as a substantial threat. New data indicates that the craft often is exposed to debris that could cause major damage. For example, debris as small as a quarter-inch in diameter could punch a hole through the wall of the crew's cabin and cause a loss of air pressure. An object that punctures the shuttle's wing could cause structural failure when the shuttle re-enters the atmosphere. Even relatively minor damage can require repairs that add significant expense and delays.

Avoiding Collisions

NASA routinely moves the shuttle out of the path of debris large enough to be tracked by ground-based sensors operated by the Department of Defense (DOD). However, estimates indicate that more than 95 percent of debris that could critically damage the shuttle is too small to be picked up by current sensors. NASA and DOD should work together to improve the collision warning system and identify ways to track smaller debris, the committee said. It also urged NASA to examine how the shuttle can be protected when it is used to support the international space station, scheduled for launch in 1998.

The study was funded by NASA. The National Research Council is the principal operating arm of the National Academy of Sciences and the National Academy of Engineering. It is a private, non-profit institution that provides independent advice on science and technology issues under a congressional charter.

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