Protecting the Space
Shuttle from Meteoroids
and Orbital Debris

Committee on Space Shuttle Meteoroid/Debris
Risk Management

National Research Council

• Notice
• Committee
• Preface
• Contents
• Executive Summary

NATIONAL ACADEMY PRESS
Washington, D.C. 1997

NATIONAL ACADEMY PRESS • 2101 Constitution Avenue, N.W. • Washington, DC 20418



NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competencies and with regard for appropriate balance.

This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

This study was supported by Contract No. NASW-4938 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, and recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the organizations or agencies that provided support for this project.

International Standard Book Number: 0-309-05829-5
Library of Congress Catalog Card Number: 97-80863




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Copyright 1997 by the National Academy of Sciences. All rights reserved.




Cover Illustration: The large picture of the shuttle orbiter was taken from the Mir space station during shuttle mission STS-71 in July 1995. The inset is a scanning electron micrograph of a 0.6 mm diameter crater found on the Solar Maximum Mission Satellite, which was recovered from space in April 1994 by the crew of shuttle mission STS-41C. Source: NASA.




Printed in the United States of America.

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COMMITTEE ON SPACE SHUTTLE
METEOROID/DEBRIS RISK MANAGEMENT


FREDERICK H. HAUCK (chair), AXA Space, Bethesda, Maryland
KYLE T. ALFRIEND, Texas A&M University, College Station
DALE B. ATKINSON, consultant, Springfield, Virginia
DALE R. ATKINSON, POD Associates, Inc., Albuquerque, New Mexico
G. TAFT DEVERE, Space Warfare Center, Falcon Air Force Base, Colorado
DONALD H. EMERO, Rockwell International (retired), Fountain Valley, California
GEORGE J. GLEGHORN, TRW Space and Technology Group (retired), Rancho Palos Verdes, California
DARREN S. MCKNIGHT, Titan Research and Technology, Reston, Virginia
WILLIAM P. SCHONBERG, University of Alabama in Huntsville, Huntsville


Aeronautics and Space Engineering Board Liaison

WILLIAM H. HEISER, U.S. Air Force Academy, Colorado Springs, Colorado


Aeronautics and Space Engineering Board Staff

PAUL SHAWCROSS, Study Director
JOANN CLAYTON-TOWNSEND, Aeronautics and Space Engineering Board Director (until July 11, 1997)
MARVIN WEEKS, Senior Project Assistant





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AERONAUTICS AND SPACE ENGINEERING BOARD


JOHN D. WARNER (chair), The Boeing Company, Seattle, Washington
A. DWIGHT ABBOT, Aerospace Corporation, Los Angeles, California
STEVEN AFTERGOOD, Federation of American Scientists, Washington, D.C.
GEORGE A. BEKEY, University of Southern California, Los Angeles
GUION S. BLUFORD, JR., NYMA, Inc., Brook Park, Ohio
RAYMOND S. COLLADAY, Lockheed-Martin Astronautics, Denver, Colorado
BARBARA C. CORN, BC Consulting, Inc., Searcy, Arizona
STEVEN D. DORFMAN, Hughes Telecommunications and Space Company, Los Angeles, California
DONALD C. FRASER, Boston University, Boston, Massachusetts
JAMES M. GUYETTE, Rolls-Royce North America, Reston, Virginia
FREDERICK H. HAUCK, AXA Space, Bethesda, Maryland
WILLIAM H. HEISER, U.S. Air Force Academy, Colorado Springs, Colorado
WILLIAM HOOVER, U.S. Air Force (retired), Williamsburg, Virginia
BENJAMIN HUBERMAN, Huberman Consulting Group, Washington, D.C.
JAMES G. O'CONNOR, Coventry, Connecticut
GRACE M. ROBERTSON, The Boeing Company, Long Beach, California
GEORGE SPRINGER, Stanford University, Stanford, California


Staff

JOANN C. CLAYTON-TOWNSEND, Director (until July 11, 1997)
GEORGE M. LEVIN,* Director (from July 14, 1997)







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*George Levin was Orbital Debris Program Manager at NASA before becoming ASEB director. To avoid a potential conflict of interest, he did not participate in any work related to this report after joining the National Research Council.



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Preface






In 1983, three days into my first shuttle mission, I noticed a small pit in one of the windows of the crew cabin. Spectrographic analysis of the residue left in this tiny pit revealed the presence of titanium and aluminum, suggesting that the orbiter had been hit by a chip of paint that had flaked off of some unknown spacecraft or rocket body. This was one of the first indications that orbital debris might pose a hazard to the space shuttle.

By 1995, the number of reported window impacts had increased dramatically, and the debris hazard had forced planners to modify plans for shuttle mission STS-73. In September 1995, the space shuttle program manager established a Space Shuttle Meteoroid and Debris Damage Team to review the environment modeling and orbiter modeling, to assess the potential for damage from meteoroids and orbital debris, and to "recommend concepts and methods to reduce risk to critical orbiter areas" (Holloway, 1995).

In 1995 and 1996, significant impacts occurred on the orbiter's payload bay door and rudder speed brake, as well as on the tethered satellite pallet. In May 1996, the manager of the space shuttle program established interim guidelines to minimize the time spent in sensitive attitudes, minimizing the probability of orbital debris impact to the wing leading edge and orbiter radiators." He further stated that "mission planning and design should be implemented with the objective of not exceeding a probability of critical penetration of 1/200 while also minimizing the exposure of the orbiter radiators to orbital debris as much as possible" (Holloway, 1996).

The allowable risk of 1/200 means that the hazard from meteoroids and orbital debris is, on some missions, the single greatest threat to the shuttle and crew, slightly larger than the hazard from ascent. To gain an independent, outside assessment of the threat, and of measures to address it, the National Aeronautics and Space Administration (NASA) asked the National Research Council (NRC) to review the space shuttle program's strategy for assessing and mitigating the threat posed by meteoroids and orbital debris. In response, the NRC formed the Committee on Space Shuttle Meteoroid/Debris Risk Management, under the auspices of the Aeronautics and Space Engineering Board. (The charge to the committee is contained in Appendix A.) The committee met in April and June of 1997 to receive briefings from NASA and NASA contractors and to deliberate on findings and recommendations. This report is the product of those meetings and of additional data gathering, writing, and discussion during the summer and fall of 1997.

The committee concurs that the threat to the shuttle from meteoroids and orbital debris is real, although the magnitude of the threat and the resulting hazard are not clear. In recent years, researchers have greatly improved models of the debris environment and conducted numerous tests and studies to assess the damage caused by the impact of meteoroids and orbital debris, but no end-to-end assessment has been made of the orbiter's survivability in the face of the meteoroid and debris hazard.

Such an assessment is needed, and needed soon. Until the magnitude of the threat—and the uncertainty of the threat assessment—are better known, program managers and mission planners will be forced to balance crew safety against cost and mission goals based on very incomplete information. The assessment will have other benefits as well—improvements in NASA's environment and impact models will benefit space activities worldwide.

NASA has developed a world-class center of expertise on the meteoroid and orbital debris hazard. Many experts from NASA and NASA contractors briefed the committee and provided us with information essential to this study. The committee thanks them for their professional and candid presentations. I extend my warm thanks to Dr. Bill Heiser, NRC Aeronautics and Space Engineering Board liaison to this project, for his active participation, his counsel, and his insightful critique of our process and text. In closing, I want to thank the members of the committee personally for their time and effort on the study and on writing this report, as well as Paul Shawcross, the study director, for his tireless efforts in bringing this project to fruition.





RICK HAUCK  
Committee Chair  




REFERENCES

Holloway, T.W. 1995. Space shuttle meteoroid and debris damage team. MA2-95-074. Houston: NASA Johnson Space Center. September 28, 1995.

Holloway, T.W. 1996. Orbital debris mission planning guidelines. MA2-96-082. Houston: NASA Johnson Space Center. May 30, 1996.

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Contents




EXECUTIVE SUMMARY


1 INTRODUCTION
  Reference


2 RISK TO THE ORBITER AND CREW
  Orbiter Susceptibility
  Orbiter Design
  Orbiter Vulnerability
  Extravehicular Activity
  References


3 RISK MANAGEMENT STRATEGY
  Current Approach
  Analysis and Findings
  Recommendations
  References


4 TOOLS FOR RISK ASSESSMENT
  Current Approach
  Analysis and Findings
  Recommendations
  References


5 COLLISION AVOIDANCE
  Current Approach
  Analysis and Findings
  Recommendations
  References


6 RISK MITIGATION
  Current Approach
  Analysis and Findings
  Recommendations
  References


ACRONYMS


APPENDICES
A Statement of Task
B Biographical Sketches of Committee Members





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List of Tables, Figures, and Boxes






TABLES

2-1 Predicted Number of Impacts on Orbiter
2-2 Damage Thresholds for Orbiter Components
2-3 Risk during EVA
3-1 Total Calculated Risk of Critical Failure




FIGURES

2-1 Survivability analysis
2-2 Comparison of meteoroid and debris flux in ISS orbit
2-3 Orbiter primary structure
2-4 Orbiter systems concentrated along the fore-aft axis
3-1 On-orbit impact analysis methodology
3-2 Relative critical risks for orbiter components after refinement of critical failure limits
4-1 Particle lifetime as a function of diameter and solar activity
5-1 The space shuttle alert and maneuver boxes
6-1 Window replacements vs. shuttle orientation
6-2 Critical penetration risk vs. shuttle orientation
6-3 Modification of radiator tube shielding
6-4 Use of remote manipulator system to survey orbiter for damage




BOXES

1-1 Meteoroids and Orbital Debris
1-2 High-Speed Collisions
5-1 NASA Flight Rule A4.1.3-6



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Have I not walked without an upward look

Of caution under stars that very well

Might not have missed me when they shot and fell?

It was a risk I had to take—and took.




"Bravado"    
Robert Frost    

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Executive Summary






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.




COLLISION AVOIDANCE

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.




RISK MITIGATION

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.