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6 End-of-Life Disposal At some point in the future, the ISS will have to be decommissioned, deorbited, and returned to Earth. The return of the ISS will require stringent safety standards to minimize third-party damage and avoid casualties. NASA has performed, and must be commended for, its detailed analyses of the requirements for and methods of end-of-life disposal of the ISS in a manner consistent with NASA's stated safety requirements (i.e., less than a 1 in 10,000 chance of a casualty from reentry operations). NASA has performed a number of analyses related to ISS end-of-life disposal, including an analysis performed for the International Space Station Alpha (ISSA), a space station configuration that preceded the current ISS design (NASA, 1995~. The analysis consisted of a risk assessment, a con- trolled deorbit analysis, a debris dispersion analysis, and a disposal area assessment. Additional analyses were also performed based on the use of a U.S. propulsion module (Thorn, 1999~. The disposal risk assessments concluded that the risk to human life from an uncontrolled ISS reentry would be unac- ceptable, ranging from a 0.024 to a 0.077 chance of a single casualty (i.e., 2 in 100 to 8 in 100~. On the assumption that a failed deorbit mission would result in an uncontrolled reentry, with about a 0.05 chance of a casualty, then even a 1-percent chance of a failure exceeds NASA's stated safety objective. According to NASA's safety guidelines, the casualty risk must be limited to a 0.0001 chance of a single casualty (i.e., 1 in 10,000~. The risk assessment, therefore, included other alternatives, such as: boosting the ISS to a much higher orbit to prolong its on-orbit lifetime; disassem- bling and returning ISS components to Earth via the Space Shuttle; and controlling and targeting reentry to a safe loca- tion in the oceans. In one analysis conducted for the ISSA, an end-of-life deorbit maneuver could begin from a circular orbit after the two Soyuz vehicles had been separated from the station. After separation of the Soyuz vehicles, the Progress vehicles) and the service module, both fully loaded with 28 propellant at the beginning of the maneuver, would be used for the deorbit burn (Thorn, 1999~. A more recent study was done with end-of-life disposal performed by a U.S. propulsion module. This study estab- lished deorbiting criteria based on the U.S. propulsion module directing the reentry to a remote ocean area to ensure that the dispersed surviving debris "footprint" would not fall within 370 km (200 nmi) of any land mass. The ISS deorbit trajectory would be designed so that natural orbit decay would lower the orbit to the point where excessive attitude control propellant would begin to be needed (i.e., about 241 to 185 km [130 to 100 timid. Solar arrays would be posi- tioned to minimize aerodynamic torque. At 222 km (120 nmi) altitude, the U.S. propulsion module would lower perigee (the lowest point in the orbit) to an altitude of ap- proximately 140 km (75 nmi) with orbit adjustments made over several orbits because of the long burn times needed to achieve the required change in orbital velocity (/\v). A final deorbit burn would then lower perigee from 140 km to 83 km (75 to 45 nmi) altitude reaching at least a 16.8 m/s (55 fps) /\v in a period of 35 minutes. Solar arrays would collapse at about 130 km (70 nmi) altitude as the ISS begins its atmospheric reentry profile. As a result of the reentry and subsequent breakup of the ISS, surviving debris would be scattered over the surface of the Earth. About 80-percent of the debris would vaporize in the atmosphere. The debris impact footprint is affected by many factors, including deorbit maneuver accuracy, range of debris ballistic coefficients, breakup altitude, breakup /\v, atmospheric density, winds, and debris aerodynamic lift. Assuming a successful reentry mission, NASA analyses have concluded that the ISS dispersed debris footprint could be as large as an ellipse measuring 300 km by 5,370 km (162 nmi by 2,900 nmi). Therefore, ocean disposal will be necessary. The largest disposal region, in the eastern Pacific Ocean, would allow the initiation of deorbit maneuvers on either of two consecutive orbital passes over the area. NASA has concluded that the U.S. propulsion module, as
END-OF-LIFE DISPOSAL presently designed, will not be able to meet the performance requirements of the ISS end-of-life deorbiting mission (Thorn, 1998~. To achieve the deorbiting mission require- ments, the U.S. propulsion module would have to burn more than 142 kg (5,000 lbs) of propellant, developing 3,556 New- tons (800 lbs) of thrust for a duration of 35 minutes to com- plete the final deorbiting burn. NASA concluded that boosting the ISS to a higher orbit is not an option because of insufficient propellant and because the orbit would gradually decay to a lower orbit, and the ISS would ultimately deorbit in an uncontrolled reentry. Another alternative, disassembly of the ISS, was considered too expensive (the ISS is not designed for disassembly). The committee concurs with NASA's conclusion that the only viable solution for ISS end-of-life disposal is controlled deorbiting of the ISS. Based on these assessments, a controlled ISS reentry to a remote ocean area would be the safest disposal option. Therefore, ISS end-of-life disposal requirements will have to be incorporated into the U.S. propulsion module design requirements. NASA believes that a deorbiting mission must have at least a 99-percent reliability. The committee believes that even this reliability level would not meet NASA' s safety goal of a less than a 1 in 10,000 chance of a casualty. Recommendation. End-of-life disposal should be accom- plished by a controlled deorbiting of the International Space Station. Sufficient onboard propulsion must be provided for this operation. The National Aeronautics and Space Admin- istration should consider upgrading the U.S. propulsion module to provide the required deorbiting capability. Recommendation. Because of the potential hazards asso- ciated with the reentry of relatively large objects, the safety requirement for International Space Station reentry should be more stringent than the requirement for other National Aeronautics and Space Administration operations (i.e., the chance of casualties should be much less than 1 in 10,000~. NASA's calculations of the probability of success of the 29 final deorbit burn do not make enough allowances for the fact that these operations will take place in the very stressful environment of reentry, which will include the heating, vibration, and collapse of subsystems. Therefore, NASA cannot ensure that the U.S. propulsion module will have a greater than 99-percent probability of success. In fact, the committee believes that the reliability of the U.S. propulsion module will have to exceed 99-percent to achieve NASA's stated safety objectives. Recommendation. The National Aeronautics and Space Administration should undertake a thorough analysis of International Space Station reentry operations, including ranges of uncertainty associated with the multiple variables of reentry operations. The analysis could take the form of a Monte Carlo simulation of reentry operations and projected impact areas to characterize the hypothetical potential for property damage or casualties. The analysis should include the sequence of operations, possible failures, and conse- quences of failures, from the initiation of reentry operations to final impact. Uncertainty variables should include, but should not be limited to, reliability characteristics, duration of burn, atmospheric density, ballistic coefficients of frag- ments, population densities, and the characterization of acceptable impact areas. REFERENCES NASA (National Aeronautics and Space Administration). 1995. End-of- Life Disposal Assessment International Space Station Alpha. May 8, 1995. Engineering Directorate, Houston, Texas, NASA Johnson Space Center, Aerospace and Flight Mechanics Division. Thorn, V. 1998. ISS End-of-Life Disposal. Presentation by V. Thorn, Mis- sion Integration Office, International Space Station, to the Committee on the Engineering Challenges to the Long-Term Operation of the Inter- national Space Station, National Aeronautics and Space Administra- tion, Washington, D.C., September 18, 1998. Thorn, V. 1999. Personal communication from V. Thorn, Mission Integra- tion Office, International Space Station, to J. Greenberg, member of the Committee on the Engineering Challenges to the Long-Term Operation of the International Space Station, March 24, 1999.
failures. The SR&M program would benefit from more trend analyses of ad data that coma have a bearing on the long-term failure rate, maintenance capabilities, and spare Parr; requirements of the ISS. The Hubble Space Telescope (MST) program, for example, now considers "trending" as its primary SR&M analysis methocI. Trend analysis for the ISS could include the foBow~ng elements: . · · ~ 1ncommg inspection reports · in-process test reports failure reports from on-orbit segments ~ maintenance records for ground and on-orbit operations Recommendation. Analyses of the incoming inspection and in-process testing data should be used to establish a s~x-sigma environment In which failures wiD be extremely rare. Analyses of failure reports and maintenance records should be used to improve on-orbit procedures and Me quality of replacement items. SPARE PATS PHILOSOPHY NASA provided the committee with information on plans to provide spare parts and logistics for the ISS (NASA, 199Sb). The committee also reviewed the same information for the HST, He onIv other On-duration U.S. sDace Rehire that hat Involved crew servicing. _ ~s ~_ T Tic ~ a ~ ~ J ~ O ~ ~ ~ - Id___ ~ ___ ~~ new The opportunities for component replacement on the ISS and the Em amer tour or five visits per year for the ISS and three or four years between v3 sits for the EAT. In terms of manufacturing lead-time for producing spare parts, however, the biggest difference is that significant lead-time is available between repair visits to the HST to secure replacement parts. ~. For the HST, solar panels, mechanical relays, and rotating devices (e.g., wheels, _ _ e -_1_ 1~ 11 ~ ~ - ~ ~ ~ ~ ~ ~ ~ gyrOS, ~D~, ma servos) were stocKpHea, DUt ~ b~1 S me yews on orbit, He electromechanical devices and the purely electronic devices have had only moderate failure rates (Styczynski, I999~. Experience win the HST project revealed that NASA could not afford to stock and maintain the extensive depot facilities and the large numbers or spare pares required Tor tne net oasea on tne ~~o~- measure of statistical mean time between failures of the hardware (KeDey, I999~. Another method of providing spare parts would be to introduce upgrades during the operational life of the ISS to reduce failure rates and thereby reduce maintenance requirements (~is 1_ ~ ~& _ ~a ~T To . ~ ~. ~ or 30