Some orbital debris may eventually reenter the atmosphere. This usually occurs through a gradual process of orbital energy removal associated with air drag. The timescale for debris orbital decay depends on orbital altitude and the ratio of surface area to mass of the debris; the larger the area-to-mass ratio, the faster the debris decays. At altitudes of 600 km, typical timescales for the smallest tracked orbital debris (10 cm) to decay is 12 to 18 months, whereas at 1,000 km the lifetime may be centuries. Note that reentry is controlled strongly by changes in air density in the upper atmosphere, which is modulated by solar activity. At times of high solar activity, the upper atmosphere is heated and expands; this tends to hasten the rate of debris orbit decay.
The vast majority of reentering debris is too small to survive reentry; it is entirely consumed in the upper atmosphere via melting and vaporization. Larger debris, especially debris made of high-melting-point materials such as titanium or stainless steel, however, may survive partially or in full to reach the ground (see Boxes 8.1 and 8.2).
Removing derelict space objects to reduce orbital debris hazards is merely a transferring of risk from space to the ground, which must also be managed. NASA Technical Standard (STD) 8719.14 dictates that the risk to people on the ground worldwide from the reentry of a piece of space hardware must not pose a hazard greater than 1 in 10,000.1 Adherence to the probabilistic casualty metric can be determined by using one of two NASA applications: the Debris Assessment Software (DAS) and the Object Reentry Survival Analysis Tool (ORSAT).
The Debris Assessment Software (DAS) is a tool developed to provide NASA programs a simple step-by-step menu-driven application for orbital debris assessments (ODAs) that are compliant with STD 8719.14. If ODAs show non-compliance, DAS provides a means to examine debris mitigation options to meet compliance standards. DAS, however, should not be confused with ORDEM, which provides the opportunity to analyze the more technical aspects of the debris environment. Evaluation of hardware reliability, shield design, and other parameters will require engineering tools such as ORDEM and BUMPER. It is important to note that STD 8719.14 contains the actual mission requirements, while DAS is only a software tool that assists in determining compliance.2
DAS produces a first-order assessment of human casualty risks associated with uncontrolled space object reentries that, by design, yields a slightly conservative result. If a program or project meets reentry risk requirements using DAS, no more calculations are required. However, if a program or project does not meet requirements for reentry risk using DAS, then ORSAT will need to be exercised.
1 NASA, Process for Limiting Orbital Debris, NASA-STD 8719.14 (Change 4), Washington, D.C., September 2009.
2 NASA, Debris Assessment Software (DAS) User’s Guide, JSC 64047, NASA Johnson Space Flight Center, Houston, Tex., November 2007.
Cosmos (Kosmos) 954
Cosmos 954 was a Soviet nuclear-powered Radar Ocean Reconnaissance Satellite (RORSAT) that launched from the Baikonur cosmodrome on September 18, 1977.1 Only 4 months later on January 24, 1978, Cosmos 954 crashed into Canada’s Northwest Territories, scattering large amounts of radioactive material across 124,000 km2, from Great Slave Lake into northern Saskatchewan and Alberta.2 Only one in a series of satellites, Cosmos 954 and the other RORSATs operated at very low altitudes to conduct their surveillance of ocean traffic with space-based radar, which required them to expend a significant amount of fuel and energy to keep their station. To produce the energy necessary for this type of operation, the RORSATs used a nuclear power source. Typically, just before the fuel is spent in a RORSAT, its operators send the satellite into a higher “graveyard” orbit between 800 km and 1,000 km. Unfortunately, Cosmos 954’s propulsion system failed for reasons unknown, causing it to reenter Earth’s atmosphere before it could be sent to the graveyard orbit. In the end, the joint U.S.–Canadian clean-up operation recovered only approximately 0.1 percent of Cosmos 954’s power source.3 This event also marked the first time that the adjudicative process built into the UN Convention on International Liability for Damage Caused by Space Objects4 was put to the test. In 1981, the Soviet Union (USSR) and Canada settled the Canadian’s claim of reimbursement, and the USSR paid the Canadians $3,000,000.5
ORSAT is a semi-empirical model that determines survivability of reentering hardware (debris, payloads, and rocket bodies).3 This tool can be used for both controlled and uncontrolled reentry. The predicted amount of material surviving to the ground is determined, the resulting impact hazard is calculated, and this hazard is compared to the impact hazard threshold.
The ORSAT model is a suite of tools that perform trajectory, atmospheric, aerodynamic, thermodynamic, and thermal/ablation physics calculations. These algorithms together determine if the space object, or any of its remnants, will survive reentry. Different object types and shapes can also be modeled with ORSAT. Both tumbling and spinning objects can be simulated in the trajectory model. Physical parameters that change during the course of reentry, such as coefficient of drag and stagnation heating rates, are determined by modifying a well-validated circular object for varying shape effects. If the absorbed heat exceeds the heat of ablation for the material, then the object is assumed to have disintegrated. (See Figure 8.1 for an example.)
Temperature-varying properties such as thermal conductivity, specific heat, and surface emissivity are included in ORSAT for nearly 100 materials. If the model predicts that an object is within a small margin of the threshold for total destruction, extra calculations are performed to consider oxidation efficiency, initial temperature, surface emissivity, number of hardware layers, dimensions, and breakup altitude. This additional examination will provide a more accurate determination of object survival to the ground. The total debris casualty ground coverage is calculated by combining ground footprint and object survival estimates. The impact casualty risk is determined by combining the predicted mass of the surviving object mass with a worldwide population distribution model. This value is then used to discern whether the space object reentry scenario is compliant with STD 8719.14.
3 R.N. Smith, J. Dobarco-Otero, K.J. Bledsoe, and R.M. DeLaune, User’s Guide for Object Reentry Survival Analysis Tool (ORSAT)—Version 6.0, JSC-62861, NASA Johnson Space Flight Center, Houston, Tex., January 2006.
Validation of Reentry Hazard Models
The NASA Orbital Debris Program Office (ODPO) has supported other NASA programs, the Department of Defense, the Federal Aviation Administration, the National Oceanic and Atmospheric Administration, the Department of Justice, and foreign entities in predicting the hazard posed by reentry of space hardware. It takes full advantage of actual reentry events to validate its reentry models to the maximum extent possible. Usually, these reentry events start with a private citizen finding an unusual object on the ground. The committee was presented with examples of 10 such objects, found in locations all over the world. The NASA website1 as well as a publication by the Aerospace Corporation includes examples of recovered reentered space hardware.2 The following recent example of a recovery analysis illustrates that it takes a certain amount of determination to add data on a piece of reentered debris into the NASA database.
In March, 2011, Robert Dunn was hiking in Moffat County, near the NW corner of Colorado. He heard a high-pitched sound that he could not identify, but it caught his attention since he was in a fairly isolated area. A short time later, he noticed a 30” diameter object on the ground within a crater about a foot deep. The object was warm when he touched it, even though he was in an area with snow on the ground.3 Feeling that the object had to have come from space, he contacted NORAD to get more information. NORAD suggested that he contact his local sheriff. Mr. Dunn ended up contacting both the local sheriff and NASA’s retired scientist, Don Kessler, both of whom pointed Mr. Dunn to NASA’s ODPO. The ODPO was able to identify the object as a spherical titanium tank originating from a Russian upper-stage rocket launched in January 2011.4 The Russian writing on the object, and a visit from a Ukrainian news crew from where the tank was designed, pretty much confirmed the origin. A follow-up search found another, smaller sphere 34 miles to the North-East.
FIGURE 8.2.1 Robert Dunn and the Russian space debris. SOURCE: Courtesy of Elizabeth Campbell.
1 See NASA Orbital Debris Program Office, “Orbital Debris Recovered Objects,” available at http://www.orbitaldebris.jsc.nasa.gov./reentry/recovered.html.
2 W. Ailor, W. Hallman G. Steckel, and M. Weaver, Analysis of reentered debris and implications for survivability, pp. 539-544 in Proceeding of the Fourth European Conference on Space Debris, ESA SP-587, European Space Agency, Paris, France, August 2005.
3 See Barr, Z., “He Knew It Fell from the Sky,” broadcast, Colorado Public Radio, April 14, 2011, available at http://www.cpr.org/article/He_Knew_It_Fell_From_The_Sky; and “Man finds part of Russian rocket in Colorado,” video, 9News. com, April 14, 2011, available at http://www.9news.com/video/default.aspx?bctid=903246061001.
4 NASA, “Russian Launch Vehicle Stage Reenters over U.S.,” Orbital Debris Quarterly News 15(2):3, April 2011.
FIGURE 8.1 Depiction, using ORSAT, of the reentry disintegration of the Upper Atmosphere Research Satellite (UARS). The demise altitude versus downrange impact point is evaluated for major UARS components. SOURCE: Courtesy of NASA-JSC.
For each surviving object, a single point estimate is provided for the mass remaining along with its terminal velocity, thus producing a damaging effect on people on the ground as determined by its kinetic energy. The dispersion of multiple fragments surviving to the ground from a single object’s disintegration provides an area over which the debris will potentially pose a hazard to people on the ground, which in turn produces a single probability of casualty on the ground for the reentry event. Despite the wide range of potential variations that could result in rentry, neither uncertainty estimates nor confidence bounds are provided for impact point, mass of surviving object, or probability of casualty.4 Uncertainty information is required for management to be able to place this risk in proper perspective with others to guide decisions about direction of work and funding.
Finding: NASA’s Object Reentry Survival Analysis Tool provides results as point estimates without confidence bounds or uncertainty estimates.
Recommendation: In regard to debris reentry risk, NASA should provide confidence bounds on and uncertainty estimates of the resulting risk levels for use in both the Debris Assessment Software and the Object Reentry Survival Analysis Tool.
ORSAT is developed by NASA and used only by Orbital Debris Program Office personnel, due to the complex model interfaces and complicated modeling processes portrayed. However, the ODPO has created a more conservative and simpler reentry demise module as part of DAS, which is available to the public. As indicated previously, if DAS determines that none of the material from a reentering piece of space hardware will survive to the ground, then this determination is sufficient to assure compliance with STD 8719.14. However, if DAS indicates that the reentry scenario is non-compliant, the situation may be reanalyzed using ORSAT, which provides
4 R.N. Smith, J. Dobarco-Otero, K.J. Bledsoe, and R.M. DeLaune, User’s Guide for Object Reentry Survival Analysis Tool (ORSAT)—Version 6.0, JSC-62861, NASA Johnson Space Flight Center, Houston, Tex., January 2006.
a more detailed and accurate assessment as described above. ORSAT has been used numerous times, providing great utility to mission programs.5
The use of ORSAT and DAS for assessing the survivability of reentering debris will increase as debris continues to reenter and concerted efforts are made to remove derelict hardware from orbit. As with other model developments, NASA uses IADC deliberations to perform cross-program comparisons of ORSAT with ESA’s equivalent model(s).6 Although mathematical results for reentry object disintegration are found to be very similar between the two models, the inconsistent definition of “casualty” between ESA and NASA makes it difficult to easily compare results. ESA considers a “casualty” to be a person who is killed by a reentering object, whereas NASA (within ORSAT and DAS) considers a “casualty” to be a person who is injured by a reentering object. As a result, ORSAT is more conservative than the ESA reentry survival model; it predicts a higher probability of a “casualty” from the same reentry events. Updating ORSAT to provide the probabilities for both injury and death as standard outputs would require only a simple coding change.
Finding: The reentry hazard programs used by NASA and the European Space Agency to determine the risk to people on the ground from reentering debris differ in how those thresholds are defined. NASA’s Object Reentry Survival Analysis Tool defines a “casualty” as personal injury, whereas ESA models equate a “casualty” with death.
Recommendation: NASA should update the Object Reentry Survival Analysis Tool so that it provides the probabilities of both injury and death as standard outputs.
5 For examples of the use of ORSAT by mission programs, see J.P. Rustick and W.C. Rochelle, Reentry Survivability Analysis of GENESIS Spacecraft Bus, LMSEAT 33557, December 2000; J.P. Rustick and W.C. Rochelle, Reentry Survivability Analysis of Earth Observing System (EOS)—Aqua Spacecraft, LMSEAT-33622, March 2001; R.N. Smith and W.C. Rochelle, Reentry Survivability Analysis of Compton Gamma Ray Observatory (CGRO), JSC-28929, NASA Johnson Space Center, Houston, Tex., March 2000; R.N. Smith, R.M. DeLaune, and J. Dobarco-Otero, Reentry Survivability Analysis of the Genesis Spacecraft Bus for Off-Nominal Trajectories, JSC-62665 Rev. A, NASA Johnson Space Center, Houston, Tex., August 2004; R.N. Smith and W.C. Rochelle, Reentry Survivability Analysis of Earth Observing System (EOS)—Aura Spacecraft, LMSEAT-33712, July 2001, p. 12; R.N. Smith, J. Dobarco-Otero, J.J. Marichalar, and W.C. Rochelle, Tropical Rainfall Measuring Mission (TRMM)—Spacecraft Reentry Survivability Analysis, JSC-29837, NASA Johnson Space Center, Houston, Tex., September 2002, p. 4; R.N. Smith, J. Dobarco-Otero, and W.C. Rochelle, Reentry Analysis of Gamma-ray Large Area Space Telescope (GLAST) Satellite, JSC-49775, NASA Johnson Space Center, Houston, Tex., July 2003, p. 6; R.N. Smith, K.J. Bledsoe, and J. Dobarco-Otero, Reentry Survivability Analysis of the Hubble Space Telescope (HST), JSC-62599, NASA Johnson Space Center, Houston, Tex., May 2004; R.N. Smith, J. Dobarco-Otero, and R.M. DeLaune, Reentry Survivability Analysis of Shuttle External Tank Debris, JSC-62683, NASA Johnson Space Center, Houston, Tex., December 2004; and R.N. Smith, J. Dobarco-Otero, and R.M. DeLaune, Reentry Survivability Analysis of the Terra Satellite, JSC-63042, NASA Johnson Space Center, Houston, Tex., June 2005, p. 3.
6 W. Rochelle et al., “Results of IADC Reentry Survivability Benchmark Cases: Comparison of NASA ORSAT 5.0 Code with ESA SCARAB Code,” presentation at the 17th IADC meeting, Darmstadt, Germany, October 1999.