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.