>From October 2000 issue of ORBITAL DEBRIS QUARTERLY NEWS, NASA JSC, http://www.orbitaldebris.jsc.nasa.gov/newsletter/v5i4/v5i4.html#news3 Note total lack of WOMBAT/FFU Reentry Survivability Analysis of Extreme Ultraviolet Explorer (EUVE) By R. O'Hara A reentry analysis of the Extreme Ultraviolet Explorer (EUVE) spacecraft was performed using the Object Reentry Survival Analysis Tool (ORSAT) - Version 5.0. The analysis was done in response to a request by NASA Headquarters and Goddard Space Flight Center (GSFC) after a preliminary assessment had shown that the EUVE reentry may produce a debris area greater than the limit set within the NASA Safety Standard 1740.14 guidelines. NASA's 3243 kilogram EUVE spacecraft was launched on June 7, 1992 from Cape Canaveral Air Station on board a Delta II launch vehicle into a 528 kilometer, 28.5 degree inclined orbit. With the spacecraft nearing its end of mission and a possible reentry into the Earth's atmosphere expected as early as October 2001, personnel at Goddard Space Flight Center performed a reentry analysis using the NASA Johnson Space Center Debris Assessment Software (DAS) - Version 1.0, in accordance with NASA Policy Directive 8710.3. In the GSFC analysis, there were 18 individual objects predicted to survive. The total casualty area calculated for these surviving objects was 12.41 m**2, which exceeds the 8 m**2 limit set in the NASA safety standard. The large debris area implies a potential human casualty risk of approximately 1 in 5,300. The EUVE spacecraft was not designed with a propulsion system and therefore cannot perform a controlled reentry. In order to mitigate the potential risk to human safety from an uncontrolled reentry of the EUVE spacecraft, a retrieval of the spacecraft using the Space Shuttle was considered. However, since DAS is a lower fidelity model and tends to produce a more conservative result, the Orbital Debris Program Office at JSC was asked to perform a more detailed reentry study using the higher fidelity NASA-Lockheed Martin ORSAT model to determine if taking such a measure would be necessary. Several sophisticated material and thermal properties are included in ORSAT but do not exist in the DAS code. These enhancements tend to result in fewer objects surviving reentry when using ORSAT as opposed to DAS for a reentry analysis. For example, the emissivity is set to 1.0 for all materials available in DAS, implying blackbody radiation for each component analyzed. Thus, objects in DAS tend to lose heat faster and are more likely to survive. In ORSAT, however, the emissivity can be adjusted based upon what type of material the object is composed of. ORSAT also considers heat of oxidation during reentry, which means that the object gains heat faster and will demise more readily. Heat of oxidation is not considered in DAS. ORSAT also allows for thermal conductivity. With this enhancement and using a layered approach to modeling the fragments, ORSAT can reduce the overall debris area by allowing for objects to partially ablate. In contrast to this method, DAS will allow the entire fragment to survive. And finally, ORSAT enables the user to supply a wall thickness for an object, making it easier to model hollow objects. DAS treats all objects as solid and therefore requires a workaround to approximate the reentry heating to a hollow object. This workaround has been validated using comparisons with ORSAT runs, though the more direct approach used by ORSAT is more reliable. In the ORSAT analysis, only the objects shown to survive with the DAS model were evaluated, and the high fidelity features of ORSAT were applied to the reentry analysis. Reentry of the EUVE spacecraft was considered to occur at an altitude of 122 kilometers with breakup occurring at 78 kilometers. All of the objects were considered exposed to reentry heating at this breakup altitude. Objects were also analyzed for possible shielding affects by other components. Any object shown to demise at the breakup altitude, but was considered shielded by other spacecraft components, was reanalyzed starting at the demise altitude for the object shielding it. This allowed for some conservatism since in reality these objects would have experienced some heating and possible ablation prior to the demise of the object shielding it. The final debris area calculated from the more sophisticated ORSAT analysis of the surviving fragments came to a total of approximately 5.95 m**2, which is well under the 8 m**2 NASA constraint. The more detailed reentry study of the EUVE spacecraft done using ORSAT has shown the future uncontrolled reentry of EUVE to be of an acceptably low risk to human safety and therefore mitigation measures are unnecessary.
