Orbital decay analysis, reentry predictions and risk assessment for the GOCE satellite

摘要

The ESAu27s GOCE satellite was launched on 17 March 2009. After mapping the geopotential with unrivalled accuracy and detail for four years from an extremely low circular polar orbit, on 21 October 2013 the low thrust ion propulsion motor used to contrast the atmospheric drag was automatically shut down when the pressure in the xenon propellant tank dropped below a critical threshold. Then the satellite entered in "fine-pointing mode" (FPM), a phase of orbital altitude decay with active fine attitude control carried out by a set of magnetotorquers. According to the pre-launch specifications, the attitude control system was expected to compensate the gravity gradient and the aerodynamic torques up to an average drag force along the orbit of 20 mN. However the system proved itself much more robust than envisaged, remaining operational until reentry, with drag forces exceeding 2000 mN. The uncommon nature of the GOCE reentry campaign, sharing an uncontrolled orbital decay with a finely controlled attitude along the atmospheric drag direction, made the reentry predictions for this satellite an interesting case study, in particular because nobody was able to say a priori if and when the attitude control would have failed, leading to an unrestrained tumbling. Therefore, even though the casualty expectancy for this reentry was slightly above the internationally recognized alert threshold of 1/10,000, i.e. about 1/5000, it presented a number of challenges and opportunities from the prediction and risk evaluation points of view. As in previous cases, ISTI/CNR was in charge of reentry predictions for the Italian civil protection authorities and exchanged information with the other agencies involved in the framework of the international reentry campaign promoted by the Inter-Agency Space Debris Coordination Committee (IADC). Considering the peculiar nature of the GOCE reentry, the definition of reliable uncertainty windows was not easy, in particular taking into account the critical use of this information for civil protection evaluations. However, after an initial period of test and analysis, reasonably conservative criteria were elaborated and applied, with good and consistent results through the end of the reentry prediction campaign. Being the attention of the Italian civil protection authorities focused on the national territory, during the last three days of satellite residual lifetime, reentries where simulated over Italy to obtain quite accurate ground tracks, debris swaths and air space crossing time windows associated with the critical passes over the national territory still included in the current uncertainty windows. Only 14 hours before reentry it was possible to exclude any residual risk of debris fall on the Italian territory and the GOCE fragments eventually plunged into the Southern Atlantic Ocean, between the Falkland Islands and the coast of Argentina, on 11 November 2013, around 00:24 - 00:40 UTC.