Adoption of the VEEGA trajectory for RTG-powered spacecraft raised an additional question about the possibility of an inadvertent Earth re-entry on one of the flyby legs. A detailed study [1] was conducted to identify requirements on trajectory design, navigation planning, and spacecraft design and operation to minimize the probability of the spacecraft re-entering the Earth's atmosphere, and to determine the probability of entry. The results of the study showed that the probability of impacting the Earth due to navigational error was infinitesimal. However, certain failure modes were addressed to determine the conditions which could put the spacecraft on an impacting trajectory with no capability of recovery. As a result of this study, the following specific actions were taken: • an additional pressure relief valve was installed in the spacecraft propulsion system to reduce the hazard due to overpressure conditions; • the trajectory was redesigned using intermediate biased aimpoints to reduce, or in several cases eliminate entirely, the risk due to spacecraft failures; and • a set of operational constraints will be instituted prior to each encounter to further ensure that no anomalous events will occur. Given these design characteristics the probability of inadvertent re-entry was calculated to be less than 5 X 10-7. The Galileo spacecraft has completed its design modifications and major testing for opportunity to conduct valuable Venus, Earth and Moon science observations, as well as observations of the asteroids Gaspra and Ida.
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