Comparative Study of Planetary Atmospheric Uncertainties and Design Rules for Aerocapture Missions

Abstract: Aerocapture uses atmospheric drag to decelerate spacecraft and achieve orbit insertion. One of the significant risks associated with aerocapture is the uncertainty in the atmospheric density, particularly for outer planets. The paper performs a comparative study of the atmospheric uncertainties and provides design rules for aerocapture missions. The atmospheres of Venus, Mars, and Titan are well-characterized for engineering purposes. At the altitude ranges relevant for aerocapture, the 3$\sigma$ density variation is approximately $\pm$30%, $\pm$50%, $\pm$30% for Venus, Mars, and Titan respectively. With no in-situ data, the atmospheres of Uranus and Neptune are not as well characterized as the other bodies. For both Uranus and Neptune, the GRAM suite provides a 3$\sigma$ density variation of approximately $\pm$30% for the relevant altitude ranges which is considered an optimistic estimate. Until in-situ data from an atmospheric probe becomes available, a more conservative global min-max estimate is recommended to accommodate the worst-case scenario. The study presents a graphical method for selection of the optimal entry flight path angle when considering the atmospheric uncertainties to ensure the on-board guidance is given the best possible initial state for targeting the desired exit state post aerocapture.