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In-Flight Modification ofthe Cassini Reference Trajectory

In July of this year, Cassini completed its four-year prime mission of Saturn exploration, and began a two-year extended mission, the Cassini Equinox Mission (CEM). During the course of the Cassini prime mission, several changes were made to the reference trajectory either out of necessity (i.e. due to spacecraft safety concerns) or to increase the mission science return by following-up on new discoveries. HE Cassini spacecraft has just completed its prime mission of a 4-year gravity-assist tour 1-3 of Saturn’s moons and carried the Huygens probe to its landing on the planet-sized moon, Titan. In doing so, it is has made many discoveries including: lakes, mountains, rivers, and sand-dune seas among the incredible variation of terrain on Titan; that Saturn’s E-ring is formed from geysers erupting from the south pole of Enceladus (geysers fed by a liquid reservoir with great astrobiological potential); and that the “spokes” seen by Voyager in Saturn’s rings are caused by electrical storms in Saturn’s very active atmosphere. In 2004, when Cassini began its trailblazing exploration of the Saturnian System, many unknowns lay ahead of it. In response to the twists and turns of the first exploration of Saturn, the Cassini-Huygens project has made 10 changes to the reference trajectory since Saturn approach. All of this occurred on a spacecraft whose complex suite of body-mounted instruments necessitated an extremely complex resequencing process whenever trajectory modifications changed the timing of events. These changes to complex sequences not only stressed the project workforce, they also added the risk of errors possibly being introduced to a sequence. These trajectory changes, i.e. tour tweaks, were made either to improve science return or to reduce mission risk. Several tweaks were made to the tour design even before Saturn arrival to improve the mission science. 4 Particularly notable among these was the addition of the Phoebe flyby, which provided science on par with a New Frontiers class mission for the cost of only 20 m/s additional ∆V. A tweak to the tour was also made to offset the effect of the Huygens relay anomaly, 5 and successfully recover the Huygens mission. This paper, however, will focus on the design of the tour tweaks that were made after Saturn arrival and particularly the challenges that arose from making these changes to the Cassini reference trajectory while flying to it. Many of the in-flight tweaks of the Cassini tour greatly improved the science return for the mission. Examples include improvement to Iapetus 6 and Tethys 7 flybys, and most notably, improvements to Enceladus flybys 8 that aided the discovery of that moon’s cryovolcanic plumes. Other tweaks reduced mission risk by responding to new information such as uncertainties in the density of Titan’s upper atmosphere and a discovery of the threat to the accuracy of the Huygens probe delivery from Iapetus perturbations. Tweaking the reference trajectory is not without consequences. The Cassini spacecraft has a diverse and capable suite of instruments. However, all of the instruments are body-fixed with very limited articulation. This results in a sequence design process that achieves measurement requirements from the instrument teams via a complex balance of competing pointing geometries.