Flying by Titan
- Paper number
IAC-08.C1.4.8
- Author
Mr. Frederic Pelletier, United States
- Year
2008
- Abstract
The multiple close-encounters of the Cassini-Huygens spacecraft with Saturn’s largest moon Titan presents new challenges for the navigation team at NASA’s Jet Propulsion Laboratory. In order to meet the numerous goals of the mission, the spacecraft has to fly several close encounters (< 1300 km altitude) with Titan. From a scientific point of view, the close encounters are the best opportunity to study Titan up close and determine, amongst other objectives, the composition of its thick atmosphere, the surface topography and the extent of its gravity field.
But getting close to the moon is not only a scientific objective, it is also a necessity for the navigation engineers. Cassini’s trajectory is designed to visit a satellite of Saturn every month on average. This high science return is possible only with the help of Titan’s gravitational pull, which is used to periodically reshape the trajectory. Gravity assists from Titan each provide Cassini with some 600 m/s change in velocity, roughly equivalent of about 600 kg of the bi-propellant fuel on board the spacecraft. For comparison, Cassini’s fuel tank was left with less than 1200 kg of bi-propellant after the Saturn orbit insertion burn.
To correctly re-aim the spacecraft after each encounter and to provide scientists with the most precise trajectory reconstruction data, navigators pay close attention to the perturbing forces acting on Cassini during an encounter. While the methodology is similar to that of other deep space missions, the uniqueness of the Cassini-Huygens mission requires more refinement in the modeling of the dynamics at Titan. To achieve suitable orbit determination results, all perturbing forces acting on the spacecraft are modeled and estimated. This paper will discuss how both science and engineering data are used to develop the models and refine them periodically.
This includes the estimation of Titan’s gravity harmonics using dedicated radio-science passes, during which the Earth is tracking Cassini through the encounter. It also involves the modeling of an atmospheric density model to determine the drag force acting on the spacecraft. The atmosphere of Titan is so dense that a flyby with a closest approach at one thousand kilometer altitude is enough to cause a velocity change of about 60 m/s. The thick atmosphere of Titan, about 10 times denser than Earth’s, is composed of 95% nitrogen, while the remainder 5% is composed of methane, cyanide and other hydrocarbons. Scientists and engineers are currently investigating the density profile around Titan using various methods. One is to measure it directly from the composition of particles that have been collected by the ion-neutral mass spectrometer instrument. The density can also be derived from the amount of thrusting the attitude control system had to perform in order to counteract the drag torque on the spacecraft.
The attitude control thrusters are themselves an important source of perturbation to the trajectory, as they also accelerate the spacecraft in addition to the desired torque. This is because the configuration of Cassini’s thrusters is such that the Z-axis thrusters are oriented the same way and therefore not coupled. The thrusters are favored over the reaction wheels during a close flyby in order to turn the spacecraft quickly and maintain the desired attitude with respect to Titan.
Figure ?? illustrates the magnitude of all perturbations during a typical encounter. The data correspond to the Titan 16 flyby, which occurred on July 22, 2006. At that time, Cassini’s closest approach altitude was 950 km. Note that for a period of 2 hours, Titan’s gravitational pull dominates over Saturn’s (see GM curves). The attitude control thrusting curve is labeled “RCS THRUST”, showing a typical erratic behavior. For a long time, a lack of accuracy in the data used to model this force has tainted the estimates of the rest of the perturbations of the same order of magnitude, namely J2 and Drag. In fact, the noise level in the models was so high that it took time to understand the whole picture. It was found that the drag perturbation ultimately needed to be taken into account, which came as a surprise to navigators. Periodic refinements in Titan’s ephemeris and gravity parameters are also a large contribution to the improvement in the models.
Predictions for the RCS thrust is derived from calibrated flight simulation data, for which navigators developed a precise acceleration model. Telemetry data is used after the fact to further refine the thrust profile, providing an accurate reconstruction of the flyby.
The Titan J2 and Drag curves in Figure ?? both increase as a function of altitude, as expected, and are modeled with their usual mathematical representations. They, however, have a clearly different profile. The drag force becomes predominant at closest approach, while the effect of the J2 perturbation is more diffuse. This allows us to estimate a drag force, with a suitable a priori model and uncertainty for the thrust and J2. The paper will discuss in mode details how the gravity harmonics and the atmospheric drag are modeled and how the data compare with science results and different encounters.
Due to the numerous aspects involved in the development of the models, it is unlikely that we would have time to present the subject in an open forum to generate a discussion that can result in constructive inputs. For that reason, the authors do not wish to open up this subject for discussion, but would rather present to the community how progress has been made.
- Abstract document
- Manuscript document
(absent)