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Down on the second floor of Building 264 and the first floor of Building 301, one can find some of the hardest working members of the Galileo Project--the Trajectory Group. These eleven individuals are responsible for all aspects of trajectory design, including developing necessary supporting software, for the Galileo mission. The group designs the path Galileo will take for both the interplanetary phase (from Earth departure to arrival near Jupiter) and the orbital phase (the Jovian satellite tour).
Major mission changes since the Project began in 1977 have caused the trajectory to be redesigned many times. The limits on the trajectory design caused by the shuttle accident and subsequent loss of the Centaur upper stage were particularly challenging.
"It wasn't straightforward," says Louis D'Amario, Trajectory Group Leader, "but at least a solution could be found. The first thing we did was to ignore some long-established Project constraints, such as not flying the spacecraft closer to the Sun than the Earth's orbit, and we considered every trajectory mode that could possibly increase trajectory energy to compensate for the reduced capability of the IUS (Inertial Upper Stage). Several brainstorming sessions took place attended by Dennis Byrnes, Roger Diehl, Bob Mitchell, and me. After discarding a number of ideas that involved various combinations of Venus, Earth and Mars flybys, the VEEGA option emerged. The next thing we did was to understand the launch-arrival characteristics of the new mission by generating lots, and lots, and lots of trajectories."
Most of the complex programs which the Trajectory Group members utilize were developed by them for Galileo. (Some of this new software is already being used for future projects such as CRAF and Cassini.) The goal of the trajectory design programs is to determine the interplanetary path to (or the satellite tour at) Jupiter that minimizes propellant usage while satisfying science, engineering, and mission operations constraints.
Using PLATO (Planetary Trajectory Optimization), group members optimize the design of interplanetary trajectories with planetary flybys, while satisfying constraints on the spacecraft's departure velocity from Earth, flyby conditions, and maneuver parameters. PLATO is the workhorse for Galileo's interplanetary trajectory design. Since PLATO can handle asteroid flybys, it is also used to evaluate potential asteroid candidates.
Without PLATO, the Project would have been unable to survive the many delays and launch vehicle changes that have redefined the mission so often. Lou D'Amario emphasizes, "PLATO allows us to consider complex interplanetary trajectories that offer the only means for satisfying restrictive launch vehicle energy constraints and limited spacecraft propellant capacity."
Designing a satellite tour is similar to designing an interplanetary trajectory, but even more complex. First, STOUR (Satellite Tour) defines the possible satellite tours given basic science requirements and various mission constraints. Producing a viable tour with STOUR requires a great deal of ingenuity from the tour designer. This is a highly constrained process, and maximizing the constraints imposed by the Project, the spacecraft design, and the laws of celestial mechanics is one of the most difficult Galileo trajectory design challenges.
Once a viable tour is put together with STOUR, then MOSES (Multiple Orbit Satellite Encounter Software) is used to minimize the total delta-V (changes in velocity) and, hence, the total propellant required. At the same time, MOSES constrains changes in satellite flyby conditions to meet requirements imposed by science, navigation, and mission operations. In addition to optimizing the STOUR design, MOSES utilizes a much better trajectory model. MOSES has two modes--a fast reasonably accurate mode and a slow highly accurate mode, which is nearly equivalent to precision numerical integration, a mathematical process for solving differential equations.
A large, complex, highly interactive program, MOSES solves an extremely difficult trajectory optimization problem. The number of optimization parameters and constraints is large, and the trajectory sensitivities of a ten-orbit satellite tour make the optimization process tricky. A great deal of skill is required to use MOSES effectively. Galileo's complex, science-intensive Jovian tour would not be possible without the optimization capability provided by MOSES because the propellant needed for the tour would greatly exceed what is available.
The final step in this design process is to reproduce the fully optimized trajectory with a numerical integration program called FAST. Although FAST has no optimization capability, it has the most accurate trajectory model.
When the Trajectory Group is asked to investigate new mission options, the analysis proceeds through a sequence of stages that, by now, has become quite familiar. First, relevant interplanetary trajectory options are identified using quick and approximate trajectory design methods. Then, PLATO is used to determine which options are feasible given spacecraft and launch vehicle performance limits.
Once a new baseline is selected, the group begins a detailed analysis by mapping out the launch-arrival space. This entails generating a large number of interplanetary trajectories with PLATO at even intervals of launch and arrival dates. These trajectories create a database for evaluating mission performance and understanding trajectory characteristics.
The group then determines the "accessible" region of the launch-arrival space, which encompasses performance constraints from both the launch vehicle (required energy within its capability) and the spacecraft (required propellant not to exceed onboard capacity). At this point, the launch period and the range of possible arrival dates at Jupiter will be known. At the same time, the Probe mission is being characterized by Larry Bright, who investigates Probe trajectory characteristics, Probe/Orbiter relative geometries, and Probe-to-Orbiter radio relay link performance as a function of arrival date.
A search then begins for asteroid flyby candidates. A large number of trajectories are checked for relatively close encounters with about 4600 asteroids and 100 comets. Dennis Byrnes and Jennie Johannesen, specialists in interplanetary trajectory design and satellite tour optimization, analyze these results to identify the best asteroids or comets for further consideration. Then, PLATO is used to evaluate the propellant cost to include such close asteroid flybys.
At about this point, one or more precise reference interplanetary trajectories are generated with FAST. These trajectories are used for navigation analysis and mission planning. Brian Nolan, in addition to assisting with interplanetary trajectory design, is responsible for integrating these trajectories.
The next phase of mission analysis focuses on arrival-date selection, for which a considerable amount of tabular data and plots are generated to support science and engineering evaluations. Any arrival date constraints based on Probe mission requirements and avoidance of solar conjunction by key mission events are identified at this time. Cynthia Wilson, co-op students Sun Hur and Brian Keller, and software developer Ed Rinderle all contribute to trajectory data generation and analysis.
During the arrival-date selection process, several satellite tours are designed (with STOUR), optimized (with MOSES), and integrated (with FAST) to demonstrate that acceptable tours exist over the range of potential Jupiter arrival dates. James Longuski and Aron Wolf are responsible for the STOUR phase of satellite tour design.
Recently, all of these efforts culminated in the selection of December 7, 1995 as Galileo's arrival date at Jupiter and Gaspra, Ida, and Beatty as the available asteroid targets. An exciting result of this effort is that for about the first two weeks of the launch period, both Gaspra and Ida can be encountered on the same trajectory.
The next major milestone for the Trajectory Group is the generation of a set of optimized precision interplanetary trajectories. These trajectories provide to the Boeing Aerospace Company the Earth-departure targets to be achieved by the IUS for each day of the launch period.
The Trajectory Group works closely with the spacecraft, navigation, and science teams to determine what Galileo can accomplish within the constraints imposed upon the mission and the conflicting requirements of various Project teams. The Trajectory Group, with the aid of programs they have developed, designs trajectories that allow for a feasible and scientifically exciting mission for Galileo.
Group members are looking forward to Galileo's launch with great anticipation. After launch, they will be responsible for reoptimizing the trajectory in order to minimize the propellant required to complete the mission, thus maximizing potential science return.
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