Design of Low-Thrust Trajectories for the Exploration of the Outer Solar System

The paper analyses the use of a Nuclear Electric Propulsion systems for missions to the outer part of the solar system and points out the possibility to deploy a probe while performing a gravity assist manoeuvre in the vicinity of Jupiter or to insert the spacecraft in a highly elliptical orbit about Pluto. The design of the trajectories has been performed with a direct transcription method by finite elements in time. As the problem presents quite a number of possible solutions dependent on launch window, transfer time and the use of gravity assist manoeuvres, a global optimization strategy has been used to procure sets of promising initial guesses. These initial guesses have been subsequently optimized using direct transcription and NLP. INTRODUCTION Currently most of our knowledge about Pluto and its moon Charon comes from indirect clues. No spacecraft has ever visited either of them, and from the Earth (or its proximity) its angular size is close about resolving limit of the most capable ground and space-based observatories. However, there is unanimity on the scientific interest of the Pluto-Charon system. Therefore a mission to the Pluto-Charon system and eventually to a Kuiper Belt Object (KBO) will significantly increase our knowledge of the formation and evolution of the Solar System as well as the origin of volatiles and organic molecules that enabled the appearance of life on our own planet. The mission therefore has a strong exobiological interest, which could be increased exponentially by adding new elements like a Europa or Titan microprobe deployment on route to the final destination, taking advantage of the opportunities provided by the gravity assist at the giant planets. In this paper some possible mission scenarii for a mission to Pluto and the Kuiper belt are proposed: these include the utilization of advanced propulsion systems (nuclear electric propulsion) and power technologies and the possibility to deploy a probe while a gravity assist maneuver in the vicinity of Jupiter is performed. If the Jupiter option is selected the possibility of a swing-by of one of the moons is investigated. In particular a swing-by of Ganymede can be performed to brake the probe while the main spacecraft continues on its way to Pluto. In addition the possibility of advanced missions using chemical propulsion ad gravity or aero-gravity assist maneuvers have been studied. Analyses available in the literature propose to carry out a very quick flyby of Pluto and Charon with a large relative velocity, thus enabling a limited science return. Another option is therefore to study the possibility to alter the mission analysis concept in such a way that the flyby velocity can be reduced. The design of the NEP trajectories has been performed with a direct transcription method by finite elements in time. However the problem presents quite e number of possible solutions dependent on launch window, transfer time and combination of planetary encounters, therefore in order to find favourable launch windows and the optimal sequence of swing-bys a global optimization strategy has been used to procure sets of promising initial guesses. Then, these initial guesses have been optimized using direct transcription and NLP. TECHNOLOGICAL AND SCIENTIFIC OBJECTIVES A mission to the Pluto-Charon system would significantly increase our knowledge of the formation and evolution of the Solar System. Of particular interest in the Pluto-Charon system is the atmospheric transfer of methane between these bodies and their compositional difference. 54th International Astronautical Congress of the International Astronautical Federation, the International Academy of Astronautics, and the International Institute of Space Law 29 September 3 October 2003, Bremen, Germany IAC-03-A.P.14 Copyright © 2003 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Released to IAF/IAA/AIAA to publish in all forms. 2 Some of the driving scientific objectives of such a mission would include: • Surface chemical composition • Surface morphology • Atmospheric chemistry • Gravimetry A strawman payload to achieve the scientific objectives within the allocated mass limits include: • Imaging X-ray Spectrometer • Wide / Narrow Field Imager • IR-Spectrometer • Radio Science Experiment The available payload mass obviously depends on the mission scenario. However it is reasonable to assume that even a limited payload mass value (e.g. 20 kg), would be sufficient to meet a significant part of the scientific goals. This should be achievable even with current technology and considering the heritage of other planetary exploration missions like SMART-1. The scientific return from a Pluto mission would increase tremendously if either the fly-by would occur at a small relative velocity or if the spacecraft could go into orbit around the PlutoCharon system. This would not only increase the coverage but also the accuracy of the scientific investigations because of the low signal to noise ratio for certain instruments partly due to the large distance from the sun. From a technological point of view, inserting a probe in orbit around such a distant planet at a reasonable propellant expenditure, poses considerable issues both in terms of propulsion and power. Furthermore the long travel results in a long waiting time before any result can be obtained. Since access to distant targets as Pluto can be effectively achieved through a swing-by of Jupiter, the delivery of a probe in the jovian system will significantly improve the scientific return of the mission offering some intermediate results while waiting for the analysis of the Pluto-Charon system. MISSION ANALYSIS Two missions have been studied: a double probe to Jupiter and Pluto, a small probe to Europa. In both cases the spacecrafts are equipped with nuclear electric propulsion (NEP) engines and will perform a number of gravity assist manoeuvres to reach the final destination. Initial Guess Generation The aim is to find an optimal sequence of transfers from the Earth to Pluto or from Earth to Jupiter passing by a predefined number of intermediate stops (actually swingbys). Even though the propulsion system is electric and not chemical, the trajectory, which minimizes the overall cost in terms of ∆v, is regarded as optimal for both means of propulsion since a further optimization with a better model of electric propulsion will be performed using DITAN (a software tool for the design of gravity assist lowthrust trajectories, developed by Politecnico di Milano under ESA contract). Each arc connecting two subsequent bodies has a deep space ∆v manoeuvre at an unknown point in time and space, each swingby is modelled collapsing the sphere of influence in one single point in space with radius rp linking the transfer arcs ri before and ro after the swing by, thus the following relation must hold: