EARTH TO HALO ORBIT TRANSFER TRAJECTORIES. A Thesis. Submitted to the Faculty. Purdue University. Raoul R. Rausch. In Partial Fulfillment of the

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1 EARTH TO HALO ORBIT TRANSFER TRAJECTORIES A Thesis Submitted to the Faculty of Purdue University by Raoul R. Rausch In Partial Fulfillment of the Requirements for the Degree of Master of Science August 2005

2 ii ACKNOWLEDGMENTS This research topic has been challenging and frustrating at times, rewarding and fulfilling at others. I want to thank my advisor Professor Howell for her continuous support and guidance and the seemingly infinite patience. The advice and recommendations she provided on reviews of this thesis, certainly surpassed anything that could have been expected. I also wish to thank the other members of my graduate committee, Professors James M. Longuski and Martin Corless for their advice and reviews of this thesis. I am grateful to all the past and present members of my research group. They have provided much support and guidance and without their contributions, this work would have been difficult to complete. They have helped me enhancing my understanding of the three-body problem and inspired me to think about the technical issues at hand more globally. Additionally, I would like to thank my parents and my wife, Nicole, for their continuous support. Their combined energy has given me at times the extra motivation, strength and confidence necessary to complete this work. Finally, I wish to thank those who have provided the funding for my graduate studies. For the last three years, I have been funded by the German section of the Purdue School of Foreign Languages and Literatures. Teaching German has been an enlightening and educational experience for me.

3 iii TABLE OF CONTENTS Page LIST OF TABLES vi LIST OF FIGURES vii ABSTRACT x 1 INTRODUCTION Problem Definition Previous Contributions Historical Overview Transfer Trajectories Present Work BACKGROUND: MATHEMATICAL MODELS Reference Frames Inertial Frame P 1 P 2 Rotating Frame Earth Centered Fixed Frame Transformations Between Different Frames Nondimensionalization Equations of Motion Singularities in the Equations of Motion State Transition Matrix Differential Corrections Particular Solutions Invariant Manifold Theory Brief Overview Periodic Orbits and Dynamical Systems Theory

4 iv Page 2.9 Computing Manifolds Transition of the Solution to the Ephemeris Model TRANSFERS FROM EARTH PARKING ORBITS TO LUNAR L 1 HALO ORBITS Stable and Unstable Flow that is Associated with the Libration Point L 1 in the Vicinity of the Earth Design Strategy Shooting Technique Investigated Transfer Types Two-Level Differential Corrector First Step - Ensuring Position Continuity Second Step - Enforcing Velocity Continuity Constraints Position and Epoch Constraints Parking Orbit Constraints v Constraints Direct Transfer Trajectories from Earth to Lunar to L 1 Halo Orbits Transfer Trajectories with a Manifold Insertion Effects of a Cost Reduction Procedure Free Return Trajectory Summary and Conclusions Numerical versus Dynamical Issues in the Computation of Transfers Discussion TRANSFERS FROM EARTH PARKING ORBITS TO SUN-EARTH LIBRATION POINT ORBITS Stable Flow from the Libration Points in the Direction of the Earth Selection of Halo Orbit Sizes Transfer Trajectories From Earth to L 1 Halo Orbits

5 v Page 4.4 Transfer Trajectories From Earth to L 2 Halo Orbits Conclusions LAUNCH TRAJECTORIES Equation of Motions with Constant Thrust Term State Transition Matrix Patch Points and Initial Trajectory Determination of the Launch Site Two-Level Differential Corrector with Thrust Two-Level Differential Corrector with Thrust Trajectory from Launch Site into Parking Orbit Challenges with the Launch Formulation Conclusion SUMMARY AND RECOMMENDATIONS Summary Recommendations and Future Work Concluding Remarks LIST OF REFERENCES

6 vi Table LIST OF TABLES Page 3.1 Transfer Costs for Two Differently Sized Halo Orbits; TTI Maneuver Constrained to the x z Plane Crossing Transfer Costs for Two Differently Sized Halo Orbits; Location of the TTI Maneuver Determined by the Differential Corrections Scheme Transfer Costs for Transfers with a Manifold Insertion for Two Differently Sized Halo Orbits Transfer Costs for Transfers from a 200 km Altitude Earth Parking Orbit to Two Differently Sized Sun-Earth L 1 Halo Orbits Transfer Costs for Transfers from a 200 km Altitude Earth Parking Orbit to Two Differently Sized Sun-Earth L 2 Halo Orbits Changes in Thrust Parameters Throughout Trajectory Arc

7 vii Figure LIST OF FIGURES Page 2.1 Geometry of the Three-Body Problem Zero Velocity Curve for C = A Stylized Representation of 1 Step Differential Corrector Location of the Libration Points in the Earth-Moon System Relative to a Synodic Frame Northern Earth-Moon L 1 Halo Orbit in the CR3BP Lissajous Trajectory at L 1 in the Sun-Earth CR3BP Stable and Unstable Eigenvectors and the Globalized Manifold for the Earth-Moon L 1 Point Stable Eigenvectors of the Monodromy Matrix for an Earth-Moon L 1 Halo Orbit Position and Velocity Components of the Stable Eigenvectors of the Monodromy Matrix for an Earth-Moon L 1 halo Various Manifolds Asymptotically Approaching the Orbit Stable (blue) and Unstable (red) Manifolds for a Sun-Earth Halo Orbit near L Stable (blue) and Unstable (red) Manifold Tube Approaching the Earth in the Sun-Earth System A Halo-like Lissajous Trajectory in a Ephemeris model with an A z Amplitude of approximately 15,000 km Stable and Unstable Manifold Tubes in the Vicinity of the Earth Minimum Earth Passing Altitudes for Trajectories on the Manifold Tubes Associated with Various Earth-Moon L 1 Halo Orbits A Stylized Representation of Level II Differential Corrector (from Wilson [1]) Transfer from an Earth Parking Orbit to a L 1 Halo Orbit (A z = 15, 000 km) in the CR3BP (Location of HOI maneuver is constrained)

8 viii Figure Page 3.5 Transfer from an Earth Parking Orbit to a L 1 Halo Orbit (A z = 43, 800 km) in the CR3BP (Location of HOI maneuver is constrained) Transfer from an Earth Parking Orbit to a L 1 Halo Orbit (A z = 15, 000 km) in an Ephemeris Model (Location of HOI Manuever Constrained) Transfer from an Earth Parking Orbit to a L 1 Halo Orbit (A z = 43,800 km) in an Ephemeris Model (Location of HOI Manuever Constrained) Transfer from an Earth Parking Orbit to a L 1 Halo Orbit (A z = 15,000 km) in the CR3BP (Location of the TTI Maneuver Determined by the Differential Corrections Scheme.) Transfer from an Earth Parking Orbit to a L 1 Halo Orbit (A z = 43,800 km) in the CR3BP (Location of the TTI Maneuver Determined by the Differential Corrections Scheme.) Transfer from an Earth Parking Orbit to L 1 Halo Orbits (A z = 15,000 km) in an Ephemeris Model (Location of the TTI Maneuver Determined by the Differential Corrections Scheme.) Transfer from an Earth Parking Orbit to L 1 Halo Orbits (A z = 43,800 km) in an Ephemeris Model (Location of the TTI Maneuver Determined by the Differential Corrections Scheme.) Transfer from an Earth Parking Orbit with a Manifold Insertion into an L 1 Halo Orbit (A z = 15,000 km) in the CR3BP Transfer from an Earth Parking Orbit with a Manifold Insertion into an L 1 Halo Orbit (A z = 43,800 km) in the CR3BP Transfer from an Earth Parking Orbit with a Manifold Insertion into an L 1 Halo Orbit (A z = 15,000 km) in an Ephemeris Model Transfer from an Earth Parking Orbit with a Manifold Insertion into an L 1 Halo Orbit (A z = 43,800 km) in an Ephemeris Model Effects of a Cost Reduction Procedure on the Transfer Arcs Initially Using the Invariant Manifold on the Near Earth Side for a Halo Orbit with an A z Amplitude of 15,000 km in an Ephemeris Model. 74

9 ix Figure Page 3.17 x y Projection of a Free Return Trajectory to a Halo Orbit with an A z Amplitude of 15,000 km in an Ephemeris Model x z Projection of a Free Return Trajectory to a Halo Orbit with an A z Amplitude of 15,000 km in an Ephemeris Model y z Projection of a Free Return Trajectory to a Halo Orbit with an A z Amplitude of 15,000 km in an Ephemeris Model Closest Approach Altitudes for L 1 Sun-Earth Manifolds Relative to the Earth Transfer from an Earth Parking Orbit to a L 1 Halo Orbit (A z = 120, 000 km) in the CR3BP Transfers from an Earth Parking Orbit to a L 1 Halo Orbit (A z = 440, 000 km) in the CR3BP Transfers from an Earth Parking Orbit to a L 1 Halo Orbit in an Ephemeris Model Transfer from an Earth Parking Orbit to a L 2 Halo Orbit (A z = 120, 000 km) in the CR3BP Transfers from an Earth Parking Orbit to a L 2 Halo Orbit (A z = 440, 000 km) in the CR3BP Transfers from an Earth Parking Orbit to a L 2 Halo Orbit in an Ephemeris Model Spherical Coordinates to Define the Direction of the Thrust Vector Launch Trajectory from Kourou to a 200 km Altitude Parking Orbit Launch Trajectory from Kourou to a 200 km Altitude Parking Orbit Launch Trajectory from Kourou to a 200 km Altitude Parking Orbit. 103

10 x ABSTRACT Rausch, Raoul R. M.S., Purdue University, August, Earth to Halo Orbit Transfer Trajectories. Major Professor: Kathleen C. Howell. Interest in libration point orbits has increased considerably over the last few decades. The Lunar L 1 and L 2 libration points have been suggested as gateways to Sun-Earth libration points and to interplanetary space. The dynamics in the vicinity of the Earth in the Earth-Moon system, where the Earth is the major primary in a three-body model, has only been of limited interest until recently. The new lunar initiative is the origin of a wide range of studies to support the infrastructure for a sustained lunar presence. A systematic and efficient approach is desirable to ease the determination of viable transfer trajectories satisfying mission constraints. An automated process is, in fact, a critical component for trade-off studies. This work presents an initial approach to develop such a methodology to compute transfers from a launch site or parking orbit near the Earth to libration point orbits in the Earth-Moon system. Initially, the natural dynamics in three-body systems near both the smaller and larger primaries are investigated to gain insight. A technique using a linear differential corrections scheme is then developed. Initial attempts to incorporate the invariant manifolds structure in designing these transfers are presented. Simple transfers in the Earth-Moon system are computed and transitioned to an ephemeris model. The methodology is also successfully applied in the Sun-earth system. Challenges are discussed. The second task involves the determination of launch trajectories. A two-level differential corrections technique incorporating a constant thrust term is developed and a sample launch scenario is computed. Limitations and extensions are considered.

11 1 1. INTRODUCTION Today, humankind continues to explore a new frontier as civilization expands into space. In the near term, plans include a potential return to the Moon with both robotic vehicles as well as human crews; an extended stay on the lunar surface or in the vicinity of the Moon is possible. In addition to the exploration of the Moon (and Mars), many other missions and observatories have been proposed that will make use of libration point orbits, such as the James Webb Space Telescope [2] (formerly known as the Next Generation Telescope), the Terrestrial Planet Finder [3] and the Europa Orbiter Mission [4]. From these various scenarios, some of the new observatories will take advantage of the prime location offered by the Sun-Earth libration points at L 1 and L 2 and of the efficient low-energy trajectories that are available throughout the solar system. These low energy pathways are defined via the manifolds associated with libration point orbits in all Sun-Planet and Planet-Moon three-body systems. These pathways can be exploited due to only minor energy differences [5] between libration point manifolds in different systems. The entire system of tunnels is made possible by the chaotic environment resulting from multiple gravity fields. As stated by Lo et al. [5]...the tunnels generate deterministic chaos and for very little energy, one can radically change trajectories that are initially close by. This statement effectively summarizes the physical basis and practicality of libration point trajectories. Future space observatories can take advantage of the proximity of the Earth-Moon libration points L 1 and L 2 and use them as inexpensive gateways [6] to the Sun-Earth libration points L 1 and L 2 and to interplanetary space. The relatively short distance between the Earth and the Earth-Moon lunar libration point L 1 makes human servicing of observatories possible [7]. A spacecraft can be delivered to this location within a week from the Earth and within hours from the Moon s surface [5]. In addition to

12 2 easy accessibility, spacecraft in an orbit near the Earth-Moon L 1 libration point can be continuously monitored from Earth. The first spacecraft to make use of a libration point orbit was the International Sun-Earth Explorer (ISEE-3) launched on November 20, 1978 [8]. The mission was very successful at monitoring the solar winds. Subsequently, the vehicle was rerouted to explore the Earth s geomagnetic tail region before shifting to a new trajectory arc that ultimately encountered the comet Giacobini-Zinner. The spacecraft was renamed the International Cometary Explorer (ICE) [9] and, after a few close encounters with both the Earth and the Moon, ICE reached the comet on schedule. Additional spacecraft have also been launched as part of successful libration point missions, including WIND [10], SOHO [11], ACE [12], Genesis [13], and MAP. The WIND, SOHO, and ACE missions were all part of the International Solar-Terrestial Physics project. Genesis was the first mission to exploit dynamical systems theory in the design and planning phases. This new analysis concept allowed the Genesis spacecraft to take advantage of the complex dynamics in a multi-body regime, that is, the Sun-Earth-Moon gravitational fields, and to fulfill the mission requirements with a completely ballistic baseline trajectory [5] that required no maneuvers. Genesis also returned the first samples of solar wind particles to the Earth from beyond the local environment. 1.1 Problem Definition The libration points L 1 and L 2 have been proposed as inexpensive gateways [5 7] for performing transfers between different three-body systems. Many different types of libration point trajectories are of current interest with the potential for inexpensive access to interplanetary space. The concept of system-to-system transfers was suggested as early as 1968 [14]. New techniques, for example, the use of the invariant manifold structure associated with three-body systems, that simplify the design of system-tosystem transfer are currently in development by various researchers [6, 15, 16]. How-

13 3 ever, servicing and repair missions to observatories in libration point orbits (LPO) must meet very specific requirements such as time of arrival and precise target orbit requirements [7]. Libration point missions involving humans must also have feasible Earth return options in case of emergency. Meeting such requirements can quickly become a nearly impossible task. Therefore, future libration point missions will require new and innovative design strategies to fulfill the mission requirements at low fuel costs while satisfying an increasingly complex set of constraints. Two types of libration point orbits commonly investigated for applications are the three-dimensional precisely periodic halo orbits and the quasi-periodic Lissajous trajectories. Much progress has been made in the last decade by developing design strategies based on these orbits and their associated manifolds. One problem that influences every spacecraft is the launch. For libration point missions, in particular, designing the launch leg, from a site on the Earth s surface to a transfer trajectory that delivers the vehicle into a libration point orbit, is challenging. Traditionally, spacecraft enroute to libration point orbits have been first launched into low Earth parking orbits, then depart along their transfer path to insert into a baseline halo orbit or Lissajous trajectory. For preliminary analysis, this approach allows the use of traditional two-body analysis tools to determine the trajectory arc from the launch site into the parking orbit. Both the Earth-Moon and Sun-Earth systems present unique challenges. Regardless of the particular three-body system, determining any solution in an ephemeris model without good baseline approximations is a tremendously complex task. Simplified models are usually employed to obtain baseline solutions. However, using a two-body model limits the solution-space considerably when the seemingly chaotic motion present in a multi-body system is lost. To expose potential non-conic solutions, a three-body model is therefore necessary. But, there are no known closedform solutions to the three-body problem (3BP). Some simplifying assumptions do result in a model that retains the significant features of three-body motions; slight modifications then offer a very useful formulation. The first assumption is one con-

14 4 cerning the masses; that is, the bodies are all assumed spherically symmetric, and thus point masses. In fact, assuming one particle to be infinitesimal is key for another approximation. The Restricted Three-Body Problem (R3BP) consists of two massive particles moving in undisturbed two-body orbits about their common center of mass, the barycenter. It is assumed that the infinitesimal particle does not affect the orbits of the primaries. If the relative two-body orbit of the primaries is assumed elliptic, the resulting model is called the Elliptic Restricted Three-Body Problem (ER3BP). If the two-body orbit of the primaries is assumed circular, the Circular Restricted Three-Body Problem (CR3BP) yields even more insight into the motion. Although the model defined as the CR3BP is the key to the solution of interest, the design process remains challenging. Efficient and effective tools for the computation of transfer trajectories from the Earth to libration point orbits in both the Sun-Earth and the Earth-Moon systems are of increasing interest. In the three-body Sun-Earth-spacecraft system, the Earth is the smaller of the two primaries whereas in the three-body Earth-Moon-spacecraft system, the Earth is the larger of the primaries. This difference is significant in the fundamental dynamical structure of the solution space. The role that a planet, here the Earth, plays in a simplified model as the larger or the smaller primary can have a large influence on the behavior and characteristics of the solutions. First, the natural flow approaching and departing libration point orbits in both cases requires investigation. A deterministic maneuver will always be required to leave the Earth parking orbit. Additional maneuvers to insert into the halo orbit, or into the transfer trajectory, may be necessary to satisfy design requirements. Finally, tools to compute thrusting arcs from the launch site to the transfer path require development.

15 5 1.2 Previous Contributions Historical Overview Investigations in the 3BP from a dynamical perspective began with Newton. His development of the laws of motion and the concept of a gravity force, first published in the Principia [17], enabled the formulation of a mathematical model. At the time, predicting the motion of the Moon was of great interest with applications in naval navigation [18]. Newton, of course, solved the two-body problem after reformulating it as a problem in the relative motion of two bodies. But, to accurately predict the Moon s orbit, perturbations had to be incorporated. This led to the formulation of the 3BP. A complete solution to the gravitational 3BP requires 18 integrals of motion. Since only a total of ten integrals of motion exist, as later determined by Euler [19], a closed-form solution was not straightforward. Six of these scalar integrals result from the conservation of linear momentum, three from the conservation of total angular momentum, and one integral results from the conservation of energy. In his investigations of the 3BP formed by the Sun, the Earth, and the Moon, Newton nevertheless managed to compute the motion of the lunar perigee to within eight percent of the observed value in Continuing Newton s work, Leonard Euler proposed the highly special problem of three bodies known as the problem of two fixed force-centers, solvable by elliptic functions, in 1760 [20]. Euler was also the first to formulate the restricted three-body problem (R3BP) in a rotating frame. The formulation was very significant. This step allowed him to predict the existence of the three collinear equilibrium points L 1, L 2, and L 3 [20]. Lagrange confirmed Euler s prediction in his memoir Essai sur le problème des Trois Corps published in Lagrange deduces the existence of the collinear points and solves for two additional equilibrium points. These additional two equilibrium points each form an equilateral triangle with the primaries and are generally labelled the equilateral libration points L 4 and L 5. The five points are commonly denoted Lagrange or libration points. For their work, Euler and Lagrange shared the Prix de l Academie de Paris in 1772 [18].

16 6 In the pursuit of additional integrals, Jacobi considered the concept of the R3BP relative to a rotating frame. In 1836, he determined a constant of the motion by combining conservation properties of energy and angular momentum [21]. The constant he discovered now carries his name [18]. In other research efforts, Hill s Lunar Theory, published in 1878, represents the result of an investigation of a satellite s orbit around a larger planet under the influence of solar and eccentric perturbations. Specifically, Hill was interested in modelling the lunar orbit with the simplifying assumptions that the solar eccentricity and parallax, as well as the Moon s orbital inclination, are all zero [21]. Hill s work was revolutionary; he used the CR3BP as the base model and was the first person to abandon a two-body analysis [22]. In his theory, Hill demonstrated that for a specified energy level, regions of space exist where motion is not physically permitted [18]. Toward the end of the 19 th century, Poincaré studied the 3BP seeking additional integrals of the motion. Poincaré predicted an infinite number of periodic solutions if two of the masses are small compared to the third [23]. Poincaré believed that the primary problem in celestial mechanics was the behavior of orbits as time goes to infinity [24] and focused on qualitative aspects of the motion. The planar R3BP was of particular interest to him because it could be formulated as a Hamiltonian-like system. Determined to investigate the problem further, he invented an analytical technique called the surface of section [24] that allowed him, in 1892, to describe the phase space of a non-integrable system. Through his contributions to dynamics and the invention of the method of the surface of section, Poincaré is widely regarded as the father of Dynamical Systems Theory (DST). In 1899, he proved that Jacobi s Constant is the only integral of motion that exists in the R3BP. Any other integral would not be an analytical function of the systems coordinates, momenta, and the time [24]. Poincaré s insight into the 3BP, his contributions to mathematics, and the eventual advent of high-speed computing makes most of today s work possible. At the time, his findings also resulted in a shift in the focus of research in the 3BP toward determining specific trajectories rather than the general behavior.

17 7 At the beginning of the 20 th century, Darwin, Plummer, and Moulton were all seeking periodic orbits in the vicinity of the libration points [23, 25 28]. In 1899, Darwin computed several approximate planar periodic orbits in the CR3BP using a quadrature method. Plummer also accomplished the same feat in 1902 using an approximate, second-order analytical solution to the equations of motion in the CR3BP. Between 1900 and 1917, Moulton developed several approximate analytical solutions to the linearized equations of motion relative to the collinear points. Moulton s solutions result in planar as well as three-dimensional periodic orbits [21]. Although a series solution to the general three-body problem was produced by Sundman in 1912 [19,29], it is useless for any practical purposes. Further exploration into Libration Point Orbits (LPO) were hindered by the computational requirements. Only with the introduction of high-speed computers in the 1960 s did significant progress occur. In the late 1960 s, interest in the three-body problem (3BP) increased significantly. In 1967, Szebehely published a book summarizing all information to date concerning the 3BP [21]. His compilation contained numerically integrated, periodic orbits in the planar CR3BP and the planar ER3BP as well as a few threedimensional notes. Motivated by new mission possibilities at NASA, Farquhar developed analytical approximations for three-dimensional periodic orbits in the translunar Earth-Moon region [14] in the late 1960 s. Farquhar coined the term halo orbits to describe periodic three-dimensional orbits in the vicinity of the collinear libration points because, when viewed from the Earth, they appear as a halo around the Moon. Whereas halo orbits are precisely periodic in the CR3BP, Lissajous trajectories are quasi-periodic. Lissajous figures, in general, are named after the French physicist Jules A. Lissajous ( ) because planar projections of these curves look similar to those studied by Lissajous in 1857 [9]. In 1972, Farquhar and Kamel developed a third-order approximation for quasi-periodic motion near the translunar libration point using a Lindstedt-Poincaré method [30]. Heppenheimer developed a third-order theory for nonlinear out-of-plane motion in the ER3BP in 1973 [31].

18 8 Two years later, Richardson and Cary [32] obtained a third/fourth order approximation for three-dimensional motion in the elliptic restricted three-body problem in the Sun-Earth/Moon barycenter system. An analytical approximation for halo-type periodic motion about the collinear points in the Sun-Earth CR3BP was published by Richardson in Breakwell and Brown numerically extended the work of Farquhar and Kamel to yield a family of numerically integrated periodic halo orbits [33]. The discovery of stable halo orbits in that family motivated future research in the halo families near all three collinear libration points by Howell [34] in collaboration with Breakwell. In 1990, Marchal published a book summarizing the more recent progress on the CR3BP [35] Transfer Trajectories Much progress has occurred over the last decade in the development of analysis strategies to determine transfer trajectories from the Earth to halo and Lissajous orbits near Sun-Earth L 1 and L 2 points. With few analytical tools, transfer design was initially dependent on numerical techniques not available until the 1960 s. The advent of high-speed computers made the computation of transfers possible and, hence, allowed the possibility of libration point missions. The first study published on transfer trajectories between a parking orbit and a libration point was by D Amario in 1973 [36, 37]. D Amario combined analytical and numerical techniques with primer vector theory to develop a fairly accurate method for the quick calculation of transfer trajectories from both the Earth and the Moon to the Earth-Moon libration point L 2. With his multiconic approach, D Amario determined families of locally optimal two-impulse and three-impulse transfers [36]. Subsequently, ISEE-3 was planned, of course, and successfully reached a Sun-Earth L 1 halo orbit. The transfer trajectory used for the ISEE-3 mission inserted the spacecraft into the halo orbit at the ecliptic plane crossing on the near-earth side. The transfer was categorized as slow because the transfer Time of Flight (TOF) was approximately 102 days versus the

19 9 approximately 35 days for expensive fast transfers. This type of transfer trajectory was selected because numerical studies had indicated that such a path is less costly in terms of V than a fast transfer trajectory to the halo orbit [8, 38]. In 1980, Farquhar completed a post-flight mission analysis of the flight data from ISEE- 3 [39]. Simó and his collaborators were the first to publish details of a methodology to use invariant manifold theory to aid in the design of transfer trajectories in A manifold approaches a periodic orbit asymptotically and so eliminates, in theory, any insertion maneuver cost. A year later, Hiday expanded primer vector theory to the 3BP and studied impulsive transfers between parking orbits and Libration Point Trajectories (LPT) near L 1 in the Sun-Earth system. An extensive numerical study using differential techniques was performed by Mains in 1993 [40]. Mains was interested in the development of approximations useful for future automated transfer trajectory determination procedures. Mains studied transfers from a variety of different parking orbits with different times of flight (TOF) including a transfer similar to that of the ISEE-3 spacecraft. Barden [28,41] later extended Mains investigations through a combination of numerical techniques and dynamical systems theory. In the mid-1990 s, Wilson, Barden, and Howell developed design methodologies used in the determination of the Genesis trajectory [42 44]. In 2001, Anderson, Guzmán, and Howell implemented an efficient procedure to investigate transfers from the Earth to Lissajous trajectories by exploiting lunar flybys in an ephemeris model [45, 46]. Thus far, most of the work has been focused on determining transfer trajectories from the Earth to the Sun-Earth or Sun-Earth/Moon libration point orbits. 1.3 Present Work The focus of this investigation is the continued development of techniques and strategies to determine transfer trajectories from the Earth to LPOs in both the Sun-Earth and the Earth-Moon systems. Most of the emphasis has been placed on the development of the methodology used in the transfer design process. A useful

20 10 dynamical element that is significant in the determination of transfer trajectories is the set of invariant manifolds approaching and departing periodic halo orbits. In the Earth-Moon system, manifolds do not pass close by the Earth (larger primary) and additional strategies are investigated. In the Sun-Earth system (where the Earth is the smaller primary) manifolds that pass in the immediate vicinity of the Earth are frequently available. All of the analysis in this work is conducted numerically and the Circular Restricted Three-Body Problem (CR3BP) is used as the fundamental dynamical model for baseline designs. The CR3BP is well-suited for qualitative analysis and the solution can easily be transferred into a more complex model making use of planetary ephemerides. A few sample transfer trajectories are presented to underline the validity of the CR3BP as the dynamical model. The first objective focuses on transfer trajectories from Earth parking orbits to halo and Lissajous trajectories located near L 1 in the Earth-Moon system. This will improve knowledge of the dynamical structure for trajectory arcs that depart from the major (first) primary. The goal is accomplished by considering both transfer arcs to manifolds as well as direct transfers into the halo orbits. A second objective is the development of a procedure to compute launch trajectories from a specific launch site on the surface of the Earth to an invariant manifold. Some initial results based on differential corrections algorithms are presented that can be used to determine the transfer arcs and launch trajectories of interest. The results are of a preliminary nature and the beginning of a more in-depth investigation. This work is arranged as follows: Chapter 2: This chapter summarizes the background material that underlies the foundations of this study. The different reference frames are introduced and the mathematical model used to represent the Earth-Moon and Sun-Earth dynamical environments, that is, the circular restricted three-body problem, is developed. Assumptions employed in this model as well as special properties are discussed, followed by a derivation

21 11 of the linear variational equations. A method to solve for periodic solutions is introduced and particular solutions of general interest are presented. Invariant manifold theory is introduced and the computation of manifolds is discussed. Chapter 3: The natural motion to and from halo orbits at L 1 in the Earth-Moon system is investigated. The numerical algorithm that forms the basis of this study is then presented and additional constraints are introduced. The numerical procedure is applied to a number of different problems and sample transfer trajectories from an Earth parking orbit to lunar L 1 LPOs are presented. Preliminary results in an ephemeris model are presented to demonstrate the validity of the obtained transfer arcs in the CR3BP. Chapter 4: The natural motion to and from halo orbits at L 1 in the Sun-Earth system is studied and a series of transfers from the Earth halo orbits at L 1 and L 2 are summarized. Chapter 5: A constant thrust term is added to the equations of motion. A modified version of the differential corrector exploits the thrust parameters to determine launch trajectories with discretely varying thrust angles. A sample launch trajectory to an Earth parking orbit is presented. Chapter 6: The work presented here is of a preliminary nature and more detailed investigations are required. A strategy for future work is presented and conclusions and recommendations for further investigations are discussed.

22 12 2. BACKGROUND: MATHEMATICAL MODELS The circular restricted three-body problem is the simplest model that can capture the seemingly chaotic motion present in a multi-body system. Since this dynamical structure is the focus in this study, the mathematical model is derived. Different reference frames are introduced and the linear variational equations are developed. The concept of differential corrections is introduced and particular solutions, such as equilibrium points, as well as periodic and quasi-periodic orbits are discussed. Some aspects of invariant manifold theory are presented and applied to illustrate the computation of invariant manifolds. The transition of solutions to an ephemeris model is also summarized. 2.1 Reference Frames A variety of different reference frames are used in this investigation and are useful for computation as well as visualization purposes. Their definitions and the associated notation is detailed here for clarity Inertial Frame Newton s laws, as stated in their most original form, are valid relative to an inertial reference frame. In this study, one inertial frame is defined to be centered at the Earth (geocentric). The Earth Centered Inertial (ECI) frame is assumed to be inertially fixed in space but, actually, is slowly moving over time. Since a truly inertial system is impossible to realize, the standard J2000 system [47] has been adopted as the best representation of an ideal, inertial frame at a fixed epoch. The shift of this frame is so slow relative to the motion of interest, it can be neglected.

23 13 The fundamental plane is the plane defined as the X Ŷ plane. The unit vector X is directed toward the vernal equinox, the unit vector Ŷ is rotated 90 degrees to the east in the ecliptic plane. The unit vector Ẑ is, then, normal to the plane defined by the X and Ŷ axes such that Ẑ = X Ŷ. (Note that a caret indicates a vector of unit magnitude.) A formulation of the problem relative to the inertial frame is not very convenient for investigations in the 3BP because the primaries are continuously moving and no constant or fixed equilibrium solutions exist P 1 P 2 Rotating Frame The P 1 P 2 rotating frame is the most convenient frame for visualization of the motion of the infinitesimal body, P 3, moving near the libration points. The barycenter of the two primaries that define the three-body system is typically used as the origin (See figure 2.1). The unit vector x is defined such that it is always parallel to a line between the primaries and is directed from the larger toward the smaller primary. The unit vector ŷ is 90 degrees from x in the plane of motion of the primaries; it is positive in the general direction of the motion of the second primary relative to the first. The unit vector ẑ is defined to complete a right handed coordinate system and is normal to the plane of motion spanned by x and ŷ. Only when the equations of motion in the CR3BP are formulated relative to the rotating frame of the primaries are the libration points the equilibrium solutions to the differential equations. This will be apparent later. Libration point orbits only exhibit their periodicity relative to this frame Earth Centered Fixed Frame The Earth Centered Fixed Frame (ECF) is fixed in the Earth as it rotates on its own axis. Thus, the ECF frame is useful when a specific launch site on the Earth surface is defined and launch trajectories are computed. The ECF origin is at the Earth center, and, thus, this geocentric coordinate system rotates with the Earth

24 14 Figure 2.1. Geometry of the Three-Body Problem. relative to the inertial frame. The primary axis of ECF is always aligned with a particular meridian, and the Greenwich meridian is very often selected. Since the coordinate system is rotating relative to the inertial frame, it is necessary to specify an epoch. In this model, the ECF frame is assumed to be aligned with the ECI frame at time t = 0. This simplification is equivalent to assuming that the Greenwich meridian is parallel to a line between the primaries at time t = 0. The rotation of the Earth is approximated as constant. The approximation assumes that the Earth completes precisely one revolution in a 24 hour period. By determining the angular velocity of the Earth, the alignment between the ECF and the inertial frame can be computed for any later time.

25 Transformations Between Different Frames Transformations between different rotating and inertial frames are critically important to correctly model this problem. The alignment between the inertial frame and the P 1 P 2 frame is illustrated in figure 2.1. Transforming the position state from an inertial frame to a rotating frame can be accomplished using the following rotation matrix, c θ s θ 0 inert A rot = s θ c θ 0, (2.1) where θ is the angle between the rotating and the inertial frame and appears in figure 2.1. The trigonometric symbols are defined as s θ = sin(θ) and c θ = cos(θ). Transforming the velocity state between a rotating and an inertial frame requires the use of the basic kinematic equation (BKE), ( ) ( ) d r d r = dt dt inertial rot + ω r, (2.2) where ω is the angular velocity vector and r is the position vector in inertial coordinates. (Note that overbars denote vectors.) The cross product ω r yields A(2, 1) A(2, 2) A(2, 3) inert Ȧ rot = θ r A(1, 1) A(1, 2) A(1, 3), (2.3) where r is the magnitude of the position vector. Thus, the transformation of the entire state vector from the rotating frame of the primaries to the inertial frame is represented rot T inert = AT 0 Ȧ T A T. (2.4) In an ephemeris model, the θ angle does not remain constant and must be computed instantaneously; the rate θ is also evaluated instantaneously. The inertial ˆX Ŷ plane represents the ecliptic plane, i.e., the plane of motion of the Earth about the Sun. The inertial axis Ẑ is normal to the ecliptic plane. The

26 16 Earth s axis of rotation is inclined relative to Ẑ. The angle that denotes the inclination of the Earth is i and is assumed to be constant such that i = 23.5 degrees. Thus, the transformation from the inertial to the inclined equatorial frame is written with T = eclip C equat 0 0 eclip C equat, (2.5) eclip C equat = 0 c i s i. (2.6) 0 s i c i This transformation matrix, i.e., equation (2.5), allows conversions of the state between the ecliptic, and the inertial equatorial plane. 2.3 Nondimensionalization To eventually generalize the derived equations, it is advantageous to non-dimensionalize and, thus, express fundamental quantities in terms of relevant system parameters. The characteristic dimensional quantities identified in the system are length, time, and mass. As illustrated in figure 2.1, the dimensional length between the barycenter and the first primary P 1 is labelled l 1 and the dimensional length between the barycenter and the second primary P 2 is labelled l 2. For the CR3BP, the reference characteristic length is defined as the distance between the two primaries, that is, l = l 1 + l 2, the semi-major axis of the conic orbit of the second primary relative to the first. The reference characteristic mass is defined as m = m 1 + m 2, where m 1 is the mass of the first primary and m 2 the mass of the second primary. This allows a definition of the characteristic time as t = where G is the dimensional gravitational constant. [ ]1 (l ) 3 / 2, (2.7) G (m 1 + m 2 ) In standard models for the restricted problem, this specific form is employed to select t such that the non-

27 17 dimensional gravitational constant G nd is equal to 1. All other quantities can now be evaluated in terms of these three characteristic values. The non-dimensional distance between the two primaries is 1. Kepler s third law yields the expression for the mean motion, or mean angular velocity, as [ ]1 G nd / (m 1 + m 2 ) 2 n =. (2.8) (l ) 3 As can be easily verified, the mean motion possesses a non-dimensional value of 1. The non-dimensional mass of the second primary is represented by the symbol µ, i.e., µ = m 2 m 1 + m 2. (2.9) As a consequence, the mass of the first primary is represented as 1 µ = m 1 m 1 + m 2. (2.10) The non-dimensional quantities corresponding to the remaining distance elements, as seen in figure 2.1, are defined as and Non-dimensional time is then defined ρ = r B3 l, (2.11) d = r 13 l, (2.12) r = r 23 l. (2.13) τ = t t. (2.14) Nondimensionalization allows a more convenient and general derivation of the equations of motion. 2.4 Equations of Motion A Newtonian approach is used to derive the equations of motion and, thus, it is necessary to begin with Newton s law of gravity. With total force acting on the

28 18 infinitesimal particle P 3 modelled in vector form, the law of motion can be written as follows, F = m 3 I r 3 = Gm 3m 1 r 2 13 r 13 Gm 3m 2 r r , (2.15) where I r 3 is the acceleration of P 3 relative to the barycenter with respect to an inertial frame and dots indicate differentiation with respect to dimensional time. Multiplying equation (2.15) by (t ) 2 /l m 3 yields: d 2 ( r 3 /l ) d(t/t ) = Gm 1 r 13 2 r13 3 l t 2 Gm 2 r 23 r23 3 l t 2. (2.16) With equations (2.11) - (2.14), the law of motion in equation (2.15) can be rewritten in the form d 2 ρ dτ = m 1 d 2 m r 13 /l 3 m 2 r m r 23 /l 3. (2.17) Using the non-dimensional quantities in equations (2.9) and (2.13), equation (2.17) can subsequently be expressed in the form where The unit vectors ˆX, Ŷ, and d 2 ρ µ) µ = (1 d dτ2 r, (2.18) d 3 r3 d = (x + µ)ˆx + yŷ + zẑ, (2.19) r = (x (1 µ))ˆx + yŷ + zẑ. (2.20) Ẑ are parallel to inertial directions as seen in figure 2.1. Then, ρ x, ρ y, and ρ z are the non-dimensional coordinates of P 3 with respect to the inertial, or sidereal, system. Recall that unit vectors ˆx, ŷ, and ẑ are parallel to directions fixed in the rotating, or synodic, system. Note that the unit vectors comprise an orthonormal triad. Then the corresponding position coordinates are x, y, and z. The coordinates in the rotating and inertial system are related by a simple rotation, i.e., ρ x ρ y ρ z c t s t 0 x = s t c t 0 y. (2.21) z

29 19 Differentiating equation (2.21), to obtain kinematic expressions for position, velocity and acceleration, and ρ x c t s t 0 ẋ y ρ y = s t c t 0 ẏ + x, (2.22) ρ z ż ρ x c t s t 0 ẍ 2ẏ x ρ y = s t c t 0 ÿ + 2ẋ y, (2.23) ρ z z where 2ẏ and +2ẋ correspond to the Coriolis terms. Then, x and y represent non-dimensional terms that result from the centripetal acceleration. From a combination of the kinematic expansion with equation (2.18), equations (2.18) and (2.23) yield the scalar, second order, nonlinear set of differential equations: (1 µ)(x + µ) µ(x (1 µ)) ẍ 2ẏ x =, (2.24) d 3 r 3 (1 µ)y ÿ + 2ẋ y = µy d 3 r, 3 (2.25) (1 µ)z z = µz d 3 r. 3 (2.26) As illustrated in Meirovitch [29], the Lagrangian of the CR3BP does not depend on time explicitly, that results in a constant Hamiltonian. It follows that the system possesses a constant of integration known as the Jacobi Constant. Physically, the gravitational forces, must be balanced by the centrifugal forces. It follows that a modified potential energy function corresponding to the differential equations in (2.24)-(2.26) can be identified, that is, U = 1 2 (ẋ2 + ẏ 2 ) + (1 µ) d + µ r. (2.27) Note that in the above definition, the potential is positive and is a convention in the formulation of the CR3BP. The equations of motion can now be expressed in terms of the following partial derivatives, ẍ = U x + 2ẏ, (2.28)

30 20 ÿ = U y 2ẋ, (2.29) z = U z. (2.30) Equations (2.28)-(2.30) represent the traditional, convenient form of the equations of motion relative to the rotating frame in non-dimensional coordinates. Jacobi identified the constant of integration associated with the differential equations that takes the following form, C = 2U (ẋ 2 + ẏ 2 + ż 2 ). (2.31) Jacobi s Constant is sometimes called the integral of relative energy [48]. It is important to note that it is not an energy integral but rather an energy-like constant partly due to the formulation of the problem relative to a rotating system. It is also notable that, in the restricted problem, neither energy nor angular momentum is conserved. The Jacobi Constant can be used to produce zero-velocity plots that identify regions of exclusion for a specific energy level. Figure 2.2 illustrates an example of a zero-velocity curve for a Jacobi Constant value of C= A particle would require an imaginary velocity to be within the region enclosed by the closed green curve. Awareness of these forbidden regions can offer much insight into the dynamics of the problem. In addition to insight, the Jacobi Constant is very often used as a method to check the accuracy of the calculations, particularly the accuracy of the numerical integration of the differential equations Singularities in the Equations of Motion The equations of motion in the CR3BP possess singularities at the centers of the two primaries. The singularities result from terms of the form 1/r 3 and 1/d 3, where r is the non-dimensional distance from the mass m 2 to the spacecraft and d is the non-dimensional distance from the mass m 1 to the spacecraft. When a transfer trajectory originates in a low altitude Earth parking orbit or a launch trajectory is computed, d or r are very small. Thus, the state is very close to a singularity that

31 Y Axis [lunar units] 0 Earth L 1 L 2 Moon X Axis [lunar units] Figure 2.2. Zero Velocity Curve for C = degrades accuracy and limits the effectiveness of the differential corrections technique in determining transfer trajectories beyond the Earth s sphere of influence. The singularity can be avoided through regularization [40] at the cost of physical insight. Previous studies [28], originally with similar difficulties, have demonstrated that regularization is generally not necessary due to the current computational capabilities. Backward integration of the transfer trajectories is therefore employed to limit the sensitivity to the initial conditions. From the differential equations, it is apparaent that the transformation [40] τ = t results in a change in the derivatives with respect to the independent variable, such that d/dτ = d/dt and d 2 /d 2 τ = d 2 /d 2 t. With this transformation, the integration

32 22 can be initiated on the halo orbit or along a manifold trajectory at time t f and computed backwards to determine the state close to the Earth at time t 0. Both transfer trajectories, as well as launch trajectories, are determined using backward integration. 2.5 State Transition Matrix Equations (2.28)-(2.30) can be rewritten as six first-order differential equations where the state vector is defined as x = [x y z ẋ ẏ ż] T. These six first-order differential equations can be linearized relative to a reference solution x ref = [x ref y ref z ref ẋ ref ẏ ref ż ref ] T by use of a Taylor series expansion and ignoring the higher order terms. Note that the reference solution can be a constant equilibrium state or a time-varying solution to the nonlinear differential equations. Define the linearized state relative to x ref as δ x(t) = [δx(t) δy(t) δz(t) δẋ(t) δẏ(t) δż(t)] T. Then, the linear state variational equation can be expressed in the form δẋ(t) = A(t)δx(t), (2.32) where A(t) is a 6 6, generally time-varying, matrix. It can be written in term of the following four 3 3 submatrices, A(t) = 0 I 3 UXX 2Ω. (2.33) Each one of the elements of the 6 6 matrix A(t) is a 3 3 matrix, where 0 represents the zero matrix, and I 3 the identity matrix of rank 3. Then Ω is defined as constant, Ω = (2.34) The matrix of second partials, UXX, is comprised of elements U xx Uxy Uxz UXX = U yx Uyy Uyz, (2.35) Uzx Uzy Uzz

33 23 where U ab = 2 U a b. (2.36) Of course, the partials are evaluated on the reference solution and it can be assumed that, if at least the first and second order derivatives are continuous, U XX is symmetric. The form of the solution to equation (2.32) is well known, assuming the state transition matrix φ(t, t 0 ) is available, that is, δx(t) = φ(t, t 0 )δx 0, (2.37) where φ(t, t 0 ) is the 6 6 state transition matrix (STM) evaluated from time t 0 to time t. The state transition matrix φ(t, t 0 ) is a linear map that reflects the sensitivity of the state at time t to small perturbations in the initial state at time t 0. Any differential corrections scheme exploits the STM to predict the initial perturbations that yield some desired change in the final state. Differentiating equation (2.37) and substituting equations (2.32) and (2.37) into the result produces φ(t, t 0 ) = A(t)φ(t, t 0 ). (2.38) This matrix differential equation represents 36 scalar equations. The initial conditions for φ are determined by evaluating equation (2.38) at time t 0. So, the initial conditions for equation (2.38) yield the identity matrix of rank 6, or φ(t 0, t 0 ) = I 6. (2.39) Adding the 6 scalar differential equations for the state yields 42 coupled scalar differential equations to be numerically integrated. A general analytical solution is not available because A(t) is time-varying. 2.6 Differential Corrections Differential corrections (DC) schemes use the STM for targeting purposes. One application is an iterative process to isolate a trajectory arc that connects two points

34 24 in solution space. Differential corrections (DC) techniques can be used to quickly obtain a solution with the desired parameters in a wide range of problems. A sufficiently accurate first guess for the the initial state is always required. When only the natural motion is considered in three-dimensional space, the total number of parameters available in the problem is 14 [1]. This number of parameters will be termed the dimension of the problem. As illustrated in figure 2.3, there are two seven-dimensional states, one at each end of the trajectory defined by the epochs, t 1 and t 2, their positions, R1 and R 2, and the velocity components, V1 and V 2 ; thus, the problem is parameterized by 14 scalar elements. This number also characterizes the sum of the fixed constraints, that is, the targets, the controls, and the free parameters in the problem. The fixed constraints are the parameters that are not allowed to vary, for example, the initial position vector R 1. The controls are the set of parameters that the DC scheme is allowed to modify to achieve the desired target states. Free parameters are additional variables not used in the DC scheme as either fixed quantities, controls, or targets. These values will likely change in a way that may not be predictable. Differential correction schemes are often used to obtain periodic solutions to the nonlinear differential equations in the CR3BP. A common assumption, making use of the symmetry in this problem, is that the desired solution is symmetric about the x z plane. Initially, the known states are given in the form x 0 = [x 0 0 z 0 0 ẏ 0 0] T and, from the symmetry properties, it is concluded that for a simply symmetric periodic orbit, at the next x z plane crossing, the trajectory Figure 2.3. A Stylized Representation of 1 Step Differential Corrector.

35 25 will be consistent with the values, i.e., x f = [x f 0 z f 0 ẏ f 0] T. Since the initial guess for the state vector x 0 is not likely to yield the necessary form of x f, the differential corrections process then uses the STM to compute the required changes in two of the initial non-zero variables to drive the velocities ẋ f and ż f to zero. There are an infinite number of periodic orbits that satisfy these conditions. One approach to isolate a specific trajectory is to fix one non-zero quantity associated with the initial state as a constant. The fixed quantity can be varied, depending on the goals [34]. The process is repeated until the desired final result is achieved, i.e., the orbit crossing the x z plane perpendicularly. Convergence to a solution is usually obtained after about four iterations. Numerically integrating equation (2.28)-(2.30) in three dimensions, with the appropriate initial conditions for one period, yields a halo orbit. 2.7 Particular Solutions In 1772, Joseph Lagrange identified five particular solutions to equations (2.28)- (2.30) as equilibrium points in the 3BP, for a formulation relative to a rotating frame. As equilibrium points, the gravitational and centrifugal forces are balanced at these locations but it is important to note that the points are still moving in a circular orbit about the barycenter relative to the inertial frame. All five points lie in the plane of motion of the primaries and the location of the points in the Earth-Moon system appear in figure 2.4. Linear stability analysis can be employed to determine that the collinear Lagrange points L 1, L 2, and L 3 are inherently unstable and that the equilateral points L 4 and L 5 are linearly stable. Placing a probe at the triangular points will result in oscillations in the vicinity of the equilibrium point. The equilibrium points obtained the name, libration points, from the oscillatory motion at the equilateral locations [20]. One type of periodic and quasi-periodic solutions that are the focus of a number of recent missions are the periodic, planar Lyapunov and three-dimensional halo orbits as well as the three-dimensional quasi-periodic Lissajous trajectories.

36 Libration Points in the Earth Moon System y [non dimensional lunar units] L Earth 3 L 1 L 2 L 4 L 5 Moon x [non dimensional lunar units] Figure 2.4. Location of the Libration Points in the Earth-Moon System Relative to a Synodic Frame. Poincaré realized the importance associated with periodic orbits. In his conjecture [49] in 1895, he stated that an infinite number of periodic orbits exist in the 3BP. For Poincaré, periodic motion appeared to be significant in nature and he considered the study of periodic orbits a matter of greatest importance. His investigations into periodic orbits in the 3BP were limited to an analytical investigation. Although considerable progress was made in approximation techniques to represent periodic orbits over the following 20 years, detailed investigations were hindered by the amount of computations involved. Lyapunov, halo, nearly-vertical orbits, and Lissajous trajectories each occur in families with similar characteristics. Lyapunov orbits are planar orbits that lie in the

37 27 plane of motion of the primaries. A pitchfork bifurcation of the Lyapunov orbits, both above and below the x y plane, results in two halo families that are mirror images across the x y plane [50]. When the maximum out-of-plane amplitude (A z ) is in the +z direction, the halo orbit is a member of the northern family (NASA Class I) and if the maximum excursion is in the z direction, the halo orbit is a member of the southern family (NASA Class II). Each member of a family corresponds to a slightly different energy level (Jacobi Constant). Halo orbits were first computed in the CR3BP and are defined as precisely periodic, three-dimensional libration point orbits. They are typically characterized by their maximum out-of-plane amplitude (A z ). An example of a halo orbit in the CR3BP appears in figure 2.5. The three-dimensional orbit is presented in terms of orthographic projections. For Lissajous trajectories, the amplitude of the in-plane motion and that of the out-of-plane motion are arbitrary and the frequencies are not commensurate [9, 51]. Orthographic projections of an example of a Lissajous trajectory appears in figure 2.6. Precisely periodic halo orbits do not exist in a more general model that incorporates additional perturbations. In an ephemeris model, Lissajous trajectories can always be generated and careful selection of in-plane and out-of-plane amplitudes yields Lissajous trajectories that are very close to periodic for a limited time interval. These orbits are generally denoted as halo orbits. Although Lissajous trajectories are not periodic, they are nevertheless bounded and exist on an n-dimensional torus [50, 51]. Another type of periodic motion, is the family of the nearly-vertical orbits first visually identified by Moulton [50]. As is true with any nonlinear dynamical system, a low order approximation does not immediately yield a continuous trajectory, however, a differential corrections procedure can result in a natural periodic solution if the initial guess is sufficiently accurate. If a linear approximation is not sufficient to create a Lissajous trajectory with the desired characteristics, the third-order analytical approximation developed by Richardson and Cary [32] is commonly deployed. Alternatively, patch points, consisting of the full six-dimensional state plus the time, along a halo orbit can be

38 28 Y [km] 3 x L X [km] x 10 5 x 10 4 Earth Moon System A x = 331,521 km A y = 25,991 km A z = 15,393 km x Z [km] L 1 Z (km) L X [km] x Y (km) x 10 4 Figure 2.5. Northern Earth-Moon L 1 Halo Orbit in the CR3BP. determined and imported into an ephemeris model to obtain a halo-like Lissajous trajectory. The patch points are corrected through a two-level differential corrections process (2LDC) developed by Howell and Pernicka [52]; details of this process are offered later. 2.8 Invariant Manifold Theory In the late 19 th century, the French mathematician Henri Poincaré searched for precise mathematical formulas that would allow an understanding of the dynamical stability of systems. These investigations resulted in the development of what is now called Dynamical Systems Theory (DST). Dynamical systems theory is based

39 29 x y [km] Sun L 1 Earth x [km] x 10 8 x 10 5 x 10 5 z [km] L 1 0 L x [km] x 10 8 z [km] y [km] 2 4 x 10 5 Figure 2.6. Lissajous Trajectory at L 1 in the Sun-Earth CR3BP. on a geometrical view for the set of all possible states of a system in the phase space [53]. Detailed background information is available in various mathematical sources including Perko [54], Wiggins [55], as well as Guckenheimer and Holmes [56]. An extensive summary appears in Marchand [51] Brief Overview For a continuous nonlinear vector field of the form ẋ = f(x), (2.40) the local behavior of the flow in the vicinity of a reference solution to the nonlinear equations can be determined from linear stability analysis for most applications. Note

40 30 that x is the entire state vector, and let x = x ref + δ x such that δ x is the vector of variations. Assuming that the above vector field is continuous to the second degree, it can be expressed in terms of a Taylor series expansion relative to this reference solution resulting in a vector variational equation of the following form δẋ(t) = A(t)x(t), (2.41) where A(t) is an n n matrix of the first partial derivatives. If the reference solution is an equilibrium solution, such as a libration point, then the A matrix is constant. In general, however, the A matrix cannot be assumed constant and is time-varying, A = A(t). However, for the moment, assume a time-invariant system and variations relative to an equilibrium point. The algebraic technique of diagonalization can be used to reduce the linear system to an uncoupled linear system. Perko [54] states the following theorem from linear algebra that allows a solution to a linear, time-invariant system with real and distinct eigenvalues: Theorem 2.1 If the eigenvalues λ 1, λ 2,..., λ n of an n n matrix are real and distinct, then any set of the corresponding eigenvectors η 1 η 2...η n forms a basis R n, the matrix P = [η 1 η 2...η n ] is invertible and P 1 AP = diag[λ 1,..., λ n ]. In the process of reducing the linear system in equation (2.41) to an uncoupled system, the linear transformation y = P 1 δx, (2.42) is defined, where P is the invertible matrix defined in theorem 2.1. Then δx = Py, (2.43) and Perko demonstrates that the solution can be written y(t) = PE(t)P 1 δx(0), (2.44)

41 31 where E(t) is the diagonal matrix E(t) = diag[e λ 1t,..., e λnt ]. (2.45) Rewriting equation (2.44) allows the more insightful expression y(t) = n c j e λjt η j, (2.46) j=1 where the c j s are scalar coefficients. Perko continues by stating The subspaces spanned by the eigenvectors η i of the matrix A determine the stable and unstable subspaces of the linear system, equation (2.41), according to the following definition, Definition 2.1 Suppose that the n n matrix A has k negative λ 1,..., λ k and n-k positive eigenvalues λ k+1,..., λ n and that these eigenvalues are distinct. Let η 1,...,η n be a corresponding set of eigenvectors. Then the stable and unstable subspaces of the linear system (2.41), E s and E u, are the linear subspaces spanned by η 1,...,η k and η k+1,...,η n respectively; i.e., E s = Span{η 1,..., η k }, E u = Span{η k+1,..., η n }. Perko completes the above definition by adding If the matrix A has pure imaginary eigenvalues, then there is also a center subspace E c. Define the complex vector w j = u j + iη j, as a generalized eigenvector of the matrix A corresponding to a complex eigenvalue λ j = a j + ib j and then let B = {η 1,..., η k, η k+1, η v+1,..., u m, v m }, (2.47) be the basis of R n. Then definition 2.2 below allows a distinction between the stable, unstable, and center subspaces.

42 32 Definition 2.2 Let λ j = a j + ib j, w j = ū j + i η j and B be as described above. Then and E s = Span { u j, η j a j < 0 }, E c = Span { u j, η j a j = 0 }, E u = Span { u j, η j a j > 0 }, i.e., E s, E c, and E u are the subspaces of the real and imaginary parts of the generalized eigenvectors w j corresponding to eigenvalues λ j with negative, zero, and positive real parts respectively. Decomposing the phase space into three separate regions is the dynamic approach [53]. The sum of the three fundamental subspaces spans the complete space R n. Selecting initial conditions carefully to ensure that certain specified coefficients c j are equal to zero in equation (2.46), results in the desired behavior inside a subspace for all time [53]. Once in a subspace, motion remains there for all time. From a linear perspective, this can be seen as only exciting the desired mode while eliminating any perturbations of the undesirable modes. Thus, solutions that originate in E s asymptotically approach y = 0 as t and solutions with initial conditions in E u approach y = 0 as t. Solutions in the center subspace E c neither grow nor decay over time. Guckenheimer and Holmes [56] relate the stable and unstable manifolds to the invariant subspaces for an equilibrium point through the Stable Manifold Theorem. Theorem 2.2 (Stable Manifold Theorem for Flows) Suppose that ẋ = f(x) has a hyperbolic equilibrium point x eq. Then there exist local stable and unstable manifolds W s loc (x eq), W u loc (x eq), of the same dimensions n s, n u as those of the eigenspaces E s and E u of the linearized system (2.41), and tangent to E s and E u at x eq. The local manifolds W s loc (x eq), W u loc (x eq) are as smooth as the function f. Let x eq be the non-hyperbolic equilibrium point, or the libration point, at L 1. Then, figure 2.7 can be used to illustrate the stable manifold theorem. In the immediate vicinity of the libration point, the eigenvectors are directed along the individual

43 33 local stable and unstable subspaces. The manifolds associated with the stable and unstable subspaces are globalized through numerical integration. In the CR3BP, the collinear libration points L 1, L 2, and L 3 possess a four-dimensional center subspace and one-dimensional stable and unstable subspaces. Figure 2.7 represents the stable and unstable eigenvectors, E s and E u, and the globalized manifolds, W s and W u, associated with the Earth-Moon L 1 point. In the case of the libration points, the position and velocity components of the eigenvectors are always parallel. For the unstable eigenvector, both position and velocity components are directed the same, i.e., away from the libration point. For the stable eigenvector, the position and velocity components of the eigenvector are oriented in opposite directions. Hence, a small displacement from the libration point along the stable eigenvector, results in motion toward the libration point. A small displacement along the unstable eigenvector, results in motion away from the libration point. Planar Lyapunov orbits and nearly vertical out-of-plane orbits are examples of periodic solutions that exist in the center subspace near L i [51,57]. As the amplitude of the Lyapunov orbits increases to a critical amplitude, a bifurcation point identifies the intersection of the planar Lyapunov orbits and the three-dimensional halo family of periodic orbits [50,51]. Note that the Lyapunov familly continues beyond the critical amplitude but the stability properties of the orbits have changed. The critical amplitude can be identified by monitoring the characteristics of the eigenvalues of the monodromy matrix Periodic Orbits and Dynamical Systems Theory For a number of applications, the state transition matrix at the end of a full revolution, also termed the monodromy matrix φ(t, 0), must be available. Consider the point along the periodic orbit that was selected as the starting and ending point. In dynamical systems, this point is denoted as a fixed point in a stroboscopic map. Then, the monodromy matrix serves as a discrete linear map near the fixed point located at the origin of the map. Such a map is also often called a Poincaré map [9,53].

44 34 x Y Axis [km] W u+ W s E s E u L 1 E s E u W s+ W u X Axis [km] x 10 4 Figure 2.7. Stable and Unstable Eigenvectors and the Globalized Manifold for the Earth-Moon L 1 Point. The eigenvalues and eigenvectors of the monodromy matrix can be used to estimate the local geometry of the phase space in the vicinity of the fixed point. It is important to note that the monodromy matrix possesses different elements for every fixed point along the periodic orbit. Thus, the eigenvectors change directions and, thus, the directions of the stable and unstable subspaces vary along the orbit. The eigenvalues, on the other hand, are a property of the orbit and remain constant as is apparent from equation (2.51). Linearizing relative to periodic orbits results in linear, periodic, differential equations. The A matrix in equation (2.41) is now time-varying but periodic, i.e., δẋ(t) = A(t)δx(t). (2.48)

45 35 According to Floquet theory [24,57], the STM can be rewritten as φ(t, 0) = F(t)e Jt F 1 (0), (2.49) where J is a normal matrix that is diagonal or block diagonal and the diagonal elements are the characteristic multipliers or Floquet multipliers. Note that F(t) is a periodic matrix. Solving for the matrices J and F that correspond to a periodic system yields φ(t, 0) = F(0)e Jt F 1 (0), (2.50) since F(T) = F(0) and e Jt = F 1 (0)φ(t, 0)F(0). (2.51) From equation (2.51), it is clear that F(0) and J contain the eigenvectors and eigenvalues of the monodromy matrix. From equation (2.51), then, λ i = e it, (2.52) i = 1 T ln(λ i), (2.53) where i are the Poincaré exponents. The Poincaré exponents are interpreted in a manner similar to the eigenvalues in a constant coefficient system. In Hamiltonian-like systems, such as the CR3BP, they must also occur in positive/negative pairs by Lyapunov s theorem. From the stability properties associated with the Poincaré exponents, conclusions about the location of the characteristic multipliers on the complex plane and, thus the stability of the fixed point and the periodic orbit, are potentially available [57]. A system with no characteristic multipliers on the unit circle is called hyperbolic and the nature of a hyperbolic system can be summarized as λ i < 1 stable y = 0 as t λ i > 1 unstable y = 0 as t If λ i = 1, no stability information can be obtained from the characteristic multipliers and they correspond to the center subspace.

46 36 Periodic orbits possess a monodromy matrix that yields at least one eigenvalue with a modulus of one [54]. The following theorem adds further significance to the eigenvalues. Theorem 2.3 (Lyapunov s Theorem) If λ is an eigenvalue of the monodromy matrix φ(t, 0) of a time-invariant system, then λ 1 is also an eigenvalue, with the same structure of elementary divisors. λ 1 = 1 λ 2, λ 3 = 1 λ 4, λ 5 = 1 λ 6. (2.54) Thus, according to Lyapunov s theorem, the six eigenvalues associated with a periodic orbit (via the map and associated monodromy matrix) also appear in reciprocal, complex conjugate pairs. So, two of the six eigenvalues of the monodromy matrix will always be precisely one. In the CR3BP, the center subspace has a dimension of four and two of the eigenvalues are real and equal to one for precisely periodic orbits. Eigenvalues with real parts smaller than 1 are considered stable eigenvalues and eigenvalues with real parts larger than 1 are considered unstable. Eigenvectors corresponding to a stable eigenvalue lie in the stable subspace and yield stable manifolds asymptotically approaching the periodic orbit as t. Eigenvectors corresponding to an unstable eigenvalue lie in the unstable subspace and yield unstable manifolds asymptotically approaching the orbit as t. 2.9 Computing Manifolds Computation of the globalized manifolds relies on the availability of initial conditions obtained from the stable and unstable subspaces, E s and E u. The eigenvectors of the monodromy matrix offer local approximations of the stable and unstable subspaces for fixed points along periodic orbits. Note that an eigenvector only indicates orientation in space. The eigenvector does not yield a specific directional sense, that is multiplying by negative one, yields a valid eigenvector in the opposite direction. This results in manifolds approaching (stable) and departing (unstable) the orbit in two

47 37 different directions. Generally, one side of the globalized manifold enters the region around the smaller primary P 2 whereas the other enters the inner region around the major primary P 1. The directions spanned by the position components for 30 points along the halo orbit(a z = 15,000 km) are illustrated in figure 2.8. The velocity (red) components relative to the position (blue) components for the stable eigenvector appear in figure 2.9. This set results in the globalized manifold approaching the halo orbit from the larger primary. The angle between the position and velocity components of the eigenvector varies between 146 and 170 degrees for a lunar L 1 halo orbit with an A z amplitude of 15,000 km. The angular range is dependent upon the orbit investigated, of course. While studying this figure, it is critical to realize that the eigenvectors only offer a linear approximation of the subspaces close to the halo orbit and, hence, only offer a valid approximation of the direction of these subspaces very close to the periodic orbit. Thus in addition to investigating the direction of the eigenvectors, it is crucial to consider the globalized manifolds to obtain a complete picture of the different subspaces. Applying a small perturbation in the direction of the eigenvector results in a local estimate of the one-dimensional manifold associated with the fixed point. After a local estimate has been determined, the trajectory on the manifold associated with the point can be globalized through numerical integration. Given the eigenvectors of the monodromy matrix, the local estimate of the stable and unstable manifolds, Xs and X u, can be computed as X s = x(t i ) + d V Ws (t i ), (2.55) where V Ws (t i ) is defined by V Ws (t i ) = Ŷ Ws (t i ) (2.56) x2 + y 2 + z2, and Ŷ Ws (t i ) = [x s y s z s ẋ s ẏ s ż s ] T is the stable eigenvector. Then, X u = x(t i ) + d V Wu (t i ), (2.57) where V Wu (t i ) is defined by V Wu (t i ) = Ŷ Wu (t i ) (2.58) x2 + y 2 + z2,

48 38 x y [km] Earth L 1 Moon x [km] x 10 5 Figure 2.8. Stable Eigenvectors of the Monodromy Matrix for an Earth-Moon L 1 Halo Orbit. and Ŷ Wu (t i ) = [x u y u z u ẋ u ẏ u ż u ] T is the unstable eigenvector. In equation (2.55) and equation (2.57), d is the initial displacement (perturbation) from the periodic orbit. Larger d values ensure an initial state along a manifold that is further advanced in departing from the periodic orbit or libration point. At the same time, the initial displacement cannot be selected arbitrarily large since the linear approximation must remain within the range of validity. In the Earth-Moon system, d values commonly range between 30 km and 70 km, whereas in the Sun-Earth system commonly used d values range between 150 and 200 km. The initial displacement can also be employed as a design parameter; changing the magnitude of d only affects the particular trajectory selected along the approaching manifold, but not the orbit

49 39 x y [km] Earth L 1 Moon x [km] x 10 5 Figure 2.9. Position and Velocity Components of the Stable Eigenvectors of the Monodromy Matrix for an Earth-Moon L 1 halo. itself. The effects of different d values are illustrated in figure Normalizing the eigenvectors relative to the position components ensures that the displacement along the eigenvector is uniform. Once the monodromy matrix is obtained for any one fixed point along the orbit, the manifolds associated with any other point along the orbit can be calculated two different ways. First, shifting the focus to another fixed point, the calculations can be repeated and a new monodromy matrix determined. Then, new eigenvectors can be computed. Alternatively, it is more efficient to exploit the state transition matrix. An eigenvector can be directly shifted along the orbit via the STM as follows, Ȳ Ws i = Φ(t i, t 0 )Ȳ Ws 0, (2.59)

50 40 Figure Various Manifolds Asymptotically Approaching the Orbit. Ȳ Wu i = Φ(t i, t 0 )Ȳ Wu 0. (2.60) This direct shift is quick and accurate. In the interest of numerical accuracy, it is also beneficial to explore the structure associated with the CR3BP to limit excessive numerical integrations of the STM as well. In the CR3BP, the monodromy matrix φ(t, 0) can be generated from the half-cycle STM φ(t/2, 0), where φ(t, 0) = G 0 I 3 I 3 2Ω φ T (T/2, 0) 2Ω I 3 I 3 0 G 1 φ(t/2, 0), (2.61) G =. (2.62)

51 41 x y [km] 0 Sun Earth x [km] x 10 8 Figure Stable (blue) and Unstable (red) Manifolds for a Sun-Earth Halo Orbit near L 1. The proof of equation (2.61) uses the fact that the STM is a simplectic matrix [33]. Two halo orbits in the Sun-Earth system appear in green in figure The halo orbits, plotted in green, possess an A z amplitude of 120,000 km. The stable (blue) and unstable (red) manifolds associated with these L 1 and L 2 halo orbits form three-dimensional tubes in the vicinity of the Earth. The Earth in figure 2.12 is not plotted to scale and only the physical location is represented by the blue sphere. For larger halo orbits, part of the manifold tubes extend below the surface of the Earth, i.e., pass less than 6378 km from Earth s center. As is illustrated in figure 2.12, the manifolds are separatrices for a given energy level. The manifold tubes separate different regions of motion in space. Additionally, if motion starts on a tube, it will not leave that tube unless the energy level is altered [58].

52 42 6 x 10 1 y [km] x [km] x 10 Figure Stable (blue) and Unstable (red) Manifold Tube Approaching the Earth in the Sun-Earth System Transition of the Solution to the Ephemeris Model Solutions in the CR3BP offer much insight into the solution space and are qualitatively similar to solutions existing in higher order multi-body models. To improve the accuracy of transfer trajectories and to illustrate their credibility and robustness, they are transferred to an ephemeris model. The ephemeris model that is employed here is the Generator software package [59] developed by Howell et al. at Purdue University. The primary motion is specified from planetary state ephemerides available from the Jet Propulsion Laboratory DE405

53 43 ephemeris files. In the ephemeris model, the Earth is used as the origin and Newtonian equations of motion are integrated with respect to the inertial frame. From a number of possible integrators, an 8/9 order Runge-Kutta integrator is selected for this application to perform the integration with respect to the Earth. In the determination of transfer trajectories in the Earth-Moon system, the Sun is also included in the force model, although any number of bodies can be incorporated. Since no assumptions about the primary motion are desired, the transformation from inertial to rotating coordinates is based on the instantaneous position of the primaries obtained from the ephemeris files. Much of the symmetry of the CR3BP is lost when solution arcs are transferred to an ephemeris model. The libration points are no longer fixed points relative to the rotating frame of the primaries. Instead, they oscillate. Precisely periodic halo orbits no longer exist, however, it is often desirable to maintain characteristics for the quasi-periodic orbits in the ephemeris model that are similar to those of the periodic halo orbits in the CR3BP. Therefore, transfer arcs that correspond to halo orbits in the CR3BP are employed as initial estimates in the ephemeris model. A two-level differential corrections (2LDC) scheme [52] (described later) computes a Lissajous trajectory, figure 2.13, resembling the original halo orbit. The halo-like Lissajous in figure 2.13 corresponds to a translunar halo orbit with A z amplitude of 15,000 km. The curve in figure 2.13 is plotted in the Earth-Moon rotating frame centered at the Moon.

54 44 5 x 104 Y [km] 0 L 1 Moon L2 Earth X [km] x x x 104 Z [km] 2 0 L 1 Moon L 2 Z [km] 2 0 Moon 2 4 Earth X [km] x Y [km] 5 x 10 4 Figure A Halo-like Lissajous Trajectory in a Ephemeris model with an A z Amplitude of approximately 15,000 km.

55 45 3. TRANSFERS FROM EARTH PARKING ORBITS TO LUNAR L 1 HALO ORBITS Efficient determination and computation of transfer trajectories from parking orbits around the larger primary to L 1 halo orbits remains a challenge. Direct insertion onto a manifold from the Earth is not possible, since the natural flow does not pass sufficiently close to the primary. The natural motion for arbitrary Earth-Moon halo orbits is presented to illustrate the difficulties in the determination of such transfers. Results in this chapter represent the most recent efforts in the analysis of this problem. The highest priority is an understanding of the difficulties associated with the computation of these transfer trajectories (including starting values) and the development of the methodology for detailed investigation. Various transfer trajectories between a low altitude Earth parking orbit and two different halo orbits are presented. A more complete understanding of the dynamical structure in this multibody environment is the foundation for the design of future trajectories originating and returning to an Earth parking orbit. A free return transfer trajectory with potential applications in human spaceflight is also included. The trajectory arcs computed here can ultimately be optimized as well. 3.1 Stable and Unstable Flow that is Associated with the Libration Point L 1 in the Vicinity of the Earth Exploration of the natural dynamics associated with invariant manifolds to and from L 1 halo orbits in the Earth-Moon system is the first step in developing an efficient design tool. Therefore, the stable and unstable manifolds have been computed for a series of different halo orbits. Although this behavior is similar in any 3BP system, the analysis presented here is limited to the Earth-Moon dynamical environment.

56 46 Figure 3.1. Stable and Unstable Manifold Tubes in the Vicinity of the Earth. The stable and unstable flow around the first (largest) primary does not allow for natural solutions, i.e., manifolds, to pass close by the primary. The trajectories corresponding to different fixed points along the halo form a tube-like surface. Each individual trajectory wraps around the manifold surface that forms the tube. These stable (blue) and unstable (red) manifold surfaces appear in figure 3.1 for an Earth- Moon halo orbit with an A z amplitude of 15,000 km. (The size of a halo orbit is typically characterized by the out-of-plane amplitude denoted A z.) The trajectories that correspond to specific L 1 halo orbits first pass the Earth at altitudes ranging between 82,200 km to 93,000 km. Although, after multiple passes, a trajectory might subsequently pass closer to the Earth, the investigation here is limited to first passes. The closest Earth passing altitudes along the trajectories that correspond to the stable manifolds for differently sized Earth-Moon L 1 halo orbits appears in figure

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