Markus Landgraf
(ESA/ESOC, Germany)

An overview is given of how the mission design for the two very different missions Herschel and Planck were performed by ESA mission analysis at ESOC, Darmstadt. The different requirements of the Herschel observatory and the Planck all-sky surveyor drive the two missions into different orbit solutions, while the ideal observation location in the vicinity of the night-side Lagrange point L2 is the same. The methods used by ESA to design a libration point mission are the exploitation of the properties of the linearised problem in a fully propagated dynamics environment. For example the nominal orbit is propagated forward in the full dynamics, while the inherently unstable trajectory is stabilised by small corrections in the unstable direction of the linear problem. The baseline design plus some, fortunately obsolete, solutions for launcher contingencies are presented together with strategies for station-keeping and the operational history of Herschel and Planck.

Toshinori Ikenaga
(JAXA, Japan)

Japan Aerospace Exploration Agency, JAXA, is now planning the next-generation infrared astronomical mission, called SPICA, utilising a halo orbit around the L2 in the Sun-Earth system. Since the L2 libration point trajectories are unstable, some form of trajectory control are necessary to keep spacecrafts close enough to their nominal paths. This paper describes the study on the stationkeeping strategy particularly applicable to the SPICA mission. In this study, although the CR3BP is generally used for libration point mission analyses, the precise force model utilising the DE405 is employed to construct the feasible stationkeeping strategy. The main stationkeeping algorithm is structured consulting the paper "Stationkeeping Method for Libration Point Trajectories" written by K.C.Howell[1993], and several functions are added to meet the SPICA mission requirements. The above-mentioned stationkeeping strategy is evaluated through some cases of orbit maintenace simulation. The attitude constraints of the SPICA mission and the frequent disturbance arised by unloading operation of Reaction Wheels are considered, and an optimal thruster allocation for the SPICA mission is suggested in this paper.

Masaki Nakamiya
(ISAS/JAXA, Japan)

Transfers from the Earth to the Quasi-halo orbits of the Sun-Earth system by exploiting the characteristic of the dynamical system of the circular restricted three-body problem, are being investigated. In our previous work, the transfers to the Halo orbits and the way of expanding the launch window by changing the size of the Halo orbit had been studied studied (See Fig. 1). In that work, we found that the launch window is limited at a certain time periods within a year. However the mission requirement of SPICA (SPace Infrared telescope for Cosmology and Astrophysics), which is the first Japanese Lagrange point mission, desires the constraint of the nominal trajectory to be loose. Therefore, for alternative option we discuss the transfer to the Quasi-halo orbits instead of to the Halo orbits in this present study. At first, the orbital conditions such as the altitude of perigee and the inclination at the given launch site for the transfers to the Quasi-halo orbits by using the characteristic of the dynamical system like stable manifolds, which are converge to the Quasi-halo orbits naturally, are shown. Next, the way to expand the launch window for the lift-off to Quasi-halo orbits is presented.

Yasuhiro Kawakatsu
(JAXA, Japan)

Discussed in this paper is a lunar transfer sequence named 'phasing orbit' (Fig.1). In this method, the spacecraft is not directly injected into the translunar orbit, but stays on a long elliptical phasing orbit for revolutions. A major merit of this method is that, by using the phasing orbit as a buffer, the translunar orbit can be fixed for acceptable width of launch window. In the phasing orbit design, the post-launch injection orbit is designed so that its perigee coincides with that of the translunar orbit. The maneuver at the first perigee passage adjusts the period of the phasing orbit to pass the perigee at the time of translunar orbit injection after revolutions. Finally, the translunar orbit injection maneuver injects the spacecraft into the translunar orbit. This design method works well under the simple model of two body approximation. The two tangential maneuvers connect the three orbits exactly. However, in the practical model, the position of the perigee is deviated during the phasing orbit by various perturbations, and the method described above cannot be applied as it is. Discussed in the paper is the design method of the phasing orbit under the practical conditions.

Mai Bando
(Kyoto University, Japan)

The trajectory design associated with the deflection of potentially hazardous asteroid(PHA) is considered. It is difficult to determine exact values of the orbital elements, mass, shape and the surface structure of the target asteroids. We investigate the robust trajectory for kinetic impactor spacecraft, including the uncertainty of orbital elements and use the low-thrust acceleration to change the terminal velocity vector to maximize the closest approach distance to the Earth. Optimal rendezvous problem using continuous low-thrust is treated as optimal control problem and H-infinity control. Using the property that both problems can be treated as two-point boundary value problem (TPVBP) of Hamiltonian system, we formulate the robust guidance control law to asteroid which minimize the quadratic cost with uncertain initial and terminal state by generating functions. The emphasis is that this approach enables us to find optimal trajectory to complicate problem not by a global search but by function evaluations. Our approach is illustrated through the study of the asteroid deflection mission. We evaluate the total delta V for intercept trajectory to the PHA to determine the departure and arrival date. Then the low-thrust trajectory is designed to maximize closest approach distance under the effect of the uncertainty of asteroid trajectory.