LOW THRUST TRANSFER TO SUN-EARTH L 1 AND L 2 POINTS WITH A CONSTRAINT ON THE THRUST DIRECTION Alexander A. Sukhanov and Natan A. Eismont Space Recearch Institute (IKI) of the Russian Academy of Sciences 84/32 Profsoyuznaya St., Moscow 117997, Russia e-mails: sukhanov@iki.rssi.ru, neismont@iki.rssi.ru Low-thrust transfers from a low Earth circular orbit (LEO) to the Sun-Earth L 1 and then to L 2 points are analyzed. A spin-stabilized spacecraft with the spin axis orthogonal to the Sun direction is considered. The thrusters provide jet acceleration along the spin axis in both directions. Thus, the thrust is always orthogonal to the Sun direction. The spiral spacecraft ascent from the LEO is considered first. Each orbit of the spiral has two thrust arcs and two coast ones. Then the spacecraft is inserted into an L 1 halo orbit in the ecliptic plane. After the operations in the halo orbit are completed the spacecraft is transferred to an L 2 halo orbit. The transfers containing zero, one, or two complete orbits around the Earth are considered. Introduction A low-thrust mission to one or two of the Sun-Earth collinear libration points was being developed in Russia a few years ago. Main mission goals were solar wind exploration and magnetic storm prediction; however, perhaps most important goal was testing of new technologies. The mission concept was the following: – a light spacecraft equipped with solar electric propulsion (SEP) was to be assembled at the International Space Station 1 ; – the spacecraft was to ascend from LEO in a spiral orbit, to be transferred to the L 1 point, and to be inserted into a halo orbit using SEP; – after the operations in the L 1 halo orbit are completed the spacecraft could transfer to an L 2 halo orbit. The mission has not been adopted, nevertheless some of its features can be of a certain interest. For instance, the following: a spin-stabilized spacecraft was considered with immovable solar arrays and thrusters what simplified the spacecraft design and control and lowered the mission cost. The spin axis is orthogonal to the Sun direction and the thrusters provide the thrust along the spin axis in both directions. Thus, the thrust is always orthogonal to the Sun direction and all the transfers and maneuvers were to be performed under this constraint on the thrust direction. This paper presents main results of the mission analysis, namely the following: – Earth to L 1 halo transfer including the spiral ascent near Earth; – L 1 halo to L 2 halo transfers with different numbers of complete orbits around the Earth. Some of the important problems of the mission (such as communication, parameters of the film solar arrays, a prolonged being in the van Allen belts etc.) are not discussed in the paper because they were outside authors’ competence. 1 Mir space station was considered for this purpose at an early phase of the project.