Mission Design for the STEREO Solar Observatories

The mission design for the National Aeronautics and Space Administration (NASA) Solar Terrestrial Relations (STEREO) solar observatories is presented. The STEREO mission features a two spacecraft formation in solar orbit to provide 3-dimensional (3D) imaging of Coronal Mass Ejections from their origination at the Sun through their interaction at the Earth. The heliocentric formation is achieved using lunar gravity assists to direct the spacecraft into solar orbit. The solar orbit is designed such that the spacecraft drift away from the Earth at a mean separation rate of 22° per year along the Earth’s orbit path near 1 astronomical unit (au). One spacecraft uses a single lunar gravity assist to escape into an Earth trailing orbit. The second spacecraft uses two lunar gravity assists to escape in the opposite direction relative the Earth’s motion into an Earth leading orbit. Use of the gravity assist technique allows the spacecraft to be launched together on a single expendable launch vehicle. This paper summarizes the science objectives, mission requirements, constraints, and spacecraft. The strategy to achieve the unique trajectory design for the mission is presented. INTRODUCTION The principle goal of NASA’s STEREO program is to provide coordinated observations of the Sun and interplanetary medium using a two spacecraft formation in heliocentric orbit. The spacecraft are being developed by The Johns Hopkins University Applied Physics Laboratory (JHU/APL) for launch in November 2005 as part of the NASA Solar Terrestrial Probes (STP) Program. The desired formation requires one spacecraft lead the Earth in its orbit, while the second spacecraft will trail behind the Earth. The spacecraft are named Ahead and Behind for the leading and trailing spacecraft, respectively. Figure 1 shows an ecliptic plane projection of the orbit relative to a fixed Sun-Earth line. From their positions ahead and behind the Earth the spacecraft are optimally located to produce 3D images of Earth-directed Coronal Mass Ejections (CMEs) and monitor the interplanetary environment near 1 au. The spacecraft are stacked and launched together aboard a 3-stage Delta-II launch vehicle. The Earth escape trajectories are achieved using lunar gravity assists to provide the desired departure energy and direction for each spacecraft. Lunar gravity assists have been effectively used on a 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.3.08 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 number of previous missions. During 1983 the third International Sun-Earth Explorer (ISEE-3) established the utility of using multiple lunar gravity assists by using three lunar encounters to explore the Earth’s geomagnetic tail region. Later in 1985, ISEE-3 renamed the International Cometary Explorer (ICE) used two additional lunar encounters to direct the spacecraft into solar orbit in order to perform a flyby of comet Giacobinni-Zinner. Lunar gravity assists have also been used extensively for the Hiten, Geotail, and WIND missions for the exploration of geospace near the Sun-Earth line within approximately 240 Earth Radii (Re) of the Earth. 4, 5 WIND has made further use of lunar gravity assists during it’s current extended mission to traverse the region perpendicular to the Sun-Earth line at distances up to 400 Re. The Wilkinson Microwave Anisotropy Probe used a lunar gravity assist to transfer to a small amplitude libration point orbit at the exterior Sun-Earth libration point, L2. The STEREO mission design uses lunar gravity assists to provide two escape trajectories in opposite directions relative to the Earth’s motion and quickly establish the desired heliocentric formation. This enables the spacecraft to be launched together on a single launch vehicle providing a significant cost savings for the program versus two separate launch vehicles.