Fundamentals of the LISA Stable Flight Formation

The joint NASA-ESA mission LISA relies crucially on the stability of the three spacecraft constellation. Each of the spacecraft is in heliocentric orbits forming a stable triangle. The principles of such a formation flight have been formulated long ago and analysis performed, but seldom presented if ever, even to LISA scientists. We nevertheless need these details in order to carry out theoretical studies on the optical links, simulators etc. In this article, we present in brief, a model of the LISA constellation, which we believe will be useful for the LISA community.