Orbit concepts at L2 for Soyuz launches from Kourou

Many of the currently planned astronomy missions will go into orbits around the L2 libration point in the Sun-Earth system. Two ESA projects at L2, GAIA (2012) and DARWIN (2016) will use a Soyuz launch from the currently constructed launch facility at Kourou. Soyuz will launch into a low circular parking orbit from which the Fregat upper stage will inject to the transfer. The nearly parabolic transfer orbits can be reached at inclinations of up to 30∘ with little mass penalty compared to low inclination orbits. This allows to reach a Lissajous orbit with a maximum Sun-spacecraft-Earth angle of 15∘ (GAIA baseline) with an insertion maneuver of less than 130 m/s for a full year launch window. Or by selecting the parking orbit near the lunar orbit plane, a lunar gravity assist can be used to reach the stable manifold of an even smaller amplitude (e.g. 7∘) Lissajous orbit without any deterministic maneuver during the transfer. Another interesting case of transfers to orbits around L2 has been studied for the DARWIN mission. DARWIN consists of four spacecraft. One studied option is to launch them in pairs, on two Soyuz launches from Kourou. The launch dates have to be at least 20 days apart, but the spacecraft have to rendezvous in an orbit around L2. It has been shown that without any phasing orbits for a given separation of the launch dates of up to 60 days, a target orbit can be defined in which the two stacks of spacecraft can be brought to the same position with a small ΔV.