ANALYSIS OF GEOSTATIONARY TRANSFER ORBIT LONG TERM EVOLUTION AND LIFETIME Alain LAMY (1), Clémence LE FEVRE (2), Bruno SARLI (3) (1)(2)(3) CNES, 18 Avenue E. Belin, 31401 Toulouse Cedex 9, France Email: Alain.Lamy@cnes.fr Abstract: The study presented in this paper deals with Geostationary Transfer Orbits (GTO) for which new French regulations (defined in the context of the French Space Act) will fully apply at the end of 2011. Geostationary Transfer Orbits are characterized by a low perigee (altitude of a few hundreds of kilometres) and a high apogee (altitude typically identical to that of geostationary satellites) among other features. The objective of the study is to analyze the dynamics of objects in geostationary transfer orbits in order to better understand what the lifetime (time during which the object remains in orbit) most depends on. Because of the high eccentricity, the orbit is strongly affected by the gravitational effects of the Sun and Moon. But because the perigee is low, drag has a strong impact too. The coupling of the two perturbations combined with the effects of the Earth potential (secular drifts mainly) makes the orbit’s evolution particularly sensitive to initial conditions and modelling errors. One key element is the initial position of the Sun (and to a lesser extent the Moon) which changes the mean altitude of the perigee, which translates into more or less drag, hence more or less decrease rate of the semi-major axis at the beginning of the lifetime. But when the semi-major axis reaches a value of around 15000km, the perigee altitude may increase or decrease strongly because the angle between the Sun and the line of apsides is then nearly constant. The paper attempts to explain all these aspects and discusses the possibility of limiting the lifetime of objects in Geostationary Transfer Orbits. Keywords: Geostationary Transfer Orbit, Long term propagation, Lifetime, French Space Act. 1 Introduction The amount of debris in orbit is a growing threat to operational satellites, as seen by the increasing number of avoidance manoeuvres performed each year. There is therefore a need for measures that will prevent this situation from becoming even worse. France has decided to take on an active role by implementing regulations (French Space Act), in line with IADC recommendations, in order to protect the most populated orbital regions (LEO, GEO...). The study presented in this paper deals with Geostationary Transfer Orbits (GTO) for which French regulations will fully apply at the end of 2011. Geostationary Transfer Orbits are mainly characterized by a low perigee (altitude of a few hundreds of kilometres), a high apogee (altitude typically identical to that of geostationary satellites), and a low inclination. At the end of their mission, objects in Geostationary Transfer Orbits will neither be allowed to cross the GEO region within one year nor to stay in orbit longer than 25 years if they cross the LEO region. No one will wait for that long to confirm that the actual lifetime has been less than 25 years. An reliable enough orbit prediction is of course necessary. It had been noticed in previous studies conducted at CNES that the selection of potentially hazardous objects in elliptical orbits (for instance objects that may pose a risk to populations) was not obvious and that no simple rule seemed to exist to easily select the objects that would re-enter in the coming months or years. Other analyses also conducted at CNES on GTO lifetime [1] have illustrated the complexity of the problem. Figure 1 shows the lifetime computed using DAS (Debris Assessment Software) as a function of day of year (x-axis) and time of day (y-axis). The red areas correspond to durations longer than 25 years, and the blue areas to durations shorter than 25 years.