REORBITING OF SATELLITES IN HIGH ALTITUDES Rüdiger Jehn(1), Alessandro Rossi(2), Tim Flohrer(1), Daniel Navarro-Reyes(3) (1)ESA/ESOC, Robert-Bosch-Str. 5, 64293 Darmstadt, Germany, Email: [email protected] (2) Istituto Scienza e Tecnologie dell’Informazione, via Moruzzi 1, 56124 Pisa, Italy, Email: [email protected] (3)ESA/ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands, Email: [email protected] ABSTRACT final decay is about 10-7 which can be considered low enough to make it an attractive re-orbiting alternative. The Inter-Agency Space Debris Coordination 1. INTRODUCTION Committee (IADC) and UNCOPUOS request that GEO spacecraft at their end-of-life are raised to a graveyard The geostationary ring is a valuable resource currently orbit with sufficient altitude clearance to operational populated by 335 operational satellites. Unlike in low satellites. Based on orbital data in ESA's DISCOS Earth orbit there is no atmospheric drag which will database the situation near the geostationary ring (GEO) remove abandoned objects over time. Therefore, it is the is assessed for a reference epoch in December 2008. An responsibility of the spacecraft operators to keep this analysis of the orbital evolution histories of 970 objects unique orbital region clean. showed that 335 of the GEO objects were controlled inside their longitude slots, 476 were drifting above, Already in 1977, Perek proposed that spacecraft should below or through GEO, 154 were captured at one of the be systematically removed from GEO at their end-of- two stable libration points, and 5 objects had an ill- mission [11]. In the same year, for the first time in space defined orbit status (e.g. due to insufficient orbit data). history, INTELSAT sent an aging satellite into a GEO Furthermore, there were 46 controlled satellites and 170 graveyard orbit. Since then a number of guidelines and uncontrolled objects with no TLE information at all. recommendations for GEO end-of-mission disposal by Thus, the total number of known objects in the national and international institutions was following as geostationary region was 1186, out of which 381 were described in [9] and [2]. In the early 1980s, the disposal controlled, 800 were uncontrolled and 5 could not be orbit guideline of NOAA (US National Oceanic and classified. Atmospheric Administration) was 300 km above GEO. During the year 2008, 12 GEO spacecraft reached their Also during the 1980s, the International Telecommuni- end of life. Seven of them were re-orbited following the cations Union (ITU) began addressing the issue of end- IADC guidelines, two were re-orbited with an of-mission disposal and super-synchronous graveyard insufficient GEO clearance and three spacecraft were orbits. Although the ITU did not explicitly recommend abandoned without any re-orbiting manoeuvre. The re- a specific super-synchronous graveyard orbit, its orbiting statistics of the years from 1997 to 2008 show definition of GSO "as a mean radius of 42,164 ± 300 km that the adherence to the IADC space debris mitigation and extending to 15º north and south latitude" indirectly guidelines is slowly improving over time. dictated a minimum perigee altitude of the disposal orbit of 300 km above GEO. Also the re-orbiting practice of GPS satellites is analysed. For 22 of the 56 launched GPS satellites a In 1989 ESA adopted a resolution on space debris, disposal manoeuvre was identified from the DISCOS including as a risk reduction measure "the re-orbiting of TLE data. Since the year 2000, the orbits of the GPS geostationary satellites at the end of their life into a satellites are usually raised by about 1000 km at end-of- disposal orbit". Subsequently 10 ESA-controlled life. geostationary satellites (all except for the failed Olympus-1 satellite) were re-orbited by at least 300 km Finally the re-orbiting options of Galileo satellites are above GEO after completion of their mission. compared. An alternative to the currently proposed orbit raising after end-of-life is the re-orbiting into a lower In 1995 the International Academy of Astronautics orbit with the highest achievable eccentricity. It will recommended to re-orbit "geostationary satellites at take more than 100 years until a satellite will finally end-of-life to disposal orbits with a minimum altitude decay. But the average risk that the satellite will increase of 300 km to 400 km above GEO depending on encounter a collision during its long journey towards its spacecraft characteristics" [1]. At the same time, _____________________________________________________ Proc. ‘5th European Conference on Space Debris’, Darmstadt, Germany 30 March – 2 April 2009, (ESA SP-672, July 2009) national space agencies like NASA, NASDA, RKA x C2: objects under longitude control (only E-W (later ROSCOSMOS), CNES and ESA developed their control) - the longitude is nearly constant but the own guidelines. All of these recommended an altitude inclination is larger than 0.3º, increase of more than 200 km above GEO. Finally, in x D: objects in a drift orbit, 1997, an international consensus was found within the x L1: objects in a libration orbit around the Eastern Inter-Agency Space Debris Coordination Committee stable point (longitude 75ºE), (IADC). The recommended minimum altitude increase x L2: objects in a libration orbit around the Western (in km) was given as stable point (longitude 105ºW), x L : objects in a libration orbit around both stable 2 1+2 H = 235km + 1000km u cR u (A/m)/(1 kg/m ) (1) points. where cR is the solar radiation pressure coefficient 3. THE CURRENT STATUS OF GEO (usually with a value between 1 and 2), A is the projected cross-sectional area and m is the mass of the Next to the 970 objects which fulfilled the orbital satellite. criteria by 31 Dec 2008, there are 216 more objects also known to be in this orbital region although no orbital In view of such guidelines and recommendations one elements are available in DISCOS. Thus, the total would expect that the geostationary ring is a well number of objects in the geostationary region is 1186. protected and unlittered space. However, only about They were classified as follows: 40% of all retired GEO satellites follow the internationally agreed recommendations. About 60% so x 381 are controlled (251 under longitude and far are re-boosted into a too low orbit that will sooner or inclination control), later return to the GEO altitude, or they are completely x 476 are in a drift orbit, abandoned in the GEO ring without any end-of-life x 154 are in a libration orbit, disposal manoeuvre. x 103 are uncontrolled with no orbital elements available, In this paper an updated survey of the GEO population, x 5 could not be classified (4 of them were recently and a summary of GEO re-orbiting practices is provided launched and are en route to their longitude slot). for the past 12 years (1997-2008). x 67 were uncatalogued but can be correlated with a launch event 2. ANALYSIS OF GEO ORBITAL DATA The basic source of information for the analysis Fig. 1 illustrates the fractional contributions of the presented here are the Two-Line Elements (TLE) different GEO categories. In ESA’s annual report generated by the US Space Surveillance Network. "Classification of Geosynchronous Objects" [8] the Updates of these TLE data are automatically status of all individual objects can be traced. In this downloaded and processed by ESA's DISCOS Database paper we confine ourselves to some statistical data. (Database and Information System Characterizing Objects in Space) on a daily basis. For the GEO Classification of geosynchronous objects Total: 1186 environment characterization one TLE per week and Status: January 2009 object is considered. The GEO objects are selected from 251 the DISCOS Database according to the following 130 criteria: 99 38 x eccentricity smaller than 0.1 17 x mean motion between 0.9 and 1.1 revolutions per sidereal day, corresponding to a radius of 42,164 170 -2,500/ +3,150 km 476 5 x inclination lower than 30° Libration around 2 points: 17 Controlled : 130 Libration around 105 W: 38 Drift : 476 970 objects met these criteria as of 31 December 2008. Libration around 75 E: 99 Indeterminate : 5 Controlled (E-W and N-S): 251 Uncontrolled (no TLEs): 170 Their orbital histories were analyzed in order to classify the objects according to six different categories: Figure 1. Number of objects in each category in Jan 2009. The 130 controlled objects consist of objects in class C (only East-West station keeping) and objects x C1: objects under longitude and inclination control 2 (E-W as well as N-S control) - the longitude is where no TLEs are available. nearly constant and the inclination is smaller than 0.3º, Fig. 2 shows the number of objects under control Objects in drift orbit (bottom bars), in drift orbit (intermediate bars), and in Status : January 2009 4000 libration orbit (top bars) as a function of the launch year. Most of the satellites launched before 1990 are 3000 meanwhile either in a drift orbit or in a libration orbit. 2000 Up to 10 objects per year were abandoned in such libration orbits (see Table 1), with a decrease in recent 1000 years to 3 or less. 0 GEO protected zone Classification of geosynchronous objects Altitude range (in km) -1000 (Objects with recently updated TLEs) Status: January 2009 -2000 45 Satellites under control Objects in drift orbit 40 Objects in libration orbit -3000 -3000 -2000 -1000 0 1000 2000 35 Semi-major axis mean deviation from the geostationary altitude (in km) 30 Figure 4. Distribution and altitude range of objects in a 25 drift orbit.