Rudiger Jehn

Observing the Geostationary Ring

Based on orbital data contained in the DISCOS database, the situation in the geostationary ring is analysed. In January 2007, from 1121 known objects populating the geostationary region, 354 are controlled within their allocated longitude slots, 448 are drifting above, below or through GEO, and 147 are in a libration orbit. For 165 objects there is no orbital information available. In the last ten years from 1997 to 2006,152 spacecraft reached their end of life; 56 were reorbited in compliance with the Inter-Agency Space Debris Coordination Committee (IADC) recommendation, 54 were reorbited below the minimum recommended altitude, and 42 were abandoned or lost without any end-of-life disposal manoeuvre. Apart from these catalogued objects, the ESA 1-m telescope has observed many smaller debris (down to about 15 cm) in this orbital region representing a non-negligable collision risk for geostationary spacecraft.

Comparison of EISCAT radar data on space debris with model predictions by the master model of ESA

Abstract In the effort to obtain low cost routine space debris observations in low Earth orbit, ESA plans to utilise the radar facilities of the European Incoherent Scatter Scientific Association. First demonstration measurements were performed from 11 to 23 February 2001. In total 16 h of radar signals were collected. Here we compare these initial measurements with the predictions of the ESA MASTER/PROOF’99 model in order to assess the sensitivity as well as the reliability of the data. We find that while the determination of object size needs to be reviewed, the altitude distribution provides a good fit to the model prediction. The absolute number of objects detected in the various altitude bins indicates that the coherent integration method indeed increases the detection sensitivity when compared to incoherent integration. In the data presented here integration times from 0.1 to 0.3 s were used. As expected, orbit information cannot be obtained from the measurements if they are linked to ionospheric measurements as planned. In addition routine space debris observations provide also useful information for the validation of large-object catalogues.

Spacecraft design impacts on the post-Newtonian parameter estimation

The ESA mission BepiColombo, reaching out to explore the elusive planet Mercury, features unprecedented tracking techniques. The highly precise orbit determination around Mercury is a compelling opportunity for a modern test of General Relativity (GR). Using the software tool GRETCHEN incorporating the Square Root Information Filter (SRIF), MPOs orbit is simulated and the post-Newtonian parameters (PNP) are estimated. In this work, the influence of a specific constraint of the Mercury Orbiter Radio science Experiment (MORE) on the achievable accuracy of the PNP estimates is investigated. The power system design of the spacecraft requires that ±35° around perihelion the Ka transponder needs to be switched off, thus radiometric data is only gathered via X band. This analysis shows the impact of this constraint on the achievable accuracy of PNP estimates. On a bigger scale, if GR shows some violation at a detectable level it inevitably leads to its invalidation.

Collision avoidance during deorbiting of satellites at end-of-life

Abstract The deorbiting of satellites at end-of-life is the only effective means to keep low-Earth orbit clean. With the success of solar-electric propulsion systems deorbiting now becomes economically feasible also for spacecraft above 1000 km altitude. The deorbiting in this case will take several months. The collision risk with 10-cm objects during this time is only of the order of 10 −6 per square meter, however, it may be used as an argument to reorbit an aging satellite above LEO rather than to deorbit it. With a proper collision avoidance concept this already low risk of an on-orbit collision with an operational satellite can almost be excluded. A 40 mN thrust system is sufficient for a 1000 kg spacecraft to keep a safe radial distance of 200 m from any target orbit.