During its mission, the ROentgen SATellite (ROSAT) spent its orbit in an elliptical pattern at distances of between 351 and 364 miles above the Earth. Since its decommissioning, atmospheric drag has caused the satellite to lose altitude. In June 2011, it was at a distance of only about 203 miles above the ground. Because ROSAT doesn't have a propulsion system, it was impossible to maneuver the satellite for a controlled re-entry at the end of its mission in 1999. When the spacecraft re-enters the atmosphere at a speed of approximately 17,400 miles per hour, the X-ray observatory will break up into fragments, some of which will burn up by the extreme heat. The latest studies show it is possible that up to 30 individual pieces weighing a total of 1.7 tons may reach the surface of the Earth. The largest single fragment will probably be the telescope's mirror, which is very heat resistant and may weigh up to 1.7 tons.

The time and location of re-entry cannot be predicted precisely. Scientists expect the X-ray satellite, which completes an orbit around Earth every 90 minutes, to re-enter around between Oct. 22 and 23. The re-entry date can only be calculated to within plus or minus one day. However, even one day before re-entry, the estimate will only be accurate to within plus or minus five hours. All areas under the orbit of ROSAT, which extends to 53 degrees northern and southern latitude could be affected by its re-entry. That area represents most of the populated areas of the world. The bulk of the debris will impact near the ground track of the satellite. However, isolated fragments could fall to Earth in a 50 mile wide path along the track.

The primary cause of uncertainty in the estimation of the date of re-entry is fluctuations in solar activity. Solar radiation heats up the Earth's atmosphere and therefore increases the atmospheric drag. Short-term fluctuations in solar activity are governed by an 11 year activity cycle. Right now, Earth is approaching the next maximum, which has turned out to be much lower than expected.

During the re-entry phase of the satellite, German scientists will be evaluating data from the US SpaceSurveillance Network (SSN). Additionally, the Tracking and Imaging Radar (TIRA), the large radar facility at the Fraunhofer Institute for High-Frequency Physics and Radar Techniques in Wachtberg near Bonn will be monitoring the descent of the X-ray satellite to further improve calculations of its trajectory. Experts will be analyzing the data obtained on behalf of the German Aerospace Center (DLR) to predict the moment of re-entry as accurately as possible.