Lessons from the end of the ROSAT mission
The German ROSAT research satellite was launched on 1 June 1990 on a Delta II launcher and successfully scanned space for X-ray sources for nine years. Its record of achievements extends from discovering countless X-ray sources to analysing galactic clusters, X-ray binary stars and black holes to discovering the reflection of the Sun's X-ray radiation by the Moon. The findings by the scientists involved in the mission have appeared in over 7000 publications. After orbiting Earth for 21 years, ROSAT re-entered the atmosphere over the Bay of Bengal. It is not known whether any parts reached the Earth's surface.
In 2007, during the first few months of my term as chairman of the DLR Executive Board, I was informed of the situation with ROSAT. At that time, it was clear that atmospheric drag, even at those great altitudes, was causing ROSAT's orbit to decay, and ultimately it would re-enter the atmosphere. On board ROSAT was an X-ray telescope with heat-resistant ZERODUR® components; this encouraged the view that larger parts could survive re-entry and might pose a potential risk to people and objects on the ground.
This concern, alongside the issue of whether potential counter-measures needed to be taken, sparked the beginning of an initially tough and then more intense debate over the following years regarding how to deal with ROSAT. Calculations of the probability of damage occurring on the ground, emergency planning and enquiries to the international spacefaring community followed. It quickly became apparent that, in particular, the major space powers such as the United States and Russia dealt with this issue in a relatively relaxed manner, based on their years of experience. However, awareness of ROSAT's compact and heat-resistant structure and the probabilities of various damage scenarios remained.
In the final days and weeks before re-entry, the satellite’s flight path was tracked very carefully from numerous locations. At DLR, we had prepared a coordination centre at our Space Administration in Bonn-Oberkassel. All the information and data was analysed and assessed here and the satellite’s final orbits were anxiously followed in cooperation with ESA/ESOC and NASA Space Debris Office. As in the previous days, numerous questions from the media and the public needed to be answered and, in doing so, a balance between concern and calmness had to be found. While collaborating closely with those German Federal authorities responsible for dealing with crises, ultimately all that could be done was to watch and wait. The dedication of the various people involved at DLR and among our partners was exemplary. They all deserve my most sincere thanks. After ROSAT came down over the Indian Ocean in the early morning of 23 October 2011, there was widespread relief.
I personally drew a number of lessons from the conclusion of the ROSAT mission:
- Responsibility for a project must encompass the entire lifespan and take every eventuality into consideration.
- National and international collaboration, regardless of whether personal or institutional, has now achieved a level that is marked by a very engaging, positive attitude and mutual trust, which must be used accordingly.
- Communication concerning projects should be as transparent as possible, but always reliable and correct in every respect. In this regard, successes and potential risks must be communicated equally.
In one of my presentations, I included an image entitled 'From space to in front of your eyes'. It was not intended as a cynical comment regarding the risks of re-entry. Rather, the intention was to illustrate another aspect of ROSAT; the grinding processes developed for manufacturing ROSAT's telescope paved the way for the successful development of varifocal spectacle lenses. ROSAT is thus also a testament to the much advocated 'non-stick pan effect'; that is, the application of spaceflight technology developments in unrelated areas. DLR cannot bring about such desirable 'side effects' on its own, but rather should develop it into an institutional system in cooperation with relevant partners. On the one hand, this secures and also drives forward research and development in our own subject areas using limited resources in an optimal way; on the other, transfer to other areas not only safeguards research, it also promotes it.
Top: The X-ray satellite ROSAT nine days before its re-entry into the atmosphere on 14 October 2011, approximately one kilometre west of Aachen on the German-Dutch border. The ROSAT telescope (main body) and the solar panels are clearly visible. Credit: R. Vandebergh / http://ralphvandebergh.startje.be/
Middle: ROSAT image of a galaxy cluster from May 1995. Source: MPE
Bottom: DLR Space Administration in Bonn-Oberkassel. The DLR coordination centre was located here during the weekend of 22/23. October 2011. Source: CC-BY-DLR.