The german ROSAT space telescope (Credits: PD-USGov-NASA).

October 19 update: In the picture below, a special false-color processing to increase certain contrasts. Note the visible shadow of the main body (the actual telescope) on the solar panels, seen from an angle as the satellite passed 51.4 degrees. The shift is cleary visible.  A very difficult observation on this small object, even at that distance.

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I have been actively trying to get some last telescopic views of the doomed ROSAT satellite in the last weeks before its reentry. There has been not many passes left, and due to the lowered orbit, the window of visibility has become narrow. When I photographed the satellite in June this year, the altitude was still 330 kilometers, just a bit lower then the minimal altitude of the International Space Station. Since then the height has decreased to less then 300 kilometers: at this height, the satellite dives sooner into the Earth Shade and the visible passes is reduced considerably.

On October 6,  I managed to observe ROSAT in a twilight pass, flying from West-Southwest into eastern direction, but it entered Earth’s shade when it was already at 68 degrees height in the Northwest. I could track it with binoculars only until 40 degrees, due to the clouds. On October 13,  I saw ROSAT again during a new observing window,  the final visible passes of the Röntgen telescope over my location in the Netherlands. It flew from West-Northwest to East with 47 degrees elevation. Sight was disturbed by clouds again, but I could see it with the naked eye between the clouds as a bright magnitude +1 object  in twilight. Angular speed was high, as anticipated. Altitude was now reduced to 240 kilometers. As far as I could observe, it looked still fairly stable, but the clouds prevented me from determining this with certainty.

Final telescopic views of ROSAT. Click for full view

On October 14 – last visible pass of the final observing window – the sky was predicted to be cloudless. The problem this time was that ROSAT would pass in very early twilight. Since I perform manual tracking,  I need a visible star to focus the camera and the question was: would I pick up the satellite in the light twilight sky? Then, while preparing the session a day earlier, I realized that it would pass at 51.4 degrees elevation through the North, as seen from my location. These coordinates are very close to the Northern pole star (Polaris). Now, with a chance to rely on a fixed position for the telescope, I knew that I could get the image, provided that everything else was fine. But of course it was not as easy as may seems; to track objects along the pole, the telescope mount has to be adjusted in a special position. But everything went fine: I picked up ROSAT in the tracking scope – with Polaris in the same field – and I captured several frames before the satellite disappeared behind a roof. The final image shows clearly the main body and the shape of the panels, with the shapes clearly defined, with increased resolution compared to the earlier images. The frames confirmed also the interpretation of the previous observations: so far, no visible sign of tumbling for this relatively small, yet massive satellite.

 

 

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About the author

Ralf Vandebergh

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Astrophotographer & independent journalist specializing in high resolution imaging of spacecraft in orbit. Captured ground-views of the last contruction years of the ISS. Took first ever ground-based image of astronauts during EVA. Sub-specialization currently is visualization of rotational/tumbling behave of (dead) satellites and debris/space junk. Writing for different newspapers and magazines about (space)science and technology.

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