Only one month since the 5.9 tonne UARS (Upper Atmosphere Research Satellite) crashed harmlessly into the Pacific Ocean another large satellite is about to hit the Earth. This time the satellite is the decommissioned German space observatory named Roentgensatellit or ROSAT. It was launched on the 1st June 1990 on a Delta II rocket from Cape Canaveral to study X-ray astrophysics, and was designed to operate for 18 months. In the end ROSAT far exceeded its original planning and actually operated for over 8 years, finally shutting down on the 12th February 1999.

Like UARS, ROSAT will re-enter the Earth's atmosphere and hit either the surface or land in one of the oceans. It is predicted that this will happen sometime between the 21st and the 23rd October 2011. As was the case with UARS, ROSAT has no propulsion system on board so there's no way to control where and when it falls. Since it is currently moving around the Earth in a nearly circular orbit (altitude about 236 kilometres or 145 miles) at an inclination of 53 degrees, virtually all inhabited areas of Earth are potentially at some risk of collision.

The total mass of ROSAT is about 2.5 tonne, which is less than half that of UARS but since the telescope mirror is made of a ultra low-expansion ceramic glass (Zerodur), it along with the mirror assembly is expected to hit Earth intact at a speed of hundreds of kilometres an hour. This could total more that 1-ton of material surviving re-entry and striking the Earth. ESA's Heiner Klinkrad said the odds of anything striking somewhere in Germany or in a comparable target area are about one in 580. He adds that: "One day before ROSAT re-enters, we will be able to predict it's landing with an accuracy of within one orbit".

For anyone who happens to be close to ROSAT when it comes finally down, they should witness a spectacular fireworks show.

**UPDATE: 24 Oct 2011 **

ROSAT has crashed to Earth somewhere in Southeast Asia but its exact whereabouts remain a mystery. U.S. military data calculations indicate that debris of the satellite must have crashed somewhere east of Sri Lanka over the Indian Ocean or as far inland as China. The satellite entered the atmosphere between 9:45pm and 10:15pm EDT on Saturday 22nd October.

**UPDATE: 27 Oct 2011 **

German Space Agency experts have confirmed that ROSAT has fallen to earth over the Bay of Bengal, in the Indian Ocean. The parts of the satellite which survived the plunge to Earth would have splashed into the ocean and avoided hitting populated areas.