Europe's ROSAT Telescope Weighs 2.4 Tons, Can't Be Steered

It looks as falling satellite debris will become a regular part of life in the 21st Century. Sometime between this Friday, and Monday, October 24, just a month after NASA's six-ton UARS fell into the Pacific, a defunct German satellite (pictured) called ROSAT will take the plunge. As was the case last month, scientists will not know until just before impact where the space junk is likely to hit the surface of the Earth.

ROSAT is an acronym for "Roentgen Satellite," named after physicist Wilhelm Roentgen, who won the first Nobel Prize for his 1895 discovery of X-rays. The satellite is an x-ray telescope which was launched on an 18-month mission in 1990, ended up providing useful functionality for more than eight years, and is credited with helping researchers map over 100,000 sources of x-ray radiation in space.

In an official statement, Heiner Klinkrad of the European Space Agency (ESA) explained, "In the final phase, ROSAT will be 'caught' by the atmosphere at which point it will not even complete an orbit around the Earth: Instead, it will go into 'free fall.'" The exact point at which that happens is a matter of chance because the satellite has no rocket propulsion capability, and its steering gyroscopes have all failed, ruling out the ability to command an attitude change for more drag to force a quicker re-entry over an ocean.

While ROSAT weighs about 2.4 tons, smaller than UARS, the German Aerospace Center predicts one-in-2,000 odds of debris hitting a human, making it a little more dangerous than UARS at one-in 3,500. There are 30 pieces of the satellite thought likely to survive the heat of re-entry.

USA Today's Dan Vergano notes that under an international treaty, unless the risk to humans is less than one-in-10,000, nations launching satellites are expected to provide some means to steer them to safe landing sites during re-entry. Both ROSAT and UARS were launched before the treaty took effect.

While it may be natural to feel anxiety over the threat of getting hit by a piece of a falling satellite, NASA Space Debris Expert Nicholas Johnson of NASA's Johnson Space Center tells the paper uncontrolled re-entry of space junk the size of ROSAT happens many times a year, while something the size of UARS drops from the sky about once a year.

By comparison, an estimated 500 meteorites reach the surface of Earth each year.

FMI: www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10424/ ; www.nasa.gov/audience/forstudents/k-4/dictionary/Meteorite.html