Satellite crashing down on Friday – Denmark barely in the danger zone – Niels Bohr Institute - University of Copenhagen

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21 September 2011

Satellite crashing down on Friday – Denmark barely in the danger zone

NASA’s defunct Upper Atmosphere Research Satellite (UARS) will crash down to Earth on Friday, September 23. Precisely where and when is not known for certain and NASA will only be able to give a more definite projection at sometime on Thursday, September 22.

NASAs defunct satellite (UARS) will crash down to Earth on Friday, September 23. The satellite weighs 6 tons, but on its way through the atmosphere it will break into smaller pieces and most will burn up before they reach the Earth surface. NASA estimates that a total of approx. 500 kg of the satellite will hit Earth. It is very unlikely that fragments of the satellite will hit Denmark. 

 

“Denmark is on the far periphery of the area the satellite could hit, so it is very unlikely that fragments of the satellite will hit Denmark. On Thursday we will probably know whether Denmark is in the danger zone at all”, explains astrophysicist Kristian Pedersen, Head of the Space Science Center at the University of Copenhagen.

The satellite was launched in 1991 with the Space Shuttle Discovery and has been in orbit 605 km above the Earth. It has 10 instruments on board and was designed to function for three years, but 20 years later, six of the instruments are still functioning.

UARS measures ozone and chemical compounds in the ozone layer. It also measures winds and temperatures in the stratosphere as well radiation from the Sun. The measurements help to define the role of the upper atmosphere in climate variations.

Why crash down?

But why is the satellite crashing down? Kristian Pedersen explains, that satellites in relatively “low” orbit around the Earth feel a bit of air resistance, so they fall gently to earth. As they descend they feel even more air resistance and you quickly get a downward spiral, which ends with the satellite crashing down. And this is the point that the UARS satellite has now reached. If possible, you make a controlled crash, where you send the satellite into the atmosphere when you know that the risk of it hitting a populated area on Earth is least likely. But it is not always possible to control a satellite and there is always some uncertainty about exactly where and when it will crash down. If possible, you try to make it crash far out to sea.

The Satellite has 10 instruments on board and was designed to operate for three years, but 20 years later, six of the instruments are functioning.

500 kilos will hit the Earth

The UARS satellite weighs 6 tons, but on the way down through the atmosphere it breaks into smaller pieces and most will burn up before they reach the Earth surface. According to NASA’s calculations there will be 26 large pieces of the satellite large enough so that they will not burn up. The largest piece weighs 158 kg before it falls through the atmosphere and NASA estimates that a total of approx. 500 kg of the satellite will hit the earth along an 800 km long track.
 

NASA regularly updates information about the satellite’s crash:
http://www.nasa.gov/mission_pages/uars/index.html

Space Science Center (www.space.ku.dk) at the University of Copenhagen is monitoring the situation and will provide new information about the risk for Denmark, as forecasts become more accurate.