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updated 2/7/2008 7:46:01 PM ET 2008-02-08T00:46:01

A dead U.S. spy satellite in a deteriorating orbit is expected to hit the Earth the first week of March, officials said Thursday.

It is not known where on Earth the satellite will hit. But officials familiar with the situation say about half of the 5,000-pound spacecraft is expected to survive its blazing descent through the atmosphere and will scatter debris — some of it potentially hazardous — over several hundred miles. The officials spoke on condition of anonymity because of the sensitivity of the matter.

The satellite is outfitted with thrusters, small engines used to position it in space, that contain the toxic rocket fuel hydrazine. Hydrazine can cause harm to anyone who contacts it.

The satellite, known by its military designation US 193, was launched in December 2006. It lost power and its central computer failed almost immediately afterward, leaving it uncontrollable. It carried a sophisticated and secret imaging sensor.

U.S. officials do not want this equipment to fall into the wrong hands.

"The Chinese and the Russians spend an enormous amount of time trying to steal American technology," said John Pike, a defense and intelligence expert. "To have our most sophisticated radar intelligence satellite — have big pieces of it fall into their hands — would not be our preferred outcome."

Where it lands will be difficult to predict until the satellite descends to about 59 miles above the Earth and enters the atmosphere. It will then begin to burn up, with flares visible from the ground, said Ted Molczan, a Canadian satellite tracker. From that point on, he said, it will take about 30 minutes to fall.

In the past 50 years about 17,000 manmade objects have re-entered the Earth's atmosphere. The largest uncontrolled re-entry by a NASA spacecraft was Skylab, the 78-ton abandoned space station that fell from orbit in 1979. Its debris dropped harmlessly into the Indian Ocean and across a remote section of western Australia.

In 2000, NASA engineers successfully directed a safe de-orbit of the 17-ton Compton Gamma Ray Observatory, using rockets aboard the satellite to bring it down in a remote part of the Pacific Ocean.

In 2002, officials believe debris from a 7,000-pound science satellite smacked into the Earth's atmosphere and rained down over the Persian Gulf, a few thousand miles from where they first predicted it would plummet.

Short-term exposure to hydrazine could cause coughing, irritated throat and lungs, convulsions, tremors or seizures, according to the Centers for Disease Control and Prevention. Long-term exposure could damage the liver, kidney and reproductive organs.

Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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