Earth-watching satellite put to rest after 14 years

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After a stellar performance, NASA has shut off the largest spacecraft dedicated to monitoring the Earth's environment.

The Upper Atmosphere Research Satellite (UARS), which was launched by Space Shuttle Discovery in 1991 with a goal to last three years, paid off on a grand scale with an additional 11 years.

One of the most valuable benefits of the satellite's unexpected longevity was that it monitored the Earth and sun through an entire 11-year solar cycle, during which the amount of activity on the sun increases and decreases.

"The solar cycle has just a huge influence on the upper atmosphere, and being able to follow that was just very useful, and that same length of time enabled us to do correlation studies," said Carl "Skip" Reber, the project scientist when UARS was launched.

In terms of political effect, the most important UARS observation was the Antarctic "ozone hole."

"In conjunction with some aircraft measurements, UARS pretty much nailed down the cause for the formation of the ozone hole in the Antarctic being chemically induced and [that] the chemicals are really confined in their location," Mr. Reber said.

NASA is now flying smaller spacecraft with fewer instruments. Several Earth observation spacecraft have been put into the same orbit, and they follow one another across the planet's surface. The concept has been nicknamed "The A-train" -- several platforms following one another like cars of a subway train.

"The A-train is a bunch of spacecraft doing somewhat similar things that all happen to be in similar orbits," Mr. Reber said.

However, the biggest limitation with the A-train is the delay of several minutes between when the first spacecraft passes over a particular point on Earth and when the last spacecraft observes that location, because the atmospheric conditions can change easily between the two observations. In contrast, UARS' multiple instruments examined the same location simultaneously.

The school-bus-size satellite carried 10 scientific instruments that monitored the Earth's atmosphere and the sun. The instruments could monitor a variety of chemicals, including carbon dioxide, ozone, chlorine, methane, nitrogen oxides and, perhaps most important, chlorofluorocarbons. The $750 million, 7-ton spacecraft has orbited the Earth more than 78,000 times.

UARS was equipped with three 50 amp-hour batteries. One battery failed in June 1997. When the second battery started to show unusual temperatures, the decision was made to shut off UARS.

On Dec. 14, the commands were radioed to shut off UARS, and NASA has no plans to retrieve the satellite. The spacecraft will continue to orbit the Earth as a dead piece of space junk until it re-enters the planet's atmosphere and burns up in 2009 or 2010. It will be the largest object launched by the space shuttle to re-enter on its own.