13-Year Mission Ends for Ocean Scanning Satellite

An artist's illustration of the Topex/Poseidon satellite in Earth orbit. CREDIT: NASA.

U.S. and French researchers announced the end of an ocean-watching satellite Thursday, concluding a 13-year mission that transformed their understanding of Earth's largest bodies of water.

Mission managers ended the TOPEX/Poseidon mission following the failure of its final reaction control wheel - used to keep the spacecraft pointed properly - that prevented future science observations, NASA officials said.

"It's definitely a happy ending," said Lee-Lueng Fu, the mission's project scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, in a telephone interview. "The mission launched in 1992 and we specified operation from three to five years, but we got 13. That's more than anybody could ask for."

A joint mission between NASA and France's Centre National d'Etudes Spatial (CNES), the TOPEX/Poseidon spacecraft watched Earth's oceans from an 830-mile (1,335-kilometer) orbit.

"The ocean is changing all the time," Fu said. "This was the first time we mapped the elevation of the ocean's surface."

The satellite used radar altimeters to track changes in sea level, map ocean tides and study ocean water mixing. Observations from the spacecraft also helped researchers to better understand global climate change and weather phenomena such as El Ni?o, NASA officials said.

"It was a revolution for oceanography," Fu said of the TOPEX/Poseidon mission. "It was the ocean equivalent of a weather map."

TOPEX/Poseidon also proved that global positioning system (GPS) measurements could be used to track orbiting satellites. The spacecraft's science mission ended after almost 62,000 orbits around Earth.

But while the mission is over, the spacecraft won't come crashing into the Earth's atmosphere any time soon due to the relatively low drag forces in its current orbit.

"It could be up to 1,000 years," Fu said of TOPEX/Poseidon's return.

A successor to TOPEX/Poseidon - dubbed Jason - launched in December 2001 and worked alongside its predecessor, allowing flight controllers to verify its function by comparison, added Fu, who also serves as its project scientist at JPL.

A third spacecraft, the Ocean Surface Topography Mission, is expected to join Jason in 2008 to continue its ocean-watching task. NASA and CNES are cooperating on both missions.

"This data stream, hopefully, will become a permanent asset of the international community," Fu said.

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