The spacecraft lost its ability to maneuver when the satellite's pitch reaction wheel, which helps keep the spacecraft in proper orbital orientation, stalled on October 9. After numerous attempts to recover the instrument, ground controllers concluded that the wheel was no longer operational.
The mission's most important achievement was to determine the patterns of ocean circulation - how heat stored in the ocean moves from one place to another. Since the ocean holds most of the Earth's heat from the Sun, ocean circulation is a driving force of climate. "TOPEX/Poseidon has given us the longest and most complete observations of surface circulation in the deep ocean," says Fu. TOPEX/Poseidon made it possible for the first time to compare computer models of ocean circulation with actual global observations and use the data to improve climate predictions.
Another of the mission's major accomplishments was to map global tides for the first time. "Tides are the most visible changes in the ocean on a daily basis," explains Fu. "They are important for navigation, they have a big role in biological activity, and they are the major source of mixing in the ocean. The mixing may be small in scale, but it has a huge effect." Before TOPEX/Poseidon, tides in the open ocean could only be estimated.
TOPEX/Poseidon was the first mission to demonstrate that the Global Positioning System could be used to determine a spacecraft's exact location and track it in orbit. Knowing the satellite's precise position, to within 2 centimeters (less than 1 inch) in altitude, was a key component in making accurate ocean height measurements possible.
"TOPEX/Poseidon revolutionized oceanography by giving us the first global ocean observing system," says Massachusetts Institute of Technology oceanographer Dr. Carl Wunsch, one the mission's architects and early champions. Oceanographer Dr. Walter Munk, Scripps Institution of Oceanography, described the joint U.S. and French mission as "the most successful ocean experiment of all times."
The ocean is a different place now than it was when TOPEX/Poseidon first set sail. The sea is warmer than it was and getting warmer faster. Global sea level is rising. Heat in the tropics is moving northward more slowly. In some regions, some currents are faster while others are slower than in the past.
"The biggest lesson from TOPEX/Poseidon is that the ocean is changing all the time," says Fu, "and it is changing rapidly."
Jason, launched in 2001, now continues the same observations begun by TOPEX/Poseidon. For the past three years, the two satellites have flown in tandem, providing twice the coverage of the sea surface and allowing scientists to study smaller features than could be seen by one satellite. A future mission, the Ocean Surface Topography Mission, is planned for 2008. After that, scientists propose to make more detailed measurements of ocean surface topography to study critical issues such as sea level rise.
Wednesday, 18 January 2006, was the final day of the TOPEX/Poseidon era. The spacecraft was terminated gracefully by a series of commands, and this remarkably successful mission has finally ended.