PARIS — Europe’s large Envisat Earth observation satellite has stopped communicating with its ground controllers and has not responded to emergency measures despite four days of effort, the European Space Agency (ESA) said April 12.

“While it is known that Envisat remains in a stable orbit, efforts to resume contact with the satellite have, so far, not been successful,” ESA said in an April 12 statement.

With a core body 8 meters long and a launch weight of 8,000 kilograms, Envisat is the largest nonmilitary satellite ever launched. Placed into low Earth orbit in 2002, the satellite on March 1 passed its 10th anniversary — more than double its expected five-year life.

But Envisat, which is equipped with 10 observing instruments, is doing more than just logging overtime. It is viewed by ESA and by the European Commission — the executive arm of the 27-nation European Union — as essential to providing data continuity as these two organizations prepare a multibillion-dollar network of Earth observation satellites as part of the Global Monitoring for Environment and Security, or GMES, program.

ESA officials in 2010 extended Envisat’s mission to 2013, time enough to await the launch of the first GMES satellites, called Sentinels.

To squeeze additional life out of Envisat, ESA lowered its orbit to around 783 kilometers, thereby saving on fuel-using adjustments the satellite would have made otherwise.

In an April 12 statement, ESA’s director of Earth observation, Volker Liebig, said: “The interruption of the Envisat service shows that the launch of the GMES Sentinel satellites, which are planned to replace Envisat, becomes urgent.”

If Envisat cannot be salvaged, the satellite will immediately be transformed from one of the world’s most sophisticated environmental monitoring platforms into one of the largest and most fragile pieces of space garbage in orbit.

In its current orbit, a dead Envisat likely will remain in place for about 150 years, according to ESA calculations. Unlike rocket stages in low Earth orbit, which are resistant to damage caused from collision with small pieces of debris, Envisat is as fragile as a heavily ornamented Christmas tree.

Envisat ground controllers performed a collision-avoidance maneuver in early 2010 to prevent a possible collision with a large rocket upper stage. ESA officials have estimated that there is up to a 30 percent chance that Envisat will collide with another piece of orbital debris before it is tugged into the atmosphere — assuming the global population of debris in low Earth orbit does not increase. Most orbital debris experts view this as a highly optimistic scenario.



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Peter B. de Selding was the Paris bureau chief for SpaceNews.