ESA has agreed to extend the operations of its large Envisat radar satellite, which had been scheduled for retirement in 2010, for three years despite the fact that Envisat is about to exhaust its fuel supply. The satellite will be operated in a slightly lower “drift” orbit that will result in a minor interruption of some services, said Volker Liebig, ESA’s Earth observation director.

Operating Envisat costs about 40 million euros per year including data distribution. Continuing its use through 2013 will increase the likelihood that users of Envisat data will see no interruption of service between Envisat and the first Sentinel satellite, Sentinel-1, which is scheduled for launch in 2012.

Envisat was designed before ESA adopted rules about de-orbiting satellites at the end of their lives to reduce the population of orbital debris. The international orbital-debris protocol asks that owners of satellites in low Earth orbit — where most Earth observation spacecraft operate — lower the altitude of their satellites at retirement so as to ensure that they re-enter the atmosphere within 25 years and burn up. Envisat’s fuel tanks are too small to accommodate regular operations and the end-of-life de-orbit maneuver. Its forerunner, the smaller ERS-2 radar satellite, has enough fuel reserves to be de-orbited. ERS-2, in orbit for 15 years and flying without stabilizing gyroscopes since 2001, will be retired in mid-2011.

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