The European Space Agency on Wednesday declared the Envisat environmental satellite lost one month after the bus-sized craft unexpectedly stopped communicating 10 years after its launch.

Artist's concept of the Envisat satellite in orbit. Credit: ESA
 
Controllers repeatedly attempted regaining communications with the satellite since it went silent April 8, but Envisat never responded. ESA officials said they will continue trying to send commands to Envisat until July.

"Although chances of recovering Envisat are extremely low, the investigation team will continue attempts to re-establish contact while considering failure scenarios for the next two months," ESA said in a written statement.

Optical, radar and laser observations of Envisat throughout April showed the satellite remained intact and in a stable orbit. The French space agency, CNES, tasked the Pleiades imaging satellite to snap photos of Envisat on April 15, producing high-resolution photos of the craft from a range of 60 miles.

A radar site in Germany also collected imagery of Envisat from the ground.

But despite success in observing Envisat from space and the ground, ESA has been unable to determine the definitive cause of the satellite's failure.

Engineers believe one of two failure scenarios likely ended Envisat's mission. One is the loss of a power regulator, blocking telemetry and commands to and from the satellite, according to ESA.

Another possible failure mode involves a short circuit, which would have automatically sent Envisat into safe mode, a condition in which the satellite would have pointed its solar panels toward the sun to charge on-board batteries and keep the craft alive.

But Envisat's transition into safe mode could have been interrupted by a second problem, potentially handicapping the satellite and leading to its loss.

The French Pleiades observation satellite captured this optical image of Envisat on April 15. Credit: CNES
 
Manfred Warhaut, head of mission operations at the European Space Operations Center in Darmstadt, Germany, told Spaceflight Now a double failure on Envisat's power bus is the more likely cause of the craft's predicament.

ESA officials hoped Envisat would continue returning valuable data on Earth's atmosphere, oceans, ice sheets and land masses for up to two more years.

Launched in March 2002, Envisat was five years beyond its planned service life, but ESA officials expected to keep the satellite operating until 2014, when it was to be removed from service when follow-on Sentinel observation craft begin launching. Envisat - a flagship-class satellite - was one of the most complex and costly civil Earth observation missions ever launched.

The spacecraft bus stretches more than 30 feet long, and its solar array spans 85 feet. The platform is outfitted with 10 instruments to probe the planet's land, oceans, ice and atmosphere.

The mission's total cost was about $3.4 billion, including the satellite's construction, launch and 10 years of operations, according to ESA.

"Envisat data were used continuously by oil spill services, by sea ice monitoring, by different types of meteorological forecasts, also sea state forecasts, by atmospheric monitoring systems, and also by terrain subsidence monitoring for many cities in Europe and elsewhere," said Volker Liebig, ESA's director of Earth observation programs.

File photo of Envisat's launch aboard an Ariane 5 rocket on March 1, 2002. Credit: CNES
 
Envisat and its predecessor satellites provided a continuous 20-year archive of research data.

"We monitored constantly the retreat of the Arctic sea ice coverage," Liebig told reporters April 13. "We found with the satellite the mobilization of the glaciers in Greenland and Antarctica. This led to extensive research from satellites and on the ground to understand what causes this mobilization."

Envisat's broad portfolio included measurements of atmospheric chemistry, rising sea levels, plate tectonics, greenhouse gas emissions, and land subsidence.

Europe is building a series of smaller Sentinel satellites to replace portions of Envisat's observation mission.

The Sentinel satellites are developed by ESA on behalf of the European Union's Global Monitoring for Environment and Security, or GMES, program.

The first Sentinel satellite is due to launch in 2013. Each Sentinel will carry instruments to fulfill part of Envisat's mission, including radar observations, high-resolution imaging, ocean surface monitoring, and atmospheric studies.