Europe's flagship Envisat observation satellite, one of the most advanced environmental spacecraft ever built, has stopped sending data to Earth after 10 years of service, the European Space Agency announced Thursday.

Artist's concept of Envisat in orbit. Credit: ESA
 
Controllers have not heard signals from the almost 9-ton satellite since Sunday, when it was supposed to pass over a ground station in Kiruna, Sweden.

"ESA's mission control team declared a spacecraft emergency and immediately called for support from additional ESA tracking stations around the world," the agency said in a statement. "A team of operations and flight dynamics specialists and engineers was quickly assembled."

The recovery team, which included industrial representatives, has scrambled to establish contact with Envisat since Sunday, according to ESA.

"While it is known that Envisat remains in a stable orbit around Earth, efforts to resume contact with the satellite have, so far, not been successful," the ESA statement said.

Envisat launched in March 2002, and its five-year baseline mission ended in 2007. The satellite has continued collecting data since then in an extended mission.

ESA planned to extend Envisat's mission through 2013 to overlap with the first launches of a series of Sentinel satellites designed to ensure data continuity from Envisat and the European Remote Sensing missions, which concluded in 2011.

Envisat's Medium Resolution Imaging Spectrometer captured this image April 8 shortly before Envisat stopped communicating. The image of Spain and Portugal was the last transmitted to the ground via ESA's Artemis Ka-band data relay satellite. Credit: ESA
 
"The interruption of the Envisat service shows that the launch of the GMES Sentinel satellites, which are planned to replace Envisat, becomes urgent," said Volker Liebig, ESA's director of Earth observation programs.

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 focused on a slice of Envisat's broad scientific portfolio, including radar observations, high-resolution imaging, ocean surface monitoring, and atmospheric studies.

European controllers lowered Envisat's orbit in 2010 to reduce the craft's fuel usage to prolong the mission until the end of 2013.

ESA officials plan to speak with reporters Friday to discuss the Envisat anomaly.

Envisat has orbited Earth more than 50,000 times at an altitude of nearly 500 miles since its launching. Outfitted with 10 instruments to probe the planet's land, oceans, ice and atmosphere, Envisat is the world's most complex Earth observation satellite, according to ESA.

The spacecraft bus stretches more than 30 feet long, and its solar array spans 85 feet. The mission cost 2.3 billion euros, or $3 billion.

More than 4,000 projects in over 70 countries were supported by Envisat data over the last decade, the ESA statement said.