PARIS — European Space Agency (ESA) officials have solicited support from the U.S. Air Force and the French and German governments to determine whether Europe’s Envisat Earth observation satellite, which stopped communicating April 8, is already dead or is still showing signs of life, ESA officials said April 13.
As of midday European time April 13, ESA officials had no explanation for why Envisat, in orbit since 2002, suddenly went silent.
In a press briefing, ESA’s Earth observation director, Volker Liebig, said the agency in the coming days would be searching for recovery possibilities and was not ready to write Envisat’s obituary.
“Our failure review board has been meeting many times since Sunday [April 8],” Liebig said. “We have not much data on which to judge. We are trying to re-establish contact and in parallel to collect more data on the satellite’s status — from ground radar images, from optical images from telescopes and also from satellites.”
In addition to scrambling a global network of ground stations to track Envisat more regularly, ESA has received precise satellite-positioning data from the U.S. Air Force’s Joint Space Operations Center and confirmed that the satellite remains in a stable orbit, according to Manfred Warhaut, head of mission operations at ESA’s European Space Operations Centre in Darmstadt, Germany.
The French space agency, CNES, was scheduled to take an optical image of Envisat when CNES’s Pleiades high-resolution optical Earth observation satellite passes 120 kilometers under the stricken spacecraft on April 15. The French Spot 5 satellite also imaged Envisat as Spot 5 flew 90 kilometers over it, Warhaut said.
From the ground, the German government’s Tracking and Imaging Radar, a 34-meter-diameter dish located near Bonn, took a radar image of Envisat that appears to confirm that the satellite has not broken apart following an in-orbit collision.
Warhaut said Envisat’s stable orbit could mean the satellite is adrift because of the lack of minor course corrections it normally would have made since April 8. But he said it was too soon to confirm that this was the case. He said there was no confirmation that Envisat had placed itself into a safe mode that would assure its solar arrays were oriented toward the sun.
Liebig said the agency has tentatively settled on two likely scenarios for what happened. The first is a failure of Envisat’s data-handling power supply, followed by a depletion of its batteries some 10 hours later. The sequence of events on April 8 lends credence to this, he said.
The second possibility is that Envisat’s platform lost power for an unknown reason.
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.
Liebig said the agency had hoped to keep Envisat in orbit long enough to calibrate some of the instruments to fly on board the Sentinel-1 satellite, the first GMES spacecraft, to be launched in late 2013.
Envisat and Sentinel 1 then would be flown in tandem until mid-2014, providing about a year of dual data streams.
Liebig said ESA had activated an existing agreement with the Canadian Space Agency to use Canada’s Radarsat-1 and Radarsat-2 satellites to fill some of the gaps in services resulting from Envisat’s loss. Other Earth observation and meteorological satellites — ESA’s Metop and the U.S.-French Jason 2 among them — can partly replace Envisat functions, he said.
In addition to Sentinel 1, the Sentinel 3 and Sentinel 5P payloads, to be launched between 2013 and 2015, together will provide a near equivalent to Envisat’s instrument suite.
But ESA has been battling with the European Commission over the status of GMES and has gone so far as to threaten to postpone the Sentinel launch in 2013 unless the commission commits to operating the Sentinel satellites.
If Envisat cannot be salvaged, the 8-metric ton satellite — measuring 26 meters by 10 meters by 5 meters in orbit — 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 would remain in place for 100 to 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.
Warhaut said ground teams have performed around a dozen collision avoidance maneuvers for Envisat in its 10-year life. He said ESA had hoped to keep a minimal Envisat operating capability for several years after its retirement in 2014 to perform similar such maneuvers until its fuel ran out.
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