PARIS — The European Space Agency (ESA) on Oct. 11 denied allegations that it could have done more to prevent its large Envisat Earth observation satellite, now dead in polar low Earth orbit, from spending the next 100-plus years as a major space-debris threat.

Responding to assertions made in a paper presented by the International Institute of Space Law (IISL), the 20-nation ESA specifically denied that it had passed up opportunities to place Envisat in a disposal orbit at the end of its life in favor of using the satellite’s remaining fuel to continue operations.

The 8-meter-long, 8,000-kilogram Envisat was launched in 2002. Because of decisions made early in its design in the late 1980s, it carried a relatively small fuel reservoir.

In its Oct. 11 statement, ESA said the Envisat design was settled at “a time when space debris was not considered to be a serious problem,” and well before debris mitigation guidelines were formulated by a group of spacefaring nations, including ESA, as part of the Inter-Agency Space Debris Coordination Committee, or IADC.

Debris mitigation guidelines were established by the United Nations in 2007. ESA said its governments adopted them in 2008 for all ESA projects.

The guidelines urge nations with low-orbiting satellites to cease operations early enough to maintain sufficient fuel to lower the orbits so that atmospheric drag pulls the satellites into destructive re-entry within 25 years.

The orbit needed to assure a re-entry within 25 years varies depending on a variety of factors including the satellite’s dimensions and solar activity.

ESA said that in Envisat’s case, the orbit required to assure re-entry within 25 years is about 600 kilometers in altitude. Envisat’s operating orbit was 780 kilometers.

“Even if controllers had lowered the satellite immediately after launch in 2002, there would not have been enough fuel to bring it down low enough … where it could reenter within 25 years,” ESA said, adding that this solution “was never an option.”

The IISL paper, presented during the 63rd International Astronautical Congress, held the week of Oct. 1 in Naples, Italy, suggests that ESA could be held liable if Envisat or pieces of it collide with an operational satellite.

The paper says ESA, in effect, turned its back on lowering Envisat’s orbit in the satellite’s final years to squeeze extra operational life from one of the agency’s most successful environment-monitoring missions. It is this allegation that ESA refutes directly.

In 2010, ESA lowered Envisat to the less-crowded orbit of 768 kilometers. This orbit permitted Envisat to continue operating, but with enough fuel to perform collision-avoidance maneuvers “for several years.” Envisat failed without warning in April and has since been given up for lost.

ESA said it now includes deorbiting capability into the design of its satellites, and is “strongly committed to reducing space debris.” The agency noted that in 2011 it lowered the orbit of its still-operating ERS-2 radar Earth observation satellite to 570 kilometers, low enough to re-enter the atmosphere “well within 25 years.”



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Peter B. de Selding was the Paris Bureau Chief for SpaceNews.