Operators of satellites in geostationary orbit are getting better about disposing of retired or crippled satellites but still are not systematically complying with international guidelines designed to limit the buildup of orbital garbage, according to European officials studying the issue.

Using data from the U.S. Space Surveillance Network and other U.S. and European ground-based sensors, officials have found that the habits of satellite operators are being modified despite the absence of a global regulatory regime dealing with orbital junk.

While the most immediate concern is low Earth orbits lower than 2,000 kilometers in altitude, it is the geostationary arc 36,000 kilometers over the equator that attracts almost all communications satellites.

How to prevent a buildup of uncontrolled satellites in this most valuable strip of orbital property was the focus of a briefing among satellite operators and European government officials Jan. 25 here at the French space agency, CNES.

Between 1997 and 2005, an estimated 159 satellites in geostationary orbit were retired, according to estimates made by the European Space Agency (ESA). Of these, only 64 complied with guidelines set in late 2002 by the 11-member Inter-Agency Space Debris Coordination Committee (IADC), whose members include all the major space faring nations.

Owners of an additional 55 satellites in the same nine-year period made at least some attempt to move their hardware out of the geostationary arc . The 40 remaining satellites retired during the period appear to have been left on station, where they ultimately could pose a threat to future spacecraft.

Heiner Klinkrad, acting director of ESA’s orbital debris department, said the figures are better than they look because the trend in the past several years has been toward greater compliance with IADC’s proposals.

“It used to be that one-third of the satellites were disposed of correctly, one-third only partly complied with disposal guidelines, and one-third did nothing,” Klinkrad said. “The trend now is for more and more operators to take the needed steps, which of course are in their interest.”

But other officials said Russian military authorities have yet to show systematic concerns for satellite disposal regulations, even if Russia’s civilian space agency, Roskosmos, is an active member of IADC.

The IADC guidelines say that the average geostationary satellite should be retired to an orbit about 300 kilometers higher. At this position, the satellite’s orbit can be controlled and it is out of the way of future satellites entering service.

From an operator’s point of view, the problem is that estimating how much fuel is remaining in a satellite is not an exact science, and no operator wants to abandon a satellite earlier than necessary.

A large telecommunications satellite that is well booked with customers can generate $50 million to $100 million in revenues per year and can cost $200 million or more to replace.

“It remains difficult to estimate how much fuel remains on a satellite, and this is an issue for operators,” said Fernand Alby, manager of the CNES orbital debris department.

In a press briefing Jan. 25, Alby said commercial operators have occasionally been faced with a choice of retiring a young satellite that has lost one of its two on board systems that permit ground teams to control it, or risking the possibility of the second, identical system failing, which would leave the satellite in geostationary orbit.

David Assemat, deputy director of CNES’ Toulouse space center, said another problem for satellite owners is that they might lose control of their satellites during an attempt to meet IADC rules.

The IADC proposes that before being shut down in a graveyard orbit, satellite fuel tanks be emptied and their batteries and other subsystems be run down so that the retired satellite cannot explode either spontaneously, or on contact with another object, decades later.

An exploded satellite or rocket stage can create hundreds of pieces of debris and multiply the threat to other satellites in the same orbit.

“The mere act of emptying a fuel tank can change the satellite’s orbit, and draining batteries takes a long time,” Assemat said. “These are delicate maneuvers.”

The IADC’s record keeping points to two explosions in geostationary orbit that created debris fields — the 1978 explosion of a Russian Ekran-2 television satellite, and the 1992 explosion of a U.S. Titan 3 rocket stage.

But Klinkrad said ESA’s space debris telescope in Tenerife, Spain, which cannot perform wide field scans like the U.S. Space Surveillance Network but is able to detect objects in geostationary orbit as small as 15 centimeters, has found at least 11 other sites that appear to be explosion-caused debris fields.

“You can see banana-shaped clusters associated with explosion events,” Klinkrad said. “We have the suspicion that there were more explosions in geo orbit than just the two that are commonly referenced.”

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