Satellite Collision Avoidance Methods Questioned After Space Crash
This computer model depicts the new debris from the Iridium-Cosmos crash (in red) on top of the existing debris (in green) in orbit today.
Credit: AGI.

STRASBOURG, France ? The Feb. 10 collision of an operational commercial satellite and a spent Russian spacecraft, which has resulted in a decades-long pollution of a widely used orbit, is raising questions about whether the company whose satellite was destroyed had done all it could to avoid the event, according to government and industry officials.

It also casts scrutiny on the way the U.S. Air Force disseminates information it collects on likely orbital collisions, these experts said.

Several officials said the collision, which produced two clouds of debris that are rapidly spreading above and below the 790-kilometer-high point of impact, might be enough to force the world's spacefaring nations to join forces to create a space traffic management agency whose data would be available to all nations with space-based assets.

Others were less optimistic. The dramatic in-orbit collision of the operational Iridium 33 satellite with the retired Russian Cosmos 2251 is the third event in two years drawing attention to the orbital debris issue in low Earth orbit. A Chinese anti-satellite test destroyed a retired Chinese satellite and unleashed thousands of pieces of debris at nearly the same altitude in early 2007, and the U.S. government sent a missile to destroy one of its own classified satellites a year later, saying the satellite's imminent atmospheric re-entry posed a hazard to people.

"Are these enough to dramatize the nature of the issues relating to space security? I'm not sure," said John M. Logsdon, chair in aerospace history at the U.S. National Air and Space Museum in Washington. "Maybe it will only happen when we face a loss of life, if debris destroys the International Space Station. Do we have to kill people to pay attention? That seemed to be the case with the [U.S. space] shuttle."

Logsdon made his remarks Feb. 20 during a space security conference here organized by the International Space University.

The U.S. Air Force is willing to help coordinate an international effort to create a space traffic management system whose goal would be to reduce the chances of a repeat of the Feb. 10 collision, said Air Force Brig. Gen. Michael J. Carey, deputy director of U.S. Strategic Command.

The Air Force has neither the resources nor the political mandate to stand watch over all commercial satellites, and the collision illustrates "the need to make information sharing more international," Carey told the conference Feb. 18. "If we don't operate safely in space then we don't have assured anything in space. We need a broader international engagement. We are actively seeking opportunities to put a coherent plan together."

But several officials questioned whether Iridium Satellite LLC has devoted sufficient resources to protecting not only its 66-satellite constellation, but the entire orbit in which it operates. The collision will render the Iridium orbit a more dangerous place to operate for all satellites, especially Iridium. One government official said a preliminary analysis suggests the several hundred pieces of debris created by the collision have roughly doubled the likelihood that another Iridium satellite will be struck.

The Iridium voice and data communications constellation is operated by Boeing Satellite Operations and Ground Systems of Leesburg, Va. Boeing officials declined to comment on the events leading up to the Feb. 10 collision.

Avoiding space crashes

Liz DeCastro, spokeswoman for Bethesda, Md.-based Iridium, said Iridium also would decline to discuss in detail what collision-avoidance measures it takes to protect its fleet and the orbital environment.

"Regarding the sequence of events that led to the collision, we may never know exactly what happened since we had no heads-up that it was coming," DeCastro said. "Our team that monitors our constellation saw that the satellite was missing when it went missing. We didn't move the satellite because we didn't know we had reason to."

Other officials cast a different light on the subject. The French space agency, CNES, which operates 15 satellites in low Earth orbit and tracks possible collision threats, reviewed the publicly available U.S. Space Surveillance Network data for the Iridium and Cosmos satellites that was released in the days preceding the collision. This same information is available to Boeing and Iridium.

Monique Moury, who works in the operational flight dynamics directorate at CNES, said the raw data alone coming from the U.S. Air Force would have been sufficient for CNES to perform a more-detailed analysis of the possible collision threat if the satellite in question had been CNES-controlled.

The U.S. Air Force data, called two-line elements, suggested that the likelihood of a collision between the Iridium and Cosmos satellites was 1 in 10,000. Given the known imprecision of the two-line-element data, that level of warning is enough to force CNES to take a second look, Moury said Feb. 20.

Unlike Iridium, CNES has at its disposal ground-based radars that, when used in conjunction with the U.S. two-line-element data, can provide a more-precise assessment of whether a given satellite faces a problem serious enough to warrant a collision-avoidance maneuver.

Whether Boeing and Iridium take two-line-element data into account in flying the Iridium constellation is unclear. Iridium's satellites were launched between 1997 and 1999 and designed to last for just five years. All are thus well past their planned service lives. Iridium officials have said in the past that they and Boeing do their utmost to save fuel, and have had to use fuel on some satellites to compensate for the loss of moment wheels, which provide satellite attitude control.

For a constellation in low Earth orbit, two-line-element data would present potential collision alerts so often that Iridium cannot respond to each one with a detailed examination and possible evasive maneuver, industry and government officials said.

"They would be reacting so often they wouldn't do much else," said Brian Weeden of the Secure World Foundation, which the week of Feb. 16 proposed a global space traffic management system to the U.N. Committee on the Peaceful Uses of Outer Space. Weeden said Feb. 20 that each operator is free to take two-line-element information into account, or not.

"There is no real industry standard for how to react to two-line data," Weeden said. "It comes down to each owner-operator's cost-benefit analysis." In addition to the use of fuel in performing an avoidance maneuver, a serious attempt to track threats for a constellation like Iridium would involve substantial spending on technical expertise, Weeden said.

Carey said that while the U.S. Air Force has compiled a catalog of objects being tracked in space, "we don't do conjunction analyses for all 18,000 objects in the catalog. Our first priority is human spaceflight. The next is our national security assets. Then you get to where you run out of opportunity and time to do conjunction analyses on all operating spacecraft. It would be a daunting task, and it's one we are not assigned to do."

But the published two-line elements are not the only database the U.S. Air Force regularly collects from its ground-based sensors. A second, more-precise database of the same population of orbital objects is also kept but is made available only in exceptional circumstances. This high-accuracy data is viewed as being too militarily sensitive to release publicly, officials said.

Commercial satellite operators may ask the Air Force to check the high-accuracy database to determine whether a given satellite faces an especially high probability of collision. Weeden said that in these cases, the Air Force takes the satellite operator's information and then performs a check against the high-accuracy data before returning a higher-reliability threat analysis.

But the procedure can take days ? too long to help operators of satellites in low Earth orbit. For these operators, the maximum time they have to react is about a week from the time they receive the basic two-line-element data until the time of potential impact.

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