Space is full of stuff, especially the space close to Earth. And the U.S. military would like to know more about what it is and where it is.

There are thousands of pieces of space junk – bolts, broken chunks of satellites, fuel tanks, rocket motors and other odds and ends from five decades of space flight.

Plus, there’s the debris from the Feb. 11 collision of an Iridium communications satellite with a Russian military satellite.

And there are about 900 other operational satellites.

Increasingly, though, the U.S. Air Force worries that today’s orbiting objects may be soon be joined by stealthy, mobile and deadly satellites that threaten U.S. assets in space.

The U.S. Air Force already keeps track of more than 18,000 objects in orbit, mainly using ground-based telescopes and radars. But the service wants to keep a sharper eye on the proliferating space clutter.

A Boeing-built satellite scheduled to be launched in April after years of delays is intended to do that.

The Space Based Space Surveillance system (SBSS) is a $425 million satellite equipped with a sophisticated 500-pound digital camera mounted on a high-speed gimbal, so it can be quickly swiveled to point and shoot pictures of space objects of interest.

Boeing promises that SBSS “will revolutionize space situational awareness.”

It’s a multimission satellite, said Marco Caceres, director of space studies at the Teal Group, a Fairfax, Va., consulting firm. SBSS is designed to “spy on spy satellites, monitor things that are launched, observe phenomena – natural or manmade – that might damage U.S. satellites and keep track of space junk,” he said.

The satellite’s launch has been delayed at least three times. At its most optimistic, the Air Force had hoped to have it up and operating in 2006.

Air Force officials say they hope eventually to be able to monitor satellites and space debris more or less in real time, much as the Federal Aviation Administration tracks commercial and military aircraft.

To do that is likely to take a constellation of four or more SBSS satellites. The tentative schedule for a constellation to be in place is about 2014.

The SBSS being launched this spring replaces an orbiting telescope, the Space Based Visible Sensor, which ceased operating last June after 12 years of service. Since then, debris and satellites have been tracked by GEODSS, the Ground-based Electro-Optical Deep Space Surveillance network of sensors in New Mexico, Hawaii and the island of Diego Garcia.

An obvious benefit of a space-based camera is that it can operate without interference from weather, atmosphere or daylight.

The Air Force calls the SBSS “essential” to achieving “full space situational awareness” – that is, knowing what and where everything in space is.

“Our adversaries recognize our overwhelming dependence on space assets, and we must have the ability to detect and track space objects – especially those that might be considered a threat,” the Air Force says in a written description of the program.

Among the threats that worry the Air Force are kinetic kill vehicles that destroy satellites by crashing into them, and killer satellites designed to collide with, explode next to or otherwise destroy U.S. satellites.

“When we see that a satellite has moved or is no longer in its current element set, SBSS can help us find that satellite and track it,” according to the Air Force Space Command.

Enemy stealth satellites are another concern.

The United States has had stealth satellites for about 15 years, said space and national security expert John Pike. Built to be invisible to radar, stealthy U.S. satellites pass over countries like China and Russia, collecting intelligence undetected. “And we want to make sure they don’t do that to us,” Pike said.

The idea is that the SBSS camera will be able to spot stealthy satellites even if radar can’t.

SBSS satellites also would provide a much wider view of space than do ground-based telescopes and radars. From the ground, sensors get a glimpse of objects in space only as they pass overhead. If they change orbits while out of range of the sensors, it is discovered only after the fact, when the satellites don’t show up where they’re expected to be on the next pass.

But a camera based in space can keep track of objects over a much wider portion of a hemisphere.

Boeing, which is reluctant to discuss SBSS in much detail, declined requests for an interview and failed to respond to written questions. The company announced in early February that its satellite had completed “end-to-end mission functionality” tests on the ground and was finally being readied for launch.

SBSS has had a troubled history.

If it gets off the ground this spring, the satellite will be three years late for the Air Force’s hoped-for launch in 2006.

By 2005, after about 18 months of SBSS development, an independent review team declared the program “not executable” as it was organized by Boeing and the Air Force. The team also determined that the program’s assembly, integration and test plan was risky and that its requirements were overstated, the Government Accountability Office reported.

As a result, the program was restructured in 2006. By then, the cost of the satellite portion of the program alone had ballooned from $189 million to $425 million. With other costs, such as the cost of the launch, the entire program grew to about $825 million. A four-satellite constellation could cost $3 billion or more, Caceres said.

Launch dates in 2007 and 2008 were also missed amid the turmoil.

Boeing is teamed with Ball Aerospace & Technologies on the SBSS program.

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