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U.S. military space plane nearing end of design life
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: November 23, 2010


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Observers tracking movements of the U.S. Air Force's X-37B secretive space plane report the spacecraft is dropping altitude, a possible sign the clandestine mission is near landing as it approaches the limit of the its design life.


Artist's concept of the X-37B space plane in orbit. Credit: Boeing
 
Air Force officials remain silent on landing and recovery plans for the reusable space plane, other than it will return to Earth at Vandenberg Air Force Base, Calif. The X-37B features stubby wings, a powerful orbit-changing engine and a landing gear for touchdown on a 15,000-foot-long runway originally built for the space shuttle.

The robotic spacecraft, also known as the Orbital Test Vehicle, launched April 22 on an Atlas 5 rocket from Cape Canaveral, Fla. The flight entered a news blackout a few minutes after liftoff, but a network of amateur satellite trackers have logged the spacecraft's journey through space with countless visual sightings.

The 11,000-pound spaceship entered an orbit more than 250 miles above Earth after launch, but four significant maneuvers have since altered that trajectory, causing observers to lose track of the X-37B for several days at a time.

An engine firing Aug. 9 raised the space plane's orbit, but three more maneuvers around Oct. 6, Nov. 1 and Nov. 12 reduced the X-37B's altitude, which is now estimated to be between 174 miles and 182 miles, according to Ted Molczan, a respected and experienced satellite observer in Canada.

The space plane, which appears from the ground as a recognizable moving dot, was rediscovered Tuesday by Greg Roberts, a skywatcher in South Africa. Another precise observation is necessary to confirm the new orbit parameters.

Molczan said the inclination of the winged spacecraft's orbit remains 40 degrees.

"At the risk of getting too far ahead of the data, I note that one feature the new orbit would have in common with all of the previous ones, is a ground track that nearly repeats every few days," Molczan told Spaceflight Now.

Such orbits permit Earth observation satellites to rapidly revisit the same location and collect imagery.

"Ground tracks that nearly repeat every two, three or four days are a common feature of U.S. imaging reconnaissance satellite orbits," Molczan said.

The X-37B's step-by-step descent comes as the spacecraft approaches the end of its design life, which the Air Force says is 270 days, or nine months. The ship will reach the design limit in mid-January.

"Assuming that the different altitudes are related to evaluating some experiment, it makes sense to first go to the highest required altitude, and then step down lower, so that in effect you are gradually coming home as you complete your experiments," Molczan said.

Before launch, Air Force officials discussed generalities of the X-37B's programmatic history and technical specifications but would not disclose any of its planned activities in space, including the duration of the mission.

Air Force officials have so far declined to discuss when the ship will land, saying there will be little more information released until after the space plane is back on the ground.


The X-37B was shrouded inside a 5.4-meter payload fairing for launch on an Atlas 5 rocket. Credit: U.S. Air Force
 
The only tip-off of a pending landing could be found in notices to airmen, or NOTAM messages, alerting pilots of hazards and restricted airspace for military operations.

When the X-37B does come in for landing, it will fire its main engine to slow its speed in a maneuver much like the de-orbit burn on the space shuttle. The space plane will plunge into the atmosphere with its nose pitched up, relying on GPS navigation and autopilot computers to guide itself toward the California coast.

The spacecraft's destruct system will be activated to terminate the flight if it veers off course and threatens populated areas, the Air Force said before launch.

The X-37B will autonomously line up with the Vandenberg runway and make a steep dive toward the landing site, pulling up in the final moments and touching down at a speed of nearly 300 mph.

The Air Force designated Edwards Air Force Base, an alternate shuttle landing site, as a backup runway for the X-37B.

The space plane was built by the Boeing Phantom Works division and is managed by the Air Force Rapid Capabilities Office. Mission operations are run by the Air Force's 3rd Space Experimentation Squadron at Schriever Air Force Base, Colo.

The craft is about one-fourth the size of a space shuttle orbiter. It measures more than 29 feet long and 9.5 feet tall, with a wingspan of nearly 15 feet, according to an Air Force fact sheet.

The mini-shuttle's payload bay can hold several hundred kilograms of payloads, but exactly what this mission is testing is classified.

The flight's demonstrations will continue after landing, when engineers will evaluate the craft's performance and condition after nearly a year in space.

Boeing and the Air Force are building a second X-37B vehicle for launch as soon as March.

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