The orbit-raising maneuver was first noticed Aug. 14 by amateur skywatcher Greg Roberts of Cape Town, South Africa, when the object failed to appear as predicted by the last known orbit. After several nights of searching, Roberts found it again on Aug. 19, which enabled the new orbit to be estimated with sufficient accuracy to easily locate the X-37B space plane on subsequent nights.
That detective work led to other sky-sleuthing detections by Alberto Rango in Rome and Brad Young in Tulsa, Okla. Their sharp-eyed skills were essential in refining the calculation of the space plane's new orbit and confidently determining the circumstances of the orbit-raising maneuvers. [Photos of the X-37B space plane.]
Also called the Orbital Test Vehicle-1 (OTV-1), the hush-hush X-37B — a robotic reusable spacecraft — was launched April 22 atop an Atlas 5 rocket from Cape Canaveral, Fla. From that point onward, mum has been the word about what it's up to.
Air Force officials have said only that the winged spacecraft is undergoing planned mission tests.
"Following the successful launch and initialization, the first flight of the X-37B Orbital Test Vehicle (OTV-1) continues to focus on checking out the on-orbit performance of the vehicle and proving the technologies required for long-duration, reusable space vehicles with autonomous re-entry and landing capabilities," Air Force spokesman Andy Roake told SPACE.com in a statement.
The X-37B had circled Earth for 123 days as of Monday. Air Force officials have said the unmanned spacecraft could stay in orbit for around 270 days.
This week the space plane is expected to fly over many North American cities and towns, according to the satellite-watching website Spaceweather.com. Skywatchers who know where to look using telescopes have a chance to see the X-37B space plane, weather permitting.
Molczan is part of the amateur skywatching group that initially spotted the space plane in May and recorded video of the X-37B in space using ground-based equipment.
Coincidence is meaningful
Molczan's analysis of the new orbit enabled him to discern that during the orbit-raising maneuvers, the space plane passed within range of several Air Force Satellite Control Network (AFSCN) facilities: the Hawaii Tracking Station at Kaena Point, Oahu; Guam Tracking Station at Andersen Air Force Base; Vandenberg Tracking Station, Vandenberg AFB, California; Colorado Tracking Station, Schriever AFB in Colorado Springs; and the New Hampshire Tracking Station, New Boston (N.H.) Air Force Station.
"To put this into perspective, these are five out of the eight fixed AFSCN tracking stations, and X-37B's orbit only passes within range of six stations," Molczan said. "Diego Garcia was the one of the six that was out of range for the orbit-raising. So it seems reasonable to conclude that the coincidence is meaningful."
The Diego Garcia Tracking Station is on the Diego Garcia atoll, part of British Indian Ocean Territory.
From launch until Aug. 9, the X-37B was in a 40-degree orbit, zipping around the Earth at a 250-mile-by-260-mile (403 km by 420 km) orbit. It maintained a nearly constant altitude by making frequent small maneuvers to counter atmospheric drag, Molczan said.
Then on Aug. 9, the space plane made two larger maneuvers, increasing its mean altitude by roughly 17 miles (27.5 km).
The mystery space plane's first orbital adjustment occurred near its apogee (high point of orbit), raising its orbit to approximately 260 miles by 276 miles (420 km by 445 km). The final maneuver occurred near the next high point of its orbit, which happened to coincide near overhead passes of Vandenberg Air Force Base in California and Schriever Air Force Base in Colorado. The resulting orbit was 268 miles by 276 miles (432 km by 445 km), Molczan noted.
As for whether the space plane's orbit change relates to its imminent return to Earth, slated to be at Vandenberg, Molczan told SPACE.com he doesn't think so.
"The spacecraft has considerable cross-range capability, so I suspect that phasing maneuvers for landing seldom would be required. Had this been a phasing maneuver, I suspect it would have been smaller, and a decrease in altitude. Very small maneuvers well ahead of a planned landing can bring about any required shift in ground track," he said.
In Molczan's view, the orbit raising may have been part of the flight testing of the military space plane. "It could also have been required by the payload. The nearly four-day repeating ground track of the original orbit — and the nearly 6 day repeating ground track of the new orbit — suggests that a reconnaissance payload is aboard. But I have no way to be certain."
Robotic landing in California
Little is known about the classified duties of the X-37B. However, Air Force officials did make it clear in pre-launch statements that the craft's first mission would focus on proving technologies necessary for long-duration reusable space vehicles with autonomous re-entry and landing capabilities.
The robotic mini-plane resembles the space shuttle and is equipped with a payload bay about the size of a pickup truck bed.
Built by Boeing's Phantom Works division, the X-37B space plane is just over 29 feet (9 meters) long and weighs about 11,000 pounds (5,000 kg). It stands slightly more than 9.5 feet (3 meters) high and has a wingspan of just over 14 feet (4.3 meters).
The spacecraft's new mission is overseen by the Air Force Rapid Capabilities Office. The X-37B is believed to be operated by contractors under the direction of Air Force Space Command's 3rd Space Experimentation Squadron at Schriever Air Force Base.
The X-37B is designed to orbit Earth at altitudes of up to 500 nautical miles, loiter in orbit for up to 270 days, then re-enter the atmosphere to make an automated landing at Vandenberg.
"After the test objectives are satisfied, we look forward to a successful re-entry and recovery at Vandenberg Air Force Base, California, and post-landing refurbishment of the vehicle," said Roake, the Air Force spokesman. "No landing date has been scheduled."
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Leonard David has been reporting on the space industry for more than five decades. He is past editor-in-chief of the National Space Society's Ad Astra and Space World magazines and has written for SPACE.com since 1999.