The U.S. Air Force's clandestine X-37B space plane will glide back to Earth as soon as Friday and land on a concrete runway in California, the military announced Tuesday.

The X-37B spaceplane sits on a runway at Vandenberg Air Force Base during prelaunch taxi tests. Credit: U.S. Air Force

The X-37B spacecraft, also called the Orbital Test Vehicle, has been circling Earth since April 22 conducting classified tests and technology demonstrations while under the watchful eye of amateur observers on the ground.

The two-paragraph statement issued by the 30th Space Wing at Vandenberg Air Force Base says the "exact landing date and time will depend on technical and weather considerations," but it is expected between Friday and Monday.

Preparations for the landing are underway at Vandenberg, the statement said.

Landing opportunities are available in the early morning Pacific time, according to multiple sources.

The winged spacecraft will fire its main engine to drop from orbit and descend through the atmosphere, braving searing hot temperatures over the Pacific Ocean under an insulating shield of blankets and ceramic tiles.

It will be the first attempted fully autonomous re-entry and runway landing from orbit in the history of the U.S. space program. The Soviet Union's Buran space shuttle accomplished the feat in a single automated test flight in 1988.

The X-37B final approach will be guided by a differential Global Positioning System precision landing system to feed navigation data into the craft's flight computer, giving the vehicle cues as it flies toward Vandenberg and lines up with the runway.

The space plane will arrive near the landing site and align with the runway for a steep final approach glide. In the last few seconds of the flight, the craft will flare its nose, deploy its tricycle landing gear and slap down on the runway.

The craft carries a destruct system to terminate the flight if it veers off course.

Vandenberg's runway is 15,000 feet long and stretches from northwest to southeast.

Edwards Air Force Base is an alternate landing site for the mission.

The Air Force did not provide any more details Tuesday, but a Pentagon spokesperson said more information could be released later this week.

Before Tuesday's terse press release, the Air Force was mum on progress of the X-37B mission since it entered a news blackout about 17 minutes after launch April 22.

An artist's concept of an X-37 re-entry when the program was under NASA management. Credit: NASA/MSFC

Ted Molczan, a respected skywatcher based in Canada, reports the space plane is now in a nearly circular orbit about 177 miles high with an inclination of 40 degrees.

A loosely-affiliated network of satellite observers has catalogued four major maneuvers by the X-37B since its launch. Once in August and October, then twice in November, observers lost track of the spacecraft only to rediscover the satellite in a different orbit.

Firings of the X-37B's powerful main engine, nearly the size of the space shuttle's primary orbit-changing thrusters, Oct. 6, Nov. 1 and Nov. 12 decreased the vehicle's altitude, a potential clue it was approaching landing.

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.

Although its outside appearance resembles the shuttle, the X-37B features modernized systems but is significantly more limited than NASA's orbiter fleet. Its payload bay is about the size of a pickup truck bed.

Instead of running on fuel cells like the shuttle, the spacecraft unfurled a small solar array to produce electricity in orbit, making the X-37B capable of on-orbit stays lasting up to 270 days.

Other technologies the space plane was supposed to test include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, and conformal reusable insulations, the fact sheet said.

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

Originally developed by NASA, then taken over by the Pentagon, the X-37 is a reusable space plane first developed as a testbed for launch vehicle technologies and a pathfinder for less expensive and more reliable access to low Earth orbit.

NASA transferred management of the program to the Defense Advanced Research Projects Agency in 2004, then the Air Force Rapid Capabilities Office took control in 2006.

The Air Force's vision for the X-37B is similar to NASA's.

Military leaders hope the system will prove to be a responsive and agile vehicle to meet up-to-the-moment demands from warfighters. Officials could plug an imaging device or other experiment into the X-37B's payload bay and launch it on an Atlas 5 or Delta 4 rocket from Cape Canaveral or Vandenberg.

But the viability of such a program hinges on the cost of readying an X-37B vehicle for another flight and launch availability.

This mission will continue after landing with a thorough check of how the space plane weathered more than 32 weeks in orbit. Engineers could then prepare the vehicle for a second mission.

Another X-37B spacecraft is under construction at Boeing Phantom Works for launch in the spring of 2011.