The U.S. Air Force's X-37B robotic space plane in orbit, as photographed by satellite tracker Ralf Vandebergh.

Skywatcher and satellite tracker Ralf Vandebergh of the Netherlands recently caught a rare glimpse of the U.S. Air Force's secretive X-37B space plane.

Vandebergh said he'd been hunting for the robotic spacecraft for months and finally managed to track it down in May. But it took a bit longer to get photos of the vehicle.

"When I tried to observe it again [in] mid-June, it didn't meet the predicted time and path," Vandebergh explained. "It turned out to have maneuvered to another orbit. Thanks to the amateur satellite observers' network, it was rapidly found in orbit again, and I was able to take some images on June 30 and July 2."

The X-37B's recent passes were almost overhead, Vandebergh added.

Related: The X-37B Space Plane: 6 Surprising Facts

Beyond expectations

The X-37B, also known as the Orbital Test Vehicle (OTV), looks like a miniature version of NASA's retired space shuttle.

"It is really a small object, even at only 300 kilometers [186 miles] altitude, so don't expect the detail level of ground-based images of the real space shuttle," Vandebergh said.

Taking this into consideration, the newly captured imagery far exceeded Vandebergh's expectations.

"We can recognize a bit of the nose, payload bay and tail of this mini-shuttle, with even a sign of some smaller detail," he said.

Vandebergh captured the photos using a 10-inch F/4,8 aperture Newtonian telescope with an Astrolumina ALccd 5L-11 mono CMOS camera. Tracking was fully manual through a 6×30 finderscope, he said.

Classified duties

The X-37B has winged past 666 days of flight on this latest mission, which is called OTV-5 because it's the fifth flight for the program.

OTV-5 began on Sept. 7, 2017, with a launch atop a SpaceX Falcon 9 booster from NASA's Kennedy Space Center (KSC) in Florida.

X-37B missions are carried out under the auspices of the Air Force Rapid Capabilities Office, and mission control for OTV flights is handled by the 3rd Space Experimentation Squadron at Schriever Air Force Base in Colorado. This squadron oversees operations of the X-37B and is tagged as the Air Force Space Command's premier organization for space-based demonstrations, pathfinders and experiment testing, gathering information on objects high above Earth and carrying out other intelligence-gathering duties.

And that may be a signal as to what the robotic craft is doing — both looking down at Earth and upward.

Related: US Air Force's Secretive X-37B Space Plane (Infographic)

Flight-duration record

Each X-37B mission has set a new flight-duration record for the program:

Most X-37B payloads are classified, and the Air Force releases few details about the spacecraft's orbit and activities. The only OTV-5 payload that Air Force officials have revealed is the Advanced Structurally Embedded Thermal Spreader, or ASETS-II.

Developed by the U.S. Air Force Research Laboratory (AFRL), this cargo is testing experimental electronics and oscillating heat pipes for long-duration stints in the space environment.

According to AFRL, the payload's three primary science objectives are to measure initial on-orbit thermal performance, to measure long duration thermal performance, and to assess any lifetime degradation.

Tarmac touchdown

Exactly when OTV-5 will end is unknown.

The last X-37B mission touched down at KSC's Shuttle Landing Facility — a first for the program. All prior missions had ended with a tarmac touchdown at Vandenberg Air Force Base in California.

Several website postings say that the sixth mission, OTV-6, is planned for this year on a United Launch Alliance Atlas-5(501) rocket. Launch would be from Cape Canaveral Air Force Station's Space Launch Complex-41.

Reusable vehicles

The classified X-37B program "fleet" consists of two known reusable vehicles, both of which were built by Boeing.

The X-37B vehicles were built at several Boeing locations in Southern California, including Huntington Beach, Seal Beach and El Segundo. The program transitioned to the U.S. Air Force in 2004 after earlier funded research efforts by Boeing, NASA and the Defense Advanced Research Projects Agency.

The military space plane is 29 feet (8.8 meters) long and 9.6 feet (2.9 m) tall, with a wingspan of nearly 15 feet (4.6 m).

The X-37B's payload bay, which measures 7 feet (2.1 m) by 4 feet (1.2 m), can be outfitted with a robotic arm. The spacecraft has a launch weight of 11,000 lbs. (4,990 kilograms) and is powered on orbit by gallium-arsenide solar cells with lithium-ion batteries.

Prior to OTV-5's launch, Randy Walden, the director of the Air Force Rapid Capabilities Office, said there were many firsts on this mission, making it a milestone for the program. "It is our goal to continue advancing the X-37B OTV so it can more fully support the growing space community," he said at the time.

The Air Force also noted that OTV-5 was launched into, and will be landed from, a higher-inclination orbit than prior missions to further expand the X-37B's orbital envelope.

Leonard David is author of the recently released book, "Moon Rush: The New Space Race" published by National Geographic in May 2019. A longtime writer for Space.com, David has been reporting on the space industry for more than five decades. Follow us on Twitter @Spacedotcom or Facebook.