The U.S. military's uncrewed X-37B space plane is nearing one year in orbit on its latest secret mission.

The robotic space plane launched atop a United Launch Alliance Atlas V rocket on May 20, 2015, kicking off the X-37B program's fourth flight. This mission, dubbed OTV-4 (short for Orbital Test Vehicle-4), remains a clandestine affair.

"I can confirm the fourth OTV mission is approaching one year on orbit," Air Force spokeswoman Capt. Annmarie Annicelli said in response to's inquiry about the X-37B's activities. [The X-37B's Fourth Mystery Mission in Photos]

Annicelli had nothing further to add.

Mysterious minishuttle

The X-37B looks like a miniature adaptation of NASA's now-retired space shuttle orbiter. The military space plane is 29 feet (8.8 meters) long and 9.6 feet (2.9 m) tall, and has a wingspan of nearly 15 feet (4.6 m). For comparison, the space shuttle was 122 feet (37 m) long, with a wingspan of 78 feet (24 m).

The X-37B spacecraft has a payload bay about the size of a pickup-truck bed. It 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.

An Air Force X-37B robotic space plane is shown after it has landed at Vandenberg Air Force Base in California. The fourth mission of the program is now underway and nearing a year in Earth orbit.
An Air Force X-37B robotic space plane is shown after it has landed at Vandenberg Air Force Base in California. The fourth mission of the program is now underway and nearing a year in Earth orbit.
Credit: USAF/Boeing
Two reusable vehicles are known to constitute the X-37B fleet. This current OTV-4 mission marks the second flight of the second X-37B vehicle, which Boeing built for the Air Force.

Boeing's involvement in the program dates back to 1999. The X-37B vehicle development falls under Boeing's Defense, Space and Security division in El Segundo, California, the firm's center for all space and experimental systems and government and commercial satellites.

Most of the X-37B's payloads and specific activities are classified, which has led to some speculation that the space plane could be a weapon of some sort. Air Force officials have always denied this claim, maintaining that they use the X-37B to explore reusable space vehicle technologies in support of long-term objectives, such as risk reduction and operations development. [The X-37B Military Space Plane Explained (Infographic)] The Air Force Rapid Capabilities Office is leading the Department of Defense's OTV initiative, by direction of the Under Secretary of Defense for Acquisition, Technology and Logistics and the secretary of the Air Force.

Four space missions

To date, the program has chalked up an impressive flight record.

According to an Air Force fact sheet, starting with the program's first launch on April 22, 2010, the first three OTV missions have spent a total of 1,367 days in orbit, "successfully checking out the X-37B's reusable flight, re-entry and landing technologies."

All three missions ended successfully with a tarmac touchdown at Vandenberg Air Force Base in California, gliding onto a landing strip on autopilot.

Arrows in its quiver

Despite the project’s general secrecy, the Air Force has been perfectly willing to release photos of the vehicle sitting on top of its Atlas V rocket at Cape Canaveral, Florida, and on the runway at Vandenberg Air Force Base in California where it lands, autonomously. The spaceplane’s dimensions are known: It’s small, about 29 feet long, with a cargo bay not much bigger than a pickup truck’s. Project officials have revealed that the X-37B’s maneuvering engine runs on hydrazine and nitrogen tetroxide, and that it uses a different kind of thermal protection than NASA’s space shuttle did. The durations of the first three missions are a matter of public record (224 days, 469 days, and 674 days), and while the orbital parameters aren’t officially disclosed, amateur astronomers have been able to spot the mini-spaceplane through telescopes and figure out that it’s been orbiting at relatively low altitudes.

At an aerospace meeting in 2011, Arthur Grantz, a chief engineer with Boeing, the company that built the X-37B, said that the program had been evaluating the vehicle’s autonomous navigation and other systems. While he didn’t say what the Air Force wanted with an operational spaceplane, he speculated that the vehicle could be modified in the future to carry passengers.

Before the most recent launch, the Air Force and NASA even revealed two of the payloads for the first time: a NASA materials science experiment and an ionizing thruster being tested for the Air Force. Those clues have led analysts to speculate with a little more confidence about the X-37B’s purpose.

It’s clear that any technologies tested on an Air Force spaceplane will have some military application, but that doesn’t narrow things down much. In space, it could mean communications, navigation, surveillance, or even anti-satellite and counter-anti-satellite operations. The smart money is on advanced surveillance sensors. The Air Force has never mentioned them directly, but everyone seems confident that they’re flying.

“I think that’s probably what they’re not telling you, that there are payloads in there that might be part of the design for future reconnaissance satellites,” says James Andrew Lewis, director and senior fellow in the Strategic Technologies Program at the Center for Strategic and International Studies. The Air Force has great interest in developing small, advanced sensors, he says, because it’s “looking to figure out how to transition from big, expensive satellites to smaller but equally capable satellites.”

The Hall thrusters on the current flight use an electric field to accelerate xenon propellant, producing a small but steady thrust that’s useful for many types of spacecraft, including military communications satellites already in orbit. Brian Weeden, technical adviser for the Secure World Foundation, thinks the Air Force might also be testing the thrusters with an eye toward placing reconnaissance satellites in lower orbits, so that imaging sensors could take higher-resolution pictures of targets on the ground.

“I think the clue is how low an orbit [the X-37B] is in,” says Weeden. The spaceplane is orbiting at an altitude of about 320 kilometers (a little under 200 miles), which is lower than the International Space Station. Low orbits require more maneuvering, and therefore more fuel, to maintain. And fuel adds weight. “One of the reasons that the traditional exquisite imaging satellites are so hard to launch is because they’re big and they’re heavy,” says Weeden. Hall thrusters could enable lighter, cheaper reconnaissance satellites to be orbited.

Right now the Air Force has two X-37B research vehicles. Are we likely to see an operational fleet, without the “X” designation? Weeden doesn’t think so: “My guess would be that [the spaceplane] itself would probably not move into an operational-type role, but that a lot of the technologies that it’s demonstrating, like the Hall effect thrusters, or whatever the sensor payloads are, are going to move into the operational role. That’s how it went with the X-planes of the 1950s and 1960s.”

If the Air Force does go for an operational fleet, “they could add different versions of the vehicle, larger versions in particular,” says Todd Harrison, a defense budget analyst at the Center for Strategic and International Studies. Harrison speculates that a bigger version of today’s X-37B, with a larger cargo bay, could conceivably bring military satellites back to Earth for maintenance or repairs, then return them to orbit.

That is, if a scaled-up vehicle fits inside a conventional rocket. Like most satellites, today’s X-37B is placed inside a protective shroud, then launched on a large Atlas V rocket. “I would guess that it’s going to be really hard to make a bigger version of the X-37B that can still fit inside a fairly standard shroud,” Weeden says.

Another option for the Air Force would be to update the two experimental spaceplanes. For example, to make them more flexible and possibly cheaper to operate, the Air Force could make them compatible with other launch vehicles, such as the SpaceX Falcon 9 or one of Orbital Sciences’ rockets.

Such modifications would depend on the budget the X-37B program receives as it moves out of the experimental phase. And those numbers are likely to remain every bit as secret as the spaceplane’s missions.

Read more: History, Travel, Arts, Science, People, Places | Air & Space Magazine
Save 47% when you subscribe to Air & Space magazine
Follow us: @AirSpaceMag on Twitter