Top-secret military spaceplanes certainly know how to make an entrance.

The U.S. military’s X-37B, an uncrewed spacecraft that looks like a miniature version of the retired space shuttles, returned to Earth over the weekend after spending nearly two years in low-Earth orbit. It sent shockwaves rippling through the air as it entered the atmosphere over Florida, producing a sonic boom loud enough to jolt people awake across the state. The Air Force, which operates the X-37B, tweeted about its return minutes later, and soon posted a flurry of images and videos of the spaceplane online. “Our team has been preparing for this event for several years, and I am extremely proud to see our hard work and dedication culminate in today’s safe and successful landing of the X-37B,” said Brigadier General Wayne Monteith, the commander of the Air Force’s Space Wing.

To which many observers said, wait, what?

The news that the military had a space shuttle quietly orbiting Earth for more than 700 days came as a surprise to some. Why didn’t we know about this thing, the reaction seemed to go. The reaction illustrated the distinct line between the country’s civilian and military activities in space, and how much the general public knows about each. People know plenty about the civilian side—the missions to other planets, the SpaceX launches, astronauts’ cool Instagram pictures from the space station. But secret military spaceplanes? You usually need a sonic boom to hear about that.

The Air Force has two X-37B spaceplanes, and they’ve flown before—three times, in fact, since 2010. Their mission is to “demonstrate technologies for a reliable, reusable, unmanned space test platform” for the Air Force, according to a press release. With each run, the Air Force has released more and more information about the highly classified program, which may be why this fourth mission received more public attention than usual.

The X-37B mission is managed by the Air Force Rapid Capabilities Office, a unit inside the Air Force that specializes in developing aircraft technology. It grew out of a project at NASA in 1998 aimed at reducing the cost of space transportation, It was transferred to the Pentagon’s Defense Advanced Research Projects Agency, or DARPA, in 2004, where it became instantly classified. The X in front of the program’s name means the spacecraft is a member of a long line of planes and rockets the U.S. has developed for experimental purposes, usually with a good amount of secrecy. The X-37B, built by Boeing, borrowed its aerodynamic design from the NASA space shuttles that ferried astronauts to and from space for 30 years, but it uses a different kind of heat shield. The X-37B weighs about 11,000 pounds at launch and has a wingspan of 14 feet. It can spend 270 days or more in orbit, and is launched into space aboard an Atlas V rocket.

The Air Force only made public two tests involved in this latest mission: an electric-propulsion device that could allow spacecraft to carry heavier payloads and a NASA experiment aimed at exposing 100 different kinds of materials samples to microgravity. This is where official information from the Air Force usually stops, and the speculation begins. Some in the military-space community believe the military might be tinkering with advanced surveillance sensors, or testing the electric-propulsion devices, known as thrusters, so it can put future reconnaissance satellites in lower orbits, where they can better see targets on the ground. Others say the military wants to use the spaceplane as a weapon capable of approaching other satellites to observe them and, perhaps, interfere.

The budget for the X-37B program is classified, but it’s likely in the hundreds of millions of dollars. NASA and the Air Force provided $125 million to the project in its development stages, and Boeing contributed about  $67 million. In late 2002, Boeing received another $301 million to develop the spaceplane.

Knowing all this, Laura Grego, a space-security expert and a senior scientist the Union of Concerned Scientists, wonders whether the X-37B is worth it. “While it can do a number of things, it doesn’t seem particularly well-designed to do any of them except for returning a space craft in a nice, tidy way,” she told me over email.

Grego said there are more efficient ways of getting things like thrusters and experiments into low-Earth orbit, like the robotic missions that transport cargo to the International Space Station. The X-37B is also not built to be especially agile in orbit, so sidling up to a satellite to check it out would be pretty difficult.

“Maneuverability depends on mass—if you have to haul around wings and landing gear with you on all your zips and zags in space, you will run out of fuel more quickly than if you were designed to be lightweight and agile and never come back to the ground,” Grego said. “This is especially true if you are trying to get close to objects that are not already in nearby orbits, that takes a lot of fuel, which needs to be launched along with the plane.”

Indeed, the most visible achievement of the X-37B program—the aspect the Air Force itself seems to want the public to pay the most attention to—is the capability to launch and land spacecraft in a way the U.S. hasn’t done since the space shuttle program ended. The landing, shown here in a video from the Air Force, looks cool whether you know what’s on board or not. But the jump from space shuttle to space weapon is, as far as the public knows, seems pretty big.