Five times now, a US Air Force X-37B space plane has traveled to orbit and returned to Earth, each time spending longer and longer in space. The most recent mission -- secret, as always -- landed at NASA's Kennedy Space Center shuttle landing facility at 3:51 a.m. ET on Oct. 27, 2019.
The Air Force likes what it's seen in the X-37B's accomplishments. "The sky is no longer the limit for the Air Force and, if Congress approves, the U.S. Space Force," said Air Force Chief of Staff Gen. David L. Goldfein in a statement.
Originally published June 14, 2012. It has been updated with details of later missions.
The fifth mission, known as OTV-5 (for "orbital test vehicle") launched from the Kennedy Space Center on Sept. 7, 2017. That means it was in space for 780 days, or more than two years. This mission started with a liftoff atop a SpaceX Falcon 9 rocket.
The Air Force doesn't reveal much about what the unmanned X-37B spacecraft do while circling the planet, but Randy Walden, director of the Air Force Rapid Capabilities Office, did say this much: "This mission successfully hosted Air Force Research Laboratory experiments, among others, as well as providing a ride for small satellites."
The preceding mission, OTV-4, landed at the Kennedy Space Center on May 7, 2017, after 718 days in orbit. Over the course of the five missions, the X-37B space planes have flown for a total of 2,865 days. (The space plane was designed for in-orbit duration of 270 days, the Air Force says.)
The Air Force says it expects to launch the next X-37B mission in 2020.
A shroud of darkness becomes the X-37B Orbital Test Vehicle. This photo shows the first X-37B, designated OTV-1, on the runway at Vandenberg Air Force Base in California on Dec. 3, 2010, just after it returned from its debut trip into space, which lasted 224 days.
Technicians in self-contained protective suits close in on the X-37B for initial checks on that December 2010 night. The space plane bears a striking resemblance to the much larger space shuttles, and for a reason -- they share a common heritage in NASA's work on lifting-body vehicles. In fact, the X-37 initially was a NASA project that ran from 1999 through 2004, though the space agency never got as far as building that orbital vehicle. The Air Force and Boeing picked up where NASA left off to build the X-37B.
There is no cockpit on the X-37B because there are no human occupants. The space plane is built to fly autonomously. When the Air Force decides it's time for the mission to end, it gets sent from the ground and the vehicle commences with autonomous re-entry, descent and landing.
Power for the X-37B while it's in orbit comes from the sun. The Air Force says the space plane is equipped with gallium arsenide solar cells and lithium ion batteries. This artist's rendering is from 2003, back when the X-37, as yet unbuilt, was still in NASA's hands (PDF).
This artist's rendering, also from a decade and a half ago, gives a sense of the X-37B's size relative to that of the space shuttle. NASA said at the time that two X-37 vehicles could fit in the cargo bay of a shuttle. (The cargo bay of the X-37B, meanwhile, is roughly the size of a pickup truck bed.)
Here's the tale of the tape for the X-37B: It's 29 feet, 3 inches long, stands 9 feet, 6 inches high, and has a wing span of just under 15 feet. Its weight at launch: 11,000 pounds.
In March 2010, back before it ever flew, the X-37B posed for this picture on a runway in Titusville, Florida. During its debut orbital mission that year, the space plane traveled some 91 million miles, according to the Air Force. It's designed to operate in low Earth orbit, or between 110 and 500 miles above Earth.
A very up-to-date design, the X-37B has no hydraulics on board, according to Boeing. Instead, flight controls and brakes are activated through electromechanical means.
The X-37B features lightweight composite materials, improved insulation on the leading edge of the wings, and heat-shield tiles that "are significantly more durable than the first generation tiles used by the space shuttle," according to a Boeing website description. "All avionics on the X-37B are designed to automate all de-orbit and landing functions."
The X-37B stands in front of part of the fairing that protects it during launch, showing off the silica tiles on its underside. On the leading edge of the wings, meanwhile, are TUFROC (toughened uni-piece fibrous refractory oxidation-resistant ceramic) tiles, which NASA named the government winner of its 2011 Invention of the Year Award.
Where the space shuttles rode exposed alongside the rockets that lifted them off the launch pad, the X-37B is nestled in a protective fairing that sits atop the rocket, which for the first flights was a United Launch Alliance Atlas V.
The OTV-1 version of the X-37B lifts off on its maiden voyage atop an Atlas V rocket, ensconced in its fairing, on April 22, 2010. "History was made in December ," Craig Cooning, head of Boeing's Space and Intelligence Systems, said in a statement marking the launch of OTV-2, "when the X-37B became the United States' first unmanned vehicle to return from space and land on its own."