The second copy of the Air Force's X-37B robotic space plane landed at Vandenberg Air Force Base in California early Saturday morning, ending a record-breaking 469-day orbital mission that began atop an Atlas rocket launched from Cape Canaveral, Florida, on March 5, 2011. The safe landing of Orbital Test Vehicle 2 after more than 15 months in space is an indisputable triumph for the US military and space industry. Much less certain is the X-37's future. Budget cuts, labor woes and the looming specter of a Chinese rival could blunt the diminutive robo-shuttle's orbital edge.

The Boeing-built X-37B, in development since the 1990s, was designed to operate nine months at a time between refueling and refurbishment. But with just two copies of the roughly billion-dollar space plane in the inventory, the Air Force wanted to get as much mileage as possible out of each. After OTV-1's proof-of-concept flight from April to December 2010, OTV-2's mission became an endurance test. "One of the goals of this mission was to see how much farther we could push the on-orbit duration," said Lt. Col. Tom McIntyre, the Air Force's X-37B program manager.

The key to the X-37's marathon flight: fuel and energy management. "It sips fuel like a Prius," one space insider said of the mini-shuttle. Even so, Air Force controllers on the ground had to pay close attention to the X-37's orbital profile and its use of engines, batteries and extendable solar panels.

Officially, the 29-foot-long X-37 is a research vehicle, meant to carry small experiments in its payload bay, which is roughly the size of a pickup truck bed. But the winged vehicle's maneuverability and flexibility mean it's capable of much more: spy missions, cargo deliveries to the International Space Station, even sneaking up on and tampering with enemy satellites. Some observers speculated that OTV-2 was monitoring China's Tiangong space station, a notion that Secure World Foundation analyst Brian Weeden dismissed. "If the US really wanted to observe Tiangong, it has enough assets to do that without using X-37B."

In any case, the X-37 partially fills a gap left by the retirement last summer of the much larger NASA Space Shuttle. "The X-37B OTV program brings a singular capability to space technology development," McIntyre said. Boeing has proposed to build a bigger X-37C version that could carry more experiments, more cargo – and even astronauts.

But it's not clear that the existing X-37s will survive much longer in the cash-strapped Air Force, which is struggling to pay for new bombers, new aerial tankers and the trillion-dollar F-35 Joint Strike Fighter program. In September, Air Force Space Command boss Gen. William Shelton questioned the space plane's worth. "It’s very flexible, in that you’ve got a payload bay in which you can launch, then it comes down, then you can launch it back – but whether or not it's cost effective is the question."

Complicating the Air Force's budgetary calculations, Boeing is in the process of shutting down the cutting-edge facility in Huntington Beach, California, where the X-37s were hand-assembled. In recent years Building 31, as the facility is known, has been a labor battleground between Boeing management and its rank-and-file engineers. The company plans to shutter the facility next year. Possible future X-37s could be built elsewhere, but the loss of Building 31's skilled workforce could drive up the cost.

Meanwhile, the US mini-shuttle could soon have competition. China is developing its own space plane called Shenlong – and apparently test-flew it for the first time in January last year. "Beijing may be entering the spaceplane era faster than many would have predicted," warns Andrew Erickson, a Naval War College analyst.

Currently the Air Force plans to launch OTV-1 on its second mission this fall, with OTV-2 possibly to follow on its own sophomore launch sometime next year. If the Air Force continues improving the X-37's performance, these coming missions could be even more amazing than the just-completed record-breaker. But that's assuming the money keeps flowing.