A U.S. Air Force X-37B UOV (unmanned orbital vehicle) landed in May 2017 under software control after a record 719 days in orbit. The air force reports few details about the X-37B but did reveal that the latest mission tested a new thruster system for mobile satellites that needed to be tried out while in orbit. Also carried were dozens of different materials, possibly including some new spy satellite components to see what the harsh environment in orbit, especially the radiation can do. Such exposure can have unpredictable effects on materials and microelectronics after prolonged time in space. Also carried were ten cubesats (very small satellites) to be released into orbit to perform various experiments.
The previous (third) X-37B mission ended in October 2014 after 675 days in orbit. The second mission landed on June 16th 2012 after 469 days in orbit. The first mission ended on December 3rd 2010 after 224 days in orbit. The official endurance of the X-37B was originally about 280 days. The real endurance appears to be nearly three times that, at least. The long endurance is largely because the X-37B carries a large solar panel, which is deployed from the cargo bay, unfolded and produces enough power to keep the X-37B up there for a long time. The air force has not made public much about what the X-37B has been doing up there for nearly 2,000 days so far. In 2011 the manufacturer revealed plans for an X-37C, which would be about twice the size of the X-37B and able to carry up to six passengers. Think of it as Space Shuttle Lite, but robotic and run by the military, not NASA. There are several other similar projects like the X-37C in the works, including one by China.
In effect the X-37B is a remotely controlled mini-Space Shuttle. The space vehicle, according to amateur astronomers (who like to watch spy satellites as well), appears to be going through some tests much of the time. The X-37B is believed to have a payload of about 227-300 kg (500-660 pounds). The payload bay is 2.1x1.4 meters (7x4 feet). As it returned to earth, it is designed to land by itself after being ordered to use a specific landing area. The X-37B weighs five tons, is nine meters (29 feet) long and has a wingspan of 4 meters (14 feet). In contrast the Space Shuttle was 56 meters long, weighed 2,000 tons and had a payload of 24 tons.
The X-37B is a classified project, so not many additional details were expected to be made available. It's been in development since 2000 but work was slowed down for a while because of lack of money. Whatever the X-37B is now doing up there has been convincing enough to get Congress to spend over a billion dollars on it. What makes the X-37B so useful is that it is very maneuverable, contains some internal sensors (as well as communications gear), and can carry mini-satellites, or additional sensors, in the payload bay. The X-37B is believed capable of serving as a platform for attacks on enemy satellites in wartime. Using a remotely controlled arm, the X-37B could refuel or repair other satellites. All this is estimates because, as a classified project, there is little confirmed information about its payload or mission (other than testing the system on its first mission). It is likely that future missions will involve intelligence work, and perhaps servicing existing spy satellites (which use up their fuel to change their orbits.) For regular satellite refueling missions a larger “X-37C” would probably be used. This is a scaled up X-37B that would have a much larger (probably over a ton) of payload. The X-37C could be quickly switched between cargo and passenger configurations. The X-37C would still be robotic and not require anyone onboard to control it.
The X-37B also demonstrated that it could not be easily tracked while in orbit although at times the X-37B could be elusive for amateur astronomers. The international collection of amateur sky watchers have proved remarkably adept at spotting orbital objects in the past, including classified ones like the X-37B. The amateur orbital observer community has concluded that one thing the X-37B tested was how well it could constantly switch positions, and stay hidden. In that respect, the X-37B was a resounding success. That's because these amateur observers are generally very good at tracking what's up there.
One notable incident occurred in 2008 when a U.S. spy satellite fell out of orbit (apparently because of a failure in its maneuvering system). The amateur astronomers were able to track it. If this had not been an American reconnaissance satellite, there would have been no media attention to this, because 4-5 satellites a month fall back to earth. Since most of the planet is ocean, or otherwise uninhabited (humans like to cluster together), the satellites tend to come down as a few fragments, and rarely is anyone, or anything manmade, hit.
Before the Internet became widely used a decade ago, you heard very little about all these injured or worn out space satellites raining down on the planet. But with the Internet, the many thousands of amateur astronomers could connect and compare notes. It was like assembling a huge jigsaw puzzle. Many sightings now formed a pattern, and a worldwide network of observers made visible the movements of hundreds of space satellites. These objects were always visible at night, sometimes to the naked eye, but unless you knew something about orbits and such, they could be difficult to keep track of. These days, a lot of the activity is posted and discussed at http://www.satobs.org/. But the X-37B has proved elusive, and sometimes became a frustrating challenge to the amateur sky watchers. This is pleasing to American air force officials, who designed the X-37B to be elusive to terrestrial observation and the dedicated (and quite effective) amateur satellite watchers gave the X-37B quite a workout.