July 6, 2016: At the end of June China revealed that a recent test of a space satellite capable of refueling other satellites was a success. Many satellites are equipped with small rockets so they can move to different orbits but the fuel only lasts so long. One reason the United States built its Space Shuttle was because it provided an opportunity to keep expensive spy satellites operational by refueling them and even replacing components that failed. But the Space Shuttle program was shut down in 2011 because of cost. The U.S. does not plan to test a similar refueling satellite until 2020.

In the meantime the United States does have the X-37 which is a remotely controlled mini-Space Shuttle. As of mid-2016 the X-37 has made four spaceflights, one off which is still underway. Three X-37s have been built, one X-37A for NASA (for testing, not for spaceflight) and two X-37Bs for the U.S. Air Force (for use in space). 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 returns to earth, the X-37B lands 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 4.5 meters (14.6 feet). Max endurance appears to be more than two years, because of the use of a solar array for power while in orbit. In contrast the Space Shuttle was 56 meters long, weighed 2,000 tons and had a payload of 24 tons. Endurance was limited to a few weeks at most because there had to be a human crew on board.

The X-37B is a classified project so not a lot of details are 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. Using a remotely controlled arm, the X-37B could refuel or repair other satellites. But X-37B is a classified project, with little confirmed information about its payload or mission (other than testing the system on its first mission). Future missions could involve intelligence work and perhaps servicing existing spy satellites, or at least practicing to do so. The X-37B is believed capable of serving as a platform for attacks on enemy satellites in wartime. It is believed that some of the missions may have also involved testing new spy satellite components in space, where the harsh environment, especially the radiation, can have unpredictable effects on microelectronics.

For regular satellite refueling missions the 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 and be able to carry up to six passengers. The X-37C would still be robotic and not require anyone onboard to control it. The Chinese are also working on something like the X-37C but have not put anything into orbit yet.