The X-37B is the US Air Force's secret space plane used to test secret tech and do other secret stuff. The spacecraft is remotely controlled and it has been in use for the last eight years. It is currently in its fifth mission in orbit, which is only a few weeks short of a full year.
The fifth mission, known as Orbital Test Vehicle-5 (OTV5) started on September 7, 2017, and we know quite a fair bit more than usual about this flight. The craft is currently being used to test some new space tech. It is exposing the Advanced Structurally Embedded Thermal Spreader, or ASETS-11, to the low-orbit space environment. The Air Force Research Laboratory developed this technology and the team is interested in understanding how experimental electronics and oscillating heat pipes would fare beyond the atmosphere as they are designed for long duration space flights.
At present, it is unclear how long the X-37B will remain in orbit. Since its first mission in 2010, which lasted 224 days, each successive one has increased the length of the previous one, with the last one, OTV-4, reaching 718 days.
The US Air Force has two X-37B vehicles, both of which they were built by Boeing. They look like mini versions of the Space Shuttle, 8.8 meters (29 feet) in length and with a wingspan of 4.6 meters (nearly 15 feet). The space plane has a cargo hold comparable to the size of a pickup truck's and it has a removable robotic arm. It is solar powered and weighs roughly about 5 tonnes.
“The X-37B Orbital Test Vehicle, or OTV, is an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the US Air Force," explains the official Air Force website.
"The primary objectives of the X-37B are twofold: reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth.”
Obviously, there are people that think the "secret" space plane is testing other technologies that the Air Force is deliberately not revealing to the public. You can’t exactly hide the fact that you put a space plane in orbit, can you? But hiding its full purpose makes senses if you are testing secret space tech. Wouldn’t be much of a secret otherwise.