Space and secrets have gone hand in hand since the beginning of astronautics. After all, the space race occurred at the height of the Cold War. The geopolitical situation might have changed but nations still send secret missions into space.
The ironically better-known one is the US Air Force Space Plane, or X-37B, currently in its fifth mysterious mission in orbit. The uncrewed vehicle, also known as the Orbital Test Vehicle (OTV), has now spent more than 400 days in orbit, testing new tech and potentially deploying small satellites.
While an impressive day count, the mission is currently second to last for duration. OTV-1, which launched in 2010, was the first and shortest mission at 224 days. Every mission since then has extended the duration of its predecessor. OTV-4 lasted 717 days and 20 hours, less than two weeks from a full two years in space. As OTV-5 started when X-37B was launched on September 7, 2017, to beat the previous record it will have to come back down sometime after August 25, 2019.
X-37B was originally a NASA project and it owes a lot of its design to the Space Shuttle orbiters, which were used in 135 missions from 1981 to 2011. The space plane itself looks like a smaller version of the iconic crew launcher. Just like the Space Shuttle, it has a cargo bay with a robotic arm, although the hold is roughly as big as a truck. It is only 8.8 meters (29 feet) in length with a wingspan of 4.6 meters (nearly 15 feet). It is solar powered and weighs approximately 5 tonnes (5.5 tons).
“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.”
Most of the payload of X-37B is unknown (come on, it’s a secret space plane after all!) but it has been announced that it's testing the Advanced Structurally Embedded Thermal Spreader II, which measures the performance of an oscillating heat pipe, as well as carrying several small satellites.