WATCH LIVE: http://original.livestream.com/spaceflightnow

Spaceflight Now is carrying the ULA webcast along with its blow-by-blow launch coverage. The launch is set for sometime between 10:45 a.m. EDT and 2:45 p.m. EDT (1445 and 1845 GMT).

The X-37B space plane is a reusable spacecraft capable of flying unmanned long-duration mission in Earth orbit. It is powered by a solar panel array packed inside a payload bay about the size of a pickup truck bed. The Air Force currently has two X-37B space planes, both built by Boeing's Phantom Works division, and began flying them on ultra-long missions in 2010. The spacecraft are also known as Orbital Test Vehicles. In October 2014, the third mission landed at Vandenberg Air Force Base after 674 days in space. 

X-37B Space Plane: US Air Force's 4th Mystery Mission in Photos

For this fourth mission, the U.S. Air Force has made some surprising revelations, with officials stating that the flight will test an innovative Hall effect space thruster during the flight. NASA is also flying a space materials experiment on the mission.