The Air Force's X-37B Orbital Test Vehicle mission 4 landed at NASA 's Kennedy Space Center Shuttle landing facility Sunday morning.
The U.S. Air Force posted about the X-37B touching down at around 8:00 a.m. EST (12:00 GMT) at the SLF after spending some 636 days on orbit. Launched on May 20, 2015, Sunday's touchdown brought the OTV-4 mission to its end at one year, 11 months and 17 days, the X-37B's longest flight to date.
The first three missions all ended with landings at Vandenberg Air Force Base in California. Local residents reported on social media of being woken by sonic booms as the vehicle decelerated through the speed of sound.
"Technologies being tested in the program include advanced guidance, navigation and control; thermal-protection systems; avionics; high-temperature structures and seals; conformal, reusable insulation, lightweight electromechanical flight systems; and autonomous orbital flight, re-entry and landing", Capt. AnnMarie Annicelli, an Air Force spokeswoman, told Space.com via email in March.
The Air Force owns two X-37B space planes, and one of them is finally back on the ground after a almost 2-year-long secret mission. In 2006, the Air Force announced it would develop what is now the X-37B, and launched the experimental space plane for the first time in 2010.
But the program operated by Boeing has consolidated its operations at two of NASA's former space shuttle hangars, called Orbiter Processing Facilities 1 and 2, enabling the mini-shuttles to launch and land in the same place.
Peaceful uses in space: The X-37B is quiet because it's a robot and doesn't talk.
"Today marks an incredibly exciting day for the 45th Space Wing as we continue to break barriers", Brig.
This flight was the longest for the X-37B program, which consists of two identical mini-shuttles, each about the size of a truck. What caused it was a secret military mission ending at Kennedy Space Center.
The space plane had been circling Earth for over 700 days.
The exact objective of the X-37B was never revealed and the secrecy surrounding the project has led to speculation that the solar-powered X-37B can be used as a spy satellite or to deliver weapons from space.
Overall, the program's spacecraft have spent 2,085 days in orbit, according to the Air Force.