US Air Force's secret unmanned space plane lands after 2-year mission

X-37B plane set the on-orbit endurance record at 718 days

The US Air Force's unmanned X-37B space plane that has been zipping around the Earth for nearly two years on a classified mission has landed successfully but woke up residents in Florida from their weekend slumber.

The X-37B, which resembles a miniature space shuttle, landed on Sunday morning at NASA's Kennedy Space Centre, the Air Force said.

But the space plane, which is also known as the Orbital Test Vehicle (OTV), sent a sonic boom that rattled east- central Florida before 8 am, waking residents from their weekend slumber, CNN reported.

"Thought somebody crashed into my garage ... It was just a sonic boom ... Thanks @NASA for the scare!" said one woman on Twitter.

The X-37B made history by landing for the first time in Florida instead of California. It also set the on-orbit endurance record at 718 days, or almost two years.

"Our team has been preparing for this event for several years, and I am extremely proud to see our hard work and dedication culminate in today's safe and successful landing of the X-37B," said Brig. Gen. Wayne Monteith in a press release.

There has been mystery surrounding this question for many years.

The Air Force maintains that the reusable plane "performs risk reduction, experimentation and concept of operations development for reusable space vehicle technologies."

But many speculate about other possible uses, including spying activities or testing of a secret space weapon.

This completed mission extends the programme to 2,085 days in space.

The Air Force is preparing to launch the fifth X-37B mission from Florida's Cape Canaveral Air Force Station later in 2017.

The other three OTV flights launched in April 2010, March 2011 and December 2012. Here are a few surprising facts about the military space plane.