An unmanned US military space plane has landed at NASA's Kennedy Space Center following a mission lasting more than two years.

The X-37B, which looks like a miniature space shuttle, touched down on Sunday morning, causing a sonic boom as it landed on a runway once used for space shuttles which have been mothballed.

Twitter users reacted to the sonic boom with one saying her house "shook" and her dog had "gone into a frenzy".

Exactly what the space plane was doing during its 718 days in orbit is not entirely clear, with the US Air Force saying the orbiters "perform risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies".

The cost of the mission - the fourth and longest so far - is classified.

The Secure World Foundation, a non-profit group that promotes the peaceful exploration of space, says the secrecy surrounding the X-37B suggests intelligence-related hardware is being tested or evaluated aboard the craft.

At 29 feet-long and with a wingspan of 15 feet, the Boeing-built craft is about a quarter of the size of NASA's now-retired space shuttles.

This mission began in May 2015, when the plane set off from nearby Cape Canaveral Air Force Station aboard an Atlas 5 rocket built by United Launch Alliance, a partnership between Lockheed Martin Corp and Boeing Co.

Its first mission was eight-months-long from April 2010, its second from March the following year lasted 15 months.

A third took off in December 2012 and ended after 22 months.

Another mission is scheduled later this year.

According to the Orlando Sentinel, sonic booms used to be common in the area during the 30 years of NASA's manned space shuttle programme, with landings at the Kennedy Space Center preceded by a loud double boom.
But the last of those shuttles landed nearly six years ago.

There is also a type of rocket - SpaceX's Falcon 9 - which produces sonic booms and these were last heard earlier this month.

But officials had refused to confirm the return date for the X-37B, so its arrival was not expected by residents.