A pioneering spacecraft belonging to the United States military has landed in Florida after completing a secret two-year mission circling the Earth.
The experimental X-37B space plane produced a sonic boom that surprised residents along the Florida coastline as it prepared to touch down at the Shuttle Landing Facility at Nasa’s Kennedy Space Centre.
At 718 days, the mission broke records for being the lengthiest conducted by the Air Force. The X-37B is one of two unmanned planes conducting such orbits.
The secrecy around the project has prompted much speculation, with some observers suggesting that the craft is some sort of space weapon.
But the Air Force has always denied this notion, insisting that it is testing technologies for future spacecraft travel.
The orbiters “perform risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies,” the Air Force has said.
The Secure World Foundation, a nonprofit group promoting the peaceful exploration of space, said the secrecy surrounding the X-37B suggests the presence of intelligence-related hardware being tested or evaluated aboard the craft.
The Air Force has refused to reveal the cost of the programme.
The office of the secretary of the Air Force heralded the landing of the X-37B plane on Sunday as proof a major advancement in the project.
“The landing of OTV-4 marks another success for the X-37B program and the nation,” said Ron Fehlen, a lieutenant colonel and X-37B program manager.
“This mission once again set an on-orbit endurance record and marks the vehicle’s first landing in the state of Florida.
“We are incredibly pleased with the performance of the space vehicle and are excited about the data gathered to support the scientific and space communities.”
The vehicles resemble a miniature version of Nasa’s now retired space shuttles. The original Space Shuttle system had carried up to eight astronauts into low Earth Orbit. The shuttle would re-enter the Earth’s atmosphere upon completing its mission and land like a glider.
It was not without its dangers: in the course of 135 missions flown, two orbiters were destroyed, killing 14 astronauts.
These new aircraft are 8.8 metres long and have a wingspan of 4.5 metres, making them about one quarter of the size of the original shuttles.
Like the space shuttle, the X-37B lands on a runway, plane-style, but does so autonomously.
A fact sheet for the vehicle states that the space plane is designed to spend up to 270 days in orbit at a time. But so far several of the missions have much exceeded that limit.
Also known as the Orbital Test Vehicle, or OTV, it first flew in April 2010 and stayed aloft for 225 days. A second mission launched in March 2011 and lasted 15 months, while a third flight took off in December 2012 and returned after 22 months.
The Boeing-built space plane blasted off for this mission in May 2015 from Cape Canaveral Air Force Station aboard an Atlas 5 rocket built by United Launch Alliance, a partnership between Lockheed Martin Corp and Boeing Company.
Experts have suggested that engineers testing technologies for future satellites want to see how well these materials survive in space environment for long periods of time.
The Air Force intends to launch the fifth X-37B mission from Cape Canaveral Air Force Station, located just south of the Kennedy Space Center, later this year.