US Air Force's X-37B space plane reaches 600 days in orbit - and its mission STILL remains a mystery

The X-37B space plane, an experimental program run by the Air Force, launched atop a United Launch Alliance Atlas V rocket on May 20, 2015. Pictured is an artist's impression

The US Air Force's mystery military space plane has now been in orbit for 600 days.

The X-37B space plane, an experimental program run by the Air Force, launched atop a United Launch Alliance Atlas V rocket on May 20, 2015.

While some details of its payload have been revealed - we still do not know exactly what's on board.

Theories have ranged from it being a space bomber, to a probe on a mission to 'take out' spy satellites.

If the spacecraft spends 74 more days in orbit, it will break the record it set during its last mission, which touched down in October 2014.

The mystery vehicle, essentially a technology test bed, is designed to orbit the Earth and then land like one of Nasa's old shuttles.

According to X-37B manufacturer Boeing, the space plane operates in low-earth orbit, between 110 (177km) and 500 miles (800km) above earth.

By comparison, the International Space Station orbits at about 220 miles (350km).

While its main mission payload is a mystery, Nasa has revealed it has a materials experiment aboard, while the Planetary Society is tagging along with a solar-sail demo.

Called LightSail, it uses a propulsion system that uses the pressure of photons from the sun, a technique known as solar sailing.

Nine other CubeSat nanosatellites are also taking a piggyback ride into orbit.

The space plane - one of two of the same design - is operated robotically, without anyone on board, and is reusable.

It is 29ft (8.8m) long — about one-fourth the size of a Nasa shuttle. The longest X-37B flight lasted about 675 days; touchdown was last October.

There is no official word on exactly how long this one will stay up, although report suggest it will return to Earth later this year.

When the plane does return, it will land in Nasa's Kennedy Space Center (KSC) in Florida.

There is no official word on exactly how long this one will stay up, although report suggest it will return to Earth later this year

A former KSC space-shuttle facility will enable the Air Force 'to efficiently land, recover, refurbish and relaunch the X-37B Orbital Test Vehicle (OTV),' Boeing representatives told Space.com.

In an unprecedented disclosure, last year the Department of Defense did reveal some details about the X-37B's main mission.

'[We] are investigating an experimental propulsion system on the X-37B on Mission 4,' Captain Chris Hoyler, an Air Force spokesman, told Space.com.

Theories have ranged from it being a space bomber, to a clandestine probe on a mission to 'take out' spy satellites. Pictured is the Atlas V rocket carrying the mysterious payload moments (left) after launch last year, and just before (right)

Like a shuttle, X-37B is blasted into orbit by a rocket. However, it lands using a runway like a normal aircraft. The X-37B is too small to carry people onboard, but does have a cargo bay similar to that of a pickup truck, which is just large enough to carry a small satellite

'The Air Force Rapid Capabilities Office will also host a number of advance materials onboard the X-37B for Nasa to study the durability of various materials in the space environment,' Hoyler added.

He added the vehicle's mission 'cannot be specified' but that it will enhance 'the development of the concept of operations for reusable space vehicles'.

Spaceflightnow.com revealed more details of the flight, which is described as a 'hall thruster electric propulsion test.'

It is intended to improve performance of the units onboard Advanced Extremely High Frequency (AEHF) military communications spacecraft, officials claim.

AEHF satellites' Hall thrusters are 4.5-kilowatt units that use electricity and xenon to produce thrust for moving satellites in space.

The benefit of using electric propulsion is that its xenon fuel weighs much less than traditional hydrazine.

This technology could help in the development of technologies to control satellites with better accuracy.

However, experts claim that refining an advanced manoeuvring thruster is probably just a small part of the vehicle's true mission set.

The US Air Force's top secret X-37B space plane was caught on camera in 2015 by a team of amateur astronomers. South African astronomer Greg Roberts captured this of the space plane in two-second–long exposures just a few weeks after launch

An infrared view of the X-37B unmanned spacecraft landing at Vandenberg Air Force Base. The purpose of the U.S. military's space plane is classified

One leading secrecy expert previously told DailyMail.com the drone is 'very likely' be used to test technologies that will increase spying capabilities of the US.

'The US government has a bottomless appetite for sensitive information,' said Steven Aftergood, a secrecy expert at the Federation of American Scientists.

'As powerful as our intelligence satellites may be, they also have their limitations - most notably the limitations imposed by their orbital parameters.

'It's conceivable that a spy plane would introduce new versatility into overhead reconnaissance.'

The X-37B space drone, otherwise known as the Orbital Test Vehicle, is blasted into orbit by a rocket. However, it lands using a runway like a normal aircraft.

The X-program has bounced between several federal agencies, Nasa among them, since 1999. The plane has been in space for a total of 674 days, far more than its two previous flights which lasted 225 and 469 days

The X-37B is too small to carry people onboard, but does have a cargo bay similar to that of a pickup truck, which is just large enough to carry a small satellite.

The X-program has bounced between several federal agencies, Nasa among them, since 1999.

The program's first mission launched in April 2010 and landed in December that year.

The second space plane took off on March 2011 and came back to Earth in June 2012. dailymail

One secrecy expert has told DailyMail.com that the drone (artist's impression ) is 'likely' to be a spy plane. The program's first mission launched in April 2010 and landed in December that year. dailymail

This December 3, 2010, image by the Vandenberg Air Force Base shows technicians examining the X-37B unmanned spaceplane shortly after landing at Vandenberg Air Force Base, California. dailymail