The US Air Force's top secret X-37B space plane has been caught on camera by a team of amateur astronomers.

The unmanned plane launched atop a United Launch Alliance Atlas V on May 20 on its fourth mission, but most of the details about its flight remain classified.

The latest image, however, provides new insights into the spacecraft's activities and has fuelled speculation that X-37B may be a spy plane.

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The US Air Force's top secret X-37B space plane has been caught on camera 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

X-37B'S MYSTERY MISSIONS The U.S. Air Force's unmanned X-37B space plane has flown four secret missions to date. Each time it has carried a mystery payload on long-duration flights in Earth orbit. The spacecraft looks similar to Nasa's space shuttle but is much smaller. The X-37B is about 29ft (8.8m) long and 9.5ft (2.9 m) tall. It has a wingspan of just less than 15ft (4.6 m). At launch, it weighs 11,000lbs (4,990kg). The craft is taken into orbit on a rocket but lands like the space shuttle by gliding down to Earth. While it's main mission payload is a mystery, Nasa has revealed it has a materials experiment aboard. The Planetary Society is tagging along with a solar-sail demo. Ten CubeSat nanosatellites are also taking a piggyback ride into orbit. Advertisement

'It's in a lower orbit than normal … had us confused for a while, as I thought it would be the standard operating orbit,' tracker Greg Roberts told Space.com.

'The inclination is also lower than before.'

Roberts captured an image of the space plane in two-second–long exposures just a few weeks after launch.

Its inclination of 38 degrees is the lowest of the X-37B program, with the first mission flying at an inclination of 40 degrees.

It also features the lowest initial altitude of the four X-37B launches at 198 miles (318km). The previous low was OTV-2, at 205 miles.

The low-altitude flight profile may be a indication of test for a new propulsion technology.

'One thing that OTV-4 has in common with each of its predecessors is that its ground track nearly repeats after every 31 revolutions, which takes two days,' Toronto-based Ted Molczan told Space.com.

He added that this could be in support of a spy mission, to permit targets to be revisited frequently.

Other theories have for the spaceplane have ranged from it being a space bomber, to a clandestine probe on a mission to 'take out' spy satellites.

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

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

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

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

TOP FOUR CONSPIRACY THEORIES The space plane is a spy plane: The leading theory appears to be that the unmanned space plane is a shuttle-shaped surveillance vehicle. It could be a space bomber: This is the least likely theory, according tot Seven Aftergood, a secrecy expert at the Federation of American Scientists. He claims the US doesn't need this capability. It is on a mission is to 'take out' satellites: This activity would be easily traceable, making it unlikely to stay a secret. The X-37B deploys spy satellites: Instead of destroying them, the theory suggests that the space plane's orbit matches up to where deployed satellites would work best for spying on other countries. Advertisement

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 long — about one-fourth the size of a Nasa shuttle. The longest X-37B flight lasted about 675 days; touchdown was last October.

There's no official word on exactly how long this one will stay up, although report suggest it will return to Earth in mid-to-late 2016.

In an unprecedented disclosure, last month month 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.

'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'.

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

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.

One leading secrecy expert previously told DailyMail.com that 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.

LIGHTSAIL BEGINS ITS VOYAGE! SOLAR SAIL CRAFT LAUNCH ON ATLAS V The Planetary Society is tagging along with a solar-sail demo called LightSail Piggybacking a ride on last month's launch was a small spacecraft designed to test solar sail technology. The Planetary Society's revolutionary LightSail vehicle took off on an Atlas V rocket on May 20, and it will later unfurl its giant sail in orbit and test how effective the sun is at pushing it. If it works, the technology could signal a breakthrough in space propulsion and will allow vehicles to explore distant destinations, such as the moon and Mars, at very low cost. At its core is a tiny 'craft' made up of three 'CubeSats', which are essentially tiny spacecraft that can be launched relatively cheaply. In total, the core of the vehicle weighs 22lbs (10kg) and is just 11.8-inches (30cm) high and 3.9-inches (10cm) wide - about the size of a loaf of bread. At the bottom of the four spacecraft on each of its four sides, though, a huge solar sail has been 'folded up'. This sail, measuring 345 square ft (32 square metres) in total in size, is made of an extremely reflective material called Mylar. When it is unfurled, photons from the sun will strike the sail and push it forwards, similar to how a sail on Earth catches the wind. The push is extremely minimal - less than holding a sheet of paper in your hand - but it is theorised that, over time, this push could build up enough to reach high speeds. This is because the force from the sun is constant, and there is no air resistance or friction holding the spacecraft back, so it will always accelerate. Advertisement

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 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 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.

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).

One secrecy expert told DailyMail.com that the drone (artist's impression pictured) is 'likely' to be a spy plane

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