- X-37B drone blasted off on a mysterious mission on May 20, 2015
- Theories range from it being a space bomber to destroying satellites
- The flight is testing a system that makes satellites easy to manoeuvre
- This is probably a small part of the vehicle's true mission, say experts
One of the most mysterious craft ever to be flown by the US military has now been in orbit for a record 677 days.
The X-37B space plane, known as as Orbital Test Vehicle-4 (OTV-4), broke the record on Saturday after surpassing 674 days.
While some details of its payload have been revealed - we still do not know exactly what's on board the spacecraft, or its mission.
Theories have ranged from it being a space bomber, to a clandestine probe designed to 'take out' spy satellites.
One of the most mysterious craft ever to be flown by the US military has now been in orbit for a record 677 days. The X-37B space plane, known as as Orbital Test Vehicle-4 (OTV-4), broke the record on Saturday after surpassing 674 days. Pictured is an artist's impression
THE X-37B MYSTERY
The U.S. Air Force's unmanned X-37B space plane has flown three previous 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 its 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.
The previous record was 674 days, set during OTV-3, which lasted from December 2012 to October 2014.
Earlier X-37B flights have been known to land at Vandenberg Air Force Base in California, and recent plans to consolidate launch and landing activities could mean vehicles touch down at the Shuttle Landing Facility at NASA’s Kennedy Space Center,Space.com reports.
But, where and when the mysterious space plane will really land remains a mystery.
‘The landing date will be determined based on the completion of the program’s on-orbit demonstrations and objectives for this mission,’ Captain Annmarie Annicelli, an Air Force Spokeswoman, told Space.com.
‘While the program has the capability to land at either KSC or Vandenberg, the landing location is determined by a variety of factors.’
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.
It's said to be one of the most mysterious craft ever to be flown by the US military.
Last month, experts claimed it may be about to finally return to Earth - almost two years later.
'The historic Shuttle Landing Facility at the Florida spaceport is preparing to once again host an end of mission landing as the Air Force's X-37B mini spaceplane prepares to return from a near two year mission on orbit,' nasaspaceflight.com said in February.
However, the Air Force claimed the movement was simply a test exercise, meaning the mission continues.
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
'The X-37 is still on-orbit.
'The program is conducting a regularly scheduled exercise this week,' said Capt. Annmarie Annicelli, media operations officer at the Pentagon's Air Force Press Desk.
Nasaspaceflight.com says even if there is a test, the landing equipment has been left in place for a possible landing.
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)
TOP 4 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.
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.
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
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.
'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'.
The US Air Force's top secret X-37B space plane was caught on camera last year 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
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.
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
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 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.'
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
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-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.
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
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