The unmanned space plane X-37B of the US Air Force UU (In the photo) is approaching a year in orbit as part of his fifth secret mission

The X-37B unmanned space plane of the United States Air Force has spent 400 days in orbit as part of its fifth secret mission.

The last trip of the aircraft, called Orbital Test Vehicle-5 (OTV-5), began on September 7, 2017.

The vessel was launched aboard one of Elon Musk's SpaceX Falcon 9 rockets from the Launch Complex 39A at the NASA Kennedy Space Center in Florida.

Officials have revealed few details about the OTV-5 mission, but there has been a suggestion that it could be part of a boost for a US Space Force. UU

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The unmanned space plane X-37B of the US Air Force UU (In the photo) is approaching a year in orbit as part of his fifth secret mission

The unmanned space plane X-37B of the US Air Force UU (In the photo) is approaching a year in orbit as part of his fifth secret mission

The space plane is 29 feet (8.8 meters) long, 9.6 feet (2.9 meters) tall and weighs about 11,000 pounds. (4,990 kilograms).

It is in orbit about 200 miles (320 km) high and is powered by solar cells with lithium-ion batteries.

Little is known about what it takes, but on board the OTV-5 payload is a US thermal spreader. UU That will prove the longevity of electronic components and heat pipes in the space environment.

In June, President Donald Trump announced that he is directing the Pentagon to a new Space Force as an independent service branch aimed at securing US dominance in space.

The president framed the space as a national security problem, and said he does not want "China, Russia and other countries to lead us."

One expert has suggested that this plane could already be part of a US Space Force. UU

"Ironically, the X-37B is exactly the kind of program, to give flexibility to US operations in space, which seems to be driving the current momentum of a Space Force, but it's already underway," he said. Joan Johnson-Freese. Professor at the Department of Homeland Security Affairs at Naval War College in Newport, Rhode Island, writes Space.com.

Four previous X-37B missions were launched with the United Launch Alliance Atlas 5 rockets.

Each time the unmanned space plane carries a mysterious payload on long-duration flights in Earth orbit.

The spacecraft (pictured) was launched from Elon Musk's SpaceX Falcon 9 rocket from the Launch Complex 39A at the NASA Kennedy Space Center in Florida

The spacecraft (pictured) was launched from Elon Musk's SpaceX Falcon 9 rocket from the Launch Complex 39A at the NASA Kennedy Space Center in Florida

The spacecraft (pictured) was launched from Elon Musk's SpaceX Falcon 9 rocket from the Launch Complex 39A at the NASA Kennedy Space Center in Florida

WHAT IS THE PLANE OF SPACE X-37B?

The unmanned space plane X-37B of the US Air Force UU It looks similar to the NASA space shuttle, but it is much smaller.

The space plane is 29 feet (8.8 meters) long, 9.6 feet (2.9 meters) tall and weighs about 11,000 pounds. (4,990 kilograms).

It is orbiting about 200 miles (320 kilometers) high.

The unmanned space plane X-37B of the US Air Force UU It looks similar to the NASA space shuttle, but it is much smaller. The space plane is 29 feet (8.8 meters) long, 9.6 feet (2.9 meters) tall and weighs about 11,000 pounds. (4,990 kilograms)

Officials have revealed few details about the OTV-5 mission (the plane's fifth), but according to the Air Force, one aboard the OTV-5 is the US thermal spreader. UU That will prove the longevity of electronics and heat pipes in the space environment.

The ship is powered by solar cells with lithium-ion batteries.

Four previous X-37B missions were launched with the United Launch Alliance Atlas 5 rockets.

Each time the unmanned space plane carries a mysterious payload on long-duration flights in Earth orbit.

"The many new features in this mission make the upcoming launch of OTV a milestone for the program," said Randy Walden, director of the Air Force's Rapid Capabilities Office at the launch last year.

"Our goal is to continue moving forward with the X-37B OTV so that it can more fully support the growing space community."

"The fifth mission of OTV continues to advance the performance and flexibility of the X-37B as a space technology demonstrator and host platform for experimental payloads," said a spokesman for the US Air Force. UU

The previous OTV-4 mission lasted 718 days.

The spacecraft built by Boeing took off in May 2015 from the nearby Cape Canaveral Air Force Station aboard an Atlas 5 rocket built by United Launch Alliance, a partnership between Lockheed Martin Corp and Boeing.

The reusable Orbital Test Vehicle (photo) returned in June after its fourth mission, concluding a mission of almost two years in orbit, according to the US Air Force. UU

The reusable Orbital Test Vehicle (photo) returned in June after its fourth mission, concluding a mission of almost two years in orbit, according to the US Air Force. UU

The reusable Orbital Test Vehicle (photo) returned in June after its fourth mission, concluding a mission of almost two years in orbit, according to the US Air Force. UU

The X-37B, one of the two in the Air Force fleet, performed unspecified experiments during its long orbit.

It was the fourth and longest mission so far for the secret program, administered by the Air Force's Rapid Capabilities Office.

The Air Force said that the orbiters "perform risk reduction, experimentation and development of the concept of operations for reusable spacecraft technologies".

The Air Force tweeted that the vehicle had landed safely, marking the conclusion of its fourth classified mission, which lasted more than 700 days.

The Air Force tweeted that the vehicle had landed safely, marking the conclusion of its fourth classified mission, which lasted more than 700 days.

The Air Force tweeted that the vehicle had landed safely, marking the conclusion of its fourth classified mission, which lasted more than 700 days.

However, the exact nature of the mission and its cost are classified.

The Secure World Foundation, a nonprofit group that promotes the peaceful exploration of space, says the secret surrounding the X-37B suggests the presence of hardware related to the intelligence being tested or evaluated aboard the ship.

The X-37B flew for the first time in April 2010 and returned after eight months.

A second mission was launched in March 2011 and lasted 15 months, while a third mission took flight in December 2012 and returned after 22 months.

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