USA: Air Force launches its fourth rocket for its classified space mission

Theories about what the US military’s X-37B is doing range from being a space bomber to a drone ship used to monitor Chinese satellites.

Iran believes it is a space warplane carrying weapons — a claim dismissed by the air force.

Another theory is it is sweeping up broken American satellites to bring them home to be fixed.

It has flown three previous secret missions to date and the air force has been slightly more forthcoming about this one.

For the first time, air force officials released details of some of the prototype equipment on board.

One is a new “thruster system” — an experimental plasma-propulsion space engine — that could be used to manoeuvre satellites in orbit.

The X-37B taxis on the flight line.Source:Supplied

Captain Chris Hoyler said the Boeing-built X-37B spacecraft — 8.8m long with a 4.5m wingspan — has room inside for an array of electronic equipment.

“[We] are investigating an experimental propulsion system on the X-37B on Mission 4,” he said.

“The air force Rapid Capabilities Office will also host a number of advance materials on-board the X-37B for NASA to study the durability of various materials in the space environment.”

air force spokeswoman Capt. Annmarie Annicelli kept up the secrecy around the mission.

“I can confirm the fourth OTV mission is approaching one year on orbit,” she told Space.com

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

By comparison, the International Space Station orbits at about 350km.

It is a quarter of the size of NASA’s retired space shuttles and is launched in the same way, vertically on a rocket, before landing like a conventional aeroplane.

The space plane is too small to carry people on-board but its cargo bay is just large enough to carry a small satellite.