A prototype spaceplane developed for the US military has been launched into orbit from Florida.

一架美国军用航天飞机在佛罗里达州发射升空。

The project could lead to the military's first operational spaceplane

The X-37B, which has been likened to a scaled-down space shuttle, blasted off from Cape¹ Canaveral Air Force Station at 0052 BST (1952 EDT).

The military vehicle is unpiloted（无人驾驶的） and will carry out the first autonomous²（自治的，自主的） re-entry and landing in the history of the US space programme.

The spacecraft can return experiments to Earth for inspection³ and analysis.

At 9m (29ft) -long, 4.5m (15ft), the reusable spaceplane is about one quarter the size of the shuttle, with a large engine mounted at the rear of the ship for orbit changing.

And while the space shuttle uses a fuel cell power system; the military vehicle is powered by a solar array and lithium-ion batteries.

The precise objectives and cost of the programme are secret. But the first few flights will allow officials to evaluate（鉴定，评估） the vehicle's performance and ensure components⁴ and systems work the way they are supposed to.

"The top priority technology demonstration⁵ on this first flight is the vehicle itself," Gary Payton, the US Air Force's deputy under secretary for space programs, told journalists on a teleconference（电话会议） this week.

"Getting it into orbit, getting the payload（有效负荷） bay doors open, the solar array（太阳电池板） deployed⁶, learning about on-orbit attitude control（姿态控制） and bringing it all back."

The X-37B was launched vertically⁷（垂直地） atop an Atlas⁸ V rocket. The Air Force (USAF) says the vehicle will be used to test advanced guidance, navigation and control, thermal⁹ protection systems, avionics（航空电子学） and high temperature structures and seals.

The Pentagon has not specified¹⁰ a duration for this mission, but the X-37B is designed to operate on orbit for up to 270 days: "In all honesty, we don't know when it's coming back for sure. It depends on the progress that we make with the on-orbit experiments, the on-orbit demonstrations," said Mr Payton.

Once the mission is complete, a command will be sent from the ground prompting the 5,000kg (11,000lb) spaceplane to fire its engine to re-enter the atmosphere.

It will then autonomously¹¹ navigate¹² its way to the 4.5km (15,000ft) -long primary runway at Vandenberg Air Force Base in California.

The X-37B started life in 1999 as a US space agency programme, but Nasa handed the project over to the Pentagon in September 2004. As such, the Air Force is in a position to talk openly about the craft's design, but its purpose remains¹³ classified.

Dr Joan Johnson-Freese, chair of national security and decision making at the US Naval¹⁴ War College in Newport, Rhode Island, said the military would be waiting to see if this project yielded（产生） new capabilities¹⁵: "It might be at this point in time that [the US Air Force is] going to roll the dice¹⁶（掷骰子） and see if something good happens.

"If it does, they'll continue with it. Otherwise, this will be another one of those [experimental] projects that goes into a bin¹⁷ somewhere."