神秘的太空飞机已经绕地球运行了719天，我们却不知道它在干什么

An uncrewed, solar-powered military plane has just broken its spaceflight-duration record, having now spent more than 719 days in orbit around Earth.

一架无人驾驶的太阳能军用飞机，刚刚打破了其航天飞行时间的记录，目前已在地球轨道上飞行了719天多。

Known as the X-37B, the space plane belongs to the US Air Force, and this is its fifth mission, Orbital Test Vehicle 5 (OTV-5). However, exactly what it's achieving on its missions remains classified.

这架航天飞机被称为X-37B，属于美国空军，这是它的第五次任务，轨道试验飞行器5(OTV-5)。然而，它在其任务中的具体成果仍然是保密的。

On Monday, the plane broke the record set by the previous mission, OTV-4, which stayed in the air for a duration of 717 days, 20 hours and 42 minutes.

周一，这架飞机打破了先前任务OTV-4的记录，它在空中停留了717天20小时和42分钟。

OTV-5 beat that record at 10:43 UTC (06:43 EDT) on August 26. We're now nearly at the end of day 719 of the plane being in orbit.

8月26日，世界时间10:43(美国东部时间06:43)，OTV-5打破了这一纪录。现在，我们已经接近第719天的尾声了。

So what's it doing up there?

那么它在上面做什么呢?

Well, no one's sure as yet. While the US Air Force doesn't avoid talking about the plane, they use very general terms.

嗯，目前还没有人确定。虽然美国空军不回避谈论飞机，但他们使用非常通用的术语。

Some things we do know - X-37B looks a little like a smaller version of NASA's old space shuttle, and the US Air Force has at least two of the solar-powered aircraft, which were built by Boeing.

我们知道的一些事情——X-37B看起来有点像美国宇航局旧航天飞机的小版本，美国空军至少有两架太阳能飞机，由波音公司制造。

This latest mission was launched on 7 September 2017 by a SpaceX Falcon 9 rocket and is still active.

这项最新任务于2017年9月7日由SpaceX猎鹰9号火箭发射，目前仍在进行中。

It's not clear when it'll come down, but when it does, it'll land on a runway using its wheels.

现在还不清楚它什么时候会降落，但一旦降落，它就会用轮子降落在跑道上。

According to the Air Force, the X-37B's primary objectives are to research reusable spacecraft technologies for America's future in space, and to conduct experiments that can be returned to, and examined, on Earth.

根据美国空军的说法，X-37B的主要目标，是为美国未来的太空研究可重复使用的航天器技术，并进行可以返回地球并在地球上进行检测的实验。

Beyond that, officials haven't given up much actual dirt on the specifics of its orbital experiments.

除此之外，官方并没有透露太多关于其轨道实验的具体细节。

In the past, that's led to fervent speculation that the military might be testing an EM Drive in space – a hypothetical fuel-less propulsion system that's been studied by NASA, and which China claims it's already testing.

在过去，这导致了狂热的猜测，军方可能正在太空中测试一种电磁驱动——一种假设的无燃料推进系统，NASA已经对其进行了研究，中国声称已经在进行测试。

Other speculation has suggested the Air Force might be using the X-37B for weapons research or orbital surveillance operations – although back in 2010 the Air Force denied that the program involved any "offensive capabilities".

其他猜测表明，美国空军可能正在使用X-37B进行武器研究或轨道监视行动——尽管早在2010年，美国空军否认该计划涉及任何“进攻能力”。

"The program supports technology risk reduction, experimentation and operational concept development," a spokesperson said at the time.

“该项目支持技术风险降低、实验和运营理念的开发，”一位发言人当时说。

Regardless of what the plane's mission is, it was initially only designed for an orbit window of 270 days, so the fact that it's been able to more than double that time is a pretty big win for solar-powered aircraft.

不管这架飞机的任务是什么，它最初只是为270天的轨道窗口设计的，所以事实上，它已经能够超过两倍的时间，这已经是一个太阳能飞机相当大的胜利。

Beyond that, we unfortunately can't tell you much, other than we'll be keeping an eye on what the space plane does next.

除此之外，不幸的是，我们不能告诉你太多，除了我们将密切关注下一步太空飞机将做什么。