天宫一号即将坠落地球，地点仍然存疑

科学家们终于确定中国第一座空间站会在什么时候掉落到地球上了。

天宫一号2011年发射升空，2016年中国宣布该空间站终止数据服务，并且预计它将于2017年下半年陨落。这个时间窗口看来已经相当大，但当时就有科学家指出，天宫一号随时都可能掉下来，而且落到哪里都有可能。

目前天宫一号仍在轨道上运行。3月初，专家将其再次进入大气层的时间划定在3月24日至4月19日之间，但无法做出更详尽的预估。

据欧洲航天局介绍，现在最具体的预测是天宫一号将在3月30日到4月6日之间掉在地球上。以追踪卫星为“副业”的考古学家马科·朗布鲁克将这座八吨重空间站从轨道跌落的时间定在3月31日前后三天。

我对天宫一号何时再入大气层的最新预期：

3月31日前后三天

昨天的地磁暴似乎并未让它出现颠簸。@SSC_NLpic.twitter.com/MdZgP1rX1R

- 马科·朗布鲁克博士（@Marco_Langbroek），2018年3月20日

预测不够具体可以理解：如果低轨道，也就是距地面高度在大约2000公里以下的物体不再自行推进，它们最终就会失速并陨落到地球上。

但在这个高度的物体不会一直受到大气层阻力的作用。因此，就目前在距地面250公里轨道上飞行的天宫一号而言，具体预测它什么时候开始下落的难度非常大。

天宫一号的近地点目前已降至215公里，而且它的高度每天都会下降2-3公里——今后两周，这个数字将迅速蹿升：pic.twitter.com/O2gQySMEO9

- 马科·朗布鲁克博士（@Marco_Langbroek），2018年3月20日

不过，在天宫一号可能重返大气层的那几天，大家不用紧张地注视天空。

欧洲航天局表示：“实际上，和一年被闪电击中一次的可能性相比，被天宫一号残骸击中的几率为前者的一千万分之一。”

虽然天宫一号残骸几乎不会对人们构成威胁，但欧洲航天局仍在设法推断这座空间站大概会掉在哪里。

该局称：“我们不可能就时间/地点做出详细预测。”

天宫一号最有可能落在北纬43度和南纬43度之间的海洋中，但在它真正陨落前，人们可以预测的基本上就是这些。

得克萨斯大学研究员利昂·温斯通在给《财富》杂志的书面评论中写道：“我们需要适应太空中有大量物体朝着我们落下来的想法。”温斯通指出，随着卫星发射数量的持续增长，卫星陨落只会变得越来越常见。

科学家仍在想办法解决卫星和其他太空残骸做自由落体运动带来的问题。到目前为止的方案中就有中国今年早些时候提出的用激光将其击落的建议。（财富中文网）

译者：Charlie

审稿：夏林

Scientists have finally determined when China’s oldest space station will crash back to our planet.

In 2016, the Chinese announced they had lost control of Tiangong-1, which was first launched in 2011, and that it would come falling to Earth sometime “in the latter half” of last year. That seems like a pretty large window, but at the time scientists noted that it could fall anywhere at anytime.

The space lab, named “heavenly palace,” is still in orbit. Earlier in March, experts put the re-entry period between March 24 and April 19, but couldn’t give a more precise estimate.

Now the best prediction is that the space station will de-orbit and crash back to Earth between March 30 and April 6, according to the European Space Agency (ESA). Marco Langbroek, an archaeologist who also tracks satellites, put the window for the eight-ton space station to fall from orbit within three days of March 31.

My latest #reentry estimate for #Tiangong1:

31 March +- 3 days

The geomagnetic storm of yesterday does seem to have given it a bump.@SSC_NLpic.twitter.com/MdZgP1rX1R

— Dr Marco Langbroek (@Marco_Langbroek) March 20, 2018

The lack of accuracy was understandable: At low orbit, below about 2,000 km above the planet’s surface, objects will eventually lose speed and fall back to Earth if they don’t continue to exert a force to propel themselves.

However, the drag from Earth’s atmosphere that acts on objects at this height is not consistent, so pinpointing exactly when the space station, which is currently orbiting at about 250 km above the planet, will begin its descent is very difficult.

#Tiangong1 perigee is currently down to 215 km. It is currently losing 2-3 km/day in altitude, and that value will rapidly increase the coming two weeks: pic.twitter.com/O2gQySMEO9

— Dr Marco Langbroek (@Marco_Langbroek) March 20, 2018

Don’t worry about checking the sky during the days the space station is likely to make its re-entry, though.

“The personal probability of being hit by a piece of debris from the Tiangong-1 is actually 10 million times smaller than the yearly chance of being hit by lightning,” the ESA said.

While there is little risk that the debris from the space station will endanger humans, the ESA is still trying to determine a ballpark idea of where on Earth the space station will make landfall.

“At no time will a precise time/location prediction from ESA be possible,” the ESA said.

It’s most likely to crash into the ocean somewhere between latitudes 43° north and 43° south, but that’s about as much as people will know before Tiangong-1 falls from orbit.

“We need to get used to the idea of things raining down on us from space,” University of Texas researcher Leon Vanstone wrote in a commentary piece for Fortune, noting that falling satellites are only going to become more common as the number of launches continues to increase.

Scientists are still searching for solutions to the problem of dealing with free-falling satellites and other space debris. Options so far include shooting them down with lasers, as China proposed earlier this year.