科學家們終于確定中國第一座空間站會在什么時候掉落到地球上了。

天宮一號2011年發射升空，2016年中國宣布該空間站終止數據服務，并且預計它將于2017年下半年隕落。這個時間窗口看來已經相當大，但當時就有科學家指出，天宮一號隨時都可能掉下來，而且落到哪里都有可能。

目前天宮一號仍在軌道上運行。3月初，專家將其再次進入大氣層的時間劃定在3月24日至4月19日之間，但無法做出更詳盡的預估。

據歐洲航天局介紹，現在最具體的預測是天宮一號將在3月30日到4月6日之間掉在地球上。以追蹤衛星為“副業”的考古學家馬科·朗布魯克將這座八噸重空間站從軌道跌落的時間定在3月31日前后三天。

我對天宮一號何時再入大氣層的最新預期：

3月31日前后三天

昨天的地磁暴似乎并未讓它出現顛簸。@SSC_NLpic.twitter.com/MdZgP1rX1R

- 馬科·朗布魯克博士（@Marco_Langbroek），2018年3月20日

預測不夠具體可以理解：如果低軌道，也就是距地面高度在大約2000公里以下的物體不再自行推進，它們最終就會失速并隕落到地球上。

但在這個高度的物體不會一直受到大氣層阻力的作用。因此，就目前在距地面250公里軌道上飛行的天宮一號而言，具體預測它什么時候開始下落的難度非常大。

天宮一號的近地點目前已降至215公里，而且它的高度每天都會下降2-3公里——今后兩周，這個數字將迅速躥升：pic.twitter.com/O2gQySMEO9

- 馬科·朗布魯克博士（@Marco_Langbroek），2018年3月20日

不過，在天宮一號可能重返大氣層的那幾天，大家不用緊張地注視天空。

歐洲航天局表示：“實際上，和一年被閃電擊中一次的可能性相比，被天宮一號殘骸擊中的幾率為前者的一千萬分之一。”

雖然天宮一號殘骸幾乎不會對人們構成威脅，但歐洲航天局仍在設法推斷這座空間站大概會掉在哪里。

該局稱：“我們不可能就時間/地點做出詳細預測?！?

天宮一號最有可能落在北緯43度和南緯43度之間的海洋中，但在它真正隕落前，人們可以預測的基本上就是這些。

得克薩斯大學研究員利昂·溫斯通在給《財富》雜志的書面評論中寫道：“我們需要適應太空中有大量物體朝著我們落下來的想法?！睖厮雇ㄖ赋?，隨著衛星發射數量的持續增長，衛星隕落只會變得越來越常見。

科學家仍在想辦法解決衛星和其他太空殘骸做自由落體運動帶來的問題。到目前為止的方案中就有中國今年早些時候提出的用激光將其擊落的建議。（財富中文網）

譯者：Charlie

審稿：夏林

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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.