《那些古怪又讓人憂心的問題》第65期:人類大遷徙(2)

Fortunately, we're saved from an infinite loop-where we add 1.3 kilograms for every 1 kilogram we add-by the fact that we don't have to carry that fuel all the way up. We burn it as we go, so we get lighter and lighter, which means we need less and less fuel. But we do have to lift the fuel partway. The formula for how much propellant we need to burn to get moving at a given speed is given by the Tsiolkovsky Rocket equation:

幸好我們有方法擺脫每增加1千克的質量就需額外加1.3千克的燃料的死循環，應該注意到我們並不需要把所有的燃料都帶到目的地，而是邊飛邊消耗燃料，這樣飛船的質量就會越來越輕，加速所需的燃料也就越來越少。但我們還是需要帶夠一定量的燃料，計算爲達到給定速度需要多少推進劑的公式就是大名鼎鼎的齊奧爾科夫斯基火箭方程：

and are the total mass of the ship plus the fuel before and after the burn, and is the “exhaust velocity” of the fuel, a number that's between 2.5 and 4.5 km/s for rocket fuels.

初始質量和末質量分別是燃燒前後飛船加上燃料的總質量，而排氣速度則是燃料燃燒後產生的氣體噴出噴口時的速度，對火箭燃料來說這個速度在每秒2.5～4.5千米之間。

What's important is the ratio between , the speed we want to be going, and , the speed that the propellant exits our rocket. For leaving Earth, we need a of upward of 13 km/s, and is limited to about 4.5 km/s, which gives a fuel-to-ship ratio of at least . If that ratio is x, then to launch a kilogram of ship, we need ex kilograms of fuel.

這個公式中最重要的是速度變化量（也就是我們想要達到的速度）和排氣速度（推進劑離開火箭時的速度）之間的比值。如果想要離開地球，那麼速度變化量在垂直方向上至少需要每秒13千米，但排氣速度最大也就約每秒4.5千米，因而所需燃料重量和飛船總重量之間的比值至少爲e（13.5/4.5）≈20。也就是說如果速度變化量和排氣速度的比值爲x，那麼飛船的質量每增加1千克，我們就需要攜帶ex千克燃料。

As x grows, this amount gets very large.

隨着x的增長，這個數字會變得非常龐大。

The upshot is that to overcome Earth's gravity using traditional rocket fuels, a 1-ton craft needs 20 to 50 tons of fuel. Launching all of humanity (total weight: around 400 million tons) would therefore take tens of trillions of tons of fuel. That's a lot; if we were using hydrocarbon-based fuels, it would represent a decent chunk of the world's remaining oil reserves. And that's not even worrying about the weight of the ship itself, food, water, or our pets.1 We'd also need fuel to produce all these ships, to transport people to the launch sites, and so forth. It's not necessarily completely impossible, but it's certainly outside the realm of plausibility.

總之，要想用傳統的火箭燃料擺脫地球的引力束縛，1噸重的飛船就需要20～50噸燃料。把所有人類（總重量大約4億噸）都發射到太空去則需要上百萬億噸燃料。這可是很大一個數字。如果我們用的是以烴類化合物爲基礎的燃料，那麼將消耗全球剩餘原油儲量的一大部分。而且現在我們還沒考慮飛船本身的重量，還有水、食物和我們的寵物的重量呢。1我們還需要許多燃料來建造這些飛船，把所有人類都運送到發射場地等。雖然並不是完全不可能，但很明顯這非常不現實。

But rockets aren't our only option. As crazy as it sounds, we might be better off trying to (1) literally climb into space on a rope, or (2) blow ourselves off the planet with nuclear weapons. These are actually serious-if audacious-ideas for launch systems, both of which have been bouncing around since the start of the Space Age.

但我們可用的並不只有火箭，還有聽上去更加瘋狂的方法，比如沿着繩子爬到太空去，或者用核武器把我們炸飛到太空去。這兩個想法其實真的是嚴肅的——即使有點太野心勃勃——發射計劃，而且自從太空時代開啓以來，人們就一直在琢磨怎麼付諸實踐。