Fortunately, we 8217;re saved from an infinite loop-where we add 1.3 kilograms for every 1 kilogram we add-by the fact that we don 8217;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:
幸好我们有方法摆脱每增加1千克的质量就需额外加1.3千克的燃料的死循环，应该注意到我们并不需要把所有的燃料都带到目的地，而是边飞边消耗燃料，这样飞船的质量就会越来越轻，加速所需的燃料也就越来越少。但我们还是需要带够一定量的燃料，计算为达到给定速度需要多少推进剂的公式就是大名鼎鼎的齐奥尔科夫斯基火箭方程：
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 8217;s between 2.5 and 4.5 km/s for rocket fuels.
初始质量和末质量分别是燃烧前后飞船加上燃料的总质量，而排气速度则是燃料燃烧后产生的气体喷出喷口时的速度，对火箭燃料来说这个速度在每秒2.5～4.5千米之间。
What 8217;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.
这个公式中最重要的是速度变化量（也就是我们想要达到的速度）和排气速度（推进剂离开火箭时的速度）之间的比值。如果想要离开地球，那么速度变化量在垂直方向上至少需要每秒13千米，但排气速度最大也就约每秒4.5千米，因而所需燃料重量和飞船总重量之间的比值至少为e（13.5/4.5）≈20。也就是说如果速度变化量和排气速度的比值为x，那么飞船的质量每增加1千克，我们就需要携带ex千克燃料。
As x grows, this amount gets very large.
随着x的增长，这个数字会变得非常庞大。
The upshot is that to overcome Earth 8217;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 8217;s a lot; if we were using hydrocarbon-based fuels, it would represent a decent chunk of the world 8217;s remaining oil reserves. And that 8217;s not even worrying about the weight of the ship itself, food, water, or our pets.1 We 8217;d also need fuel to produce all these ships, to transport people to the launch sites, and so forth. It 8217;s not necessarily completely impossible, but it 8217;s certainly outside the realm of plausibility.
总之，要想用传统的火箭燃料摆脱地球的引力束缚，1吨重的飞船就需要20～50吨燃料。把所有人类（总重量大约4亿吨）都发射到太空去则需要上百万亿吨燃料。这可是很大一个数字。如果我们用的是以烃类化合物为基础的燃料，那么将消耗全球剩余原油储量的一大部分。而且现在我们还没考虑飞船本身的重量，还有水、食物和我们的宠物的重量呢。1我们还需要许多燃料来建造这些飞船，把所有人类都运送到发射场地等。虽然并不是完全不可能，但很明显这非常不现实。
But rockets aren 8217;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.
但我们可用的并不只有火箭，还有听上去更加疯狂的方法，比如沿着绳子爬到太空去，或者用核武器把我们炸飞到太空去。这两个想法其实真的是严肃的——即使有点太野心勃勃——发射计划，而且自从太空时代开启以来，人们就一直在琢磨怎么付诸实践。