《那些古怪又讓人憂心的問題》第86期:軌道速度(2)
Getting to space is easy. It's not, like,something you could do in your car, but it's not a huge challenge. You couldget a person to space with a rocket the size of a telephone pole. The X-15aircraft reached space just by going fast and then steering up.
進入太空很簡單。雖然你確實沒法開車進入太空，但這並不是一個大問題。僅靠一個電話線杆大小的探空火箭就能把一個人送入太空。要知道美國的X-15飛機就是因爲飛得非常快，然後機頭往上一拉進入太空的。
You will go to space today, and then youwill quickly come back.
你這是來也匆匆，去也匆匆。
But getting to space is easy. The problemis staying there.
進入太空容易，要留在太空中就沒那麼簡單了。
Gravity in low Earth orbit is almost asstrong as gravity on the surface. The Space Station hasn't escaped Earth'sgravity at all; it's experiencing about 90 percent the pull that we feel on thesurface.
近地軌道處的地球引力幾乎和在地面上一樣強。即使是空間站也沒能逃離地球引力的魔爪，它受到的引力大小約是在地面上的90％。
To avoid falling back into the atmosphere,you have to go sideways really, really fast.
要想不從太空掉回來，你必須要繞地球飛得足夠快。
The speed you need to stay in orbit is about8 kilometers per second. Only a fraction of a rocket's energy is used to liftup out of the atmosphere; the vast majority of it is used to gain orbital(sideways) speed.
要想保持在繞地軌道的高度你需要達到每秒8千米的速度。火箭的能量只有很少一部分用來將負載帶到大氣層外，大部分的能量都被用於加速到軌道上側着走的速度。
This leads us to the central problem ofgetting into orbit: Reaching orbital speed takes much more fuel than reachingorbital height. Getting a ship up to 8 km/s takes a lot of booster hing orbital speed is hard enough; reaching orbital speed while carryingenough fuel to slow back down would be completely impractical.
於是我們就看到了入軌的重點：達到軌道速度要比進入軌道高度花費多得多的燃料。要想把一艘飛船加速到每秒8千米需要許許多多的助推器。達到軌道速度已經很難了，還要攜帶足夠的燃料把速度再降下來根本就是不切實際的。
These outrageous fuel requirements are whyevery spacecraft entering an atmosphere has braked using a heat shield insteadof rockets-slamming into the air is the most practical way to slow down. (Andto answer Brian's question, the Curiosity rover was no exception to this;although it used small rockets to hover when it was near the surface, it firstused air-braking to shed the majority of its speed.)
如此驚人的燃料需求也是爲什麼飛船在再入大氣層時都用熱防護罩而不是反推火箭來減速——一頭栽入大氣層是最可行的減速方式。（至於布賴恩問的那個問題，“好奇號”也是這麼減速的，它攜帶的反推火箭只是在接近地面時才啓動，之前已經通過火星大氣制動降低了一大部分的速度了。）
How fast is 8 km/s, anyway?
那麼，每秒8千米有多快呢？
I think the reason for a lot of confusionabout these issues is that when astronauts are in orbit, it doesn't seem likethey're moving that fast; they look like they're drifting slowly over a bluemarble.
我覺得人們之所以會有開頭的那些迷惑，是因爲在太空中的航天員看起來移動得並不快，他們看起來就像在慢慢飄過一顆藍色星球。
