《那些古怪又讓人憂心的問題》第18期:太空潛水艇(2)

Temperature is a measure of the average kinetic energy of a collection of particles. In space, individual molecules have a high average kinetic energy, but there are so few of them that they don't affect you.

溫度描述的是一堆粒子的平均動能的大小。在太空中，一個個分子都有着很高的分子平均動能，只是它們數量太少影響不到你。

When I was a kid, my dad had a machine shop in our basement, and I remember watching him use a metal grinder. Whenever metal touched the grinding wheel, sparks flew everywhere, showering his hands and clothes. I couldn't understand why they didn't hurt him-after all, the glowing sparks were several thousand degrees.

在我還小的時候，我家地下室裏有一個機械小作坊。我記得我曾經看着我爸使用一臺金屬研磨機，每當金屬接觸砂輪時都會有四散飛濺的火花，許多火花飛到了他的手上或衣服上。我當時不能理解爲什麼我爸不會受傷，因爲那畢竟都是幾千攝氏度的火花。

I later learned that the reason the sparks didn't hurt him was that they were tiny; the heat they carried could be absorbed into the body without warming anything more than a tiny patch of skin.

後來我才知道火花不會傷人的原因是它太小了，它所攜帶的那一丁點兒的熱量會被人體吸收，隻影響到極小一部分皮膚。

The hot molecules in space are like the sparks in my dad's machine shop; they might be hot or cold, but they're so small that touching them doesn't change your temperature much.1 Instead, your heating and cooling is dominated by how much heat you produce and how quickly it pours out of you into the void.

太空中那些“熱”分子就像我爸作坊裏的火花，它們可能很熱也可能很冷，但它們的體形過於微小，就算碰到了也不會對你產生多大影響。1相反，潛艇裏是變熱還是變冷則取決於產生多少熱量，以及你能多快地把這些熱量排出去。

Without a warm environment around you radiating heat back to you, you lose heat by radiation much faster than normal. But without air around you to carry heat from your surface, you also don't lose much heat by convection.2 For most human-carrying spacecraft, the latter effect is more important; the big problem isn't staying warm, it's keeping cool.

現在你的周圍沒有一個溫暖的環境能把熱量輻射回來，因此你通過輻射流失的熱量會比在正常情況下快許多。不過沒有了周圍的空氣從你體表帶走熱量，你因對流而流失的熱量也同時減少了。2對於大多數載人飛船來說，後者更值得關注。他們要面臨的最大問題不是保持溫暖，而是保持涼爽。

A nuclear submarine is clearly able to maintain a livable temperature inside when the outer hull is cooled to 4°C by the ocean. However, if the submarine's hull needed to hold this temperature while in space, it would lose heat at a rate of about 6 megawatts while in the shadow of the Earth. This is more than the 20 kilowatts supplied by the crew-and the few hundred kilowatts of apricity3 when in direct sunlight-so they'd need to run the reactor just to stay warm.4

在水溫爲4℃的海洋中，核潛艇顯然有能力在艙內維持一個可供生存的溫度。不過要想在太空中維持這個溫度，處於地球的陰影中時，潛艇流失熱量的功率約爲6兆瓦，而艇員們散發熱量的功率只有20千瓦，就算加上太陽直射時的幾百千瓦日暖3也不夠，所以他們必須打開反應堆來供暖。

To get out of orbit, a submarine would need to slow down enough that it hit the atmosphere. Without rockets, it has no way to do this.

爲了脫離地球軌道，潛艇需要把速度降到足夠低的程度，這樣才能重返大氣層。但沒有火箭，它無法完成。