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Burning up on re-entry Wed, Jan 31, 2018
There's a lot of confusion regarding why certain objects burn up on re-entry into earth's atmosphere.  There was confusion on my part until I figured out how to explain it to myself, if that makes any sense.  So here's the deal:  it doesn't happen because the atmosphere is hot.  In fact, the atmosphere gets thinner and thinner (i.e. less air) as altitude increases, so it's actually quite cold near space. 

Objects burn up on re-entry because they're going ridiculously fast.  And when a fast-moving object comes into contact with a stationary or slow-moving object (in this case, air), its speed energy gets converted to heat energy.  It's kind of like jumping into a pool.  Your speed slows down because you went from air (low density, easy to travel through) to water (higher density, harder to travel through).  But when you jump in a pool, you don't feel a change in temperature because you probably weren't traveling that fast on the way in. 

It's kind of hard to compare it to something that happens in normal life, because we don't experience those types of speed differences, like ever.  Objects in orbit around the earth are traveling tangentially to earth's surface at about 17,000 miles per hour.  The reason we don't know much about objects traveling at 17,000-ish miles per hour is because they would burn up in our atmosphere!  Some objects in space are moving much faster than that (such as meteors and asteroids that have been flung out of some other orbit).  In that case the speed difference can be much bigger, which tends to make things explode. 

There was a question online about why Felix Baumgartner, the guy who rode a balloon into space and jumped off, didn't burn up on re-entry.  The answer is simply that he wasn't traveling that fast because he didn't start from orbit.  Sure, he sped up on the way down, but only as fast as the air around him allowed.  He didn't start at 17,000 miles per hour.  He started at roughly zero. 

Finally, the way objects can avoid burning up on re-entry is to enter the atmosphere slowly over time instead of all at once.  That's achieved by choosing the right re-entry angle.  There's that line from Apollo 13:  "The re-entry corridor is in fact so narrow ... that if this basketball were the Earth ... the crew would have to hit a target no thicker than this piece of paper." All that's saying is that the angle at which you enter the atmosphere has to be pretty precise, but since the earth is much bigger than a basketball, the angle is much bigger than the thickness of paper.  Sometimes making things sound simple makes them sound more complicated. #science

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