Tuesday, September 17, 2013

Another Interesting "What IF"

XKCD’s “What If” broaches an interesting subject this week.  It talks about the difference between going to space, and going into orbit, which are two different things.  Actually getting to space just means that you need to have enough power to fly 100 kilometers up.  The common misconception is that when you are in space, in Orbit around the Earth, you haven’t managed to break free of Earth’s gravitational pull – it’s still there, just about as strong as it is when you’re on the surface of Earth, itself.  You can fly straight up into space until your rocket runs out of fuel, and then you’ll just fall back down to Earth via gravity.  The trick to staying in space is achieving a speed fast enough to gain orbit.  This is where the common misconception about space comes from – objects in Earth’s orbit have not transcended Earth’s gravitational pull.  Like anything else on Earth that is suspended in air with forward momentum, objects in orbit are moving in what is called a ballistic trajectory, which is the same path as a bullet or a thrown baseball.  Neither of these are defying gravity, either.  A ball or bullet thrown or fired parallel to the Earth’s surface will fall to Earth just as fast as a ball or bullet dropped from the same height, it’s just a matter of how far, laterally, each travels due to their velocity, before they hit the ground.  The trick is that the object in orbit is moving so goddamned fast;  In pretty simplistic terms, you’re falling back to Earth while in orbit, just like the aforementioned bullet or ball, but at the same time, you’re moving so fast that you miss Earth entirely and just keep falling around it. 

And when I say fast, I mean FAST.

In fact, in order for the ISS to be going the same speed as a bullet fired out of a high powered rifle, the ISS would have to slow down to 1/10th its current speed!

That is why space craft heat up during re-entry – it isn’t some intrinsic quality of space travel, it’s just that the craft is moving balls-out fast as hell, and as it descends into the Earth’s atmosphere at such dazzling speeds, the friction of the air moving past the craft, and being compressed in front of the craft, is such that it creates a lot of heat.  If you were to fly straight up into space and not achieve orbit, and then just fall back to Earth, you would not experience such dramatic heating (I’m relatively certain that at first, you’d fall pretty fast without air resistance to slow you down, so you would probably experience SOME heating, but not of the likes to melt your craft into plasma like you get on orbital re-entry).  

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