why is atmosphere reentry such an issue? If it is just because of speed hitting the atmosphere why can’t the spacecraft just slow down before and synchronise with earth rotation?
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The problem is how you slow down before you hit the atmosphere.
On Earth we mostly slow down due to friction against the earth itself in some way or the other… we kinda dig in our heels against the planet directly or indirectly (our car brakes apply friction against the tires which then apply friction against the road).
But… what if you are in space? NOTHING to brake against? How do you slow down???
You do it by throwing stuff away from you as hard as you can.
Think about two ice skaters that push off from each other. They’ll move in opposite directions.
Now imagine that the two skaters are moving QUICKLY in the same direction. If they push off each other one of them will be moving faster in the original direction, but the other will be moving more slowly in that direction.
That’s basically the only way you can change velocity in space (not going into slingshot stuff here).
So… we can’t just throw equal masses in the opposite direction to reduce our velocity since that means shedding a lot of our total spacecraft all the time. Instead, we burn fuel to throw much smaller masses out at large velocities.
To slow down a lot we need either a BUNCH of mass or a way to accelerate a smaller mass a LOT. Right now that means using small masses that interact explosively so that the small mass can slow us down a lot.
With enough fuel you can slow down as much as you want… but our problem is that you have to get all that fuel (mass) to space first… and that means spending a LOT of fuel/mass/energy to get it up there to being with.
Maybe in the future we’ll discover some better way to travel to and from space, but right now it’s all about mass and momentum… and that’s some very costly physics!
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