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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To launch you need to accelerate to some particular speed if you want to get into orbit and even faster if you want to go to the moon.
After your trip you need to reverse that acceleration back down to a stop.
You *could* let the ground do that for you but then you’d need a new spacecraft and fresh astronauts for every mission.
The alternative you mentioned would be to use a rocket to slow you down just like you used a rocket to speed you up. Intuitively this makes sense. Use half your tank to speed up and half your tank to slow down. Done, right?
The problem with this analogy is that cars are mostly car by weight (fuel is about 1-2%) while a rocket is mostly fuel by weight (the usefuly stuff is about 1-2%) so “a tank twice as big” actually means “a rocket twice as big”. So, ok then, let’s do that, right?
Not quite. The problem with rockets is that twice the amount of fuel does not get you twice as far. Because a rocket is mostly fuel you spend most of your fuel, speeding up the rest of the fuel. To get to the moon you need about 100x as much fuel than spaceship. So a spaceship that lands back on Earth with rockets would not be twice as big, it would be 100x as big.
So using the atmosphere to slow down is less of “an issue” and more of a hugely convenient cheat. Landing on Earth is so much easier than landing on Mars, precisely *because* Earth has a dense atmosphere.
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