# 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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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 atmosphere is much denser than space. You literally have to shove into it, and then gravity grabs you. Friction causes everything to heat up like crazy, and they couldn’t afford the extra weight to have leftover gas for re-entry.

The extreme speeds they’re at is the only thing keeping them in orbit. The moment they start decelerating, they start to fall towards Earth, reentering the atmosphere. So there’s no way to slow down *before* reentering the atmosphere, because slowing down is what *causes them* to reenter the atmosphere.

It’s significantly easier to just make anything re entering heat resistant not to mention probably cheaper

Heat is kinetic energy; hitting air fast enough is indistinguishable from hitting hot air. And hitting the atmosphere *is* how it slows down.

> can’t the spacecraft just slow down before [hitting the atmosphere]

By burning fuel? The exponential nature of the rocket equation; it would take about 10x as much fuel to reach a speed and then get back to zero (if you can’t refuel inbetween) as just reaching that speed. Rockets are already 90+ percent fuel, [almost all of which goes towards accelerating to the ludicrously fast speeds involved](https://what-if.xkcd.com/58/).

You could totally do that. The problem is that this would take a lot of fuel. Like A LOT. I don’t know the exact math, but ballpark is you need to slow down by the same amount that the rocket needed to accelerate in order to make orbit in the first place. Using the atmosphere to slow down is a neat trick to get free aerobraking.
If the earth didn’t have an atmosphere, ot would be quite difficult to land again.