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 speed to remain in orbit is something like 7,000 miles per hour. Slowing down from that speed would take an immense amount of fuel – the mass of that fuel and the size of storing it would require even more fuel to slow it down. Re-entry relies on the density of the atmosphere to bleed off most of the speed due to friction, which creates a lot of heat – the heat is either bled off with ablative surfaces, or insulated (like the Shuttle tiles) with a very complex series of dips and turns to control how hot the exterior gets.
Apollo 10 gets the re-entry speed record, at (IIRC) 24,000-ish MPH.
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