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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Because that takes an immense amount of fuel. Spacecraft decelerate substantially during re-entry without expending any fuel. It’s much cheaper and easier to engineer a spacecraft that can handle re-entry than it would be to haul a huge amount of excess fuel, throughout the entire course of a spacecraft’s mission, just so it can get a smoother re-entry.
Optimizing fuel consumption is critical to making spaceflight economical. Adding fuel to a spacecraft quickly leads to diminishing returns because additional fuel means additional weight, which means you need even more fuel than you first thought to bring the extra fuel into orbit.
That means each additional ounce of fuel adds less “delta-v” (the ability of a spacecraft to change its velocity) than the previous ounce.
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