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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Slowing down to the speed of ground with a rocket engine requires an enormous amount of propellant.
You can see the size of the rocket that is required to launch stuff from earth. Around 80% of the acceleration is for the vertical speed and 20% to overcome air resistance and gain altitude.
The mass of a Falcon 9 at launch is around 549 tonnes. it can launch 16.7 tonnes to low earth orbit and be reused. So you launch 3% of the initial mass to earth orbit.
The Dragon 2 space ship launch to ISS has a mass of around 15 tonnes with max cargo. So all of the Falcon 9 capacity is use for it.
If you would be able to slow it down with thrusters you need a Falcon 9-sized rocket up there and dock with it or residue the payload. 3% of the Dragons 2 is only 450 kg, which is not a lot for astronauts and the capsule that land. The re-entry capsule has an empty mass of 7,7 tonnes.
The result is that you instead have a small thruster so you can change the orbit and intersect the earth’s atmosphere. You need to reduce the speed by less than 2% of the orbital speed to do that. You can let the atmosphere slow you down. You need some heat shield and another structural part to handle that but the mass of them is minuscule compared to if you use thrusters to slow down.
Compare it to if you jump from an airplane or for the military to drop cargo from the. You use parachutes to land not just a rocket engine to slow the object down. A parachute is light than a rocket engine and can do the same job.
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