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.
It’s significantly easier to just make anything re entering heat resistant not to mention probably cheaper
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.
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.
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.
Slowing down for reentry means carrying fuel for the slowdown and the maneuvering. That fuel is spend at the end of the mission, hence it must be carried and accelerated around throughout the mission as well. The fuel mechanics are a big big problem for space flight.
Surfing on the atmosphere, while dissipating kinetic energy through heat, seems the most fuel efficient way to slow down.
If you really want to acquire an intuitive appreciation for how this stuff works, I recommend playing Kerbal Space Program.
Economics. For example, look at the size of the space shuttle compared to the rocket that it sits on that puts it into orbit. You’d need a second rocket of the same size to provide the reverse experience except then you’d also need an even bigger launch rocket to propel the shuttle and the re-entry rocket.
Alternatively, you can just have one rocket and use the atmosphere itself to slow you down for re-entry.
To mention one thing unmentioned I think.
The speed things are going up there is very fast. There is mo atmosphere so you’d need a fuel source for to slow down quickly. And fuel is heavy/expensive so it’s not financially logical to “stop”.
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