Atmospheric reentry is notoriously a difficult and HOT endevour.
Could this thermal energy be used as an alternative energy source on earth?
Could this be used through some sort of orbital charging station?
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Reentry heat is not some magically created energy - it's all dissipation of energy that was painstakingly pumped into the orbital vehicle on ascent, so you will never break even with what you spent on propelling it to the orbit (plus propelling its second stage and its fuel to suborbital speed, plus propelling that fuel to nearly orbital speed, plus all that energy spent on launch and dissipated with the reaction mass). Never mind delivering it to Earth.
But that doesn't mean you can't tap into that energy. You won't recover nearly as much as you'd spent, but you can still recover aplenty.
On descent, on suborbital trajectory it would be extremely difficult due to sheer amount of energy created, and a very short time it takes to land. When moving in LEO, dissipating/collecting even a little of energy will send you into suborbital trajectory, and the above situation. But if you're approaching the body "from outside" - on a very elongated, elliptical trajectory, like most Martian probes - there's a lot of energy to be dissipated during the periapsis pass, and it lowers the apoapsis and orbital speed (while hardly affecting periapsis), meaning much more gentle reentry, or much less delta-V for circularization into a low orbit. It's normally always wasted - multiple aerobraking passes through upper Mars atmosphere, heating the probe somewhat, but not enough to mean damage or tap into the ablator supply, dissipated during the flight through apoapsis, repeatedly, often for many months, until the probe is in low orbit.
Now I don't have any good ideas just how to tap into that energy, but it should be perfectly possible - each pass through periapsis heating the craft, then the craft cooling when it leaves the atmosphere. Possibly a thermoelectric layer, or such. This energy is not "free" - it was bought by burning a lot of fuel to get there - but it's up for taking; it can be recuperated/recovered, and e.g. supply the craft's own needs and (considering extended solar panels don't take well to aerobraking), it could be a good alternative to solar, especially for missions to distant planets where sunlight is too weak to sustain a probe.
No: The heat produced by atmospheric reentry isn't a happy side effect of returning to the earth, it's a byproduct of the fact that your satellite/orbiter has enough kinetic energy to be circling the earth every 90 minutes and you want it to stop doing that and come down.
To have something you've made for the purpose of harvesting energy reenter the atmosphere it must first leave it. Due to drag (gravity, atmospheric) and inefficiencies in however you're lifting it you're always going to spend more energy launching your device out of the atmosphere than it can possibly encounter on the way back in.
Also, there's nothing special about the outer atmosphere: if you got something moving that fast at sea level you'd produce way more heat due to the density of the atmosphere, but you'd also be pumping even more energy into achieving/maintaining that speed.
The Space Shuttle remains the best by "harvesting the atmoshere" to act as an air cushion for descent. Once you start applying "ablative cooling" and "drogue chutes" and "parachutes" and "retro rockets" and "fancy seats" you're really failing to harvest all that wonderful energy that Mother Earth is giving to you for free.
In other words the "air" represents "brake horsepower" and any heat or fire generated and "absorbed" is energy wasted both in form of inefficiency since absorbing the heat does not slow you down and materially since basically your just "burning off" perfectly good and probably expensive "stuff" that then has to be put right back on only to be burned right back off again.