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?
Could thermal energy be collected by dragging a thermoconductive device against the outer atmosphere
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1$\begingroup$ That is the equivalent of trying to power your house using your car's regenerative brakes. $\endgroup$– AronMay 12, 2016 at 1:50
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1$\begingroup$ Thermal energy, in isolation, can't do anything. It must flow (the movement of thermal energy is called "heat") in order to be convertible to any other form of energy (mechanical, electrical, etc.). $\endgroup$– Nick TMay 13, 2016 at 17:03
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3 Answers
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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.
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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.
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5$\begingroup$ OTOH you might collect the energy for use in the orbital craft, on approach to a planet "from outside". Say, Martian probes that perform numerous aerobraking runs to circularize in Mars Low Orbit after arriving from Earth - harvesting this energy might be tricky, but it's "up for taking". $\endgroup$– SF.Apr 26, 2016 at 8:45
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$\begingroup$ True. Energy is extracted from the flow of heat (typically by boiling water and passing it through a turbine on Earth, though there are other methods), so you'd have some part of your craft gaining heat as you pull energy out of the reentry plasma, but if you can manage that you might be able to reclaim a useful amount of energy. $\endgroup$– 1337joeApr 26, 2016 at 14:23
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$\begingroup$ If you could turn that heat from re-entry into useful energy, you could use it on the next trip up, but the device that does so would have to be pretty darned light to be worth the energy you took putting in orbit in the first place. $\endgroup$– corsiKaApr 26, 2016 at 14:49
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$\begingroup$ @SF. Put it up as an alternative answer. $\endgroup$ Apr 26, 2016 at 21:07
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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.
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$\begingroup$ You aren't getting the energy for free. Rather, you paid for it when you climbed up the walls of the gravity well. Besides, you do realize that the space shuttle did a deorbit burn too, right? It could probably have deorbited from atmospheric drag alone on a lot of its missions, but it would have been a far longer process and the touchdown point would not have been anywhere near as predictable... $\endgroup$– userMay 12, 2016 at 9:47
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$\begingroup$ Energy can neither be created nor destroyed...this is an empirical fact. What powered the Space Shuttle? Merely the reconnection of albeit liquid hydrogen with albeit liquid oxygen...otherwise known as H2O. $\endgroup$ May 15, 2016 at 7:41
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$\begingroup$ I would argue there were serious caviatation issues with the Shuttle Orbiter Program making the use of liquid hydrogen and liquid oxygen very dangerous since chemically speaking their molecular structure could "collapse" just by shaking. $\endgroup$ May 15, 2016 at 7:44
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