We all know spacecraft reentry causes extreme heat - plasma, ablator, flaming trail, all that jazz. I'd like to know just what level of heat are we dealing with - could someone throw some numbers, like what's the maximum temperature occurring in the air or on the heatshield surface, or in the hottest place during reentry generally? Just how many Celcius degrees are we dealing with?
Maximum temperatures are estimated at around 3,200 Kelvin or 2900 degrees C at the surface. It should be noted that the entry probe had no re-entry data recording so this measurement was estimated from spectroscopic examination of the heat shield as it descended, which must have been an interesting day's work.
The spectroscopic measurement was taken through the glowing plasma surrounding it, and the range of the measurement will mean that the temperature is an average over the whole shield. As a result this doesn't represent a direct measurement from the hottest point on the heat shield, but it's interesting reading nonetheless.
CFD simulations show the air in the bow shock of the stardust probe reached temperatures of around 50,000°K at 71km, falling to 10,000°K at 51km (thin red line). It must be remembered that the air is extremely thin at these altitudes, fortunately resulting in poor heat transfer to the craft.
The surface temperature was much lower, as mentioned in Andy's answer, due to ablative cooling. The surface is designed to burn away, so the surface temperature largely depends on the decomposition temperature of the ablative material.
"The most difficult atmospheric entry ever attempted" was by the Galileo Probe. Temperature could refer to the plasma temperature or heat-shield temperature, but the latter generally caps out because a) nothing will remain solid past ~4000 °C, and b) many heat shields are designed to ablate, vaporizing in order to absorb some of the thermal energy.
Anyways, the Galileo Probe had to endure a 230-250 g deceleration. Citations claim that it endured "15,500 °C", which I don't quite understand given the above, but it did go on a rapid weight-loss program, shedding 80 kg of mass in about 2 minutes. Some other stats visible on the first page of this pay-walled paper (full paper available through Marcia McNutt's favorite website, though most of it is technical details about how they measured the ever-decreasing thickness of the heat shield) include:
- 30 kW/cm2 heat flux
- "300,000 suns" (300k × the solar insolation at Earth's surface)
- 300 kJ/cm2 heat load
- Entry speed of Mach 50 (47.4 km/s)
So technically you're not generating any "heat" let alone "g's" because as stated above all you are doing is a: falling and b:slowing down.
So what is happening when you literally fall to earth from space you are simply gaining massive amounts of pressure by entering an "atmosphere" (a region rich with gases which in the case of Earth highly flammable nitrogen and oxygen.)
So basically unless you're the Space Shuttle the "air around you" catches on fire meaning the only thing you're trying to do is sustain the pressure ... the same thing you must do to survive in the vacuum of space.
Since in a capsule you immediately falling really, really fast there are in fact an innumerable ways to "gain atmospheres" to compensate for lighting the air on fire during descent. The X37b Space Plane is a wonderful option actually...far better than a capsule as youre using the "atmospheres" to your advantage by simply gliding down...meaning allowing these gaseous states to act as an "air cush" thus obviating the need for all this complex cooling, chuting, rocket firing malarkey. The problem with the Shuttle Orbiter was how massive it was. A slightly larger X37 (the "c" variant) is all that is needed.
Space capsules are really dumb in my view...and if I were SpaceX I would ditch the second stage and the "storage box" entirely and just build a huge fairing to enclose a man rated X37 to get to and from the Space Station with full reusability and the ability to land either at the Cape or on the West Coast or both...or anywhere in between actually.
A space capsule does have the advantage of being very light so there might be a cost savings in that.
But all in all I see needless complexity plus the disability of not going for full reusbility as far as Earth to Space Station is concerned. Travelling to the Moon or Mars is totally different since they have almost nil in the way of "pressure". The lunar landings were "soft" because while large it's still very close so once you place yourself in orbit you just fire a few bursts of propellant and suddenly you're moving VERY slow.
This is not an option on Mars which is a Planetary Body with nil for atmosphere and will cause a very rapid descent right to impact no matter what you do. In short there are no material "atmospheres" to "cushion the blow." Instead youre just falling really, really, fast and barely decelerating....generating almost zero g's the whole way down with nothing more than a Sudden Impact.
The Viking Missions did this very well though I must say..."they just fell." That's not an option for something human rated though.