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)