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?
4$\begingroup$ Earth only, and crewed spacecraft only? For example the galileo probe that entered Jupiter's atmosphere survived outrageous temperatures and accelerations.... $\endgroup$– AndyApr 28, 2016 at 11:20
$\begingroup$ @Andy: The best answer would contain an overview of these. $\endgroup$– SF.Apr 28, 2016 at 15:14
$\begingroup$ (but yes, Earth only, not necessarily manned.) $\endgroup$– SF.Apr 28, 2016 at 19:37
2$\begingroup$ At atmospheric re-entry speeds, it may not be useful to talk in terms of a single temperature. Non-equilibrium effects in the gas means there may be a translational temperature (what we normally think of as a temperature) as well as vibrational and electronic temperatures, all of which may be different due to the high speeds, high energies, and rarefied air. See this page for a discussion of the different models used. $\endgroup$– tpg2114Apr 28, 2016 at 19:39
1$\begingroup$ @Andy: I'm exactly thinking about what -minimal- protection would be needed so that it wouldn't happen. $\endgroup$– SF.Apr 29, 2016 at 12:10
The Stardust sample return probe had an interesting re-entry to Earth's atmosphere. Returning from a solar orbit the maximum deceleration has been reported as 34g.
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.
7$\begingroup$ The older medium range ballistic missiles could get even hotter: temperatures up to 12000 °F (6650 °C) were measured at the stagnation zone. The NASA eBook 'Coming Home: Reentry and Recovery from Space' gives some fascinating insights in the challenges encountered. $\endgroup$– AaganrmuApr 29, 2016 at 8:02
The Space Shuttle thermal protection system is rated for temperatures of up to 1510 °C.
There's a boundary layer of air just above the TPS, outside that temperatures can reach 5500 °C. NASA used HYTHIRM to make thermal images of the orbiter during reentry:
3$\begingroup$ The shuttle has a really reentry low temperature compare to other spacecrafts. $\endgroup$– AntziApr 28, 2016 at 11:41
1$\begingroup$ @Antzi Yes, because it has a huge surface area. Remember, reentry is all about losing speed, it's not inherent to space travel. Slamming into the atmosphere is just still the cheapest way of losing orbital speed. Since the shuttle has a huge surface area, it can afford a less drastic approach. At the same time, it doesn't use ablative shielding, so it must have lower reentry temperatures - the heat is not disposed of, just "stored", and that limits the capacity (compare cooling with liquid water with cooling via evaporation / ice melting). It's all about the reusability. $\endgroup$– LuaanApr 29, 2016 at 9:21
$\begingroup$ It also uses its plane-like characteristics to stay up high for longer, reducing the heat flux due to the lower air density. This is not unique to the shuttle (the soyuz does the same thing, to reduce G-forces), but it has a better L/D ratio than capsules. $\endgroup$– AI0867Jan 13, 2020 at 21:46
This article says the recent Orion test experienced 2200 deg C, and this old Apollo fact sheet says 5000 deg F (2760 deg C) on Apollo 4 (a test at lunar return speed).
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.
$\begingroup$ Good figures that remind us our notion of "temperature" (and it's extremes) deviate from our usual senses in more exotic environments. And maybe to subsequently ask about "peak heating" or "peak heat flux" :) $\endgroup$– Nick TMay 13, 2016 at 3:43
1$\begingroup$ @uhoh thanks for the edit, but the use of the degree symbol with Kelvin has been incorrect since 1967 physics.nist.gov/cuu/Units/kelvin.html en.wikipedia.org/wiki/Kelvin The edit is inconsequential so I will not bother to roll it back. $\endgroup$ Aug 26, 2017 at 9:40
$\begingroup$ Wow, you are right! I needed the commas to see how big the numbers were because I was born before the degrees symbol for Kelvin was incorrect and the SE font is so small :) I should have stopped after that. It makes sense,
Kis a unit all by itself so it doesn't need the
°helper. Thanks for the information! $\endgroup$– uhohAug 26, 2017 at 10:20
"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)