At five times the speed of sound a shock wave stops being a pressure ridge and starts being a furnace. Below Mach 5 you can still pretend the air is a simple gas: compress it, and it gets hot in the ordinary way, the way a bicycle pump gets warm in your hand. Above Mach 5 the pretending stops. The shock is so strong, the compression so abrupt, that the kinetic energy of the vehicle doesn't just warm the air — it tears it apart.
The first casualty is the assumption of a single temperature. A gas molecule has several ways to hold energy: it can move (translation), it can spin (rotation), it can flex (vibration), and eventually it can lose an electron (ionization). Under gentle heating these modes share the energy fairly and a single temperature describes all of them. A hypersonic shock heats faster than the modes can equilibrate with each other. The translational temperature — literally how fast the molecules are flying — spikes first and hardest, sometimes to tens of thousands of kelvin in a boundary layer a few centimeters thick, while the vibrational modes lag behind, still catching up. The gas is described not by one temperature but by a pair, T and Tv, locked in a race the flow doesn't give them time to finish. This is thermal nonequilibrium, and it is the normal condition of hypersonic flight, not an edge case.
The second casualty is the molecule itself. Enough energy dumped into a vibrational mode eventually just breaks the bond: diatomic oxygen and nitrogen dissociate into atomic O and N in the shock layer, turning the "air" around a hypersonic vehicle into a different gas than the one it took off in — one with different chemistry, different heat capacity, different everything. Push further and atoms start losing electrons outright: thermal ionization forms a thin sheath of weakly ionized plasma clinging to the vehicle's windward surface, which is why hypersonic reentry vehicles go radio-silent — the plasma sheath absorbs and reflects the very radio frequencies a spacecraft would use to talk to the ground. The blackout isn't a metaphor. It's literal opacity, an ionized fog the signal can't get through.
The third casualty is the wall. All that dissociated atomic oxygen and nitrogen wants very badly to recombine into diatomic molecules again, and if it does so right at the vehicle's surface — catalyzed by the material itself — it releases the recombination energy as heat, right into the thermal protection system. This is surface catalycity, and it means the choice of heat-shield material isn't just about melting point; a "non-catalytic" surface that discourages recombination can run measurably cooler than a catalytic one exposed to the identical flow, because it refuses to let the atoms give their energy back at the worst possible place to receive it.
And radiation joins the balance sheet too — a hot enough shock layer glows, genuinely radiates energy away in the UV and IR, and at the very highest reentry speeds (lunar-return, Mars-return velocities) that radiative heat transfer to the vehicle rivals or exceeds the convective heating everyone worries about first. The Apollo capsules were shaped partly to manage a heating problem most people don't know existed: the glow of their own shock layer, cooking them from outside the boundary layer as much as within it.
None of this fits in a Mach number alone. A vehicle at Mach 8 in thin upper atmosphere and a vehicle at Mach 8 lower down, in denser air, face entirely different chemistry — because the reaction rates depend on density as much as temperature, and a slower, denser encounter can dissociate more air than a faster, thinner one. Engineers track this with a binary duty they call the Reynolds-number-and-real-gas problem: you cannot simply scale a wind tunnel test up to flight condition, because no ground facility reproduces the actual chemistry of a real atmospheric reentry — the energy density is too high, the timescales too short, the electrons too willing to leave. Hypersonic vehicles are still designed substantially on computation, because the wind tunnel, that century-old workhorse of aerodynamics, simply cannot build the furnace the sky builds for free.
Seed: Hypersonic aerothermodynamics — thermal nonequilibrium, dissociation, plasma blackout, surface catalycity, radiative heating.