Are shockwaves responsible for heating in very sparse atmospheres?

Question

I recall learning about hypersonic flow in high atmosphere. I think one of the modelling assumptions used is that the relative air flow is so fast, and the average space between molecules is so great, that you can treat each molecule as individually hitting air/spacecraft surfaces and imparting momentum.

Question 1 is if my memory of this assumption is correct?

Question 2, if this is the case then do shockwaves form only when the air is sufficiently dense?

Question 3, is it shockwaves that are responsible for heating (of flow) in this scenario, or simply molecules colliding with an air/spacecraft?

Answer 1

you can treat each molecule as individually hitting air/spacecraft surfaces and imparting momentum.

This is true for satellites in low orbits, but not for airplanes which get their lift from conventional wings. This kind of flow is called rarefied as opposed to continuous flow for aerodynamic lift.

do shockwaves form only when the air is sufficiently dense?

Yes and no. Shockwaves are happening in continuous flow, but the pattern of energy and speed changes of molecules in rarefied flow are very similar to those in continuous flow, except that the energy at orbital speeds will cause ionization and complex chemical reactions. Note that Newton's impact theory, while not suitable for slower flows, starts to become a good approximation for hypersonic flow. Note as well, though, that concepts based in statistic means like pressure and temperature do not apply to rarefied flows.

is it shockwaves that are responsible for heating (of flow) in this scenario, or simply molecules colliding with an air/spacecraft?

The change in heat through a shock wave is caused by the rise in pressure and, especially in the steep middle of a detached shock, by friction within the shock. The skin temperature of a hypersonic airplane is caused by friction as well as heat transfer between air and skin. Since the direction of the flow is changed by an oblique shock, air molecules do not simply bump into the airplane but are redirected by the surrounding pressure field. Now you might say that they now bump into the pressure field, but that is not like an impact between solids. Instead, many numbers of collisions between molecules make them all change speed and direction. Of course the pressure field around an airplane is in turn the result of zillions of individual impacts between airplane and air molecules.

Answer 2

Imparting momentum (actually the aircraft hits the air) in relative sense can be described by molecules hitting the aircraft, but this is more aptly described as mv/s, aka drag.

Heating occurs when energy from impact of individual air molecules going very fast is enough to increase the rate of movement of molecules on the surface of the aircraft. What we know as "heat" is actually a measure of how fast the molecules of a solid are moving, and if sufficient energy is applied, the solid melts or even vaporizes.

Heating occurs at very high altitudes where the True Airspeed is extremely fast, as in multiple Mach. This was referred to in the early days as the "thermal" barrier, beyond the sound barrier.

If a shock wave builds in front of the aircraft, then the collisions will be absorbed by it, with the potential of heating it to an ionized plasma, literally breaking the air molecules into atoms. This phenomena actually lead to a miscalculation of the center of lift during the first Space Shuttle re-entry. The resulting nose-up had to be corrected by dropping a flap on the rear of the delta wing.

To summarize: heating is the result of high energy collisions that input energy faster than it can dissipate.