The article that was HTML linked was a mixed bag of insight and ignorance. Responding to it:

>However, it is not friction that heats
>them. Think of it this way: a space
>shuttle's tiles are extremely delicate;
>they crumble easily in your hand.

This is mostly baloney. Carbon Phenolic, RCC and TUFFI are hard as rock. SIRCA and PICA are like pumic. SLA-561V is about as strong as cork. Heat shield material is made strong where it needs to be strong.

>If they were heated by friction as the
>shuttle de-orbits and enters the
>atmosphere at Mach 25, the tiles would
>disintegrate. That's not a very good
>design characteristic.

Mach 25 is a typical entry Mach number from low Earth orbit. When at Mach 25, the spacecraft is still in the upper atmosphere where there is no significant heating or dynamic pressure. As the vehicle descends into the atmosphere the Mach number goes down as the heating and dynamic pressure goes up.

>In reality, it isn't friction, but ram
>pressure that heats the meteoroid. When
>a gas is compressed it gets hot, like
>when a bicycle pump is vigorously used >to inflate a tire. A meteoroid, moving
>at 33,500 mph (15 kilometers a second) >or more compresses the air in front of >it violently. The air itself gets very >hot, which is what heats the meteoroid.
>That's the fact, not friction.

Again this is insight mixed with ignorance.

Leonid meteors enter at 70 km/sec.

It is true that free stream atmospheric gas gets processed by the bow shockwave which results in a gas shock layer having high pressure and temperature. The rule-of-thumb is the velocity in km/sec. gives a shocklayer temperature in 1000s of deg.K, e.g. 5 km/sec gives 5000 deg.K. However the way we model heat flux on the end of a spacecraft is through the Fay-Riddell equation. Gas viscosity is one of the parameters used in the Fay-Riddell equation. If the gas viscosity is set to zero then the heat flux goes to zero. This of course is impossible but it demonstrates that viscosity (friction) plays a role in heat flux. The main mechanisms in heating the surface of a reentering spacecraft are:

convection
radiation
surface catalysis

convection: The heat in the shocklayer gas is convected to the surface of the vehicle. This is done through the surface boundary layer which involves the diffusion and viscosity of the gas in the boundary layer.

radiation: The radiatiant heat of the gas in the shock layer illuminates the surface of the vehicle thus heating it. This was the main mechanism of heating for the Pioneer Venus probes and the Galileo Probe.

surface catalysis: The shocklayer gas dissociates from molecules into atoms and ions. These dissociated atoms recombine into molecules when they touch the vehicle's surface (the surface acts as a catalyst). The process of recombination releases heat from chemical energy.

Don't trust what you read on the Internet.

Gary