Re-entry from Moon vs. re-entry from low Earth orbit

Question

What is the difference between re-entry from the Moon and Re-entry from low Earth orbit?

Answer 1

I suspect you're asking about low earth orbit versus a lunar return trajectory. (Strictly speaking, that lunar return trajectory was an orbit). The difference between the two is velocity. The Shuttle reentered about about 7.7 kilometers per second. The Apollo spacecraft, at about 11 kilometers/second.

That extra 3.3 kilometers per second doesn't sound like much, but it is. Since kinetic energy is proportional to the square of velocity, that means that energy per unit mass for a lunar return is more than double that of a reentry from low earth orbit. That doubling in energy in turn mandates drastically different protection against the temperatures that build up due to reentry.