IIRC, they also discuss the heat of earth re-entry from such a high speed descent is greater than any current heat shield protection would allow.
Dragon's heat shield is already designed for Mars re-entry. Using PICA or PICA-X it's not much of a problem.
The entry velocity would be between 15 and 21 km/sec, versus a lunar entry velocity of 11 km/sec or a low-orbit entry velocity of 7.9 km/sec, and the re-entry window is easy to hit even with mid-1960's technology.
1966 NASA report on re-entry from Mars
The "blogger" suggested it was still quite a problem.
[/QUOTE]Besides life support for the crew, one of the biggest challenges would be the return into the Earth's atmosphere. Heat shielding for a high speed re-entry hasnít been tested. NASA isnít even testing its new system on the Orion spacecraft until next year at the earliest. Orion is in development to take astronauts back to the moon and on to Mars.[/QUOTE]