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Apart from testing the ability of the Mars landing craft to make a landing on another body, verifying that the Mars habitat and power supply can be reliably run for a period of two to four years afterwards with autonomy, and testing of crew environmental support solutions, I see no other purpose for lunar missions.
It's obvious that no spacecraft with a few weeks worth of supplies onboard is ever going to take humans to Mars. Degrading the capability of the service module to hit mass targets for Ares I severely restricted its capabilities for lunar missions. Apart from taking humans to ISS, Orion/SLS has no practical utility at this point with the Altair program cancellation and a Block I SLS configuration that can't put that much mass into TLI anyway, so it's no better than the Space Shuttle in terms of cost and can't even perform the mission it was built for.
If NASA insists on launching Orion on a flying pork barrel, I mean SLS, then there was no reason to retire the Space Shuttle. The cost of an Orion/SLS mission will be the same as a STS mission and there's no landing craft to land on anything with, so what's the point?
NASA's Space Shuttle was not unreliable, the humans involved simply ignored the problems and the result was destruction of flight hardware.
Given the cost differential between Orion/SLS and Dragon/Falcon 9, I'd rather NASA did the following:
- Continue work on SLS, absent any other launch system with comparable capabilities; SpaceX BFR is a paper rocket at this point
- Work with SpaceX to man rate Dragon, a system proven sufficient for LEO taxi service; if Orion is man rated before Dragon I'll be suitably impressed
- Cancel Orion, as it's not required and there's no compelling reason to drag a capsule all the way to the moon or Mars and back
- Restart work on Altair and EDS with an eye towards lander and/or EDS reusability and refueling from propellant depots
- Use SLS to launch the Altair/EDS combination and propellant depots
- Put a small inflatable habitat module atop Altair instead of a confining tin can
- Work on EOR with Altair for lunar missions
If there's still money burning a hole in NASA's pockets:
- Composite tanks for SLS
- Composite SRM casings