Larry wrote: 1. We need to develop, test, and prove effective closed life support systems capable of supporting the crew without outside resources for at least a year. The system needs to be able to carry the crew outbound to Mars and allow for abort scenarios in case they don't make it. It does no good to send people to Mars if they arrive dead. There has been research on closed life support systems but none has been built, much less tested in space.

Chris writes: Larry, you can't both have the Moon be close enough for quick resupply and far enough away to justify a true closed loop life support system. Ask yourself exactly why doesn't ISS have a closed loop system? It could, but at a higher cost than needed and that will apply to the Moon as well.

Larry wrote: 2. We need to build the Mars landing vehicle. Developing the Lunar Module was perhaps the most difficult challenge of the Apollo program. Read the history - the program was filled with enormous challenges, long delays, and significant overruns. Developing the Mars landing vehicle will be a considerably tougher job because it will have to survive an atmospheric entry and liftoff. Aerodynamic braking will help during the landing phase but you have to allow for heat shielding and parachutes. You also have to allow for aerodynamic drag on liftoff, which means some form of streamlining will probably be necessary. These things add to weight. In addition, the Mars lander will have to support the crew on the surface for months instead of 3 days like the LM, provide for lab space, provide a clean room airlock, carry any rover vehicles, and a host of other things not required by Apollo.

Chris writes: You seriously need to either read the NASA reference mission documents or read Zubrin's "The case for Mars." You use two different craft one an Earth Return Vehicle and the other a Habitat Module. You can send several ERV's before sending a single Hab.

Larry wrote: 3. We need to develop space systems that are both reliable and easy to service when they break (and they WILL break). To date, such systems aren't available. These will include propulsion, electrical, life support, avionics, thermal, and structural systems.

Chris writes: We haven't done this in 40 years plus and we will now start doing it for the Moon...because?

Larry wrote: 4. We need to develop high efficiency propulsion systems with long storage lives. Today, the only such systems involve highly toxic and corrosive hypergolic propellants. If we want something with higher efficiency, then we have to consider long term storage of cyrogenic propellants (not currently available for space systems) or perhaps nuclear propulsion.

This list is by no means comprehensive, but it should serve to point out that there is a lot of R&D to do before a Mars mission has any chance of success. Developing and proving these systems will take time and money. This is where those incremental steps you're dismissing come to play. Those incremental missions like exploring an NEO allow a good proving run for the systems that can also be used to go to Mars. Only, proving the systems first increases the chances that the crew will actually make it to Mars and back alive.

Chris writes: And non of this has to be done at NEO's or the Moon and would even for those mission probably take place on Earth or LEO anyway. And as I said before other aerospace engineers disagree with your assessment on all these things.