Responding to Raoul's questions:

>The Mars direct spacecraft, will it use its engine before it reaches the Mars
>atmosphere? It seems to me that the spacecraft will first need a heat shield ,
>then parachutes, then the engine.

I'm not going to respond about Zubrin's Mars direct proposal. Instead I'll comment about generic transport to Mars issues. Three big issues concerning the design of human to Mars transportation systems are:

1) Human beings are not well adapted to interplanetary space due to microgravity and ionizing radiation. Therefore trip time from Earth to Mars needs to be minimized.

2) The atmosphere of Mars is thin.

3) People should not be exposed to greater than 4 gravities of deceleration after being in zero gravity for more than 2 months.

Problem #1 is best solved by using a space station in an Earth/Mars cycler orbit (trip times can be kept under 120 days one way). The down side to cyclers is the delta-V between the cycler and the Martian atmosphere can be up to 11 km/sec. Killing off 11 km/sec with rocket propulsion is not practical. Therefore the best way to get from the cycler orbit to the Martian surface is to use "aerocapture" (dump the kinetic energy into the atmosphere of Mars). In order to stay below the 4 gravities deceleration limit, a "lifting entry" must used for aerocapture. A lifting entry requires an entry vehicle that is shaped to provide both lift and drag. For a variety of reasons, an axisymmetric biconic shape is typically proposed (easier to control and fewer issues with radiative heating). The vehicle's lift vector is pointed towards Mars. This causes the vehicle to fly along the curvature of Mars as it dumps orbital kinetic energy. At this point the various design proposals diverge. One approach is to go all the way in with the biconic and land. Another approach is to skip off the atmospere into a Mars centered orbit. The biconic shape is then shed and the colonists reenter the atmosphere in 70 deg. sphere-cone shaped vehicles (optimized for drag with almost no lift). The actual landing always involves a combination of a parachute and rockets. The parachute kills off most of the the velocity. Unfortunately they best terminal velocity possible with a practical parachute is about 50 meters/sec which is way too high. Consequently a rocket is required to kill off that last bit of energy.

>Will it have been tested before the actual use (how about the heat shield
>then?).

The vehicle shape could be tested in the Earth's atmosphere or a scaled down unmanned model tested in the Martian atmosphere. We floated a proposal for a flight test of a Martian lifting entry vehicle but it never got past the conversation phase.

>The interesting thing about powered descent on the Moon is that it allows for
>extreem precision landing while saving lots of fuel (the difference between
>Apollo11 and Apllo 15 is stricking in that aspect). Starting Powered descent
>can be done at a safe distance from the Moon, so in case there's a problem,
>the crew can be saved as they would still be in Lunar orbit. But landing on
>Mars with a parachute.....(with the winds!).

When we send people to Mars there will almost certainly be some sort of GPS system in place so navigation will not be an issue. The Martian winds are not an issue in terms of navigation because the atmosphere is so tenuous. The winds are a slight problem in terms of causing lots of dust to be in the air that could cause heat shield erossion. A coworker of mine actually wrote his Ph.D. thesis on this very topic.

>About colonizing Mars: well, have we colonized our own oceans, yet?
>It's loaded with resources and it can even be environmentally friendly.
>There are untold numbers of life forms in the oceans we're not even aware
>of. And even underground, I've read there are many times more life forms
>than on the surface.

The "ocean" is really only the part that does not descend below 100 meters which is almost always near the coast of continents (it would be cheaper to colonize Mars than the oceanic abyss). Also a reason why one goes to Mars is to avoid single planet catastrophe, e.g. long period comets, nuts with nukes, designer viruses, etc. A future bin Laden could easily exterminate an ocean colony with a single nuclear depth charge. Getting away from murdering bastards is another big plus for Mars colonies.

Gary