Defining the problem :
Mars Direct vs. Earth orbit

The problem must first be defined in order to start analyzing different alternatives for the propulsive system. We know for sure that we want to get from Earth to Mars, land there and come back to our home planet. But where will be launch from?

The two alternatives have been extensively studied. Advocates of the Mars Direct and Semi Direct, led by Dr. Robert Zubrin call for a giant booster in stages to propel the spacecraft directly from the Earth's surface onto Mars.

The more traditional approach considers an excursion vehicle launching from Earth orbit and possibly assembled in the Space Station, thus requiring less thrust in terms of propulsion. The Mars Direct plan does without the Space Station, partly through skepticism on its operability that would retard the development of a Mars mission by many years.

At this point, it would be very difficult to select an approach that would be technically justified as superior to the other, for it would involve economic and cost related considerations that are beyond the goals of our simulation.

However, both approaches share common features. Even in the Mars Direct plan, or any plans that involve a direct trip from Earth surface to Mars, the mission would be staged and probably stay in Earth orbit for a while in order to check all systems. The final Mars insertion burn will have to be accomplished by a stage of the initial giant rocket or by the excursion vehicles engines themselves, similarly to launching that excursion vehicle from the Space Station.

So in fact, our task would consist in designing an excursion vehicle (last stage of the Mars Direct plan or unique spacecraft to be launched from the Space Station) that would take off from LEO to Mars. If it is finally decided to adopt a direct approach, then the spacecraft will be integrated into the giant booster. Placing a payload in Earth orbit is not a technological problem, and the excessive weight problem could be solved by Earth orbit rendezvous of the different parts.

• Mars Direct: A Simple, Robust, and Cost Effective Architecture for the Space Exploration Initiative from Dr. Zubrin at the Mars Direct Headquarters