The first human expedition from Terra to Mars was selected on the theory that the greatest danger to man in space was man himself.  At that time, only eight Terran years after the founding of the first human colony on Luna, any interplanetary trip made by humans necessarily had to be made in weary free-fall orbits, doubly tangent semi-ellipses - from Terra to Mars, two hundred fifty-eight days, the same for the return journey, plus four hundred fifty-five days waiting at Mars while the two planets crawled slowly back into relative positions which would permit shaping the doubly-tangent orbit - a total of almost three Earth years.