Going to Mars would be daunting. The planet never comes closer than 80 million kilometers to ours; a round-trip would take years. But scientists and engineers say they have solutions to the main technological challenges that a human mission would entail. The biggest obstacle is simply the enormous cost.

Cost estimates for a Mars mission boil down to one crucial number: the mass of the spacecraft. Lighter spacecraft need less fuel, which is the greatest single expense of a spaceflight. The history of Mars mission planning is largely an effort to minimize weight without unduly compromising safety or science. In 1952 rocket pioneer Wernher von Braun envisioned an armada of spaceships propelled by conventional chemical rockets and weighing 37,200 tons on departure. Just to haul such a fleet into Earth orbit would cost hundreds of billions of dollars. Since then, planners have wrung economies by using more efficient nuclear or electromagnetic rockets, scaling back the number of astronauts or the level of redundancy, and manufacturing fuel on Mars itself [see chart at right].

Today the barest-bone mission is the Mars Direct plan, with an estimated price tag of $20 billion in start-up costs, spread out over a decade, plus $2 billion per mission [see "The Mars Direct Plan," on page 52]. The National Aeronautics and Space

WEIGHT of proposed Mars missions on departure from Earth orbit— a proxy for cost—has slowly come down. Each weight estimate includes both crew and cargo flights for one team of astronauts.

Administration's own plan, the "design reference mission," has adopted many of the ideas of Mars Direct but costs roughly twice as much, in return for extra safety measures and a larger crew (six rather than four).

In its most recent version, NASA's plan [see illustration on opposite page] calls for three spacecraft: an unmanned cargo lander, which delivers an ascent vehicle and propellant plant to the Martian surface; an unoccupied habitat lander, which goes into Martian orbit; and a crew transfer vehicle (CTV), which, if the first two arrive successfully, sets out when Mars and Earth come back into alignment, 26 months after the von Braun (1952 Stuhlinger et al. (1966 Boeing (1968 von Braun (1969 Jenkins (1971 NASA 90-day study (1989 Soviet all-solar plan (1989 Mars Direct-chemical (1990 Mars Direct-nuclear (1990 NASA reference v1 (1993 NASA reference v4 (1999 VASIMR (2000

Mass in Low Earth Orbit (tons)

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