Clearly, space travel is more expensive and more difficult than the amateurs imagine; indeed, it is often more difficult than the seasoned professionals imagine. Comparisons between the spectacular success of the Apollo Project and the steadily plodding space shuttle ignore the fact that one mission to the moon out of seven failed (Apollo 13).

If Apollos 18 through 20 had flown, would we have seen a major disaster? Would the public have continued a space exploration program if it had left corpses on the moon?

Expenditures on space exploration approaching one percent of America's gross domestic product could not have continued, especially since the race to the moon was won so decisively. Budgets were slashed. Hence there was a need to scale back. One Saturn left over from the Apollo project was used to launch Skylab, an earth orbit station of the sort the Soviets were building.

To replace enormous rockets that were used once and thrown away, an experiment to build a reusable space vehicle-the space shuttle-was undertaken. The promise of cheap, frequent access to space was so tantalizing that the Soviets poured billions into a similar vehicle, suggesting that the decision was neither as deceptive nor as foolish as some NASA critics have suggested.

Subsequent developments gradually-over twenty-five years-have shown that that particular type of vehicle has yielded neither more frequent space access, nor cheaper launch capacity.

Expendable launch vehicles have improved since then, but their failure rate rarely has dropped below one per hundred launches, and their costs remains high; usually several thousand dollars for every kilogram placed in low earth orbit.

Since the political will to replace the shuttle with a heavy lift vehicle has been absent, the International Space Station has been launched by the shuttle one small piece at a time, an expensive approach requiring many launches and extensive assembly.

Currently there is no demand for a heavy lift vehicle except for sending humans to the moon or Mars; expendable launch vehicles capable of placing up to twenty-four tonnes in low earth orbit are adequate for commercial purposes. Unless a military project of the scale of a Star Wars is approved, no heavy lifter is likely to be built any time soon.

Where does that leave manned space flight? While it is true that a station in low earth orbit is not necessary for flights of astronauts to the moon or Mars, it is not very likely such a station can be abandoned.

Even though the International Space Station (ISS) has not yet yielded much science, the promise of zero-gravity materials and medical research remains tantalizing and has its lobby, and the need for low earth orbital study of this planet remains strong and persuasive.

The ISS may eventually become more productive. Furthermore, it provides a station in low earth orbit for European and Japanese astronauts and their equipment that those places could not have afforded otherwise.

The ISS requires many tonnes of consumables per person-year to operate-especially water-a requirement that will have to be reduced considerably if humans are to be sent even to the moon, let alone Mars.

The ISS is an ideal facility to test advances in life support systems. The ISS requires two and a half of its three personnel to be maintained; no one wants to send three people to Mars and have only half a person of time to devote to exploring that world!

Clearly, maintenance has to be automated and streamlined for deeper penetration into space. The international nature of ISS is also a crucial test bed for the political and diplomatic mechanisms that will be essential for a moon or Mars mission, because it is unlikely that any single nation will undertake such an expensive project alone.

NASA's current plans for future exploration of space, though limited, are encouraging. The Orbital Spaceplane will represent an advance in the safety and reliability of transportation of astronauts to low earth orbit.

Since it will not transport significant quantities of cargo, the development of better systems for launching supplies and new modules to the ISS will be stimulated; the Russians, Europeans, and Japanese are all looking at such systems.

NASA's interest in nuclear power represents another important development necessary for deeper exploration into space, for stations on the moon and Mars will be far safer and easier to construct with nuclear-supplied electricity (although new breakthroughs in more efficient solar cells could prove as effective and cheaper in the long run, in the inner solar system).

Nuclear power will also allow the use of ion engines for the robotic exploration of the outer solar system and will raise the data transmission rate, which is essential for sophisticated scientific exploration of Titan or the Uranian and Neptunian systems.

Ion engines and related electrical propulsion systems such as VASMR also promise a great improvement in space transportation closer to home. It requires more delta-vee to place a satellite in geosynchronous orbit from low earth orbit (4.1 km/sec) than to launch it to the moon (3.2 km/sec) or Mars (3.8 km/sec).

Currently, less than a third of the payload launched into low earth orbit can end up in geosynchronous orbit. Solar-powered electrical propulsion could boost the percentage significantly, making transportation to geosynchronous orbit cheaper (though decreasing the need for boosters in the 24-tonne range).

Electrical propulsion systems also promise to make human transportation to the moon and Mars feasible with commercial-scale boosters. In his 1997 "A Lunar Reference Strategy", Michael Duke points out that astronauts can return to the moon using the shuttle as the primary launch system (though his observation would apply to any booster able to lift 24 tonnes to low earth orbit).

Duke's system involves solar-powered ion engines to lift cargo from low earth orbit to a lagrange point, one of the points where the gravities of the Earth and Moon balance each other (Duke uses L2, which lies beyond the moon).

An eight-tonne solar electric propulsion system, he estimates, can be launched with sixteen tonnes of cargo and transport the latter to the lagrange point over a six-month period. (If the solar-electric vehicle returned to low earth orbit for reuse, the next twenty-four tonne launch presumably could involve eighteen or nineteen tonnes of payload and five or six tonnes of propellant for the ion engine.)

The sixteen tonnes carried to the lagrange point in turn could include eight tonnes of payload for the lunar surface and a reusable eight-tonne lunar-based vehicle (including the hydrogen and oxygen fuel) to transport it there.

Duke's first launch would put an eight-tonne fuel making plant near the lunar North or South Pole, including a one-tonne nuclear reactor or solar power system able to make 25 kilowatts of power.

The plant would ingest ice-laden regolith, heat it, extract the water, convert the water to hydrogen and oxygen, liquefy them, and store them back in the reusable lunar-based vehicle.

The lunar-based vehicle would be filled with sixteen tonnes of hydrogen and oxygen fuel in a few months, enough to launch itself back to the lagrange point with eight tonnes of fuel left over to bring another eight tonnes of cargo to the moon.

A second launch from earth would land a second lunar-based vehicle on the moon with an eight-tonne habitat and supplies. That lunar-based vehicle would also be refueled in a few six months. Then one of the lunar-based vehicles would be launched to the lagrange point to await a crew.

The third launch from Earth would lift an eight-tonne crew vehicle and a sixteen-tonne stage to propel it quickly through earth's Van Allen radiation belts to the lagrange point. It would dock to the waiting lunar-based vehicle, which would land the crew vehicle on the surface and be refueled with the hydrogen and oxygen in the other lunar-based vehicle.

The crew would breathe oxygen and drink water made from lunar ice. Once the crew completed routine maintenance of the fuel-making plant and explored the area, the lunar-based vehicle would launch the crew vehicle back to the lagrange point, where a small engine firing would send it back to the earth's atmosphere for aerobraking and rendezvous with the International Space Station.

Duke's proposal is perhaps the simplest and most elegant proposal for sending astronauts back to the moon that has been make in recent years. Three launches of the space shuttle (or of equivalent, much cheaper expendable boosters, costing perhaps a half billion dollars total) would establish a reusable transportation system to the moon.

An eight-tonne fuel-making system on the moon would allow one or two manned flights to the moon per year. The lunar-based vehicles would be designed for ten uses before a solar electric vehicle would have to bring them back to low earth orbit for refurbishment (if a facility for such refurbishment were built).

It is easy to imagine other eight-tonne cargo flights (or perhaps ten-tonne cargo flights, if subsequent launches can dock to an existing solar-electric vehicle in low earth orbit) transporting a second or third fuel-making plant to the lunar surface, scientific equipment, pressurized rovers, etc.

Lunar fuel could be brought back to low earth orbit to launch future crews to lagrange. A human presence could be developed on the moon gradually and extended toward the equator from the temporary station at the lunar north or (more likely) south pole.

Duke also noted that the resulting transportation system is roughly one quarter the size of one needed to send astronauts to Mars. One can imagine a Mars transportation system about the size of Robert Zubrin's "Mars Direct" proposal being launched into low Earth orbit by three or four expendable boosters.

Payload destined for the Martian surface-an inflatable habitat similar to the eight-tonne habitat used on the moon, supplies, a pressurized rover, scientific equipment-could be lifted to the lagrange point, where a lunar-based vehicle would dock to it and accelerate it to Mars, then return to the moon for refueling.

Each twenty-four tonne launch from Earth could land between eight and fourteen tonnes of supplies on Mars, depending on how the lunar transportation system were used. Another twenty-four tonne launch could put an earth return vehicle into orbit, which could be lifted to the lagrange point by a previously fueled solar electric vehicle and pushed to Mars by a lunar-based vehicle.

Two to four automated cargo landers and an earth-return vehicle would be sent to Mars two years before a crew was sent. The crew would be accompanied by a second set of automated cargo landers in case they landed far from the first landing zone, and possibly by an interplanetary habitat to give them a comfortable home while cruising to Mars (and to give them life-support redundancy in case of an Apollo-13 style accident).

The earth return vehicle would bring along the hydrogen it needs to make methane and oxygen fuel for the return flight, just as proposed by the "Mars Direct" plan.

A system such as this could send astronauts to Mars and the moon using relatively little new technology and at a reasonable expense (Duke estimated his system could be developed for $1.7 billion). Expendable rockets already developed for commercial purposes would be sufficient. Surface vehicles for a Mars expedition could be tested extensively on the lunar surface.

Both the lunar and Mars lobbies could cooperate in supporting the establishment of such a system rather than competing against each other and undermining each other.

The electric propulsion system would be an expansion of existing systems and would have a variety of uses. Solid-core nuclear rockets-extremely expensive to develop and test on the Earth's surface-would not be needed.

Interestingly, a report by a NASA Exploration Team (NExT) for advanced exploration may have been thinking along these lines. In September 2002 it recommended as a possible next step the establishment of "Gateway Station" at the L1 Lagrange point between the Earth and moon, where large telescopic arrays could be serviced and missions to the moon and Mars could be launched (Reference).

No timeline was proposed, but one can imagine that such a facility (and the electric propulsion necessary to get it there) cannot be developed until the International Space Station reaches a mature stage and the orbital spaceplane is flying.

Thus 2010 appears to be the earliest date when plans to move beyond low earth orbit could commence. It seems likely that humans could return to the moon by 2019, the fiftieth anniversary of Apollo 11. Mars exploration would follow about a decade later.

It is disappointing to think that humans might be absent from the moon for half a century before returning permanently, and that Mars might not be visited in the lifetime of most of those who watched Neal Armstrong place the first footprints in the dust of the Sea of Tranquility.

But it appears that unless a breakthrough occurs that drastically reduces space transportation costs or a compelling reason emerges to send humans to either world sooner, a step-by-step process will be followed that maximizes safety while minimizing risks and incremental costs.

The process will also probably maximize international cooperation while minimizing aspects that will disturb the tax-paying public, such as nuclear power. Inevitably, the process will have to accommodate the conflicting demands of different lobbies as well.

When the history of the effort is written, in 2050 or so, it may reveal that the process was neither the safest nor the cheapest; the space shuttle already teaches us that lesson. But history cannot prevent us, today, from pursing false hopes or following temporary detours.

What it does suggest is that the path will entail tragedy, it will never be as easy or as cheap as we wish, that grandiose plans may not be sustainable long term, but that with perseverance and efforts to involve the public, humanity can permanently extend itself beyond its home planet and begin to inhabit other worlds. Perhaps that lesson will have to sustain us as we wait.

Robert Stockman has a Masters degree in planetary geology from Brown University (1977) and served as a graduate student assistant on the Viking mission. He has since obtained a doctorate in religious studies from Harvard University (1990) and teaches world religions at DePaul University in Chicago, but has retained his interest in space flight and scientific study of Mars. Robert H. Stockman: rstockman @ usbnc.org

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