The other rationale, repair of satellites in orbit, dissolved long ago -- for with current technology, manned in-orbit repair of satellites is far more expensive than simply building and launching a replacement.

The repair missions to the Hubble Telescope have been the only exception -- but Hubble's successors will use new technologies to cut their costs to only a small fraction of Hubble's cost, and most of them will be launched to the "Sun-Earth L-2 point" fully 1.5 million kilometers from Earth anyway, in order to greatly increase the cost-effectiveness of their scientific observation programs.

As for zero-G manufacturing experiments: they can be run not only vastly more cheaply, but even more scientifically effectively, on recoverable (and perhaps even reusable) unmanned satellites.

Even the slight vibrations produced on any manned vehicle -- including the Station -- by the motions of the crew themselves can significantly interfere with such tests; and while live monitoring and equipment repair by an on-site crew can sometimes salvage a malfunctioning experiment, the cost of such monitoring is so huge that any experiment thus lost could be reflown not just once but a multitude of times on unmanned satellites for the same cost.

The same thing applies to the vast majority of experiments on the biological effects of zero-G on various living things -- animals, plants, microbes and tissue samples.

The only scientifically valid reason for keeping a long-duration manned satellite like the Station in orbit is simply to study the medical effects of very prolonged spaceflight on human beings themselves.

The one kind of mission that unavoidably requires long-term human spaceflight and exposure to low gravity environments is manned deep-space flight into the Solar System -- to Mars and Near Earth Asteroids, or long-term stays on a manned lunar base at only 1/6 G. But the rationale any time soon for these missions is very weak other than for the simple reason of human curiosity and an urge to explore.

Moreover, aside from the huge cost of a manned Mars mission -- certainly far higher, even at the best, than the $90 billion total cost of the International Space Station -- there's a massive "Catch-22" where manned Mars exploration is concerned.

The one scientific goal that could possibly justify such a huge expenditure is the discovery, by earlier unmanned probes and sample-return vehicles, of evidence for present-day or fossil life on Mars -- but any manned ship that lands on Mars will instantly and unavoidably contaminate the area around it with large amounts of biological debris from Earth that will very seriously interfere with any studies or samples that it tries to obtain and may even allow living Earth bacteria to start slowly spreading from the landing area to the rest of Mars.

This problem of compromising exobiosphere does not apply to manned lunar and asteroid missions -- but the scientific rationale for them will be limited to specialized geological studies whose expense relative to their paltry scientific return would be enormous.

All the worlds in the Solar System, for a long time to come, can be explored not only far more cheaply, but also more thoroughly and scientifically effectively, by sending out an armada of vastly cheaper unmanned craft.

And when the time finally does come to start considering deep-space manned scientific missions, any manned ship to Mars or an asteroid will certainly have to first undergo very extensive flight testing in Earth orbit.

The best way to do that is simply to build a copy of its habitation module first, and then fly that in Earth orbit for the months or years needed for the actual flight, to test both the medical effects of such prolonged spaceflight (which are already largely known, thanks to Mir) and the ability of the ship's life support systems to keep the crew healthy without any new supplies or spare parts arriving from Earth.

The International Space Station is a complete red herring where such tests are concerned. It is $90 billion of dead-end expenditure without any sane justification whatsoever -- unless you count all that massively expensive pork, carefully distributed by NASA among politically strategic states, which is the critical motivating force for Congressional and White House support over the three decades since the triumph of Apollo.

Then there are the "intangible" reasons for continuing manned space flight: the ones that appeal emotionally to all space enthusiasts including myself and all those infuriated by the arguments I am putting forward here.

In his Feb. 3 opinion piece on SpaceDaily, John Carter McKnight responded to my initial Columbia commentary, saying my "facts are right" and my "criticism of NASA's performance as a government agency is unassailably accurate" -- but he thinks I've completely overlooked the spiritual, poetic component of manned space travel.

"We don't go for efficiency. We go for reasons almost impossible to express to those who don't feel the pull into the unknown, who don't see risk and danger as matters to be controlled and then accepted, rather than avoided at any cost," writes McKnight.

And he quotes Tennyson's famous poem in which the elderly Ulysses and his crew set off on one final voyage of exploration:

"To sail beyond the sunset, and the baths Of all the western stars, until I die."

It sounds very inspiring -- at first. But consider what he's really saying:

Those of us who favor unmanned space exploration whenever possible are tempted by "the petty sniping of the dreamless", and "call for us to stay safely, affordably home and send out robots to do the bloodless work of science and engineering they yet call 'exploration' ".

But absolutely no one has ever become interested in space exploration without feeling something more than cold scientific interest. We, too, feel awe. I have never lost that feeling since 1964.

He's saying that we should risk human lives unnecessarily in space exploration; that we should embrace danger and risk entirely for their own sakes -- even when we could carry out such exploration, and such commercial exploitation of space, not only far more safely but tremendously more cheaply using robots.

And he's saying that the taxpayers should be forced to subsidize such unnecessary risk-for-its-own-sake. This is like saying that taxpayers should be forced to subsidize recreational mountain climbers or the Flying Wallendas (before they started working with a net).

Charles Krauthammer's Feb. 4 Washington Post column, "It's Time to Dream Higher" -- although I disagree with much of it -- is closer to the truth.

Krauthammer excoriates the "troglodytes who want to give up manned space flight altogether" and he thinks the appropriate response is an immediate, multi-hundred billion dollar effort to start sending major manned expeditions to the Moon and Mars.

I think this is destructive nonsense at this point in our history -- but I agree entirely with him when he emphasizes that manned spaceflight, even at its safest, will always be a very risky enterprise, and that we should therefore risk it only when it has a goal genuinely worthy of the risk and achievable in no other way. And this also means that manned ships should always be designed to eliminate those risks that cxan practically be avoided -- something the Shuttle does not do, thanks to NASA's use of it and the Station as giant cash cows.

In all the history of the human race, we are the very first explorers who have been given, by our technology, the power to send our minds and eyes into the unknown without the great expense and risk of having to take our bodies along as well. We are fools if we don't utilize that new power to the maximum.

At some point, manned scientific expeditions into the Solar System really will become necessary, simply because of the barrier produced by the speed of light and the fact that it produces an unavoidable delay of hours between the data sent from a robot and the new orders sent to it.

But it will be decades before that factor alone justifies the huge cost of manned planetary spaceships -- and, meanwhile, the continuing march of technology will continue to reduce their cost.

And using manned spaceships when we DON'T need to do so tremendously reduces the economic efficiency with which we can both explore and exploit space -- and thus tremendously reduces the amount of such exploration and exploitation that we do. Using robots whenever possible not only allows us to "sail beyond the sunset" with far less risk. It allows us to sail much farther.

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