ials to generate oxygen to breathe； water to drink； and rocket propellants to get home； whether you land using the thin Martian atmosphere for aerobraking； the degree of redundancy in equipment thought prudent； the extent to which you use closed ecological systems or just depend on the food； water； and waste disposal facilities you've brought from Earth； the design of roving vehicles for the crew to explore the Martian landscape； and how much equipment you're willing to carry to test our ability to live off the land in later voyages。
Until such questions are decided； it's absurd to accept any figure for the cost of the program。 On the other hand； it was equally clear that SEI would be extremely expensive。 For all these reasons； the program was a nonstarter。 It was stillborn。
There was no effective attempt by the Bush Administration to spend political capital to get SEI going。
The lesson to me seems clear： There may be no way to Send humans to Mars in the paratively near future—despite the fact that it is entirely within our technological capability。 Governments do not spend these vast sums just for science； or merely to explore。 They need another purpose； and it must make real political sense。
It may be impossible to go just yet； but when it is possible； the mission； I think； must be international from the start； with costs and responsibilities equitably shared and the expertise of many nations tapped； the price must be reasonable； the time from approval to launch must fit within practical political timescales； and the space agencies concerned must demonstrate their ability to muster pioneering exploratory missions with human crews safely； on time； and on budget。 If it were possible to imagine such a mission for less than 100 billion； and for a time from approval to launch less than 15 years； maybe it would be feasible。 （In terms of cost； this would represent only a fraction of the annual civilian space budgets of the present spacefaring nations。） With aerobraking and manufacturing fuel and oxygen for the return trip out of Martian air； it's now beginning to look as if such a budget and such a timescale might actually be realistic。
The cheaper and quicker the mission is； necessarily the more risk we must be willing to take with the lives of the astronauts and cosmonauts aboard。 But as is illustrated； among countless examples； by the samurai of medieval Japan； there are always petent volunteers for highly dangerous missions in what is perceived as a great cause。 No budget； no timeline can be really reliable when we attempt to do something on so grand a scale； something that has never been done before。 The more leeway we ask； the greater is the cost and the longer it takes to get there。 Finding the right promise between political feasibility and mission success may be tricky。
IT'S NOT ENOUGH to go to Mars because some of us have dreamt of doing so since childhood； or because it seems to us the obvious long…term exploratory goal for the human species。 If we're talking about spending this much money； we must justify the expense。
There are now other matters—clear； crying national needs—that cannot be addressed without major expenditures； at the same time； the discretionary federal budget has bee painfully constrained。 Disposal of chemical and radioactive poisons； energy efficiency； alternatives to fossil fuels； declining rates of technological innovation； the collapsing urban infrastructure； the AIDS epidemic； a witches' brew of cancers； homelessness； malnutrition； infant mortality； education； jobs； health care—there is a painfully long list。 Ignoring them will endanger the well…being of the nation。 A similar dilemma faces all the spacefaring nations。
Nearly every one of these matters could cost hundreds of billions of dollars or more to address。 Fixing infrastructure will cost several trillion dollars。 Alternatives to the fossil…fuel economy clearly represent a multitrillion…dollar investment worldwide； if we can do it。 These projects； we are sometimes told； are beyond our ability to pay。 How then can we afford to go to Mars？
If there were 20 percent more discretionary funds in the U。S。 federal budget （or the budgets of the other spacefaring nations）； I probably would not feel so conflicted about advocating sending humans to Mars。 If there were 20 percent less； I don't think the most diehard space enthusiast would be urging such a mission。 Surely there is some point at which the national economy is in such dire straits that sending people to Mats is unconscionable。 The question is where we draw the line。 plainly such a line exists； and every participant in these debates should stipulate where that line should be drawn； what fraction of the gross national product for space is too much。 I'd like the same thing done for 〃defense。〃
Public opinion polls show that many Americans think the NASA budget is about equal to the defense budget。 In fact； the entire NASA budget； including human and robotic missions and aeronautics； is about 5 percent of the U。S。 defense budget。 How much spending for defense actually weakens the country？ And even if NASA were cancelled altogether； would we free up what is needed to solve our national problems？
HUMAN SPACEFLIGHT In general—to say nothing of expeditions to Mars—would be much more readily supportable if； as in the fifteenth…century arguments of Columbus and Henry the Navigator； there were a profit lure。* Some arguments have been advanced。 The high vacuum or low gravity or intense radiation environment of near…Earth space might be utilized； it is said； for mercial benefit。 All such proposals must be challenged by this question： Could parable or better products be manufactured down here on Earth if the development money made available were parable to what is being poured into the space program？ Judging by how little money corporations have been willing to invest in such technology—apart from the entities building the rockets and spacecraft themselves—the prospects； at least at present； seem to be not very high。
* Even then it wasn't easy。 The Portuguese chronicler Gomes Eanes de Zurara reported this assessment by Prince Henry the Navigator： 〃It seemed to the Lord Infante that if he or some other lord did not endeavor to gain that knowledge； no mariners nor merchants would ever dare to attempt it； for it is clear that none of them ever trouble themselves to sail to a place where there is not a sure and certain hope of profit。〃
The notion that rare materials might be available elsewhere is tempered by the fact that freightage is high。 There may； for all we know； be oceans of petroleum on Titan； but transporting it to Earth will be expensive。 Platinum…group metals may be abundant in certain asteroids。 If we could move these asteroids into orbit around the Earth； perhaps we could conveniently mine them。 But at least for the foreseeable future this seems dangerously imprudent； as I describe later in this book。
In his classic science fiction novel The Man Who Sold the Moon； Robert Heinlein imagined the profit motive as the key to space travel。 He hadn't foreseen that the Cold War would sell the Moon。 But he did recognize that an honest profit argument would be difficult to e by。 Heinlein envisioned； therefore； a scam in which the lunar surface was salted with diamonds so later explorers could breathlessly discover them and initiate a diamond rush。 We've since returned samples from the Moon； though； and there is not a hint of mercially interesting diamonds there。
However； Kiyoshi Kuramoto and Takafumi Matsui of the University of Tokyo have studied how the central iron cores of Earth； Venus； and Mars formed； and find that the Martian mantle （between crust and core） should be rich in carbon—richer than that of the Moon or Venus or Earth。 Deeper than 300 kilometers； the pressures should transform carbon into diamond。 We know that Mars has been geologically active over its history。 Material from great depth will occasionally be extruded up to the surface； and not just in the great volcanos。 So there does seem to be a case for diamonds on other worlds—on Mars； lied not the Moon。 In what quantities； of what quality and size； and in which locales we do not yet know。
The return to Earth of a spacecraft stuffed with gorgeous multicarat diamonds would doubtless depress prices （as well as the shareholders of the de Beers and General Electric corporations）。 But because of the ornamental and industrial applications of diamonds； perhaps there is a lower limit below which prices will not go。 Conceivably； the affected industries might find cause to promote the early exploration of Mars。
The idea that Martian diamonds will pay for exploring Mars is at best a very long shot； but it's an example of how rare and valuable substances may be discoverable on other worlds。 It would be foolish； though； to count on such contingencies。 If we seek to justify missions to other worlds； we'll have to find other reasons。
BEYOND DISCUSSIONS OF PROFITS and costs； even reduced costs； we must also describe benefits； if they exist。 Advocates of human missions to Mars must address whether； in the long term； missions up there are likely to mitigate any of the problems down here。 Consider now the standard set of justifications and see if you find them valid； invalid； or indeterminate：
Human missions to Mars would spectacularly improve our knowledge of the planet； including the search for present and past life。 The program is likely to clarify our understanding of the environment of our own planet； as robotic missions have already begun to do。 The history of our civilization shows that the pursuit of basic knowledge is the way the most significant practical advances e about。 Opinion polls suggest that the most popular reason for 〃exploring space〃 is 〃increased knowledge。〃 But are humans in space essential to achieve this goal？ Robotic missions； given high national priority and equipped with improved machine intelligence； seem to me entirely capable of answering； as well as astronauts can； all the questions we need to ask—and at Maybe 10 percent the cost。
It is alleged that 〃spinoff〃 will transpire—huge technological benefits that would otherwise fail to e about—thereby improving o