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logram of gold。 This is surely a major reason we have yet to stride the ancient shorelines of Mars。 Multistage chemical rockets are the means that first took us into space； and that's what we've been using ever since。 We've tried to refine them； to make there safer； more reliable； simpler； cheaper。 But that hasn't happened； or at least not nearly as quickly as many had hoped。
So maybe there's a better way： maybe single…stage rockets that can launch their payloads directly to orbit； maybe many small payloads shot from guns or rocket…launched from airplanes； maybe supersonic ramjets。 Maybe there's something much better that we haven't thought of yet。 If we can manufacture propellants for the return trip from the air and soil of our destination world； the difficulty of the voyage would be greatly eased。
Once we're up there in space； venturing to the planets； rocketry is not necessarily the best means to move large payloads around； even with gravity assists。 Today； we make a few early rocket burns and later midcourse corrections； and coast the rest of the way。 But there are promising ion and nuclear/ electric propulsion systems by which a small and steady acceleration is exerted。 Or； as the Russian space pioneer Konstantin Tsiolkovsky first envisioned； we could employ solar sails—vast but very thin films that catch sunlight and the solar wind； a caravel kilometers wide plying the void between the worlds。 Especially for trips to Mars and beyond； such methods are far better than rockets。
As with most technologies； when something barely works； when it's the first of its kind； there's a natural tendency to improve it； develop it； exploit it。 Soon there's such an institutional investment in the original technology； no matter how flawed； that it's very hard to move on to something better。 NASA has almost no resources to pursue alternative propulsion technologies。 That money would have to e out of near…term missions； missions which could provide concrete results and improve NASA's success record。 Spending money on alternative technologies pays off a decade or two in the future。 We tend to be very little interested in a decade or two in the future。 This is one of the ways by which initial success can sow the seeds of ultimate failure； and is very similar to what sometimes happens in biological evolution。 But sooner or later some nation—perhaps one without a huge investment in marginally effective technology—will develop effective alternatives。
Even before then； if we take a cooperative path； there will e a time—perhaps in the first decades of the new century and the new millennium—when an interplanetary spacecraft is assembled in Earth orbit； the progress in full view on the evening news。 Astronauts and cosmonauts； hovering like gnats； guide and mate the prefabricated parts。 Eventually the ship； tested and ready； is boarded by its international crew； and boosted to escape velocity。 For the whole of the voyage to Mars and back； the lives of the crew members depend on one another； a microcosm of our actual circumstances down here on Earth。 Perhaps the first joint interplanetary mission with human crews will be only a flyby or orbit of Mars。 Earlier； robot vehicles； with aerobraking； parachutes； and retrorockets； will have set gently down on the Martian surface to collect samples and return them to Earth； and to emplace supplies for future explorers。 But whether or not we have pelling； coherent reasons； I am sure—unless we destroy ourselves first—that the day will e when we humans set foot on Mars。 It is only a matter of when。
According to solemn treaty； signed in Washington and Moscow on January 27； 1967； no nation may lay claim to part or all of another planet。 Nevertheless—for historical reasons that Columbus would have understood well—some people are concerned about who first sets foot on Mars。 If this really worries us； we can arrange for the ankles of the crew members to be tied together as they alight in the gentle Martian gravity。
The crews would acquire new and previously sequestered samples； in part to search for life； in part to understand the past and future of Mars and Earth。 They would experiment； for later expeditions； on extracting water； oxygen； and hydrogen from the rocks and the air and from the underground permafrost—to drink； to breathe； to power their machines and； as rocket fuel and oxidizer； to propel the return voyage。 They would test Martian materials for eventual fabrication of bases and settlements on Mars。
And they would go exploring。 When I imagine the early human exploration of Mars； it's always a roving vehicle； a little like a jeep； wandering down one of the valley networks； the crew with geological hammers； cameras； and analytic instruments at the ready。 They're looking for rocks from ages past； signs of ancient cataclysms； clues to climate change； strange chemistries； fossils； or—most exciting and most unlikely—something alive。 Their discoveries are televised back to Earth at the speed of light。 Snuggled up in bed with the kids； you explore the ancient riverbeds of Mars。
CHAPTER 16 SCALING HEAVEN
Who； my friend； can scale heaven？
—THE EPIC OF GILGAMESH （SUMER； THIRD MILLENNIUM B。C。）
What？； I sometimes ask myself in amazement： Our ancestors walked from East Africa to Novaya Zemlya and Ayers Rock and Patagonia； hunted elephants with stone spearpoints； traversed the polar seas in open boats 7；000 years ago； circumnavigated the Earth propelled by nothing but wind； walked the Moon a decade after entering space—and we're daunted by a voyage to Mars？ But then I remind myself of the avoidable human suffering on Earth； how a few dollars can save the life of a child dying of dehydration； how many children we could save for the cost of a trip to Mars—and for the moment I change my mind。 Is it unworthy to stay home or unworthy to go？ Or have I posed a false dichotomy？ Isn't it possible to make a better life for everyone on Earth and to reach for the planets and the stars？
We had an expansive run in the '60s and '70s。 You might have thought； as I did then； that our species would be on Mars before the century was over。 But instead； we've pulled inward