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The Challenges of Mars


 
written by Chris Ferenzi on March 30, 2003 | contact me
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Mars Direct
Mars Direct
Credit: Robert Zubrin
President John F. Kennedy, to inaugurate the historic undertaking of landing a man on the moon, boldly put forth, “ ‘Space is there, and we are going to climb it’ ” (qtd. in Zubrin 46). Now, at the dawn of a new millennium, the space exploration initiative that was present in the minds of Americans forty years ago no longer endures. Without the Cold War to provide us with the competition that got our nation to the moon, a new motivational focus must be established. This focus is clearly Mars. As demonstrated by the Mars Pathfinder Mission web site’s 566 million visits, public interest in the exploration of the Red Planet is high (Zubrin 51). Mars Direct, a $20 billion program proposed by Robert Zubrin to open the Martian Frontier to human settlement, now looks as if it may make the transition from the drawing boards into reality. Examining Mars Direct proves that with today’s technology, the concept is plausible, possible, and economically viable.

In 1989, celebrating the 20th anniversary of the manned moon landing success, President Bush revived our nation’s interest in the space program by calling upon the National Aeronautic and Space Administration (NASA) to devise a plan for sending humans to Mars. Needing no further encouragement, NASA set straight to work. Ninety days later, they released an elaborate mission outline calling for the construction of orbital hangars, space docks, lunar bases, transportation fleets, and Mars spacecraft rivaling the size of those on the popular TV show Battlestar Galactica (Raeburn 43). In theory, the time lapse between the industrialization of space and the landing of explorers on the Martian surface would have been approximately thirty years. But in the same way that all ambitious government programs carry a high price tag, the “90 Day Report” (43) had a fantastic minimum funding estimate of about $450 billion. Nauseated by the staggering figure, Bush scrapped the proposal. As stated quite frankly by Paul Raeburn, “It was an utterly unrealistic plan” (43).

Although NASA and President Bush had both rejected the “90 Day Report,” that did not necessarily mean that the prospect of landing explorers on Mars was pure fantasy. The $450 billion price was so high for one reason: NASA envisioned a mission in which a massive rocket was sent to Mars loaded with all the fuel and supplies needed for its voyage home. The weight specifications in such a strategy came out staggeringly high (Raeburn 43), therefore requiring the construction of a massive new launch vehicle. In response to this approach, Robert Zubrin, the head of Pioneer Astronautics aerospace engineering firm of Lakewood, Colorado, proposed a mission in 1990 that relied more on existing technology. But what held the most weight in Zubrin’s plan, entitled Mars Direct (44), was the fact that it did not require a giant spacecraft to haul along to Mars the 320 tons of fuel needed for the return voyage. Due to that simple fact, its price tag was an affordable $20 billion. Zubrin maintains, “the key to success lies with the same strategy that served the earliest explorers of our own planet: travel light and live off the land” (Zubrin 46). Living off of the land, when in all actuality the land is a desert, may sound outlandish, even downright foolish. But the technology being utilized for this has already been tested by the Martin Marietta Astronautics Company nearly a decade ago (Zubrin and Wagner xvii). In-situ propellant production is the one property of the Mars Direct program that makes it so much different from all the other space exploration proposals. What this technology allows NASA to do is manufacture the fuel needed for the crew’s return voyage on the Martian surface itself. It begins when a mere twenty tons of hydrogen is sent to the Martian surface. Then, using an atmospheric compressor and chemical processing unit, the hydrogen reacts with the carbon dioxide in Mars’s atmosphere (Zubrin 46). This process produces methane and water. In another process called electrolysis, the water created by the previous reaction is split into its two chemical components, hydrogen and oxygen (Zubrin and Wagner 308). Once combined, the methane and oxygen are a powerful rocket propellant.

The hydrogen byproduct of electrolysis is then sent through the cycle of fuel production again, thereby creating a continuous loop capable of producing as much fuel as needed. Thus, out of the twenty tons of hydrogen brought from Earth, the 320 tons of return-trip propellant will be manufactured with the simple system of “living off the land.” As an added bonus to the electrolysis process, the oxygen extracted from the water can be used by the life support systems in the crew’s habitation module (Zubrin 47). The first launch of the Mars Direct mission will conform to something along these lines: a single heavy-lift booster is launched toward Mars with an unmanned payload. Instead of the four-person crew that will launch on a subsequent rocket, the cargo will consist of an Earth Return Vehicle (ERV), twenty tons of liquid hydrogen, an automated chemical processing unit, a set of atmospheric compressors, and a small nuclear fission reactor mounted on a rover (Zubrin 46). Once the ERV arrives at Mars six months later it uses a method of deceleration, called aerobraking, in which the Martian atmosphere is used to slow down the craft. After the ERV lands, the rover which houses the nuclear reactor is driven a distance of a few hundred meters away from the landing site. There it is activated and used as the sole power source for the atmospheric compressors and chemical processing unit. The ERV now begins the task of manufacturing its return propellant (46). After NASA verifies that the ERV has successfully produced an adequate amount of fuel for its return journey, two more rockets are launched from Earth. One contains a manned habitation unit with the first crew of four and three years of provisions. The second rocket houses a second ERV whose main purpose would be to function as a future crew’s ride home from the Red Planet (Raeburn 47). But in the event of a catastrophic malfunction with the first ERV, it would act as a backup for the first crew (47). In the fashion of sending two boosters every other year, one to dispatch a four-person crew and the other with an ERV to open up a new landing site for a future crew, Mars would officially be open to exploration (48).

The habitation unit in which the Mars crew will spend their entire journey living will consist of an exercise room to keep the crew in shape for the long on-foot explorations ahead of them, sleeping quarters, a bathroom, and a laboratory, as well as a galley, lounge, and a “digital library” for recreation (Zubrin 48-49). Overall, if all goes as planned, the Mars Direct crew will have spent a total of two and a half years away from Earth: six months for the journey there, 500 days on the Martian surface, and six months for the trip home (Raeburn 47). And if the United States holds fast to the average Martian rocket launch rate of one rocket per year, NASA will only be launching at fifteen percent of the rate it currently launches the shuttle (Zubrin 48). Indeed, Mars Direct is the most economically sound method for space exploration to date. In the early years of manned Mars exploration, Robert Zubrin is not in favor of using such an advanced propulsion technology as nuclear fusion, mainly due to the fact that it still requires years of research and development, and a larger spacecraft (Raeburn 46). As Zubrin states it, “ ‘Columbus did not wait to develop trans-Atlantic sailing vessels before wandering off in search of a new world [he used what was available to him at the time] . . . Once he did that, once people knew there was someplace to go, then you had the development of three-masted frigates and steamships and ocean liners and Boeing 747's. Destinations drive transportation.’ ” (qtd. in Raeburn 46-47) Eventually, however, Zubrin does expect newer technologies to emerge and take the place of today’s chemical rocket systems.

Although as it may seem, Robert Zubrin has polished the Mars Direct program up enough so that no more thought needs to be put into it, that is far from the truth. Zubrin is the first to admit that many things need to be accomplished before the first rocket is even constructed. In the summer of 2000, The Mars Society, formed by Zubrin in an attempt to gain public support for his Mars Direct plan (Zubrin 51), will set up the Mars Arctic Research Station, or MARS, in the Arctic Circle (“Mars Arctic” 1). At a cost of about $1.2 million, the MARS project will be “the first practical attempt to solve many of the problems facing those wishing to build habitats that will one day be deployed on Mars” (2). This endeavor will also enable scientists, engineers, and astronauts to test the equipment and technology (transportation, life support, recycling, etc.) that will be used on the future manned missions (1). Careful thought and planning have been put into the selection of a location for the Mars Arctic Research Station. To be constructed in Haughton Crater on Devon Island within Canada’s Queen Elizabeth Island group, the crew and the habitation unit will be subject to the same “polar desert” conditions that the planet Mars is best known for (“Mars Arctic” 2). The crater remains snow and ice-free for most of the year and has areas of significant geological interest for the crew to explore and study (3). The 27-foot diameter prototype habitation unit to be used in the Arctic will accommodate a crew of six, housed on its three floors of living space. Alongside the unit, a piece of experimental technology will begin its testing phase: the inflatable greenhouse. This greenhouse consists of a rugged plastic outer shell that can be pressurized to support human and plant life. On the Martian surface, such a lightweight structure as this will be incredibly beneficial. The lighter they are, the cheaper it would be to ship them to the Martian surface. The energy for the unit will be supplied by a solar-cell array. An interesting point about the solar panels is that they will receive less sunlight energy in the Arctic than they would if placed on the Martian surface. Thus, if the array provides sufficient power for the habitation unit on Devon Island, it will provide more than an ample supply on Mars (“Mars Arctic” 4).

The dominant method of space exploration aside from the Space Shuttle program will be robotic orbiters and landers, which will continue their rein over NASA well into the first decade of the new millennium with missions centered around Mars. NASA has planned the launch of an unmanned Mars craft every 26 months designed to study the planet’s soil and atmosphere and to test new technologies related to the manned exploration of the planet (“Mars, Water” 1). Essential to Mars Direct, these robots will help NASA decide whether life had existed or still exists on Mars today, as well as what technologies it needs to further develop in order to successfully send humans to what Paul Raeburn calls “the frozen red sands of Mars” (Raeburn 43). In December of 1998, the Mars Climate Orbiter launched into Earth orbit with a Mars trajectory. When it arrives in late 1999, its mission will be to study the planet’s atmosphere and to assess Mars’ current water resources. The Climate Orbiter will also map the Martian surface (“Mars Climate” 1). A month later, second in the two-launch series, the Mars Polar Lander rocketed towards Mars with the goal of answering four extremely important questions once it lands: How did Mars become dry and airless? Did life evolve and then die out? Did life leave behind a fossil record? Will the same thing that happened on Mars happen on Earth? Once these questions are answered, NASA will have a much clearer view on the forthcoming manned mission (“Mars Polar” 1). In 2001, if all goes as planned, NASA will launch another batch of two robotic probes to Mars. The first being the 2001 Orbiter, and the second being the 2001 Lander (“Mars Surveyor” 1). The orbiter will mainly function complimentary to the lander, acting as a relay station between it and the Earth (1). The 2001 Lander will have three experiments on-board, designed for testing the crucial design elements that will be incorporated into Mars Direct. Both the Mars Environmental Compatibility Assessment, or MECA, and the Martian Radiation Environmental Experiment, or MARIE, will determine if the Martian environment poses any danger to human explorers, such as soil contaminants and surface radiation. What is likely to be the most important experiment payload on the lander, the Mars In-Situ Propellant Production, or MIP, will determine the feasibility of producing rocket fuel from the chemical components in Mars’s atmosphere (3).

Two years later, in 2003, yet another battery of launches will take place. This time, however, NASA will begin a series of sample return missions designed to drill into the Martian soil, collect samples, and send them to Earth for analysis in NASA laboratories. NASA hopes this will provide much more detailed information as to the presence of life, past or present, on Mars (“Where to” 1). The missions in 2003 will also test schemes for more precise landings which will be crucial to the Mars Direct rockets’ ability to accurately land at the designated touch-down coordinates (Raeburn 47). The unmanned robotic missions NASA currently has scheduled for the years between 2005 and 2013 are basically a repeat of the 2003 missions, designed primarily to bring Martian soil to Earth for study (1). After a decade of successful Martian exploration via robotic presence, the long-anticipated moment will finally arrive: the first crew that has been trained to live on another world will embark on the journey to Mars. Most people tend to think that the six-month transit between Earth and Mars and the one and a half year stay on the Martian surface are full of dangers, such as radiation exposure, zero gravity, psychological stresses, and once on Mars, fierce dust storms. While they do exist and warrant some mild concern, these dilemmas pose no serious risk to the crew if properly dealt with. Radiation, as seen by most people, is the main threat to a Mars crew. But what most people do not know is that “the radiation dosage on a two and a half year Mars mission imposes a statistical cancer risk of about one percent, or the same as a person who smokes for the same time period” (Zubrin 49). A one percent cancer risk is hardly what anyone would expect on a mission to another planet. Although the percentage of cancer is low, the radiation still needs to be shielded. To shield the habitation unit, all that needs to be done is to incorporate a radiation-absorbing material, such as lead, into its hull, which can be done rather inexpensively (49).

The lack of gravity is the second peril of living in space for an extended period of time. Studies aboard the Russian space station Mir and the Space Shuttle have concluded that it causes muscle and bone deterioration (Zubrin and Wagner 121). But with a rigorous exercise regimen, astronauts have spent up to eighteen months in zero gravity conditions and returned to Earth in good health (121). Fortunately for the Mars crew, artificial gravity will be utilized on the Mars mission, so the dangers that the lack of gravity poses are inconsequential. In much the same way a child is able to swing a bucket of water around without losing any of the water, the habitation unit will be spun around with the assistance of a counterbalance, the rocket’s expended upper stage, to produce a gravity equal to that of Mars’, or .38 of Earth’s (Zubrin and Wagner 121). Psychological stresses are another concern of the critics to a manned mission to Mars. They fear that the astronauts will succumb to cabin fever due to the isolation and cramped quarters. Zubrin remarks, “compared with stresses dealt with by previous generations of explorers, the hand-picked Mars crew should have no problems with isolation” (Zubrin 50). As for the cramped quarters, the habitation unit will have over 1,083 square feet of floor space and once they land on Mars, more space will be available through the use of inflatable habitats (Zubrin and Wagner 127). The final danger to the crew, this time on the surface of Mars itself, are Martian dust storms. The storms’ wind speeds can reach upwards to 200-kph, but because Mars has an atmospheric density comparable to one percent that of Earth’s, the winds will only have the strength equal to a 20-kph terrestrial wind. Certainly, these winds are a problem that can be overcome. Hence, these storms present little danger to the crew when they are on the planet’s surface. Landing the craft during one of these storms, however, is not something that should be attempted. The Soviets have lost two robotic probes in that fashion (Zubrin 50). As chief engineer at the Johnson Space Center, Ken Joosten remarks, “ ‘We’re only in space for six months . . . six months is something we have experience in. What we need to do is learn how to live on Mars’ ” (qtd in Raeburn 45).

After the six-month journey through interplanetary space to Mars is completed, the crew will set down on the Martian surface. There they will begin the tasks which have brought them: to search for evidence of life and to scout out possible mining resources for use by future colonies and the Earth. The human crew will have the leading edge over robotic explorers, due mainly to their ability to roam, follow hunches, and pursue leads (Raeburn 45). Those three characteristics are what makes the chances of finding evidence of life realistic. Robots can dig and prod in the soil, but only humans can traverse the terrain knowing that if they find life, their view on the universe will be forever changed. The key to finding Martian life is to understand the history of Martian water resources (“Mars, Water” 1). Accordingly, the astronauts will begin their search for life within the permafrost of Mars’ soil lying a few meters below the surface. Research as to how and where life thrives here on Earth has shown us that it can thrive in extreme environments, far from what we consider to be optimal surroundings (1). Maybe life still exists today on the cold, barren Mars, maybe it doesn’t. But the only way we’ll find out is if we send astronauts there to vigorously search for it. Only then will we understand how unique or how commonplace of an occurrence the creation of life truly is. Because of the similar geological history between Mars and Earth, Mars will most likely contain an abundance of concentrated mineral ores beneath its surface. On Earth, we are running out of concentrated mineral ore, so if we mine the materials from Mars and then export them to Earth, a multi-billion dollar industry may blossom (Zubrin and Wagner 223).

Among the iron, gold, and other substances that exist on Mars, there is a much more promising material that may be the key to unlock a $60 billion industry and the long-awaited nuclear fusion power: Deuterium. Deuterium’s main application would be as fuel in the nearly waste-free, safe, thermonuclear fusion reactors. However, the research that would make the fusion reactor possible has been allowed to stagnate here on Earth simply because we are too involved in the production of “dirty energy” through the process of burning fossil fuels. If on Mars the material was mined, the Martian colonists would be much more determined to get fusion online because it would provide an inexhaustible source of energy for their colony (“Significance of” 6). Once the fusion reactor has been established and is working to specifications, fusion power will most likely lead to fusion propulsion, an extremely swift means of space transportation. According to Zubrin, “Fusion propulsion would make possible craft that could carry hundreds of passengers and thousands of tons of cargo to Mars and back rapidly” (“Significance of ” 6). With that type of high- speed transit available, the population growth rate of Mars colonists would be about one fifth that of Colonial America’s during the 17th and 18th centuries (Zubrin and Wagner 232). But the intriguing aspect is that fusion propulsion “would not just cause Earth-Mars travel time to shrink from months to weeks, but to the outer solar system, from years to months and voyages to the stars, from millennia to decades” (“Significance of ” 6). This technology, which will most likely be developed entirely by Martian colonists, would increase the rate of emigration to Mars exponentially. The mining of Mars will act as a technological gateway, opening not just itself up to our exploration and colonization, but the solar system and eventually the universe (6). The first astronauts will only spend a total of 18 months on the Martian surface, then head back to Earth. But as a base develops, astronauts may choose to stay longer - four, maybe six years. Remaining on the planet not just to conduct scientific research, these astronauts will most likely receive monetary bonuses for opting to stay on the planet. This does not come as a surprise, however, because most of the mission expense of Mars Direct is transporting the astronauts back and forth between Earth and Mars (Zubrin and Wagner 215).

The initial Martian bases will not be massive domes or cities. Instead, they will consist of interconnected networks of the Mars Direct “tuna-can” habitats that have been left behind on previous missions. To connect the habitation modules, wheels would be attached to their sides and with the aid of some type of cable system, be pulled across the terrain to be mated up with the other modules directly or through inflatable tunnels (Zubrin and Wagner 173). As these smaller, interconnected bases grow and develop and more people move in to colonize, it is inevitable that children will be born on Mars. These children, in a sense, will be the first ‘true’ Martians (Oberg 187) . With this population boom, new types of habitation structures will need to be constructed because the smaller “tuna cans” will no longer be sufficient. There are two such structures under consideration at this time: large, inflatable, geodesic domes (Zubrin and Wagner 177) and pressurized dwellings composed of Martian-native brick (174). The former, geodesic domes, would be capable of opening huge amounts of land to habitation and agriculture (Zubrin and Wagner 177). Fabricated from the super-strong material Kevlar, these domes would first have to be imported from Earth. But as the manufacturing technology on Mars steadily increases, the materials needed to make the Kevlar structures could be extracted from the Martian soil. The latter, however, could be made entirely on the Martian surface to begin with. Due to the fact that creating brick from the Martian soil is incredibly simple, the brick dwellings could be built in high numbers, at multiple locations, and at a swift rate. Although a brick structure may seem impossible to pressurize, once they are covered with a few meters of soil, the process is quite simple. The soil would provide the downward force pushing on the structures’ roofs and when the oxygen is pumped into the dwellings, the gas would act as the opposing force, pushing upwards. The combination of the Martian soil and pressurized dwellings is capable of maintaining a very safe and comfortable internal environment (175).

In what will be the final step of Martian colonization, the human race will attempt something that has never been done before: the terraformation of another world. Terraformation is essentially the transformation of a non-Earth-like planet into an Earth-like planet (McKay 52). This process would do for Mars what geological evolution did for Earth. Scientists have been studying the concept for nearly half a century and they have come to the conclusion that it is very much possible. Their climate models indicate that humans could alter the Martian ecosystem enough that it would change from the barren wasteland it is today into a lush green and blue planet, all using technology available today (53). Why terraform Mars and not another planet in our Solar System? Four billion years ago, Mars was warm and wet, possibly teeming with life. If we were to revert the planetary climate to what it once was, it would give us the unprecedented opportunity to study on a grand scale how biospheres grow and evolve (McKay 53). Aside from that, Mars and Earth have strikingly similar features. Both have roughly the same axis tilt, which provides seasonal shifts, and both have the same length day. This allows for plant life to better adapt than if we terraformed a planet with a six hundred hour day and no seasonal climate changes (53). The only way to begin the global terraformation of Mars would be to first envelop the planet in a thick carbon dioxide atmosphere. Currently, Mars does have a thin atmosphere, but it is nowhere near the density needed to trap solar energy (McKay 54). Once the atmosphere begins to thicken, the trapped solar heat would cause the polar ice caps and subterranean permafrost to melt. Carbon dioxide would then diffuse from the melting soil and help thicken the atmosphere more. But in order for the atmosphere to become thick enough to melt the permafrost, super greenhouse gases, such as Perflurocarbons (PFCs) would have to be pumped into it. PFCs can trap solar energy with thousands the times efficiency than any other gas can, causing global warming (55). On Earth, that would be devastating, but on Mars, it would be just what the doctor ordered. Calculations suggest that if only a few particles per million of super greenhouse gases were present in Mars’s atmosphere, the temperature would rise forty degrees Fahrenheit (56).

The only problem with using PFCs is that they can’t be imported from Earth; they would have to be manufactured on the Martian surface out of native atmospheric gases. But Mars does contain an abundance of fluorine, carbon, and sulfur, the three main ingredients of PFCs (McKay 56). Once the miniature factories that extract the gases and convert them into PFCs are constructed, hundreds would need to be distributed over the Martian surface. Either they would be powered by their own fusion reactors if the technology is available to them, or solar panels (56.) If the PFCs trap solar energy in the Mars atmosphere at once hundred percent efficiency, then Mars could be transformed into a water and carbon dioxide rich planet in sixty years (McKay 56). But being realistic, trapping the sun’s energy at ten percent efficiency, Mars would gain a thick carbon dioxide atmosphere in one hundred years and become a water-rich planet in little over six hundred years. While that may seem like a long time, if transforming Mars into a water-rich planet would have taken a million years, then it would have been impossible (56). Transforming Mars into an oxygen-rich planet, however, will take much longer. With the technology we have available to us today for converting carbon dioxide into oxygen, mainly plant life, the oxygen-rich Mars could be achieved in about a million years; Earth took two billion to attain an oxygen-rich atmosphere (57). Terraformation may have once seemed like a fantasy, something only to be dreamt of. But, as proved here, it is possible to achieve the global climatic change of Mars utilizing current technology.

Examining the Mars Direct mission proves that with today’s technology, the concept of Martian colonization is plausible, possible, and economically viable. In 1990, when Robert Zubrin reacted to NASA’s $450 billion Mars mission plan by creating his own $20 billion mission, many high ranking NASA officials took his ideas as ludicrous and unworkable. But as they realized that the ‘living off the land’ method he proposed was the only viable way to complete a manned Mars mission economically, his mission plan became the one they were going to use. Now, at the dawn of a new millennium, NASA is finally testing the technologies that Mars Direct requires through experiments on robotic landers. Landing a human team on Mars is no longer bound to the pages of fiction, but continually evolving in the real world of science to become a reality within the next twenty years. If we as a race do not take this opportunity to spread our presence to Mars and throughout our solar system, then in fifty years Earth could possibly become a polluted, dying world with over fifteen billion in population. However, if we embrace Mars Direct and begin to colonize the Red Planet, overpopulation will not occur on Earth and many of the other problems plaguing it will disappear. Today, the power rests in our hands to determine whether or not the human race continues to expand tomorrow. The question is, will we act on that power?

Works Cited:

1) “CFM Mission Scenario.” http://spot.colorado.edu/~marscase/cfm/cfm84/cfm84plan.html May 13, 1999.
2) Dyson, Marianne. “To Mars, Young Human!” Odyssey 5.5(May 1996): 30-31.
3) “Mars Arctic Research Station.” http://www.marssociety.org/researchstation.asp March 22, 1999.
4) “Mars Charts.” http://www.marssociety.org/mars_charts.asp May 13, 1999.
5) “Mars Climate Orbiter.” http://www.exploringmars.org/missions/mco/ April 25, 1999.
6) “Mars Direct Image Gallery.” http://www.nw.net/mars/gallery.html May 13, 1999.
7) “Mars Exploration Artwork.” http://spot.colorado.edu/~marscase/emmart.html May 13, 1999.
8) “Mars Polar Lander.” http://www.exploringmars.org/missions/mpl/ April 25, 1999.
9) “Mars Surveyor 2001.” http://www.exploringmars.org/missions/2001/ April 25, 1999.
10) “Mars, Water and Life.” http://www.exploringmars.org/missions/water_life.html April 25, 1999.
11) McKay, Christopher P. “Bringing Life to Mars.” Scientific American Presents 10.1(Jan. - May 1999): 52-57.
12) Oberg, James. Mission To Mars. Harrisburg: Stackpole Books, 1982.
13) Raeburn, Paul. “Manned Mission to Mars.” Popular Science 254.2(Feb. 1999): 40-47.
14) “Significance of the Martian Frontier.” http://www.nw.net/mars/docs/m_fronti.txt May 1, 1999.
15) “Where to Next?” http://www.exploringmars.org/missions/future.html April 25, 1999.
16) Zubrin, Robert. “Sending Humans to Mars.” Scientific American Presents 10.1(Jan. - May 1999): 46-51.
17) Zubrin, Robert and Richard Wagner. The Case For Mars: The Plan to Settle the Red Planet and Why We Must. New York: The Free Press, 1996.

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