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Allernative Mission Concepts The Committee on Human Exploration of Space examined NASAs approaches to the HEI and a number of alternatives. However, a wide range of possibilities for program architectures and mission configurations exists that is yet to be examined in detail. The scope of early HEI missions can be defined, but, given the scientific and engineering unknowns, it is too early in the process to focus upon a single, final plan for a permanent return to the Moon and voyages to Mars. The need to bring innovative ideas to the planning process is widely recognized. THE SPACE STATION AS A FIRST STEP The President's policy incorporates Space Station Freedom (SSF) as the first step to achieving the ultimate goals of Moon settlement and Mars exploration. Although it is technically feasible to go to the Moon and Mars without the intermediate step of establishing a permanent station in low Earth orbit (LEO), most of the mission architectures under consideration employ a station for assembly of vehicles for travel beyond Earth, for storing fuel and supplies, and as a human transfer facility. In the near term, a facility in LEO is essential for conducting research on human performance and well-being in zero or fractional gravity as well as long-term confinement in zero gravity. It also can serge as a testbed for the life support system that eventually will be needed on the Moon and Mars, and can house experiments with artificial gravity, should that become a requirement for the journey to Mars. 5

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6 HUAL4N EXPLORATION OF SPACE Thus, the first major step to be taken in human space exploration is establishment of a station In LEO. The current station could meet some, but not all, of the requirements of the HEI. Proceeding with the HEI, therefore, would require continued development of the station to meet the demands of the initiative, including the conduct of life science research and use of the station as a spaceport. In the long term, the compatibility of the many functions to be performed on the station may be a serious question. A more precise interpretation of the goals surrounding the return to the Moon and the advance to Mars, as defined by the social and political decision-making process, will help to determine the nature, magnitude, and pace of the lunar and Mars ventures. The question even arises whether an additional station, complementary to the first and designed as a trans- portation node, will eventually become necessary to accommodate the later, more demanding missions. THE NASA REPORT OF THE 90-DAY STUDY ON HUMAN EXPLORATION OF THE MOON AND MARS The NASA Report of the 90-Day Study provided descriptions of five reference approaches: Approach A is formulated to establish human presence on the Moon in 2001, and the Moon is used as a learning center to develop the capability to move on to Mars. An initial nuclear power unit and lunar oxygen production demonstration hardware are added in 2003 to reduce lunar logistics requirements. Research is planned in geologic and geophysic exploration, geophysical and particle physics, and astronomy, as well as in the life sciences. The first Mars expedition is a 30-day stay on the surface, followed by a 600-day visit beginning in 2018, during which many scientific experiments are foreseen. This scenario involves advancing completion of SSF to 1997, requiring a heavy lift launch vehicle. Approach B is basically the same as Approach A, except that it ad- vances the first human Mars landing to 2011 and limits the degree to which lunar experience could affect the design of the Mars transportation and surface systems. It delays lunar science activities, but advances those on Mars. Approach C is akin to Approach A, except it advances to 2005 the date by which lunar oxygen is available, requiring earlier development of nuclear power system capabilities. By accelerating lunar activities, the knowledge learned can be applied to Mars missions. Approach D is also based on Approach A, except that all milestones are delayed two to three years, with a return to the Moon in 2004. Approach D does not require accelerating SSF and permits incorporation of new technology developments in plans for Mars excursions.

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ALTERNATIVE MISSION CONCEPTS 7 Approach E envisions a scaled-down, human-tended lunar base, and does not require that SSF be advanced In time. It includes a 600-day Mars simulation activity on the Moon. A lunar outpost is established In 2004, and three human expeditions to different locations on Mars begin In 2016, proceeding establishment of a permanent base. The reference approaches in the NASA 90-Day Study are largely variations on a theme and have certain common features: They depend on heavy lift vehicles to LEO and on SSF for assembly in LEO and as a transportation node. They employ unmanned robotic precursor missions, reusable transfer vehicles to lunar and Martian orbits, and excursion vehicles at surface bases. Each features sequential Moon and Mars programs, assumes zero gravity in transit to Mars and requires a decade or more of research on adaptability of humans to low or zero gravity, depends on aerobraking (using atmospheric drag to slow a vehicle for capture in the planetary gravity field), and requires new chemical propulsion engines using cryogenic fuels. Proceeding from initial habitats to constructible bases, these reference approaches all provide an extensive, reusable orbital transfer capability and infrastructure designed for permanent occupancy of the Moon and Mars. The approaches described in the NASA study are relatively low in risks, in that each would proceed in methodical steps after earlier steps have been proven and after scientific and engineering questions inherent in the architecture (e.g., nuclear propulsion, aerobraking, Mars surface habitability) are answered. The study recognizes the need for substantial advances in technologies such as those relating to the life sciences and nuclear power and propulsion. The committee believes the reference missions provide a useful background of possible mission configurations against which new ideas and concepts can be compared, and against which various cost and schedule scenarios can be analyzed. The architectures of the reference approaches, however, were built upon the presumed objective of returning to the Moon permanently and establishing bases on the surface of Mars. Therefore, the reference missions do not provide explicitly for an option that may entail less risk: a habitable station in orbit around Mars from which exploration initially could be conducted by telerobotics and later by human excursions to the surface. Considerable energy is required to transfer mass to Mars orbit from the surface, so it would be prudent to minimize the need for such transfer. In this context, it appears that a station in Mars orbit requires a less demanding infrastructure than a surface base and might serve a useful purpose in the early stages of human space exploration. An important aspect of the NASA Mars reference approaches is the reliance on aerobraking, a technology that has not yet been demonstrated

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8 HUA0V EXPLORATION OF SPACE in the dilute Martian atmosphere. An aerobraking vehicle will require large surfaces, new materials, and precise controls to avoid descending too rapidly or deflecting from the atmosphere back into space. The final decision regarding aerobraking should await technology demonstration and further knowledge about the Martian atmosphere, as well as information regarding the weight trade-offs between successful aerobraking materials and fuel for propulsive braking, especially were nuclear propulsion to be available. Aerobraking has the potential for reducing the initial mass of a spacecraft by 20 to 50 percent, however, and demonstrations are needed to bring this technology to fruition. Last, in these reference scenarios, extensive extravehicular activity (EVA) is implied for space construction and assembly. Human experience in space suggests that less EVA means safer missions, owing to the limited maneuverability and flexibility of astronauts in currently available space suits. Emphasis on teleoperations or more synergistic human/machine interactions can provide substitutes for extensive EVA But to facilitate a wide range of human activities in space, it seems desirable to develop an improved space suit for necessary EVA tasks. THE GREAT EXPLORATION The most characteristic features of The Great Exploration concept are its success-oriented pace, the estimated low total costs projected by its pro- ponents (permanent bases on the Moon and Mars by the year 2000 at an estimated cost of $10 billion to the launch of the Mars excursion vehicle), and the use of essentially identical, preassembled, inflatable structures for an Earth-orbiting space station, for propellant storage, and for structures for the Moon and Mars. The technology of space-based inflatables has been studied extensively, but has not been demonstrated In space. It also appears in the NASA study, in less critical applications. Clearly, prior to commitment to the use of such structures, there would be a need for advanced development and demonstration of space-based inflatables and of specific techniques for incorporating the necessary expandable hardware and fixtures in such structures. The potential advantage of inflatables is the reduced requirement for lifting mass to LEO, perhaps even reducing the requirements for a new heavy lift launch vehicle. However, if preassembled inflatable modules prove not to be useful for one or more of the applica- tions envisioned in this mission architecture, modules of traditional, rigid construction would have to be substituted, presumably with considerable effect on the mission concept. A number of critiques have been performed on The Great Exploration concept and its proponents have prepared responses. The committee's judgments have benefitted from this exchange of information and analyses.

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ALTERNATrVE MISSION CONCEPTS 9 However, it might be noted that the projected economies of time and cost proposed for The Great Exploration depend, in part, on using off-the-shelf technologies and "standard terrestrial machinery and equipment." The committee is not convinced that off-the-shelf, terrestrial technology will perform as required in the environment of space, the Moon, and Mars, nor that the technology meets requirements for reliability that should govern human-rated space systems. For example, the development of machines and apparatuses and their operation must take into consideration the adhesive and abrasive nature of the lunar soil, which is well known from earlier lunar landings. Further, The Great Exploration proposes no robotic precursor missions to learn more about the environments of the Moon or Mars or to identity safe or scientifically interesting landing sites. The committee believes The Great Exploration underestimates the many engineering and operational challenges involved in bringing its technical concepts to practical realization. The Great Exploration strategies are self-described as intentionally "maximally time~ompressed" and "reward- and risk-intensive" to achieve the ultimate goals as quickly as possible, on the premise that "there has never been a successful 25-30 year Federal technology program." Special priority procurement processes and waivers that are not otherwise available in unclassified civilian programs are required in order to meet the demand- ing schedule. Such procedures may not be acceptable in an open project, especially if there are international partners. Nevertheless, the committee believes there may be technologies in this alternative to the NASA approaches that should be further investigated, for example, the use of space-based inflatables for at least some of the required functions and modules. OTHER ARCHITECTURES FOR THE HUMAN EXPLORATION INITIATIVE Many other approaches exist for accomplishing the HEI. A concept was presented that featured advanced bases on the lunar surface and in Mars orbit essentially identical to SSF core modules. Modules, assembled on SSF as complete bases, would be mated with expendable propulsion systems to be launched intact and unmanned from LEO. The concept relies on the existing space shuttle for transport to orbit of relatively lightweight, valuable cargo and personnel and on a heavy lift launch vehicle for fuel and major unit (modules, nodes, spacecraft) transport. Mars orbit is selected as a preferred base from which to explore the planet, in the belief that it would be a more predictable and controllable environment than one of the Martian moons, and less dangerous than the planet's surface. The principal obstacle to going to the surface is that the space station

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10 HUAL4N EXPLORATION OF SPACE replica would not survive Mars atmospheric entry. Separate vehicles, also chemically propelled, are required for rapid transport of personnel. In this concept the similarity of the space station, lunar, and Mars modules implies less development time and cost and indicates that Mars might be reached sooner for early exploration than if the Mars flights had to await technology validation by aerobraking research and development and nuclear propulsion demonstrations. The committee did not study this concept in great detail, but it appears that the principal unknown in this scenario concerns the stability of such modules in transit. Among other early architectures that might be considered are the NASA baseline concept, with the initial Mars base in orbit rather than on the surface and the NASA baseline with separate cargo and crew transport systems, the latter with high-speed, staged chemical propulsion and even Earth launch of lunar and Mars missions. The committee is convinced that other alternatives will arise as concept development proceeds. The report of the National Commission on Space, Pioneering the Space Frontier, for example, contains stimulating discussions of future approaches to human exploration of space. From the committee's brief exploration of these alternative concepts, it appears likely that the eventual choice of mission architecture will incor- porate ideas from a variety of concepts, some that now exist and possibly some new ones. While the scenarios thus far described vary substantially in schedules, technologies, and in the need for research and development, all would benefit from advances in space transportation and in technologies critical to the support of humans in space. The committee found imagi- native and worthwhile components in all of the presentations, and, at the same time, recognizes the value, at this time, of encouraging the search for new ideas.