Mars Exploration Strategy

8/3/2019 Mars Exploration Strategy

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AIAA 93-4212

MARS EXPLORATION STRATEGIES:

A REFERENCE PROGRAM ANDCOMPARISON OF ALTERNATIVE

ARCHITECTURES

David B. Weaver *NASA Lyndon B. Johnson Space Center, Houston, Texas

and

Michael B. Duke*NASA Lyndon B. Johnson Space Center, Houston, Texas

Abstract

The human exploration of Mars has been historically placed as an objective to beconsidered only by the grandchildren of today's generation of planetary explorationenthusiasts. The basis for this grim prediction centers around the high projected costsof such missions, the necessity of first establishing a permanent presence in low-Earthorbit and then establishing an outpost on the Moon, and the seemingly insurmountabletechnical obstacles that such an endeavor presents. This paper presents an overview ofthe results of a year-long NASA-wide effort to consider innovative strategies for thehuman exploration of Mars. The result of these efforts has been the development of adesign reference mission (DRM) that directly challenges the prevailing obstaclesgenerally associated with human missions to Mars and creates a strategy that couldenable humans to explore the mysteries of Mars within this generation.

In developing the Mars DRM, the study team employed a strategy of focusing

programmatic and technical resources on the systems required to support a crew onthe surface of Mars, rather than focusing all resources on the trip to and from theplanet. This strategy enhanced the mission return, improved the safety of the crew,and reduced or eliminated many of the obstacles associated with conventionalstrategies for the human exploration of Mars. In particular, the Mars DRM describedhere permits crews to explore the surface of Mars for nearly 500 days on the first andall subsequent missions, while limiting their exposure to the interplanetary space
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environment to periods near those of US. experience on Skylab and well withinRussian experience from Mir. This surface-oriented philosophy emphasized thedevelopment of high-leveraging surface technologies in lieu of concentratingexclusively on space transportation technologies and development strategies. Thus,the DRM relies on the in-situ production of CH4/O2 propellant for the crew's ascentvehicle and surface mobility, as well as the necessary water and life support gases forthe crew's entire surface stay, thereby permitting a significant decrease in the amountof material that must be delivered to Mars from Earth, while simultaneouslyincreasing the productivity of the crew on the surface and their safety. Byemphasizing a robust suite of surface capabilities and high-leverage technologies, theMars study team was able to consider abort-to-the-Mars-surface as a an alternative tothe traditional trajectory abort options. The DRM, consistent with this risk-mitigation philosophy, also employs a single common habitat design for use during theinterplanetary transits to and from Mars, as well as to support the crew during their~500 day stay on the planetary surface. As a result of this balanced approach tomission and crew risk, element commonality, and technology develop-ment, humanmissions to Mars can be accomplished without the need for complex assemblyoperations in low-Earth orbit. Both cargo and human missions are launched direct toMars with rendezvous occurring on the planetary surface. The associated reduction intotal launch mass allows the first crew of six to explore Mars within a total of fourlaunches of a Saturn VII launch vehicle, and continuing missions to Mars to beconducted with only three Saturn VII launches every Mars opportunity (every 26months).

I. Introduction

The known is finite, the unknown infinite; intellectually we stand on an islet in themidst of an illimitable ocean of inexplicability. Our business in every generation is toreclaim a little more land, to add something to the extent and solidity of ourpossessions.

- THOMAS HENRY HUXLEY on the Reception of the "Origin of Species" (1887)

Mars has long captured the imaginations of scientists, engineers, and explorers.[1] However, conventional approaches to mounting human expeditions to the fourthplanet from the Sun have presented formidable engineering and fiscal challenges thathave created the presumption that the human exploration of Mars is a goal for futuregenerations, if achievable at all. Conventional plans[2,3,4] for these missions haveincluded many or all of the characteristics presented in Table 1.
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High costs (e.g. $100s of Billions) Development of numerous, costly, but terrestrially inapplicable technologies Complex Assembly Operations in LEO involving multiple HLLV launches Long-term cryogenic propellant storage and transfer in LEO Long-duration exposure of the crew to both the zero-g and space radiation

environments Short stay times (e.g. 30 days) for the crew at Mars, with low mission return Extensive prerequisite experience with human physiology at a LEO space

station Predecessor lunar outpost experience High HLLV launch rates

Table 1 - Conventional Mars Exploration Program Characteristics

The Mars Exploration Study Project was undertaken by the Exploration ProgramsOffice (now, Planetary Projects Office) in response to the strategic planning initiativesof the Associate Administrator for Exploration at NASA Headquarters, in the summerof 1992. The purpose of the study, as viewed by the Associate Administrator forExploration, was to establish a vision for the human exploration of Mars that wouldserve as a mechanism for understanding program and technical requirements thatwould be placed on existing and planned Agency programs, including a precursorlunar exploration program, which was being developed by the Office of Exploration(The First Lunar Outpost - FLO). Emphasis was to be placed on determining potentialcommonality between Mars and Moon exploration programs so that total programcosts for both programs might be minimized and so the lunar program would notcontain dead ends which would be difficult or expensive for the Mars program tocorrect if both programs were to be carried out sequentially.

The study team chose to take an approach that emphasized the important aspects ofMars exploration without consideration of an assumed predecessor lunar capability.Because Mars exploration is inherently more complex than initial lunar exploration programs, it was considered important to identify the characteristics required forMars, then the evolution of a FLO-like lunar program could be planned that wouldoptimize the programmatic interactions. The result, toward which the study has

progressed, is a coherent view of Mars exploration, which has value in its own right,as well as being useful for the integrated programmatic view.

II. Human Exploration of Mars: Rationale & Objectives


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In August, 1992, the first workshop of the Mars Study Team was held at the Lunarand Planetary Institute, in Houston, Texas. It was the purpose of that workshop toaddress the "whys" of Mars exploration, to provide the top-level requirements fromwhich the Mars exploration program could be built (Duke and Budden, 1992).

The workshop attendees identified the major elements of a potential rationale for aMars exploration program as:

Human Evolution - Mars is the most accessible planetary body beyond theEarth-Moon system where sustained human presence is believed to be possible.

Comparative Planetology - The scientific objectives of Mars exploration are tounderstand the planet and its history, in part to better understand Earth. Thetechnical objectives of Mars exploration are to understand what would berequired to sustain a permanent human presence beyond Earth.

International Cooperation - The political environment at the end of the ColdWar may be conducive to a concerted international effort that is appropriateand may be required for a sustained program. There would be political benefitsfrom a cooperative program.

Technology Advancement - The human exploration of Mars lies at the raggededge of achievability. The technical capabilities are available or on the horizon,such that commitment to the program will both effectively exploit previousinvestments, but also contribute to advances in technology.

Inspiration - The goals of Mars exploration are grand, will motivate our youth, benefit technical education goals, and excite the people and nations of theworld.

Investment - In comparison with other classes of societal expenditures, the costof a Mars exploration program is modest.

Many of the benefits of a human exploration program are indirect or intangible.Further analysis is needed to quantify the benefits of Mars exploration, so thatcompelling arguments can be made to the public and leaders of those nations whomight participate in a Mars exploration program, to justify the expenditures that willbe required.

Reflecting the conclusions of the workshop, the following technical goal was adoptedfor the Mars exploration program:

Verify a way that people can inhabit Mars.

Derived from this goal are three objectives: (1) Conduct human missions to Mars; (2)Conduct applied science research to use Mars resources to augment life-sustainingsystems; and (3) Conduct basic science research to gain new knowledge about the


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solar system's origin and history. Conducting human missions to Mars is required toaccomplish the exploration and research activities, but contains the requirements forthe safe transportation, maintenance on the surface of Mars and return of a healthycrew to Earth. The surface exploration mission envisions approximately equal priorityfor applied science research - learning about the environment, resources, andoperational constraints that would allow humans eventually to inhabit the planet; andbasic science research - exploring the planet for insights into the nature of planets, thenature of Mars' atmosphere and its evolution, and the possible past existence of life.These more detailed objectives are shown in Table 2 and form the basis for definingthe required elements and operations for the Mars exploration program.

Conduct Human Missions to Marsa. Land people on Mars and return them safely to Earthb. Demonstrate the capability of people to effectively perform useful work

on the surface of Marsc. Demonstrate the ability to support people on Mars for two years or more

at a timed. Demonstrate the ability to support people in space for periods of time

consistent with Mars mission opportunities (2-3 years)e. Demonstrate that space operations capabilities including

communications, data management, and operations planning canaccommodate both routine and contingency mission operationalsituations; and understand abort modes from surface or spacecontingencies

f. Determine the characteristics of space transportation and surfaceoperations systems consistent with sustaining a long term program ataffordable cost

Conduct Applied Science Research to Use Mars Resources to Augment Life-Sustaining Systems

a. Catalogue the global distribution of life support, propellant, andconstruction materials (hydrogen, oxygen, nitrogen, phosphorous,potassium, magnesium and iron) on Mars and of the Moons Phobos andDeimos

b. Demonstrate effective system designs and processes for utilizing in-situ

materials to replace products that otherwise would have to be providedfrom Earth Conduct Basic Science Research to Gain New Knowledge About the Solar

System's Origin and Historya. Demonstrate the capacity for robotic and human investigations to gain

significant insights into the history of the atmosphere, the planet'sgeological evolution, and the possible evolution of life


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b. Determine whether Mars, the Martian system, or Earth-Mars transits aresuitable venues for other science measurements

Table 2 - Mars Exploration Program Technical Objectives

III. Reference Mission Overview

1. Purpose of the Reference Mission

The reference mission serves several purposes. First, it provides a mechanism fordiverse technical personnel to collectively integrate their definition and design effortsaround a baseline strategy. This allows people with innovative concepts to comparetheir approach on a direct basis. However, it is particularly important to establish a setof mission accomplishments that must be met by alternative scenarios. This is a majorstep in documenting the expected benefits of such an exploration program. Second,establishing a reference mission allows the formulation of a technically credibleapproach, with appropriate documentation of the technical and programmatic risks,which can form the basis for defensible cost estimates for the program. Previousstudies of Mars missions have been associated with rather high costs, but with littlevisibility into the assumptions and approaches to developing the costs. Developing thereference mission provides a starting point for cost analysis, which can identifyimportant programmatic or technical problems whose solution can reduce the overallcost and risks of the program. Likewise, the reference mission provides a basis for

analyzing the importance of technology development and new data which can begathered in advance of the human exploration mission design. Finally, the referencemission provides a basis for understanding potential international cooperativeapproaches to conducting the mission.

2. Reference Mission Goals

The goal of the Mars Study Team in developing the Mars DRM was:

Create a baseline strategy enabling the earliest and most cost-effective program for thehuman and robotic exploration of Mars while addressing fundamental sciencequestions and demonstrating the ability for humans to inhabit Mars.

From this goal, several study objectives were developed:

1. Challenge the notion that the human exploration of Mars is a 30-year programthat will cost hundreds of billions of dollars.


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2. Challenge the traditional technical obstacles associated with sending humans toMars.

3. Identify relevant technology development/investment opportunities.

3. Reference Mission Overview

The approach taken in developing the reference mission adopted for this study was to:(1) Define a robust planetary surface exploration capacity, capable of safely andproductively supporting crews on the surface of Mars for 500-600 days each mission.This is in contrast to previous mission studies that have adopted short stay times forthe first or first few human exploration missions and focussed their attention principally on space transportation; (2) Limit the length of time the crew iscontinuously exposed to the interplanetary space environment. In doing this, the physiological and psychological degradation to the crew is reduced, therebyenhancing both crew safety and mission return. In addition, the associated life scienceconcerns are partially mitigated, reducing the requisite scope of any crew certificationprogram; (3) Provide an operationally simple mission approach that emphasizes thejudicious use of common systems. Because an integrated mission in which a singlespacecraft is launched from Earth and lands on Mars to conduct the long explorationprogram is not feasible, it is necessary to determine the simplest and most reliable setof operations in space or on the surface of Mars to bring all of the necessary resourcesto the surface where they are to be used. A strategy emphasizing multiple uses for asingle system development potentially enhances not only the total program costs, butalso crew safety and system maintainability; (4) Provide a flexible implementationstrategy. Mars missions are complex, so that multiple pathways to the desiredobjectives have considerable value in insuring mission success; (5) Balance technical,programmatic, mission, and safety risks. Mars exploration will not be without risks;however, the risk-mitigation philosophy will be a critically important element intechnical and fiscal feasibility of these missions, as well as the acceptability of themission concept to the public and its elected leaders. Mars is not "three days away"and overcoming the temptation to look back to Earth to resolve each contingencysituation may be the most challenging obstacle to be overcome in embarking upon thehuman exploration of Mars.

The provision of a robust surface capability is fundamental to the reference mission philosophy employed in this study. Assets are focused at the planetary surface because that is where the goals of Mars exploration can be achieved. Althoughefficient and reliable space transportation elements are a critical component of anyplanetary exploration strategy, the exploration goal adopted in this study suggests theneed to be able to "live off the land." Thus, the surface capability must provide acomfortable, productive, reliable, and safe place for the crew. This, in turn, changes


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the risk perspective with respect to previous studies by relieving the pressure on thespace transportation systems to resolve any and all contingencies. Whereas in previous studies, many mission contingencies resulted in trajectory aborts (directreturns to Earth),[5,6] another option exists in this reference mission, namely, abort toMars' surface. This allows the mission design to focus on the surface capability, noton the provision of costly propulsive performance increases and redundant systems tobe used in the unplanned and relatively improbable event of system failure in flight.Unlike in Apollo and other strategies for returning humans to the Moon, free-returnabort and powered abort maneuvers do not come for free at Mars. The goal of thehuman portion of the space transportation function should be to deliver the crew toand from Mars with the least reasonably achievable exposure to the hazards of thespace environment. Trajectory aborts, far from being presumed requirements forhuman missions to Mars, should have to fight their way into a reference mission as alast resort. By emphasizing the capabilities available to the crew on the surface ofMars, it, not the interplanetary space environment, becomes the most secure,reachable place for the crew in the Solar System after the completion of the TMI burn.

The robust surface capability is implemented through a split mission concept, inwhich cargo is transported in manageable units to the surface and checked out inadvance of committing crews to their missions. Each cargo and human flight to Marsis launched in a single FLO-class (i.e. ~240 t. to 500 km. circular LEO) heavy-liftlaunch vehicle which contains a nuclear thermal propulsion (NTP) upper stage thatinjects the entire HLLV payload on a direct trajectory to Mars. This approachprovides a basis for continued expansion of capability at the outpost through the

addition of modules to the original systems. The split mission approach also allowsthe crews to be transported on faster, more energetic trajectories, minimizing theirexposure to the space environment, while the vast majority of material sent to Mars,including the crew's Earth-return stage, is sent on minimum energy trajectories.

For system design purposes, the Mars DRM assumes the use of the Earth-Mars launchopportunities occurring in 2007 and 2009. There were two reasons for selecting thesedates. First, 2009 represents the most difficult opportunity in the 15-year Earth-Marscycle. By designing the space transportation systems for this opportunity, particularlythose associated with the human flights, the systems can be flown in any opportunity

with either additional performance margins or with improved performance (e.g. fastertransit times for the crew or increased payload delivery capacity). For example, wherethe trans-Mars and trans-Earth crew transfers in the 2009 opportunity are 180 days, inthe easier 2018 opportunity they are 120 days each way for the same vehicle. Thesecond reason for designing to these opportunities is that they lie ~15 years in thefuture, a time considered by the authors to be a reasonably robust goal for committinghumans to Mars. As an additional design consideration, the Mars Study Team adopted
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the ground rule that the study would examine three human missions to Mars. Eachmission returns to the site of the initial mission, with missions two and threelaunching in the 2012 and 2014 launch opportunities, respectively. This approachpermitted the Team to consider an evolutionary establishment of capabilities on theMars surface and was fundamentally more consistent with the stated goals for thehuman exploration of Mars. Although it is certainly arguable that science return couldbe enhanced by a strategy where each human mission went to a different surface site,the goal of understanding how humans could inhabit Mars seems more logicallydirected toward a single outpost approach.

The reference mission is depicted in Figures 1, 2, and 3. In the first opportunity,September 2007, three cargo missions are launched on minimum energy trajectoriesdirect to Mars (i.e. without assembly or fueling in low Earth orbit). Table 3 identifiesthe overall manifest of these three ETO launches. The first launch delivers a fully-fueled Earth-return stage (ERV) to Mars orbit. The crew will rendezvous with thisstage and return to Earth after completion of their surface exploration in October2011. The second launch delivers a descent vehicle to Mars orbit which will deliverits payload of a dry Mars ascent stage and crew module (MAV), a propellantproduction module, a nuclear power plant, liquid hydrogen (to be used as a reactant toproduce the ascent vehicle propellant), and approximately 40 metric tons of additionalsurface payload to the surface. After the descent stage lands on the surface in lateAugust, 2008, the nuclear reactor autonomously deploys itself several hundred metersfrom the ascent vehicle and the propellant production facility begins to produce fromthe Mars atmosphere the nearly 30 metric tons of oxygen and methane that will be

required to deliver the crew to Mars orbit in October 2011. As Figure 3 illustrates, thisproduction is completed within approximately one year - several months prior to thefirst crew's scheduled departure from Earth in mid-November 2009. The third launchin the 2007 opportunity, delivers a second descent vehicle to Mars orbit which willdeliver its payload of a surface habitat/laboratory, non-perishable consumables for asafe-haven, and a second nuclear power plant to the planetary surface. It descends tothe surface in early September 2008, landing near the first descent vehicle. Thesecond nuclear power plant autonomously deploys itself nearby the first plant. Each ofthe two plants provides sufficient power (i.e. 160 kWe) for the entire mature surfaceoutpost, thereby providing complete redundancy within the power function.


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Figure 1. Mars Exploration Reference Mission Overview

2007 Launch Opportunity - Cargo Missions

Figure 2. Mars Exploration Reference Mission Overview2009 Launch Opportunity - Cargo & Human Missions


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Figure 3. Mars DRM Timeline

Table 3: General Launch Manifest - 2007 Launch OpportunityFlight 1: Cargo Flight 2: Cargo Flight 3: Cargo

Surface Payload

none

Surface Payload

Ascent Capsule Empty Ascent Stage LOX/CH4

Production Plant LH2 Propellant Seed Power Supply

(nuclear-160kW) Utility Truck Pressurized Rover

Additional Payload

Surface Payload

SurfaceHabitat/Laboratory

non-perishableConsumables

Power Supply(nuclear-160kW)

Utility Truck Spares

Teleoperable ScienceRover

Mars Orbit Payload

Earth Return Vehicleo Fueled (LOX/CH4)

TEI Stageo Transit Habitat

o Earth Return Capsule

Mars Orbit Payload

none

Mars Orbit Payload

none

Space Transportation Vehicles

NTR Transfer Stage

LOX/CH4 TEI Stage w/Mars

Space TransportationVehicles

NTR Transfer Stage LOX/CH4 Descent


NTR Transfer Stage


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aerobrakeStage w/Marsaerobrake

LOX/CH4 AscentStage

LOX/CH4 DescentStage w/Marsaerobrake

In the second opportunity, October 2009, two additional cargo missions and the crewmission are launched. All assets previously delivered to Mars are checked out and theascent vehicle is verified to be fully fueled before either the crew or the additionalcargo missions are launched from Earth. The two cargo missions described in Table 4are launched prior to the crew mission. The first launch is a duplicate of launch onefrom the 2007 opportunity, delivering a fully-fueled Earth-return stage to Mars orbit.The second launch similarly mirrors the second launch of the 2007 opportunity,delivering a second ascent dry ascent stage and propellant production module. Thesesystems provide backup or extensions of the previously deployed capabilities. Forexample, the second Mars ascent vehicle and second Earth return vehicle provide the2009 crew with two redundant means for each leg of the return trip. If, for somereason, either the first ascent stage or the first Earth-return stage become inoperableafter the first crew departs Earth in 2009, the crew can use the systems launched in2009 instead. They will arrive in plenty of time to be available for the crew's departurefrom Mars in October 2011. If the ascent and Earth-return vehicles delivered in 2007operate as expected, then the systems delivered in 2009 will support the second crewof six that will launch to Mars early in 2012. Subsequently, one piloted mission andtwo cargo missions can be launched at each opportunity, resulting in a consistent

launch rate of 3 HLLVs per opportunity.Table 4: General Launch Manifest - 2009 Launch Opportunity

Flight 4: Cargo Flight 5: Cargo Flight 6: First Crew

Surface Payload

none

Surface Payload

Ascent Capsule Empty Ascent Stage LOX/CH4 Production

Plant

LH2 Propellant Seed Bioregenerative Life

Support OutfittingEquipment

Science: 1km drill Science Equipment

Additional

Surface Payload

Crew Surface Habitat Consumables Spares

EVA Equipment

Science Equipment


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Payload/Spares

Mars Orbit Payload

Earth Return Vehicleo Fueled (LOX/CH4)TEI Stage

o Transit Habitat

o Earth ReturnCapsule

Mars Orbit Payload

none

Mars Orbit Payload

none


NTR Transfer Stage

LOX/CH4 TEI Stagew/Mars aerobrake


NTR Transfer Stage LOX/CH4 Descent

Stage w/Marsaerobrake

LOX/CH4 AscentStage


NTR Transfer Stage

LOX/CH4 DescentStage w/Marsaerobrake

The first crew of six departs for Mars in mid-November 2009. They leave Earth afterthe two cargo missions launched in 2009, but because they are sent on a fast transfertrajectory of only 180 days, they will arrive in Mars orbit approximately two months prior to the cargo missions. The crew rides out to Mars in a surface habitat

substantially identical to the habitat/laboratory previously deployed to the Marssurface. The transit habitat sits atop an identical descent stage as those used in the2007 opportunity. After capturing into a highly elliptic Mars orbit (250 x 33793 km),the crew descends in the transit habitat to rendezvous on the surface with the otherelements of the surface outpost (see figure 3). There is no required rendezvous inMars orbit prior to the crew descent. This is consistent with the risk philosophyinherent in the Mars DRM. Once the TMI burn has been completed, the crew mustreach the surface of Mars. The crew carries with them sufficient provisions for theentire 540 day surface stay in the unlikely event that they are unable to rendezvous onthe surface with the assets previously deployed.

After their stay on Mars, the crew uses the previously landed ascent vehicle to returnto orbit, rendezvous with the Earth return vehicle, and return to Earth. Like theoutbound transit leg, the crew rides in a surface habitat on the inbound transit leg.This habitat is part of the Earth-return stage deployed in 2007, and as figure 4illustrates, has been in an untended mode for nearly four years prior to the crewarrival. As more fully discussed below, this strategy for habitation permits the


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development of only one major habitable element, thereby reducing costs, lesseningspares/logistics quantities, limiting the number of unique systems with which the crewmust be familiar, and enhancing maintainability by the crew. Figure 4 presents apictorial overview of the Mars DRM.

The major distinguishing characteristics of the design reference mission, compared toprevious concepts, include: (1) No extended low-Earth orbit operations, assembly orfueling; (2) No rendezvous in Mars orbit prior to landing; (3) Short transit times toand from Mars (180 days or less) and long surface stay times (500-600 days) for thefirst and all subsequent crews exploring Mars; (4) A heavy lift launch vehicle, capableof transporting either crew or cargo direct to Mars, and capable of delivering allneeded payload with a total of 4 launches for the first human mission and threelaunches of cargo and crew for each subsequent opportunity; (5) Exploitation ofindigenous resources from the beginning of the program, with important performance benefits and reduction of mission risk; (6) Availability of abort-to-Mars'-surfacestrategies, based on the robustness of the Mars surface capabilities and the cost oftrajectory aborts; (7) Common transit/surface habitat design; (8) A common set ofspace transportation vehicles and heavy lift launch vehicles, capable of transportingeither crew or cargo to Mars; and (9) No presumed reliance on a previously emplacedlunar outpost. These features are more completely discussed below.

4. Mission Profile

A great deal has been written in the past about the necessity of achieving quick triptimes to Mars in order to reduce the crew's exposure to the zero gravity and spaceradiation environments. Two such options have been proposed for achieving thesequick trip times: opposition-class missions and fast transfer conjunction classmissions. Mars mission classes are generally characterized by the length of time in theMars system and the total round-trip mission time.[7,8] The first of these is typifiedby short Mars stay-times (typically 30-90 days) and relatively short round-trip missiontimes (400-650 days). This is often referred to as an opposition-class mission,although the authors have adopted the terminology "short-stay" mission. Thetrajectory profile for a typical short-stay mission is shown in Figure 5. This class hashigher propulsive requirements than the long-stay missions and typically requires a

gravity-assisted swingby at Venus or the performance of a deep-space propulsivemaneuver in order to reduce total mission energy and constrain Mars and/or Earthentry speeds. Short-stay missions always have one short transit leg, either outbound orinbound, and one long transit leg, the latter requiring close passage by the sun (0.7 AUor less). The second mission class consists of long-duration Mars stay-times (as muchas 500 days at Mars) and long total round-trip times (approximately 900 days). Thismission type is often referred to as conjunction-class, although the authors have


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previously adopted the more descriptive terminology "long-stay" mission. Theserepresent the global minimum-energy solutions for a given launch opportunity. Thetrajectory profile for a typical long-stay mission is shown in Figure 6.

Figure 5: Typical short-stay Mission Profile

Figure 6: Typical long-stay Mission Profile

Within the long-stay category of missions the option exists to dramatically decreasethe transit times to and from Mars through moderate propulsive increases.[3] The totalround-trip times remain comparable to those of the minimum-energy, long-staymissions, but the one-way transits are substantially reduced, in some cases to less than100 days, and the Mars stay times are increased modestly (to as much as 600 days).The round-trip energy requirements of this class, referred to as a "fast-transit"mission, are similar to the short-stay missions, even though the trajectories areradically different. The profile for a typical fast-transit mission is shown in Figure 7.


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Figure 7: "Fast Transit" Mission Profile

Three factors make the selection of the trajectory class critical to a reference mission.First, the selection must be consistent with achieving the Mars exploration goals and

objectives. Second, the selection must be consistent with the risk philosophy of theMars DRM. Finally, for programmatic reasons, the trajectory class selection mustprovide the flexibility to conduct missions in all Earth-Mars opportunities within the15-year cycle and for conducting missions supporting the evolution of Marsexploration objectives and implementation strategies.

The applicability of each of the previously discussed mission types to the humanexploration of Mars has been the subject of much debate. The opinion has generallybeen held that the initial flights should be short-stay missions performed "as fast aspossible" (so-called "sprint" missions), ostensibly to minimize crew exposure to the

zero-gravity and space radiation environment, to ease requirements on systemreliability, and to enhance the probability of mission success. However, whenconsidering "fast" Mars missions, it is key to distinguish whether one is referring tofast round-trip or fast transit missions. In fact, past analyses have shown thatdecreasing round-trip mission times for the short-stay missions does not equate to fasttransit times (i.e., less exposure to the zero-gravity and space radiation environment)as compared to the long-stay missions. Indeed, fast transit times are available only forthe long-stay missions. This point becomes clear when looking at Figure 8 whichgraphically displays the transit times as a function of the total round-trip missionduration. Although the short-stay mission has approximately half the total duration of

either of the long-stay missions, over 90% of the this time is spent in transit,compared to 30% for the fast-transit mission.


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Figure 8: Round-trip Mission Comparisons

The Mars Study Team adopted the use of the fast transit missions based on the factorslisted above. First, verifying the ability of people to inhabit Mars requires more than abrief stay of 30 days at the planet. In addition, the low return on investment associated

with a 30 day stay at Mars (of which significantly less than 30 days would actually beproductively spent on the Mars surface due to the crew adaptation to the Mars gravity,crew preparations for Mars departure, etc.) was considered unacceptable. The MarsStudy Team concluded following the August 1992 Workshop that a "Plant the Flag"mission objective was not a tenable rationale to support the substantial investmentinvolved.

The Mars DRM risk philosophy mandated limiting the crew's exposure to theinterplanetary space environment, looking instead to the facilities on Mars to safelysupport the crew for the majority of the mission duration. Substantial information

exists in the literature concerning the concerns with the space radiation environment(the interplanetary ionizing radiation environment of concern to mission plannersconsists of two components: GCR and solar particle events (SPEs)) and the zero-gravity environment and their effects on human physiology.[9] They will not bediscussed in detail in this paper. However, a brief comparative discussion of thesecritical environmental issues and their impact on mission class is warranted. Figure 9illustrates a relative comparison of the representative galactic cosmic radiation (GCR)exposure experienced by a crew on each of the three trajectory options.[10] Becauseboth long Mars stay options separate the inbound an outbound transit legs by a 500+day stay on the Mars surface (where the GCR fluence is attenuated by 75% due to the

Mars atmosphere and the planet itself), crews on these trajectories would not exceedthe estimated 50 rem annual dose limits. Conversely, since the crew in a short Marsstay mission spends virtually the entire mission duration in the GCR environment, acrew on these missions could receive radiation doses which not only exceed theannual dose limits, but also exceed the total dose received by a crew on a fast transitmissions. It is prudent to stress the preliminary nature of the dose calculationsreported above. It is best to consider a relative comparison of the trajectory types.


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Calculations are very dependent on shielding assumptions, the mission opportunity,and the current state of knowledge regarding the interaction and transport of GCRthrough material and the human body. Indeed, although none of the mission doseestimates violate the current NASA career standards, recent studies have suggestedthat the associated cancer mortality may have been underestimated by a factor of 3-4.[11] If so, these limits may well become more restrictive in the future, unlessdispensation is given for exploration missions.

Figure 9: Radiation Exposure Comparisons for Various Mission Classes

A similar analysis of mission classes is involved in considering the crew's exposure tothe zero-gravity environment during transits to and from Mars. Significantphysiological changes occur when zero-gravity time begins to be measured in months.Bone decalcification, immune and cardiovascular system degradation, and muscular

atrophy are a few of the more unpleasant effects. Research on the effects of long-termzero-gravity on the human body is in an elementary stage. The longest US mission,Skylab 4, was 84 days in duration and the longest Soviet mission was 366 days. Inneither case were crews exposed to zero-g/partial-g/zero-g sequences similar to thatprojected for Mars missions. Upon arrival on the Martian surface, the crew mustspend some time readapting to a partial-gravity field. Current data indicates thatrecovery in a 1-g environment can be fairly rapid (on the order of a few days), butdevelopment of full productivity could require significantly more time. This may beof concern for the short-stay missions where a substantial portion of the surface staytime could be consumed by crew adaptation to 0.38 g's. Conversely, ample time willbe available for the crew to regain stamina and productivity during the long surfacestays associated with the minimum-energy and fast-transit missions.

Several potential solutions to the physiological problems associated with zero-gravitytransits to and from Mars may exist, including: countermeasures (exercise, body fluidmanagement, lower body negative pressure), artificial-gravity spacecraft, and reducedtransit times. The usefulness of countermeasures to reduce some of the zero-gravity


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effects is still unknown. Soviet long-duration crews have experienced physiologicaldegradation even when rigorous exercise regimens have been followed. However,most of these effects seem to be quickly ameliorated upon return to a 1-genvironment, at least when immediate medical aid is available.

Rotating the Mars Transfer Vehicle (MTV) is a method of providing an artificialgravity environment for the crew and is most often associated with low-performancepropulsion systems, or the short-stay class of trajectories (since both require longtransit times). Studies have indicated that the MTV design mass penalties are on theorder of 5-20% if artificial-g is incorporated.[11] Depending upon the specificconfiguration, there may also be operational complications associated with artificial-gspacecraft including EVA, maintenance, and the spin-up/spin-down required for mid-course maneuvering and rendezvous/docking.

Figure 10 illustrates some example transit times for minimum-energy, fast-transit, andshort-stay missions. Note that all one-way transits are within the Soviet zero-gravitydatabase.

Figure 10: Microgravity Comparison for Various Mission Classes

However, the inbound and outbound transits for short-stay missions are typicallyseparated by only one to three months. It is questionable whether such a short timespent in a 0.38-g field will counteract five months of outbound zero-gravity exposure.In contrast, the one-way trip times of representative fast-transit missions are nearly

within the current U.S. zero-gravity database, which will certainly be augmented bynormal Space Station operations prior to executing human interplanetary missions.Also, note that the fast-transit mission's zero-gravity transfer legs are separated by asubstantial period of time in the Martian gravitational field. This long period on thesurface of Mars should prove sufficient to ameliorate the physiological effects of therelatively short outbound transit.


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Finally, the selection of trajectory type depends upon its ability to flexibly respond tomission opportunities and implementation strategies. The higher energy, short Mars-stay missions have a significant variation in both propulsive requirements and round-trip flight times across the 15 year Earth-Mars cycle.[12] Additionally, these missionsgenerally require the use of a Venus swingby maneuver in order to keep propulsiverequirements within reason. However, these swingbys are not always available on thereturn transit leg and must be substituted in the outbound transit leg. Since the transitleg with the Venus swingby is the longer of the two, the result is to have the crewspending up to 360 days on the trip to Mars, with any associated physiologicaldegradation occurring at the beginning of the mission - i.e. prior to the crew's arrivalat Mars. These variations can result in significant configurational impacts to theEarth-Mars transportation elements for different Earth-Mars opportunities.Programmat-ically, such a result is extremely unattractive. In contrast, the minimum-energy long Mars-stay missions exhibit very little variation, while the fast-transit longMars-stay missions reflect only moderate variations across the same 15-year cycle. Inaddition, neither of these missions require a Venus swingby. Indeed, neither missionrequires the crew to travel inside the Earth's orbit around the Sun.

For the above reasons, the Mars Study Team selected the fast transit, long Mars-stayclass trajectories. However, it was decided that the amount of reduction sought in theEarth-Mars and Mars-Earth transit times must be balanced with the otherconsiderations involved in the DRM. Reductions below 180 days in the one-waytransit times (for the 2009 opportunity) would have required either significantpropulsive capability improvements, or would have necessitated much larger IMLEOs

for the human missions, thereby requiring assembly/docking in LEO and higherHLLV launch rates. Indeed, others have demonstrated that reductions in trip timesreach a point of diminishing returns from the space transfer vehicle designperspective.[13] Thus, a C3 (C3 is the square of the velocity of departure from a planet) leaving Earth of 20-25 km2/sec2 appears to be appropriate for humanmissions. This results in maximum Earth-Mars transit times of approximately 180days (2009 opportunity) and minimum transit times of approximately 120 days (2018opportunity). Similarly, a C3 leaving Mars of ~16 km2/sec2 appears to be appropriatefor human missions, resulting in similar Mars-Earth transfer times for theseopportunities.

5. Space Transportation

The space transportation system consists of a trans-Mars injection (TMI) stage, abiconic aerobrake for Mars orbit capture and Mars entry, a descent stage for surfacedelivery, an ascent stage for crew return to Mars orbit, an Earth-return stage fordeparture from the Mars system, and an Earth crew capture vehicle (ala Apollo) for


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Earth entry and landing. As mentioned earlier, the reference program splits thedelivery of elements to Mars into cargo missions and human missions, all of whichare targeted to the same locale on the surface and must be landed in close proximity toone another. The transportation strategy adopted in the Mars DRM eliminates theneed for assembly or rendezvous in low-Earth orbit of vehicle elements and requires arendezvous in Mars orbit only for the crew in preparing to leave Mars. Thetransportaiton strategy also emphasized the use of common elements in order to avoiddevelopment costs and to provide operational simplicity. Thus, a modular spacetransportation architecture resulted. A complete detailed description of the spacetransportation architecture would be beyond the scope of this paper. Instead, below isdescribed an overview of each of the major elements in the space transportationfunction. References are provided to the more detailed system descriptions, whereavailable.

TMI Stage

The TMI stage (used to propel the spacecraft from low Earth orbit onto a trans-Marstrajectory) employs nuclear thermal propulsion. Nuclear thermal propulsion wasadopted for the TMI burn because of its performance advantages, its advanced,previously demonstrated state of technology development, its operational flexibility,and its inherent mission and crew risk enhancements. A single TMI stage wasdeveloped for both piloted and human missions. The stage is designed for the moreenergetically demanding 2009 human mission and then used in the minimum energycargo missions to throw the maximum payload possible to Mars. In the humanmissions, the TMI stage uses four 15,000 lb. thrust NERVA derivative (NDR) engines(Isp = 900 seconds) to deliver the crew and their surface habitat/descent stage onto thetrans-Mars trajectory. After completion of the two-perigee burn Earth departure, theTMI stage is disposed of in interplanetary space on a trajectory that will not re-encounter Earth or Mars over the course of 106 years. The TMI stage used with thecrew incorporates a shadow shield between the NDR engine assembly and the LH2tank in order to protect the crew from the radiation from the engines that build upduring the TMI burns.

As shown in figure 11, the same TMI stage is used in all cargo missions, where thetransportation system can deliver approximately 65 metric tons of useful cargo to thesurface of Mars or nearly 100 tons to Mars orbit (250 x 33,793 km) on a single launchfrom Earth atop a heavy lift launch vehicle that has the capability of lifting 240 metrictons to low Earth orbit (407 km). The TMI stage for cargo delivery only requires theuse of three NDR engines, so for cost and performance reasons one engine is removedfrom the piloted mission stage, as is the shadow shield as it is not required in theabsence of the crew on these flights. For a thorough description of the TMI stage andthe trades associated with its use in the Mars DRM, see Borowski.[14]


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Mars Orbit Capture and Descent Stage

Mars orbit capture and the majority of the Mars descent maneuver is performed usinga single biconic aeroshell. The decision to perform the Mars orbit capture maneuverwas based upon the fact that an aeroshell will be required to perform the Mars descent

maneuver, no matter what method is used to capture into orbit about Mars. Unlikepast mission concepts employing aerocapture, however, where the Mars entry speedshave been high, and the mission profile required a post-aerocapture rendezvous inMars orbit with another space transportation element, the Mars DRM has neither ofthese features. Thus, the strategy employed was to drive toward the development of asingle aeroshell development that can be used for both the MOC and descentmaneuvers. Given the demands on a descent aeroshell of the Mars entry and landingrequirements, the delta's to permit aerocapture are considered to be modest.

Figure 11.

The descent stage itself, employs four RL10-class engines, modified to burnLOX/CH4, to perform the post-aerocapture circularization burn and to perform thefinal ~500 m/se. of descent prior to landing on the Mars surface. The use ofparachutes has been assumed to reduce the descent vehicle's speed after the aeroshellhas ceased to be effective and prior to the final propulsive maneuver. A singlecommon descent stage has been assumed for the delivery of both the surface/transithabitats as well as the ascent vehicle and other surface cargo. The descent vehicle iscapable of landing ~65 metric tons of cargo on the Mars surface. When deliveringcrew, this number is reduced because of the limitations of the TMI stage to deliver the

same payload to the higher-energy trajectory required for the crew.

Ascent Vehicle

The ascent vehicle is delivered to the Mars surface atop a cargo descent stage. It iscomposed of an ascent stage and an ascent crew module. The ascent stage is deliveredwith its propellant tanks empty. However, the descent stage delivering the ascentvehicle includes several tanks of seed hydrogen for use in producing the nearly 30metric tons of LOX/CH4 propellant for the nearly 5,600 meters/second required forascent to orbit and rendezvous with the previously deployed Earth-return vehicle. The

ascent vehicle also uses two RL10-class engines, modified to burn LOX/CH4. Thecrew rides into orbit in the Earth Crew Capture Vehicle (ECCV) or in a dedicatedascent capsule. The ECCV is similar to an Apollo Command Module and iseventually used by the crew to enter Earth's atmosphere and deliver the crew safely toa land landing. An ECCV would have the necessary heat shield for Earth re-entry.Thus, as in Apollo, it would be heavier than a dedicated ascent module for deliveringthe crew to Earth orbit. However, unlike Apollo, the ascent propellant is produced in


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situ, thereby substantially muting the impact of the heavier ECCV for ascent. Theadvantages of using the ECCV for ascent lies in the ability to eliminate a separatesystem development and the safety/maintainability associated with the crew havingaccess to the ECCV during their entire surface stay, as well as their Earth-returntransit.

Earth-Return Vehicle

The Earth-return vehicle is composed of the TEI stage, the Earth-return transit habitat,and the ECCV (if the ECCV is not the ascent crew module). The TEI stage isdelivered to Mars orbit fully fueled, where it loiters for nearly four years before beingused by the crew in returning to Earth. It uses two RL10-class engines, modified toburn LOX/CH4. Again, these are the same engines developed for the ascent anddescent stages, thereby reducing engine development costs and improvingmaintainability. The return habitat is effectively a duplicate of the outbound

transit/surface habitat used by the crew in going to Mars, less the substantial stores ofconsumables in the latter habitat.

6. Mars Habitation System

A more detailed description of the Mars Habitation System is beyond the scope of thispaper but is available from L.Weaver/(713)483-3748.

Overview

At the core of meeting the two primary Mars exploration goals, science and humanpresence, the Mars habitation system seeks to provide a robust surface safety systemfor the crew and an environment conducive to high levels of crew self sufficiencywhile focusing on enabling historically cost saving decisions. Following the moremundane requirements of maintaining the required atmosphere, water and food tosustain the life of six crewmembers, a human mission to Mars creates a myriad ofcomplex issues requiring consideration at the earliest stages of mission development.Of primary concern, the crew's physiological response to various gravityenvironments and severe isolation. Even considering the potential windfall of humansystems information from the space station program, critical data may remain

substantially undefined and must be addressed in the development of the Mars DRM.Accordingly, the reduction of the crew transit time, specifically the exposure to zero-gravity and the space radiation environment, has been limited to the duration ofprojected space station missions, about four to six months. Although countermeasuressuch as exercise and diet, should continue to be enhanced and more thoroughlycomprehended, current measures have been unable to counteract 100% ofphysiological degradation and minimizing crew exposure is still the best solution.


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Figure 12. The Crew Exercise Facility is a Critical Component of a CountermeasuresSystem Designed to Inhibit Crew Degradation from Exposure to Reduced GravityEnvironments

A secondary consideration, sending the crew from the surface of Earth to the surfaceof Mars in the same habitation element provides an additional, and perhapsmandatory, safety feature by eliminating a rendezvous, docking and transferprocedure in Mars orbit and by allowing the crew to remain in a fully functional andoutfitted habitation element for the duration of their adaptation period to the Martian

gravity environment. As each individual reacts differently to these situations, andexperience with partial gravities is almost non-existent, the exact time required forcomplete adaptation will probably remain an unknown. Eliminating the requirementfor immediate post landing physical activity provides substantial benefits to themission.

Surface Safety

As previously discussed, the options provided by planetary alignments and varioustrajectories, as well as the availability of key resources, make the surface of Mars the

safest place for the crew. Therefore, the Mars habitation system must be designed toincorporate fundamental safety features, such as isolatable pressurized elements andcontinuous access to Extravehicular Activity suits. At the core of this discussion is thedecision to physically connect [pressurized crew access] each habitable element onthe surface of Mars or to leave each habitable element as a distinct unit separated byapproximately 500 meters. As expected, the impacts on both sides of this issue aresubstantial. However, from a crew safety perspective, interconnection of the


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pressurized habitable elements is considered to be a primary requirement. Crew accessto alternate pressurized elements substantially increases the risk mitigation optionsreducing an otherwise severe dependence on EVA systems and rovers.

Figure 13. EVA Suit Stowage Locations Play A Critical Role in A Robust CrewSafety System

Additional benefits of physically connecting the surface habitable elements includethe option to share functions and systems between elements and missions, the ability

to selectively locate sensitive consumables and hardware and a reduction of crew timedevoted to logistics transfer and access to functions located in isolated habitationelements. Surface mobility becomes the technological challenge of implementingphysically connected habitation elements and is discussed in the following section.

Crew Self-Sufficiency

Following completion of the recently constructed space station mission controlbuilding, the assumption can be made that station missions, like Shuttle missions, willbe very closely monitored and controlled from Earth. Intricate timelines and a host of

monitors accompany each Shuttle mission as the crew completes well rehearsed tasksand experiments. This philosophy must change to provide the crew every opportunityto successfully complete a three year mission to Mars. Crew self-sufficiency mustbecome the adopted philosophy of long duration exploration missions. Clearly, therole of mission control will not be eliminated. However, communications delays andunexpected situations will shift reliance away from Earth to the Mars crew.


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A simple example of increasing crew self-sufficiency is exchanging the philosophy of providing on-orbit replacement units, or spares, for each system to providing acapable machine shop with the requisite supply of raw materials as well as therequired training. Additional examples range from providing extensive computerbased libraries, such as the current CD-ROM technology affords to growing freshfood in greenhouse environments.

Commonality

Historical cost data indicates that commonality between large components of a systemcan substantially reduce development and production costs. Additionally, systemcommonality maintains crew familiarity and confidence thereby reducing training forboth operation and maintenance. The reference mission developed by the Mars StudyTeam includes a singular habitation element which must function in various gravityenvironments. There are two gravity environments of any duration to consider in

designing a habitation element, zero-gravity for four to six months during the transitphases and 3/8ths gravity for 500 days on the Martian surface. The primary issueconcerning commonality and the design of habitation elements is, can a commonhabitation element function efficiently in both the transit and surface phases?

As commonality has substantial mission benefits, this study began with theassumption that a single, largely common element can effectively function in allphases of the Mars DRM, and subsequently set about the task of identifying any andall serious drawbacks to habitation element commonality. To date, no show-stopperstowards commonality have been identified. Development of some subsystems,

particularly water systems such as ECLSS and personal hygiene systems, havesubstantially different requirements for zero versus partial gravity environments. Asgravity can greatly simplify the design of a water system, a trade study to identify thecost to benefit ratio of duplicating a more complex zero-gravity system or providingtwo separate systems must be conducted.

Commonality can exist at many levels. Complete commonality, sometimes referred toas "cookie cutter" habitation elements, is a desirable cost goal but is not considered tobe feasible for this mission. At an obvious level, the duplication of the galley fourtimes, or the crewquarters four times, is not the most efficient use of the totalavailable pressurized volume on the surface of Mars. In summary, commonality hasgreat benefits and should be achieved at all possible levels. Serious drawbacks toduplicating habitation elements throughout the current DRM have not yet beenidentified and commonality continues to be a key driver in the development of a Marshabitation system.

Habitation Element Description


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The structural cylinder, 7.5 meters in diameter, bi-level, and vertically oriented, wasderived from a series of volume, mass and mission analysis. Primary drivers included:

1. Commonality2. Crew size and mission duration

3. Repeat visits to the same site4. Physical connection of all pressurized elements5. Mass landing capability to the surface of Mars [estimated at 60 -75 mt]6. Mass capability of the trans-Earth injection stage [est. at 40 - 50 mt]

Each habitation element will contain substantially identical primary and secondarystructure, windows, hatches, docking mechanisms, power distribution systems, lifesupport, environmental control, safety features, stowage, waste management,communications, airlock function and crew egress routes. From this point, there existsan endless array of feasible internal architecture designs. Each solution involves atrade of resources derived from a specific set of goals, which at this level of detail, arevery open for discussion. The following brief description of the four primaryhabitation elements was developed for early costing purposes and in direct support ofthe Mars DRM analysis.

Figure 14. Conceptual Mars Habitation Module - Wardroom Design

Sent out, landed and verified prior to the launch of any crewmembers, the MarsSurface Lab, which will operate only in 3/8ths gravity, contains a large,. non-sensitivestowage area with crew support elements, such as waste management, on one leveland a second level devoted entirely to the primary science and research lab. Future


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development of this element include retrofitting the stowage level into a greenhouseas consumables and resources are consumed and free volume is created[15].

The Mars Transit/Surface Habitation Elements must contain the required consumablesfor the Mars transit and surface duration of approximately 700 days as well as all the

required elements for the crew during the 180 transfer trip. This is the critical elementthat must effectively operate in both zero and partial gravity. Once on the surface ofMars, this element will be physically connected with the previously landed SurfaceLab doubling the pressurized volume, to approximately 1,000 cubic meters, availableto the crew for the 500 day surface mission.

The Earth Return Habitation Element, functioning only in zero-gravity and requiringthe least amount of volume for consumables, will be volume rich but must be massconstrained to meet the limitations of the trans-Earth injection stage. As little activityis projected for the crew during this phase of the mission, mass and radiationprotection were the key drivers to the internal architecture concepts created.

Figure 15. Conceptual Design of an Airlock Sized for Two Suited Crewmembers andattached to a Maintenance/Dust Control Area

The airlock system, although integral with the habitation system, was developed as anindependent element capable of being "plugged" or located as the mission requires.EVA will be a substantial element of any planetary surface mission and will have amajor impact on the internal architecture of each element. EVA systems provide both


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a primary operational element as well as a critical component of the crew safetysystem and must be integrated into the design of a habitation system during the veryearly stages.

The habitation system is a core element of the Mars DRM developed to illustrate the

economic and technical feasibility of early Mars Missions. Complex systems capableof endless design solutions, the habitation system developed for this Mars missionfocused on three critical factors: Cost savings, providing a robust safety system on thesurface of Mars and enabling high levels of crew self-sufficiency.

Habitation Element Mass Estimates

The following charts summarize the mass breakdowns generated for each of theprimary habitation elements in support of early costing exercised. The masses wereestimated based on historical data but were adjusted for technology advances such as

aluminum lithium structures.

This habitation element was estimated for the transit of the second and third crews.The mass of the Mars Transit/Surface Element for the first crew was conceived to belower due to the off-loading of 600 days of non-critical consumables.

Mass Breakdown of estimate for Mars Transit/Surface Habitation Element

Subsystems:Subsys Mass

(mt)

Consumables Subtot

(mt)

Dry Mass Subtot

(mt)

Phys/Chem Life support: 6.00 3.00 3.00Plant growth: 0.00 0.00 0.00

Crew accommodations: 22.50 17.50 5.00

Health care: 2.50 0.50 2.00

Structures: 10.00 0.00 10.00

EVA: 4.00 3.00 1.00

Electrical power distribution:

0.50 0.00 0.50

Comm and Info Mgmt: 1.50 0.00 1.50

Thermal control: 2.00 0.00 2.00

Power generation: 0.00 0.00 0.00

Attitude control: 0.00 0.00 0.00

Spares/growth/margin: 3.50 0.00 3.50


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Radiation shielding: 0.00 0.00 0.00

Science: 0.00 0.00 0.00

Crew: 0.50 0.50 0.00

Total estimate: 53.00 24.50 28.50

Table 5. Mars Transit/Surface Habitation Element

Mass Breakdown of estimate for Mars Surface Lab Element


(mt)

Consumables Subtot

(mt)

Dry Mass Subtot

(mt)

Phys/Chem Life support : 4.00 2.00 2.00

Plant growth: 3.00 1.00 2.00


Health care: 0.00 0.00 0.00

Structures: 10.00 0.00 10.00

EVA: 1.50 1.00 0.50

Electrical power distribution:

0.50 0.00 0.50







Science: 3.00 Unknown 3.00

Crew: 0.00 0.00 0.00

Total estimate: 38.50 11.50 27.00

Table 6. Surface Lab Habitation Element

Mass Breakdown of estimate for Earth Return Habitation Element


(mt)

Consumables Subtot

(mt)

Dry Mass Subtot

(mt)


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Life support : 4.00 2.00 2.00


Health care: 1.50 0.50 1.00

Structures: 8.50 0.00 8.50EVA: 1.00 0.50 0.50

Electrical power distribution: 0.50 0.00 0.50





Crew: 0.50 0.50 0.00



Science/greenhouse/misc.: 0.00 0.00 0.00

Total estimate: 29.50 8.50 21.00

Table 7. Earth Return Habitation Element

7. Surface Systems

Surface Mission Overview

The principle was established at the beginning of the Mars Exploration Study that thetechnical benefits of Mars exploration would be heavily weighted toward those thingsthat people could constructively accomplish on the surface of the planet. Although thetrip there and back will be rigorous and will require substantial planning and good useof technology to reduce risk, the vast majority of the important exploration tasks arethose that are accomplished on the planet's surface. For that reason, emphasis in thisstudy has been placed on the definition of the surface system. As few previous studies

have addressed these surface mission issues in depth, surface mission concepts are notas advanced as space transportation issues. But the resolution of the surface missionissues is essential also to the space transportation question, because they tend todominate the requirements for transportation of hardware and crew to Mars' surface.

(a) Implications of Mission Objectives


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There are typically a set of difficulties that arise in defining and justifying a particularset of surface mission activities. These arise from an interaction of what is desiredversus what is feasible. This requires that the final definition be approached eitherfrom both perspectives simultaneously or iteratively. Both techniques will be used inthis study. Probably, at this point in the reference mission design, the set of surfaceactivities is too demanding, and will have to be scaled back somewhat. The first stepin this process is to analyze in more detail the implications of the mission objectivesthat have been adopted (Table 2).

(1) Conduct human missions to Mars

From the point of view of the surface mission, this implies that the capability forhumans to live and work effectively on the surface of Mars must be demonstrated,with several sub-objectives. These include defining a set of tasks of value for humansto perform on Mars and providing the tools to carry out the tasks; supporting thehumans with highly reliable systems; providing a risk environment that will maximizethe probability of accomplishing mission objectives; and providing both the capabilityand the rationale to continue the surface exploration beyond the first mission. Thisthen requires a set of functional capabilities on the surface, including habitats, surfacemobility systems, and supporting systems such as power and communicationssystems.

(2) Conduct applied science research to use Mars resources to augment life-sustainingsystems

This objective will require that an assessment be made of the location and availabilityof specific resources, such as water; and that effective systems designs be developedand demonstrated to extract and utilize indigenous resources, including operating thesystems beneficially. As demonstrations, there are opportunities to use indigenousresources in the life support system, in energy systems as fuel or energy storage, andas propellant for spacecraft. These may develop into essential systems for thepreservation of the outpost as the outpost evolves. To the support facilities identifiedin the previous paragraph must be added exploration systems (orbital or surface),resource extraction and handling systems and additional systems for recycling waterand air and producing food.

(3) Conduct basic science research to gain new knowledge about the solar system'sorigin and history

This will require that a variety of scientific explorations and laboratory assessments becarried out on the surface of Mars, both by humans and robots. The science problemswill not be assessed completely at any one site, so this requirement implies


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considerable crew member mobility and transportation systems to support exploration,as well as the specialized tools required outside the outpost to collect and documentmaterials and the facilities inside the outpost to perform analyses.

(4) Surface System Definition Philosophy - Safety Philosophy

The ability to define a robust surface capability that supports the reference missionobjectives requires that a design approach be accepted which balances performance,risks, and costs. It is evident that the priorities that must be established for the surfacemission, as for the entire mission are: (1) The health and safety of the crew is the top priority for all mission elements and operations; Life-critical systems are thoseabsolutely required to insure the crew's survival. Life-critical systems will have twobackup levels of functional redundancy; if the first two levels fail, the crew will not bein jeopardy, but will not be able to complete all mission objectives; (2) Completingthe mission as defined, to a satisfactory and productive level (mission-critical).Mission critical objectives will have one backup level; and (3) Completing additional,possibly unpredicted (mission-discretionary) tasks which add to the total productivityof the mission. Mission discretionary systems will not jeopardize the crew if they fail,but need not have a backup. The backup systems may be provided by either realredundancy (multiple systems of the same type) or functional redundancy (systems ofdifferent type which provide the required function). Recoverability or repairability bythe crew will provide yet additional safety margins.

This risk approach provides a framework for defining the overall surface system,which is robust with respect to safety and performance. The strategy adopted for theprincipal Life-Critical systems of the reference mission is shown in Table 8.

Primary Backup #1 Backup #2

Habitable volume Habitat #1 Habitat #2 Pressurized Rover

Air and water Life Support System #1 Life Support System #2 Consumable Cache

Power Power Unit #1 Power Unit #2 Power Unit #3

Food/food preservation Supply #1 Supply #2 Emergency Supply

Table 8. Principal Functions of Life-Critical Systems and Safety Strategy

In the reference mission, a habitat and pressurized rover are delivered and checked outprior to the departure of the crew on the first human mission. The crew arrive in asecond habitat. Each habitat is equipped with a life support system capable ofproviding for the entire crew for the duration of their surface stay. The concept of alife-support cache is derivative from the objective/assumption that indigenous


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resources will be extracted and utilized in the strategy from the beginning of theprogram. The reference mission thereby utilizes a system to produce methane, oxygenand other consumables from Martian resources, and verifying these caches prior to thecrew departing from Earth. In the reference mission, all food is brought from Earth.An experimental bioregenerative life support system capable of producing a smallamount of food is included as a mission-critical element; however, the crew will notdepend on it for their sustenance. In earlier versions of the reference mission, anenergy cache was considered as the second backup to the power system. However,such a backup apparently requires too large an initial power system, if it is to bemanufactured on the required schedule, and has therefore been replaced by aredundant power system.

Principal Elements of the Surface Mission

(a) Surface Mission Objectives

The principal science objectives for Mars exploration is determining:

Is Mars a home for life - in the past, present or future?

This set of objectives will combine field and laboratory investigations in geology, paleontology, biology and chemistry. The underlying assumption is that theseproblems will not have been solved by previous robotic Mars exploration programsand the optimum manner to solve them is through judicious use of humans at Mars asfield geologists and laboratory analysts.

What are the origins of the planet Mars and what does it tell us about Earth?

This set of objectives involve geology and geophysics, atmospheric science,meteorology and climatology, and chemistry. They will also require iterative samplingof geological units as well as monitoring of a global network of meteorologicalstations. The global network will most likely be established by robotic elements of theprogram.

What resources are available on Mars?

The location and general accessibility of resources on Mars will be determined by theseries of robotic missions; however, in detail, understanding the extent and utility ofthe resources may require the presence of humans. The first missions will require thatresources be extracted only from the atmosphere, which is well-enough known forthat purpose. Subsequent missions may utilize other resources, including indigenous


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water. The resource discovery and verification of accessibility will requireinvestigations in geology, atmospheric science and chemistry.

The targeted investigations to be carried out from the Mars outpost depend on anincreasing range of accessibility from the outpost by humans and automated

rover/sample collectors. A general geological map of the region of the outpost siteshould have been prepared by robotic missions prior to selecting and occupying theinitial site. Field investigations carried out by crews on Mars will address detailedquestions requiring access to varied terrain and rock types. The reference missionincludes provision for two pressurized rovers, eventually allowing traverses of up to500 kilometers range from the outpost. It also includes two smaller, instrumentedrovers which can be teleoperated from the outpost as necessary to document andcollect samples for analysis in the outpost laboratory or for return to Earth.

The habitation objectives of the Mars outpost include:

Insuring that Martian habitability has no fundamental limitations due touniquely Martian characteristics such as low gravity, absence of a magneticfield, soil toxicity, or the radiation environment.

Demonstrate that self-sufficiency can be achieved on the local scale of a Marsbase. This includes the provision of a reasonable quality of life and reasonablylow risk for the human crews.

Determine the potential for expansion of base capabilities using indigenousresources.

Investigate the biological adaptation to Mars over multiple generations ofrepresentative plant, animals and microbial species.

Assay the volatile inventory of Mars in regions easily available to humanactivities.

These habitation objectives are aimed at establishing the feasibility and approachrequired to move beyond the exploratory phase toward the development of long-termactivities on the planet. They influence the selection of elements that are included inthe surface systems, including habitats, mobility, life support, power andcommunications systems.

(b) Human Factors and Crew Size

Humans are the most valuable mission asset for the Mars exploration program, andmust not become the weak link. The requirement that humans spend on the order of600 days on the surface places unprecedented requirements on the people and theirsupporting systems. Once committed to the mission on launch from Low Earth Orbit,the crew must be prepared to complete the full mission without further resupply from


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Earth. Their resources are either with them or have already been delivered to orproduced on Mars. No further resupply is available and return to Earth in substantiallyless time than the nominal mission is not possible. Crew self-sufficiency is requiredbecause of the long duration of their mission and by the fact that their distance fromEarth impedes or makes impossible communications and control by controllers onEarth. The crews therefore need their own resources (skills, training) and specializedsupport (systems) to meet the new challenges of the missions. However, unlimitedresources can not be provided within the constraints of budgets and mission performance, so tradeoffs must be made between cost and comfort, as well asperformance and risk. Because the objectives of the missions are to learn about Marsand its capability to support humans in the future, there will be minimum level ofaccomplishment below which a viable program is not possible. Survival of humans onthe trip there and back is an insufficient program objective.

Basic human survival factors for the crew include adequate shelter, includingradiation protection; breathable, controlled, uncontaminated atmosphere (in habitats,suits, and pressurized rovers), food and water, medical services, psychologicalsupport, and waste management. In the 4-6 month transits to Mars, the chief problemswill be on maintaining interpersonal relationships needed for crew productivity, andmaintaining physical and mental conditioning in preparation for the surface mission.On the surface, the focus of crew concerns will turn to their productivity in a new andhazardous environment. The transit environment is likely to be a training andconditioning environment, the surface environment is where the mission-critical taskswill be done. Mental health as well as physical health will be crucial to accomplishing

the mission.For long-duration missions, with inevitably high stress levels, the trade-off betweencost and crew comfort must be weighed with special care. The development of highquality habitats and environmental design features are critical to assuaging stress andincreasing crew comfort - conditions that will greatly increase the likelihood ofmission success. Providing little more than the capability to survive invites missionfailure.

Not all amenities need be provided on the first mission. The program should be

viewed as a sequence of steps which, over time, will increase the amount of physicalspace on the surface, increase the amount of free time by the crew, increase theamount of crew autonomy, improve the quality of food, increase access to privacy,increase the quality and quantity of communications with Earth. In addition,experience in Mars surface operations may reduce some of the stresses associatedwith the unfamiliarity of the environment.


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The quality of life can be facilitated by access to indigenous resources. In the nearterm, use of indigenous resources reduces some of the mission risks (creation ofcaches, use of local resources for radiation shielding). In the long term, use of localresources may allow more rapid expansion of usable space. Achieving the capabilityto produce water and oxygen may have physical and psychological benefits overcontinued recycling. For example, reducing limitations on water utilization forhygiene purposes will be psychologically supportive. The ability to grow food on sitealso has a psychological effectiveness. The psychological impacts of thesedevelopments is difficult to quantify, however real the effects may be.

The number of crew to be taken to Mars is an extremely important parameter formission design, as many of the systems used (e.g. habitats) will scale directly to thenumber of crew. A progress report was given on the Ames Research Center's study ofthe minimal size crew needed to achieve the combined science and habitabilityobjectives of the Mars surface mission. For this study it was assumed that crew healthand safety are of first priority in successfully achieving the mission objectives and thatthe surface system design requirements for operability, self-monitoring, maintenanceand repair will be consistent with the identified minimum number of crew persons.This was done in a top-down manner (objectives => functions => skills => number ofcrew members + system requirements) as the systems have not been defined in abottoms-up manner based on an operational analysis of the system.

A workload analysis was carried out assuming that the crew's available time would bespent either in scientific endeavors or in habitation-related tasks. From these analyses,lists of required skills were developed. At a summary level, the five most relevanttechnical fields required by the exploration and habitation requirements includemechanical engineer, electrical and electronics engineer, geoscientist, life scientist,and physician/psychologist. It is assumed that these are important enough that theyshould be represented by a specialist, with at least one other crew person being cross-trained as a backup. A wide variety of tasks would have to be handled by each crewmember, including support tasks as well as tasks of command and communications. Itis assumed that technical individuals would be cross-trained for these responsibilities.

The result of the functional analysis indicates that the surface mission can be

conducted with a minimum crew size of five, based on technical skills required.However, loss or incapacitation of one or more crew could significantly jeopardizemission success. Therefore, a minimum crew size of seven or eight may be required toaddress the risk issues. Currently, the reference mission is built on the assumption of acrew of six.

There is an immature understanding of the manner in which the crew would besupported by intelligent robots and automated systems. The work load analysis


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indicates that the total amount of time spent in the field (on EHA by foot or in a rover)by a crew scientist will be 10-20% of the amount of their time on Mars. Thus, itappears that automated or teleoperated rovers, capable of extending the effective fieldtime by crew members, will be a good investment from the point of view of totalmission productivity. Progress being made currently in telerobotic operations of arover in the Antarctic environment can be translated directly to Mars explorationcapability.

(c) Life Support Systems

The life support system for the Mars surface is an integral part of the architecture ofthe mission, and must be viewed both in term of its requirement to maintain the healthand safety of the crew as well as to prepare for eventual self-sufficiency of a Marsoutpost. Solutions to design issues must also keep in mind limitations of the deliverysystems. The life support system for extended duration systems must minimizeconsumable supply and resupply from Earth. Approaches that address thisrequirement include the utilization of indigenous resources and creation of caches ofconsumables, and highly regenerative systems that reuse consumables brought fromEarth. The availability of consumables in the Martian atmosphere, and potentiallyfrom surface or subsurface deposits, can influence the degree of closure that isadopted for the system.

Indigenous resources (oxygen, nitrogen, water) are extracted from the Martianenvironment and provide caches of consumables for the life support system as well asproviding fuel and oxidizer for the space transportation system. For the first mission,all food and a supply of hydrogen will be transported from Earth. An experimentalbioregenerative life support system capable of providing a fraction of the food couldallow some of the food brought from Earth to be retained in the food cache.

The proposed approach to producing oxygen and other gases from the atmosphereconsists of pumping Martian atmosphere through a reactor, removing the inertnitrogen and argon (about 2% of the t

Mars Exploration Strategy

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