AIAA 93-4212

MARS EXPLORATION STRATEGIES:
A REFERENCE PROGRAM AND COMPARISON 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 be considered only by the grandchildren of today's generation of planetary exploration enthusiasts. The basis for this grim prediction centers around the high projected costs of such missions, the necessity of first establishing a permanent presence in low-Earth orbit and then establishing an outpost on the Moon, and the seemingly insurmountable technical obstacles that such an endeavor presents. This paper presents an overview of the results of a year-long NASA-wide effort to consider innovative strategies for the human exploration of Mars. The result of these efforts has been the development of a design reference mission (DRM) that directly challenges the prevailing obstacles generally associated with human missions to Mars and creates a strategy that could enable 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 on the surface of Mars, rather than focusing all resources on the trip to and from the planet. This strategy enhanced the mission return, improved the safety of the crew, and reduced or eliminated many of the obstacles associated with conventional strategies for the human exploration of Mars. In particular, the Mars DRM described here permits crews to explore the surface of Mars for nearly 500 days on the first and all subsequent missions, while limiting their exposure to the interplanetary space environment to periods near those of US. experience on Skylab and well within Russian experience from Mir. This surface-oriented philosophy emphasized the development of high-leveraging surface technologies in lieu of concentrating exclusively on space transportation technologies and development strategies. Thus, the DRM relies on the in-situ production of CH4/O2 propellant for the crew's ascent vehicle and surface mobility, as well as the necessary water and life support gases for the crew's entire surface stay, thereby permitting a significant decrease in the amount of material that must be delivered to Mars from Earth, while simultaneously increasing the productivity of the crew on the surface and their safety. By emphasizing a robust suite of surface capabilities and high-leverage technologies, the Mars study team was able to consider abort-to-the-Mars-surface as a an alternative to the traditional trajectory abort options. The DRM, consistent with this risk-mitigation philosophy, also employs a single common habitat design for use during the interplanetary 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 to mission and crew risk, element commonality, and technology develop-ment, human missions to Mars can be accomplished without the need for complex assembly operations in low-Earth orbit. Both cargo and human missions are launched direct to Mars with rendezvous occurring on the planetary surface. The associated reduction in total launch mass allows the first crew of six to explore Mars within a total of four launches of a Saturn VII launch vehicle, and continuing missions to Mars to be conducted with only three Saturn VII launches every Mars opportunity (every 26 months).


I. Introduction

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

- 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 fourth planet from the Sun have presented formidable engineering and fiscal challenges that have created the presumption that the human exploration of Mars is a goal for future generations, if achievable at all. Conventional plans[2,3,4] for these missions have included many or all of the characteristics presented in Table 1.

Table 1 - Conventional Mars Exploration Program Characteristics

The Mars Exploration Study Project was undertaken by the Exploration Programs Office (now, Planetary Projects Office) in response to the strategic planning initiatives of the Associate Administrator for Exploration at NASA Headquarters, in the summer of 1992. The purpose of the study, as viewed by the Associate Administrator for Exploration, was to establish a vision for the human exploration of Mars that would serve as a mechanism for understanding program and technical requirements that would be placed on existing and planned Agency programs, including a precursor lunar exploration program, which was being developed by the Office of Exploration (The First Lunar Outpost - FLO). Emphasis was to be placed on determining potential commonality between Mars and Moon exploration programs so that total program costs for both programs might be minimized and so the lunar program would not contain dead ends which would be difficult or expensive for the Mars program to correct if both programs were to be carried out sequentially.

The study team chose to take an approach that emphasized the important aspects of Mars 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 for Mars, then the evolution of a FLO-like lunar program could be planned that would optimize 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

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

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

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 that compelling arguments can be made to the public and leaders of those nations who might participate in a Mars exploration program, to justify the expenditures that will be required.

Reflecting the conclusions of the workshop, the following technical goal was adopted for 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-sustaining systems; and (3) Conduct basic science research to gain new knowledge about the solar system's origin and history. Conducting human missions to Mars is required to accomplish the exploration and research activities, but contains the requirements for the safe transportation, maintenance on the surface of Mars and return of a healthy crew to Earth. The surface exploration mission envisions approximately equal priority for applied science research - learning about the environment, resources, and operational constraints that would allow humans eventually to inhabit the planet; and basic science research - exploring the planet for insights into the nature of planets, the nature 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 defining the required elements and operations for the Mars exploration program.

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 for diverse technical personnel to collectively integrate their definition and design efforts around a baseline strategy. This allows people with innovative concepts to compare their approach on a direct basis. However, it is particularly important to establish a set of mission accomplishments that must be met by alternative scenarios. This is a major step in documenting the expected benefits of such an exploration program. Second, establishing a reference mission allows the formulation of a technically credible approach, with appropriate documentation of the technical and programmatic risks, which can form the basis for defensible cost estimates for the program. Previous studies of Mars missions have been associated with rather high costs, but with little visibility into the assumptions and approaches to developing the costs. Developing the reference mission provides a starting point for cost analysis, which can identify important programmatic or technical problems whose solution can reduce the overall cost and risks of the program. Likewise, the reference mission provides a basis for analyzing the importance of technology development and new data which can be gathered in advance of the human exploration mission design. Finally, the reference mission provides a basis for understanding potential international cooperative approaches 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 the human and robotic exploration of Mars while addressing fundamental science questions 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 program that will cost hundreds of billions of dollars.
  2. Challenge the traditional technical obstacles associated with sending humans to Mars.
  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 and productively 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 for the first or first few human exploration missions and focussed their attention principally on space transportation; (2) Limit the length of time the crew is continuously exposed to the interplanetary space environment. In doing this, the physiological and psychological degradation to the crew is reduced, thereby enhancing both crew safety and mission return. In addition, the associated life science concerns are partially mitigated, reducing the requisite scope of any crew certification program; (3) Provide an operationally simple mission approach that emphasizes the judicious use of common systems. Because an integrated mission in which a single spacecraft is launched from Earth and lands on Mars to conduct the long exploration program is not feasible, it is necessary to determine the simplest and most reliable set of operations in space or on the surface of Mars to bring all of the necessary resources to the surface where they are to be used. A strategy emphasizing multiple uses for a single system development potentially enhances not only the total program costs, but also crew safety and system maintainability; (4) Provide a flexible implementation strategy. Mars missions are complex, so that multiple pathways to the desired objectives 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 in technical and fiscal feasibility of these missions, as well as the acceptability of the mission 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 contingency situation may be the most challenging obstacle to be overcome in embarking upon the human 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. Although efficient and reliable space transportation elements are a critical component of any planetary exploration strategy, the exploration goal adopted in this study suggests the need to be able to "live off the land." Thus, the surface capability must provide a comfortable, productive, reliable, and safe place for the crew. This, in turn, changes the risk perspective with respect to previous studies by relieving the pressure on the space transportation systems to resolve any and all contingencies. Whereas in previous studies, many mission contingencies resulted in trajectory aborts (direct returns to Earth),[5,6] another option exists in this reference mission, namely, abort to Mars' surface. This allows the mission design to focus on the surface capability, not on the provision of costly propulsive performance increases and redundant systems to be 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-return abort and powered abort maneuvers do not come for free at Mars. The goal of the human portion of the space transportation function should be to deliver the crew to and from Mars with the least reasonably achievable exposure to the hazards of the space environment. Trajectory aborts, far from being presumed requirements for human missions to Mars, should have to fight their way into a reference mission as a last resort. By emphasizing the capabilities available to the crew on the surface of Mars, 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, in which cargo is transported in manageable units to the surface and checked out in advance of committing crews to their missions. Each cargo and human flight to Mars is launched in a single FLO-class (i.e. ~240 t. to 500 km. circular LEO) heavy-lift launch vehicle which contains a nuclear thermal propulsion (NTP) upper stage that injects the entire HLLV payload on a direct trajectory to Mars. This approach provides a basis for continued expansion of capability at the outpost through the addition of modules to the original systems. The split mission approach also allows the crews to be transported on faster, more energetic trajectories, minimizing their exposure 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 launch opportunities occurring in 2007 and 2009. There were two reasons for selecting these dates. First, 2009 represents the most difficult opportunity in the 15-year Earth-Mars cycle. By designing the space transportation systems for this opportunity, particularly those associated with the human flights, the systems can be flown in any opportunity with either additional performance margins or with improved performance (e.g. faster transit times for the crew or increased payload delivery capacity). For example, where the trans-Mars and trans-Earth crew transfers in the 2009 opportunity are 180 days, in the easier 2018 opportunity they are 120 days each way for the same vehicle. The second reason for designing to these opportunities is that they lie ~15 years in the future, a time considered by the authors to be a reasonably robust goal for committing humans to Mars. As an additional design consideration, the Mars Study Team adopted the ground rule that the study would examine three human missions to Mars. Each mission returns to the site of the initial mission, with missions two and three launching in the 2012 and 2014 launch opportunities, respectively. This approach permitted the Team to consider an evolutionary establishment of capabilities on the Mars surface and was fundamentally more consistent with the stated goals for the human exploration of Mars. Although it is certainly arguable that science return could be 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 logically directed 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 trajectories direct to Mars (i.e. without assembly or fueling in low Earth orbit). Table 3 identifies the 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 this stage and return to Earth after completion of their surface exploration in October 2011. The second launch delivers a descent vehicle to Mars orbit which will deliver its payload of a dry Mars ascent stage and crew module (MAV), a propellant production module, a nuclear power plant, liquid hydrogen (to be used as a reactant to produce the ascent vehicle propellant), and approximately 40 metric tons of additional surface payload to the surface. After the descent stage lands on the surface in late August, 2008, the nuclear reactor autonomously deploys itself several hundred meters from the ascent vehicle and the propellant production facility begins to produce from the 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, this production is completed within approximately one year - several months prior to the first crew's scheduled departure from Earth in mid-November 2009. The third launch in the 2007 opportunity, delivers a second descent vehicle to Mars orbit which will deliver its payload of a surface habitat/laboratory, non-perishable consumables for a safe-haven, and a second nuclear power plant to the planetary surface. It descends to the surface in early September 2008, landing near the first descent vehicle. The second nuclear power plant autonomously deploys itself nearby the first plant. Each of the two plants provides sufficient power (i.e. 160 kWe) for the entire mature surface outpost, thereby providing complete redundancy within the power function.

Figure 1

Figure 1. Mars Exploration Reference Mission Overview
2007 Launch Opportunity - Cargo Missions

Figure 2

Figure 2. Mars Exploration Reference Mission Overview
2009 Launch Opportunity - Cargo & Human Missions

Figure 3

Figure 3. Mars DRM Timeline

Table 3: General Launch Manifest - 2007 Launch Opportunity
Flight 1: CargoFlight 2: CargoFlight 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
  • Surface Habitat/Laboratory
  • non-perishable Consumables
  • Power Supply (nuclear-160kW)
  • Utility Truck
  • Spares
  • Teleoperable Science Rover
Mars Orbit Payload
  • Earth Return Vehicle
    • Fueled (LOX/CH4) TEI Stage
    • Transit Habitat
    • Earth Return Capsule
Mars Orbit Payload
  • none
Mars Orbit Payload
  • none
Space Transportation Vehicles
  • NTR Transfer Stage
  • LOX/CH4 TEI Stage w/Mars aerobrake
Space Transportation Vehicles
  • NTR Transfer Stage
  • LOX/CH4 Descent Stage w/Mars aerobrake
  • LOX/CH4 Ascent Stage
Space Transportation Vehicles
  • NTR Transfer Stage
  • LOX/CH4 Descent Stage w/Mars aerobrake

In the second opportunity, October 2009, two additional cargo missions and the crew mission are launched. All assets previously delivered to Mars are checked out and the ascent vehicle is verified to be fully fueled before either the crew or the additional cargo missions are launched from Earth. The two cargo missions described in Table 4 are launched prior to the crew mission. The first launch is a duplicate of launch one from 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. These systems provide backup or extensions of the previously deployed capabilities. For example, the second Mars ascent vehicle and second Earth return vehicle provide the 2009 crew with two redundant means for each leg of the return trip. If, for some reason, either the first ascent stage or the first Earth-return stage become inoperable after the first crew departs Earth in 2009, the crew can use the systems launched in 2009 instead. They will arrive in plenty of time to be available for the crew's departure from Mars in October 2011. If the ascent and Earth-return vehicles delivered in 2007 operate as expected, then the systems delivered in 2009 will support the second crew of six that will launch to Mars early in 2012. Subsequently, one piloted mission and two 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: CargoFlight 5: CargoFlight 6: First Crew
Surface Payload
  • none
Surface Payload
  • Ascent Capsule
  • Empty Ascent Stage
  • LOX/CH4 Production Plant
  • LH2 Propellant Seed
  • Bioregenerative Life Support Outfitting Equipment
  • Science: 1km drill
  • Science Equipment
  • Additional Payload/Spares
Surface Payload
  • Crew
  • Surface Habitat
  • Consumables
  • Spares
  • EVA Equipment
  • Science Equipment
Mars Orbit Payload
  • Earth Return Vehicle
    • Fueled (LOX/CH4) TEI Stage
    • Transit Habitat
    • Earth Return Capsule
Mars Orbit Payload
  • none
Mars Orbit Payload
  • none
Space Transportation Vehicles
  • NTR Transfer Stage
  • LOX/CH4 TEI Stage w/Mars aerobrake
Space Transportation Vehicles
  • NTR Transfer Stage
  • LOX/CH4 Descent Stage w/Mars aerobrake
  • LOX/CH4 Ascent Stage
Space Transportation Vehicles
  • NTR Transfer Stage
  • LOX/CH4 Descent Stage w/Mars aerobrake

The first crew of six departs for Mars in mid-November 2009. They leave Earth after the two cargo missions launched in 2009, but because they are sent on a fast transfer trajectory 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 Mars surface. The transit habitat sits atop an identical descent stage as those used in the 2007 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 other elements of the surface outpost (see figure 3). There is no required rendezvous in Mars orbit prior to the crew descent. This is consistent with the risk philosophy inherent in the Mars DRM. Once the TMI burn has been completed, the crew must reach the surface of Mars. The crew carries with them sufficient provisions for the entire 540 day surface stay in the unlikely event that they are unable to rendezvous on the surface with the assets previously deployed.

After their stay on Mars, the crew uses the previously landed ascent vehicle to return to orbit, rendezvous with the Earth return vehicle, and return to Earth. Like the outbound 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 4 illustrates, has been in an untended mode for nearly four years prior to the crew arrival. As more fully discussed below, this strategy for habitation permits the development of only one major habitable element, thereby reducing costs, lessening spares/logistics quantities, limiting the number of unique systems with which the crew must be familiar, and enhancing maintainability by the crew. Figure 4 presents a pictorial overview of the Mars DRM.

The major distinguishing characteristics of the design reference mission, compared to previous concepts, include: (1) No extended low-Earth orbit operations, assembly or fueling; (2) No rendezvous in Mars orbit prior to landing; (3) Short transit times to and from Mars (180 days or less) and long surface stay times (500-600 days) for the first and all subsequent crews exploring Mars; (4) A heavy lift launch vehicle, capable of transporting either crew or cargo direct to Mars, and capable of delivering all needed payload with a total of 4 launches for the first human mission and three launches of cargo and crew for each subsequent opportunity; (5) Exploitation of indigenous resources from the beginning of the program, with important performance benefits and reduction of mission risk; (6) Availability of abort-to-Mars'-surface strategies, based on the robustness of the Mars surface capabilities and the cost of trajectory aborts; (7) Common transit/surface habitat design; (8) A common set of space transportation vehicles and heavy lift launch vehicles, capable of transporting either crew or cargo to Mars; and (9) No presumed reliance on a previously emplaced lunar 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 trip times to Mars in order to reduce the crew's exposure to the zero gravity and space radiation environments. Two such options have been proposed for achieving these quick trip times: opposition-class missions and fast transfer conjunction class missions. Mars mission classes are generally characterized by the length of time in the Mars system and the total round-trip mission time.[7,8] The first of these is typified by short Mars stay-times (typically 30-90 days) and relatively short round-trip mission times (400-650 days). This is often referred to as an opposition-class mission, although the authors have adopted the terminology "short-stay" mission. The trajectory profile for a typical short-stay mission is shown in Figure 5. This class has higher propulsive requirements than the long-stay missions and typically requires a gravity-assisted swingby at Venus or the performance of a deep-space propulsive maneuver in order to reduce total mission energy and constrain Mars and/or Earth entry speeds. Short-stay missions always have one short transit leg, either outbound or inbound, and one long transit leg, the latter requiring close passage by the sun (0.7 AU or less). The second mission class consists of long-duration Mars stay-times (as much as 500 days at Mars) and long total round-trip times (approximately 900 days). This mission type is often referred to as conjunction-class, although the authors have previously adopted the more descriptive terminology "long-stay" mission. These represent the global minimum-energy solutions for a given launch opportunity. The trajectory profile for a typical long-stay mission is shown in Figure 6.

Figure 5

Figure 5: Typical short-stay Mission Profile

Figure 6

Figure 6: Typical long-stay Mission Profile

Within the long-stay category of missions the option exists to dramatically decrease the transit times to and from Mars through moderate propulsive increases.[3] The total round-trip times remain comparable to those of the minimum-energy, long-stay missions, but the one-way transits are substantially reduced, in some cases to less than 100 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 are radically different. The profile for a typical fast-transit mission is shown in Figure 7.

Figure 7

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 the Mars DRM. Finally, for programmatic reasons, the trajectory class selection must provide the flexibility to conduct missions in all Earth-Mars opportunities within the 15-year cycle and for conducting missions supporting the evolution of Mars exploration objectives and implementation strategies.

The applicability of each of the previously discussed mission types to the human exploration of Mars has been the subject of much debate. The opinion has generally been held that the initial flights should be short-stay missions performed "as fast as possible" (so-called "sprint" missions), ostensibly to minimize crew exposure to the zero-gravity and space radiation environment, to ease requirements on system reliability, and to enhance the probability of mission success. However, when considering "fast" Mars missions, it is key to distinguish whether one is referring to fast round-trip or fast transit missions. In fact, past analyses have shown that decreasing round-trip mission times for the short-stay missions does not equate to fast transit 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 for the long-stay missions. This point becomes clear when looking at Figure 8 which graphically displays the transit times as a function of the total round-trip mission duration. 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.

Figure 8

Figure 8: Round-trip Mission Comparisons

The Mars Study Team adopted the use of the fast transit missions based on the factors listed above. First, verifying the ability of people to inhabit Mars requires more than a brief 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 be productively spent on the Mars surface due to the crew adaptation to the Mars gravity, crew preparations for Mars departure, etc.) was considered unacceptable. The Mars Study Team concluded following the August 1992 Workshop that a "Plant the Flag" mission objective was not a tenable rationale to support the substantial investment involved.

The Mars DRM risk philosophy mandated limiting the crew's exposure to the interplanetary space environment, looking instead to the facilities on Mars to safely support 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 planners consists of two components: GCR and solar particle events (SPEs)) and the zero-gravity environment and their effects on human physiology.[9] They will not be discussed in detail in this paper. However, a brief comparative discussion of these critical environmental issues and their impact on mission class is warranted. Figure 9 illustrates a relative comparison of the representative galactic cosmic radiation (GCR) exposure experienced by a crew on each of the three trajectory options.[10] Because both 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 exceed the estimated 50 rem annual dose limits. Conversely, since the crew in a short Mars stay mission spends virtually the entire mission duration in the GCR environment, a crew on these missions could receive radiation doses which not only exceed the annual dose limits, but also exceed the total dose received by a crew on a fast transit missions. It is prudent to stress the preliminary nature of the dose calculations reported above. It is best to consider a relative comparison of the trajectory types. Calculations are very dependent on shielding assumptions, the mission opportunity, and the current state of knowledge regarding the interaction and transport of GCR through material and the human body. Indeed, although none of the mission dose estimates violate the current NASA career standards, recent studies have suggested that 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, unless dispensation is given for exploration missions.

Figure 9

Figure 9: Radiation Exposure Comparisons for Various Mission Classes

A similar analysis of mission classes is involved in considering the crew's exposure to the zero-gravity environment during transits to and from Mars. Significant physiological 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-term zero-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. In neither case were crews exposed to zero-g/partial-g/zero-g sequences similar to that projected for Mars missions. Upon arrival on the Martian surface, the crew must spend some time readapting to a partial-gravity field. Current data indicates that recovery in a 1-g environment can be fairly rapid (on the order of a few days), but development of full productivity could require significantly more time. This may be of concern for the short-stay missions where a substantial portion of the surface stay time could be consumed by crew adaptation to 0.38 g's. Conversely, ample time will be available for the crew to regain stamina and productivity during the long surface stays associated with the minimum-energy and fast-transit missions.

Several potential solutions to the physiological problems associated with zero-gravity transits to and from Mars may exist, including: countermeasures (exercise, body fluid management, lower body negative pressure), artificial-gravity spacecraft, and reduced transit times. The usefulness of countermeasures to reduce some of the zero-gravity effects is still unknown. Soviet long-duration crews have experienced physiological degradation even when rigorous exercise regimens have been followed. However, most of these effects seem to be quickly ameliorated upon return to a 1-g environment, at least when immediate medical aid is available.

Rotating the Mars Transfer Vehicle (MTV) is a method of providing an artificial gravity environment for the crew and is most often associated with low-performance propulsion systems, or the short-stay class of trajectories (since both require long transit times). Studies have indicated that the MTV design mass penalties are on the order of 5-20% if artificial-g is incorporated.[11] Depending upon the specific configuration, there may also be operational complications associated with artificial-g spacecraft 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, and short-stay missions. Note that all one-way transits are within the Soviet zero-gravity database.

Figure 10

Figure 10: Microgravity Comparison for Various Mission Classes

However, the inbound and outbound transits for short-stay missions are typically separated by only one to three months. It is questionable whether such a short time spent 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 by normal Space Station operations prior to executing human interplanetary missions. Also, note that the fast-transit mission's zero-gravity transfer legs are separated by a substantial period of time in the Martian gravitational field. This long period on the surface of Mars should prove sufficient to ameliorate the physiological effects of the relatively short outbound transit.

Finally, the selection of trajectory type depends upon its ability to flexibly respond to mission 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 missions generally require the use of a Venus swingby maneuver in order to keep propulsive requirements within reason. However, these swingbys are not always available on the return transit leg and must be substituted in the outbound transit leg. Since the transit leg with the Venus swingby is the longer of the two, the result is to have the crew spending up to 360 days on the trip to Mars, with any associated physiological degradation occurring at the beginning of the mission - i.e. prior to the crew's arrival at Mars. These variations can result in significant configurational impacts to the Earth-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 long Mars-stay missions reflect only moderate variations across the same 15-year cycle. In addition, neither of these missions require a Venus swingby. Indeed, neither mission requires 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-stay class trajectories. However, it was decided that the amount of reduction sought in the Earth-Mars and Mars-Earth transit times must be balanced with the other considerations involved in the DRM. Reductions below 180 days in the one-way transit times (for the 2009 opportunity) would have required either significant propulsive capability improvements, or would have necessitated much larger IMLEOs for the human missions, thereby requiring assembly/docking in LEO and higher HLLV launch rates. Indeed, others have demonstrated that reductions in trip times reach a point of diminishing returns from the space transfer vehicle design perspective.[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 human missions. This results in maximum Earth-Mars transit times of approximately 180 days (2009 opportunity) and minimum transit times of approximately 120 days (2018 opportunity). Similarly, a C3 leaving Mars of ~16 km2/sec2 appears to be appropriate for human missions, resulting in similar Mars-Earth transfer times for these opportunities.

5. Space Transportation

The space transportation system consists of a trans-Mars injection (TMI) stage, a biconic aerobrake for Mars orbit capture and Mars entry, a descent stage for surface delivery, an ascent stage for crew return to Mars orbit, an Earth-return stage for departure from the Mars system, and an Earth crew capture vehicle (ala Apollo) for Earth entry and landing. As mentioned earlier, the reference program splits the delivery of elements to Mars into cargo missions and human missions, all of which are targeted to the same locale on the surface and must be landed in close proximity to one another. The transportation strategy adopted in the Mars DRM eliminates the need for assembly or rendezvous in low-Earth orbit of vehicle elements and requires a rendezvous in Mars orbit only for the crew in preparing to leave Mars. The transportaiton strategy also emphasized the use of common elements in order to avoid development costs and to provide operational simplicity. Thus, a modular space transportation architecture resulted. A complete detailed description of the space transportation architecture would be beyond the scope of this paper. Instead, below is described an overview of each of the major elements in the space transportation function. References are provided to the more detailed system descriptions, where available.

TMI Stage

The TMI stage (used to propel the spacecraft from low Earth orbit onto a trans-Mars trajectory) employs nuclear thermal propulsion. Nuclear thermal propulsion was adopted 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 was developed for both piloted and human missions. The stage is designed for the more energetically demanding 2009 human mission and then used in the minimum energy cargo missions to throw the maximum payload possible to Mars. In the human missions, 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 the trans-Mars trajectory. After completion of the two-perigee burn Earth departure, the TMI 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 the crew incorporates a shadow shield between the NDR engine assembly and the LH2 tank in order to protect the crew from the radiation from the engines that build up during the TMI burns.

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

Mars Orbit Capture and Descent Stage

Mars orbit capture and the majority of the Mars descent maneuver is performed using a single biconic aeroshell. The decision to perform the Mars orbit capture maneuver was 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. Unlike past mission concepts employing aerocapture, however, where the Mars entry speeds have been high, and the mission profile required a post-aerocapture rendezvous in Mars orbit with another space transportation element, the Mars DRM has neither of these features. Thus, the strategy employed was to drive toward the development of a single aeroshell development that can be used for both the MOC and descent maneuvers. Given the demands on a descent aeroshell of the Mars entry and landing requirements, the delta's to permit aerocapture are considered to be modest.

Figure 11.

The descent stage itself, employs four RL10-class engines, modified to burn LOX/CH4, to perform the post-aerocapture circularization burn and to perform the final ~500 m/se. of descent prior to landing on the Mars surface. The use of parachutes has been assumed to reduce the descent vehicle's speed after the aeroshell has ceased to be effective and prior to the final propulsive maneuver. A single common descent stage has been assumed for the delivery of both the surface/transit habitats as well as the ascent vehicle and other surface cargo. The descent vehicle is capable of landing ~65 metric tons of cargo on the Mars surface. When delivering crew, 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 is composed of an ascent stage and an ascent crew module. The ascent stage is delivered with its propellant tanks empty. However, the descent stage delivering the ascent vehicle includes several tanks of seed hydrogen for use in producing the nearly 30 metric tons of LOX/CH4 propellant for the nearly 5,600 meters/second required for ascent 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. The crew rides into orbit in the Earth Crew Capture Vehicle (ECCV) or in a dedicated ascent capsule. The ECCV is similar to an Apollo Command Module and is eventually used by the crew to enter Earth's atmosphere and deliver the crew safely to a 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 delivering the crew to Earth orbit. However, unlike Apollo, the ascent propellant is produced in situ, thereby substantially muting the impact of the heavier ECCV for ascent. The advantages of using the ECCV for ascent lies in the ability to eliminate a separate system development and the safety/maintainability associated with the crew having access to the ECCV during their entire surface stay, as well as their Earth-return transit.

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 is delivered to Mars orbit fully fueled, where it loiters for nearly four years before being used by the crew in returning to Earth. It uses two RL10-class engines, modified to burn LOX/CH4. Again, these are the same engines developed for the ascent and descent stages, thereby reducing engine development costs and improving maintainability. 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 of consumables in the latter habitat.

6. Mars Habitation System

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

Overview

At the core of meeting the two primary Mars exploration goals, science and human presence, the Mars habitation system seeks to provide a robust surface safety system for the crew and an environment conducive to high levels of crew self sufficiency while focusing on enabling historically cost saving decisions. Following the more mundane requirements of maintaining the required atmosphere, water and food to sustain the life of six crewmembers, a human mission to Mars creates a myriad of complex issues requiring consideration at the earliest stages of mission development. Of primary concern, the crew's physiological response to various gravity environments and severe isolation. Even considering the potential windfall of human systems 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 of projected space station missions, about four to six months. Although countermeasures such as exercise and diet, should continue to be enhanced and more thoroughly comprehended, current measures have been unable to counteract 100% of physiological degradation and minimizing crew exposure is still the best solution.

Figure 12

Figure 12. The Crew Exercise Facility is a Critical Component of a Countermeasures System Designed to Inhibit Crew Degradation from Exposure to Reduced Gravity Environments

A secondary consideration, sending the crew from the surface of Earth to the surface of Mars in the same habitation element provides an additional, and perhaps mandatory, safety feature by eliminating a rendezvous, docking and transfer procedure in Mars orbit and by allowing the crew to remain in a fully functional and outfitted habitation element for the duration of their adaptation period to the Martian gravity environment. As each individual reacts differently to these situations, and experience with partial gravities is almost non-existent, the exact time required for complete adaptation will probably remain an unknown. Eliminating the requirement for immediate post landing physical activity provides substantial benefits to the mission.

Surface Safety

As previously discussed, the options provided by planetary alignments and various trajectories, 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 to incorporate fundamental safety features, such as isolatable pressurized elements and continuous access to Extravehicular Activity suits. At the core of this discussion is the decision to physically connect [pressurized crew access] each habitable element on the surface of Mars or to leave each habitable element as a distinct unit separated by approximately 500 meters. As expected, the impacts on both sides of this issue are substantial. However, from a crew safety perspective, interconnection of the pressurized habitable elements is considered to be a primary requirement. Crew access to alternate pressurized elements substantially increases the risk mitigation options reducing an otherwise severe dependence on EVA systems and rovers.

Figure 13

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

Additional benefits of physically connecting the surface habitable elements include the option to share functions and systems between elements and missions, the ability to selectively locate sensitive consumables and hardware and a reduction of crew time devoted to logistics transfer and access to functions located in isolated habitation elements. Surface mobility becomes the technological challenge of implementing physically connected habitation elements and is discussed in the following section.

Crew Self-Sufficiency

Following completion of the recently constructed space station mission control building, the assumption can be made that station missions, like Shuttle missions, will be very closely monitored and controlled from Earth. Intricate timelines and a host of monitors accompany each Shuttle mission as the crew completes well rehearsed tasks and experiments. This philosophy must change to provide the crew every opportunity to successfully complete a three year mission to Mars. Crew self-sufficiency must become the adopted philosophy of long duration exploration missions. Clearly, the role of mission control will not be eliminated. However, communications delays and unexpected situations will shift reliance away from Earth to the Mars crew.

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 a capable machine shop with the requisite supply of raw materials as well as the required training. Additional examples range from providing extensive computer based libraries, such as the current CD-ROM technology affords to growing fresh food in greenhouse environments.

Commonality

Historical cost data indicates that commonality between large components of a system can substantially reduce development and production costs. Additionally, system commonality maintains crew familiarity and confidence thereby reducing training for both operation and maintenance. The reference mission developed by the Mars Study Team includes a singular habitation element which must function in various gravity environments. There are two gravity environments of any duration to consider in designing a habitation element, zero-gravity for four to six months during the transit phases and 3/8ths gravity for 500 days on the Martian surface. The primary issue concerning commonality and the design of habitation elements is, can a common habitation element function efficiently in both the transit and surface phases?

As commonality has substantial mission benefits, this study began with the assumption that a single, largely common element can effectively function in all phases of the Mars DRM, and subsequently set about the task of identifying any and all serious drawbacks to habitation element commonality. To date, no show-stoppers towards commonality have been identified. Development of some subsystems, particularly water systems such as ECLSS and personal hygiene systems, have substantially different requirements for zero versus partial gravity environments. As gravity can greatly simplify the design of a water system, a trade study to identify the cost to benefit ratio of duplicating a more complex zero-gravity system or providing two separate systems must be conducted.

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

Habitation Element Description

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

  1. Commonality
  2. Crew size and mission duration
  3. Repeat visits to the same site
  4. Physical connection of all pressurized elements
  5. 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 secondary structure, windows, hatches, docking mechanisms, power distribution systems, life support, environmental control, safety features, stowage, waste management, communications, airlock function and crew egress routes. From this point, there exists an endless array of feasible internal architecture designs. Each solution involves a trade of resources derived from a specific set of goals, which at this level of detail, are very open for discussion. The following brief description of the four primary habitation elements was developed for early costing purposes and in direct support of the Mars DRM analysis.

Figure 14

Figure 14. Conceptual Mars Habitation Module - Wardroom Design

Sent out, landed and verified prior to the launch of any crewmembers, the Mars Surface Lab, which will operate only in 3/8ths gravity, contains a large,. non-sensitive stowage area with crew support elements, such as waste management, on one level and a second level devoted entirely to the primary science and research lab. Future development of this element include retrofitting the stowage level into a greenhouse as consumables and resources are consumed and free volume is created[15].

The Mars Transit/Surface Habitation Elements must contain the required consumables for 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 element that must effectively operate in both zero and partial gravity. Once on the surface of Mars, this element will be physically connected with the previously landed Surface Lab doubling the pressurized volume, to approximately 1,000 cubic meters, available to the crew for the 500 day surface mission.

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

Figure 15

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

The airlock system, although integral with the habitation system, was developed as an independent 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 a major impact on the internal architecture of each element. EVA systems provide both a primary operational element as well as a critical component of the crew safety system and must be integrated into the design of a habitation system during the very early 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 capable of endless design solutions, the habitation system developed for this Mars mission focused on three critical factors: Cost savings, providing a robust safety system on the surface 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 the primary habitation elements in support of early costing exercised. The masses were estimated 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 be lower 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:6.003.003.00
Plant growth:0.000.000.00
Crew accommodations:22.5017.505.00
Health care:2.500.502.00
Structures:10.00 0.0010.00
EVA:4.00 3.001.00
Electrical power distribution:0.500.000.50
Comm and Info Mgmt:1.500.001.50
Thermal control:2.000.002.00
Power generation:0.00 0.000.00
Attitude control:0.00 0.000.00
Spares/growth/margin:3.500.003.50
Radiation shielding:0.000.000.00
Science:0.000.000.00
Crew:0.500.500.00
Total estimate:53.0024.5028.50

Table 5. Mars Transit/Surface Habitation Element

Mass Breakdown of estimate for Mars Surface Lab Element
Subsystems:Subsys Mass (mt)Consumables Subtot (mt)Dry Mass Subtot (mt)
Phys/Chem Life support :4.002.002.00
Plant growth:3.001.002.00
Crew accommodations:7.507.500.00
Health care:0.000.000.00
Structures:10.00 0.0010.00
EVA:1.501.000.50
Electrical power distribution:0.500.000.50
Comm and Info Mgmt:1.500.001.50
Thermal control:2.000.002.00
Power generation:0.000.000.00
Attitude control:0.000.000.00
Spares/growth/margin:5.500.005.50
Radiation shielding:0.000.000.00
Science:3.00Unknown3.00
Crew:0.000.000.00
Total estimate:38.5011.5027.00

Table 6. Surface Lab Habitation Element

Mass Breakdown of estimate for Earth Return Habitation Element
Subsystems:Subsys Mass (mt)Consumables Subtot (mt)Dry Mass Subtot (mt)
Life support :4.002.002.00
Crew accommodations:7.505.002.50
Health care:1.500.501.00
Structures:8.500.008.50
EVA:1.000.500.50
Electrical power distribution:0.500.000.50
Comm and Info Mgmt:1.500.001.50
Thermal control:2.000.002.00
Power generation:0.000.000.00
Attitude control:0.000.000.00
Crew:0.500.500.00
Spares/growth/margin:2.500.002.50
Radiation shielding:0.000.000.00
Science/greenhouse/misc.:0.000.000.00
Total estimate:29.508.5021.00

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 the technical benefits of Mars exploration would be heavily weighted toward those things that people could constructively accomplish on the surface of the planet. Although the trip there and back will be rigorous and will require substantial planning and good use of technology to reduce risk, the vast majority of the important exploration tasks are those that are accomplished on the planet's surface. For that reason, emphasis in this study 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 not as advanced as space transportation issues. But the resolution of the surface mission issues is essential also to the space transportation question, because they tend to dominate the requirements for transportation of hardware and crew to Mars' surface.

(a) Implications of Mission Objectives

There are typically a set of difficulties that arise in defining and justifying a particular set of surface mission activities. These arise from an interaction of what is desired versus what is feasible. This requires that the final definition be approached either from both perspectives simultaneously or iteratively. Both techniques will be used in this study. Probably, at this point in the reference mission design, the set of surface activities is too demanding, and will have to be scaled back somewhat. The first step in this process is to analyze in more detail the implications of the mission objectives that 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 for humans 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 humans to perform on Mars and providing the tools to carry out the tasks; supporting the humans with highly reliable systems; providing a risk environment that will maximize the probability of accomplishing mission objectives; and providing both the capability and the rationale to continue the surface exploration beyond the first mission. This then requires a set of functional capabilities on the surface, including habitats, surface mobility systems, and supporting systems such as power and communications systems.

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

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

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

This will require that a variety of scientific explorations and laboratory assessments be carried out on the surface of Mars, both by humans and robots. The science problems will not be assessed completely at any one site, so this requirement implies considerable crew member mobility and transportation systems to support exploration, as well as the specialized tools required outside the outpost to collect and document materials 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 mission objectives 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 surface mission, 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 those absolutely required to insure the crew's survival. Life-critical systems will have two backup levels of functional redundancy; if the first two levels fail, the crew will not be in jeopardy, but will not be able to complete all mission objectives; (2) Completing the 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 productivity of 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 real redundancy (multiple systems of the same type) or functional redundancy (systems of different type which provide the required function). Recoverability or repairability by the 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 the principal Life-Critical systems of the reference mission is shown in Table 8.

PrimaryBackup #1Backup #2
Habitable volumeHabitat #1Habitat #2Pressurized Rover
Air and waterLife Support System #1Life Support System #2Consumable Cache
PowerPower Unit #1Power Unit #2Power Unit #3
Food/food preservationSupply #1Supply #2Emergency 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 out prior to the departure of the crew on the first human mission. The crew arrive in a second habitat. Each habitat is equipped with a life support system capable of providing for the entire crew for the duration of their surface stay. The concept of a life-support cache is derivative from the objective/assumption that indigenous resources will be extracted and utilized in the strategy from the beginning of the program. The reference mission thereby utilizes a system to produce methane, oxygen and other consumables from Martian resources, and verifying these caches prior to the crew departing from Earth. In the reference mission, all food is brought from Earth. An experimental bioregenerative life support system capable of producing a small amount of food is included as a mission-critical element; however, the crew will not depend on it for their sustenance. In earlier versions of the reference mission, an energy 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 be manufactured on the required schedule, and has therefore been replaced by a redundant power system.

Principal Elements of the Surface Mission

(a) Surface Mission Objectives

The principal science objectives for Mars exploration is determining:

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

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

The location and general accessibility of resources on Mars will be determined by the series of robotic missions; however, in detail, understanding the extent and utility of the resources may require the presence of humans. The first missions will require that resources be extracted only from the atmosphere, which is well-enough known for that purpose. Subsequent missions may utilize other resources, including indigenous water. The resource discovery and verification of accessibility will require investigations in geology, atmospheric science and chemistry.

The targeted investigations to be carried out from the Mars outpost depend on an increasing range of accessibility from the outpost by humans and automated rover/sample collectors. A general geological map of the region of the outpost site should have been prepared by robotic missions prior to selecting and occupying the initial site. Field investigations carried out by crews on Mars will address detailed questions requiring access to varied terrain and rock types. The reference mission includes provision for two pressurized rovers, eventually allowing traverses of up to 500 kilometers range from the outpost. It also includes two smaller, instrumented rovers which can be teleoperated from the outpost as necessary to document and collect samples for analysis in the outpost laboratory or for return to Earth.

The habitation objectives of the Mars outpost include:

These habitation objectives are aimed at establishing the feasibility and approach required to move beyond the exploratory phase toward the development of long-term activities on the planet. They influence the selection of elements that are included in the surface systems, including habitats, mobility, life support, power and communications systems.

(b) Human Factors and Crew Size

Humans are the most valuable mission asset for the Mars exploration program, and must not become the weak link. The requirement that humans spend on the order of 600 days on the surface places unprecedented requirements on the people and their supporting 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 Earth. Their resources are either with them or have already been delivered to or produced on Mars. No further resupply is available and return to Earth in substantially less time than the nominal mission is not possible. Crew self-sufficiency is required because of the long duration of their mission and by the fact that their distance from Earth impedes or makes impossible communications and control by controllers on Earth. The crews therefore need their own resources (skills, training) and specialized support (systems) to meet the new challenges of the missions. However, unlimited resources can not be provided within the constraints of budgets and mission performance, so tradeoffs must be made between cost and comfort, as well as performance and risk. Because the objectives of the missions are to learn about Mars and its capability to support humans in the future, there will be minimum level of accomplishment below which a viable program is not possible. Survival of humans on the trip there and back is an insufficient program objective.

Basic human survival factors for the crew include adequate shelter, including radiation protection; breathable, controlled, uncontaminated atmosphere (in habitats, suits, and pressurized rovers), food and water, medical services, psychological support, and waste management. In the 4-6 month transits to Mars, the chief problems will be on maintaining interpersonal relationships needed for crew productivity, and maintaining 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 and hazardous environment. The transit environment is likely to be a training and conditioning environment, the surface environment is where the mission-critical tasks will 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 between cost and crew comfort must be weighed with special care. The development of high quality habitats and environmental design features are critical to assuaging stress and increasing crew comfort - conditions that will greatly increase the likelihood of mission success. Providing little more than the capability to survive invites mission failure.

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 physical space on the surface, increase the amount of free time by the crew, increase the amount 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 associated with the unfamiliarity of the environment.

The quality of life can be facilitated by access to indigenous resources. In the near term, use of indigenous resources reduces some of the mission risks (creation of caches, use of local resources for radiation shielding). In the long term, use of local resources may allow more rapid expansion of usable space. Achieving the capability to produce water and oxygen may have physical and psychological benefits over continued recycling. For example, reducing limitations on water utilization for hygiene purposes will be psychologically supportive. The ability to grow food on site also has a psychological effectiveness. The psychological impacts of these developments is difficult to quantify, however real the effects may be.

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

A workload analysis was carried out assuming that the crew's available time would be spent 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 relevant technical fields required by the exploration and habitation requirements include mechanical engineer, electrical and electronics engineer, geoscientist, life scientist, and physician/psychologist. It is assumed that these are important enough that they should 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 crew member, including support tasks as well as tasks of command and communications. It is 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 jeopardize mission success. Therefore, a minimum crew size of seven or eight may be required to address the risk issues. Currently, the reference mission is built on the assumption of a crew of six.

There is an immature understanding of the manner in which the crew would be supported by intelligent robots and automated systems. The work load analysis 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, it appears that automated or teleoperated rovers, capable of extending the effective field time by crew members, will be a good investment from the point of view of total mission productivity. Progress being made currently in telerobotic operations of a rover in the Antarctic environment can be translated directly to Mars exploration capability.

(c) Life Support Systems

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

Indigenous resources (oxygen, nitrogen, water) are extracted from the Martian environment and provide caches of consumables for the life support system as well as providing 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 experimental bioregenerative life support system capable of providing a fraction of the food could allow 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 atmosphere consists of pumping Martian atmosphere through a reactor, removing the inert nitrogen and argon (about 2% of the total) and reacting the carbon dioxide with hydrogen brought from Earth to produce methane and oxygen. Enough methane and oxygen is produced to propel the ascent vehicle from the surface to orbit. Table 9 gives an overview of the total potential consumables, modeled according to a requirement to provide for a 6 person crew for 600 days on the surface, assuming an open-loop LSS.

The Sabatier Process, well known on Earth, follows the reaction CO2+4H2--->CH4+2H2O. Water produced in the reaction would be either stored in the cache or dissociated to form hydrogen and oxygen. Oxygen would be stored in the fuel or life support cache, and hydrogen would be recycled into the Sabatier reactor. The table shows that there is a general balance of the requirements. If enough water is produced to provide for the potable water cache and a 600 day reserve of breathing gas, and the oxidizer requirements for the transportation system, enough byproduct nitrogen and argon should be available for the consumable cache. It may not be feasible to have a complete cache of hygiene water available at the start of the first mission.

For the long-term system, a hybrid physio-chemical and bioregenerative life support system is proposed. The BR can produce food and can provide a buffer with significant inertia. The P/C can be modulated to provide short term control and can be used concurrently with the BR system when caches require filling. The combination of the two provides flexibility and introduces independent design diversity.

* The range depends on the atmospheric pressure adopted for surface habitats and the inert/oxygen ratio. Requirements for inert gases are significantly less if low pressures are adopted.
Quantity Required
(metric tons)
Amount of Atmosphere Required
(metric tons)
Methane, fuel5.616
Oxygen, oxidizer19.426.6
Oxygen, air0.81.1
Nitrogen and Argon, air1.2 - 3.2*50 - 150*
Potable water14---
Oxygen, for potable water12.417.4
Water, hygiene120---
Oxygen, hygiene water106146
Hydrogen, methane4.2Earth
Hydrogen, potable water1.5Earth
Hydrogen, hygiene water13Earth
Food, dry2.2Earth

Table 9. Atmospheric Volatiles for Propulsion And Consumables for the Life Support Cache

There are costs associated with the robustness of the life support system. Table 10 gives mass and volume comparisons for various combinations of P/C and BR systems and ISRU systems. The analysis includes tankage, expendables, spares and integration. Masses and volumes for P/C system are based on Space Station Freedom level technology. A packing density of 70 kg/m^3 was assumed, unless otherwise known. Consumables, spares and expendables were sized for 800 equivalent days (33% contingency).

* Power above open loop case.
ArchitectureFunction Redundancy LevelsMass (Mt)Volume (m3)Power (kW)*
Open loop11802900
100% P/C with ISRU cache2604707
100% BR with ISRU cache26041060
Hybrid 100% P/C and 100% BR with ISRU cache38060067
50% P/C and 50% BR with ISRU cache +200 days consumables from Earth2+9054037

Table 10. LSS Architecture Mass, Volume and Power Comparisons

The large power requirements associated with the bioregenerative system are due to the assumption that all lighting will be artificial. In general, it is believed that satisfactory P/C or bioregenerative systems can be made available for the Mars outpost. The major question is whether adequate power can be made available within the limitations of the space transportation system. Because of the power requirements of the bioregenerative life support system, it may be necessary to defer the complete activation of that system to later missions. It would be important to begin the testing of the BR system on the first mission, but perhaps at a 10% of food requirements level.

For the reference mission, the following characteristics were assumed:

(d) Mobility

Mobility on several scales is required by people operating from the Mars outpost. Any time the crew is outside of the habitat, they will be in pressure suits, and will be able to operate at some distance from the habitat, determined by their capability to walk back to the outpost. They may be served by a variety of tools, including rovers, carts and wagons. On a local scale, perhaps beyond a kilometer from the outpost and less than ten, exploration will be implemented by unpressurized wheeled vehicles. Beyond the safe range for exploration on foot, exploration will be undertaken utilizing pressurized rovers, allowing explorers to operate for the most part in a shirtsleeved environment.

The requirements for long range surface rovers were presented at the workshop. Requirements to be placed on the rover included: (1) a radius of action of up to 500 km in exploration sorties that allow 10 work days to be spent at a particular remote site, and (2) the speed of the rover should be such that less than half of the excursion time is appropriated for travel. Each day, up to 16 person-hours would be available for extravehicular activities. The rover is assumed to have a nominal crew of two persons, but be capable of carrying four in an emergency. Normally, the rover would be operated only in the daytime, but could conduct selected investigations at night.

Two major issues have emerged. The first is whether to power the rover with a nuclear system (dynamic isotope power system) or a chemical system. The power requirement of 40 kWe for the moving rover and 5700 kWe-hrs total is within the range of DIPS; however, there is no requirement for continuous power (when the rover is not in use) and there may be hesitancy to proliferate nuclear systems. Three chemical systems have been considered (H2-O2; CH4-O2; CO-O2). All appear to be feasible. Methane-oxygen engines possess the best combination of mass and volume. The second issue is whether wheels or legs or tracks should be used. Wheels are the leading option, representing a compromise between mass, efficiency, technical risk and operational safety. Propulsion efficiency does not uniquely discriminate between options. Tracks are not particularly valuable on Mars, based on terrain types observed in Viking images. The theoretical attributes of legs are attractive, but not yet demonstrated technologically.

(e) EVA Systems

The Mars missions will place new, unique requirements on ways of doing EVA and will thus require new innovative approaches and strategies for accomplishing EVA exploration to meet these requirements. Mars EVA system objectives are twofold:

EVA tasks consist both of constructing and maintaining the habitat, and conducting a scientific exploration program encompassing geologic field work, sample collection, and deployment, operation and maintenance of instruments. Any EVA system must perform these tasks with the critical functional elements of a pressure shell, atmospheric and thermal control, communications, monitoring and display, and nourishment and hygiene. Balancing the desire for high mobility and dexterity against accumulated risk to the explorer is a major design driver on a Mars EVA system.

Because of the unique challenges for a Mars EVA program, (frequency and duration of sorties, quick turn-around times, necessary dexterity and mobility, dust and abrasion) a fresh strategy for interfacing the crew with the surface is required.

(f) Power Systems

A number of issues guide the selection of a power systems strategy. These include the risk considerations, which require that mission critical functions have two level redundancy and life critical functions have three. The surface power systems should have 15+ year lifetimes, to allow them to service the three mission opportunities with good safety margins. Transportation power systems should have 6+ year lifetimes to minimize the need for replacement over the program lifetime. There are logistics objectives, including reducing the deployment and setup time of power systems, reducing the power system maintenance tasks, and providing interconnectability between power elements delivered on different flights. The power requirements for producing and maintaining life support caches can be met early, which reduces the boiloff of imported hydrogen and leaves the power system to meet the requirements of the habitat life support systems. The mobile systems ultimately require power systems capable of providing 1000 km (out and back) mobility.

The strategy adopted for the reference mission includes a primary and backup SP-100 class nuclear reactor with dynamic conversion. Each of these systems is capable of producing 100 kWe. The habitat is provided with a portable generator, a 15 kWe DIPS, capable of supplying a third level of redundancy for life critical functions. This could begin at 15 kWe and grow to 30 kWe over the three missions.


IV. Conclusions

The Mars DRM presented in this paper proposes a series of strategies for enabling the near-term human exploration of Mars. By focusing the resources of the mission on providing a robust planetary surface exploration capability, the mission return is enhanced, the safety of the crew is improved, and the cost of the mission should be reduced. It is worth pointing out that the Mars DRM requires approximately 850 metric tons be delivered to LEO via 4 launches of a HLLV. Of this, nearly 200 metric tons was delivered to the surface of Mars as usable exploration payload. This is in marked contrast to previous reference mission strategies that required the delivery to LEO of nearly 1,200 tons of material, mostly propellant, in order to send a crew to Mars for 30 days. Of this material, less than 100 tons was useful exploration equipment.

A preliminary cost assessment of the Mars DRM is currently underway. Early results are encouraging.

Acknowledgments

The authors wish to express their appreciation for the work of the members of the Mars Study Team over the last year. The team had to operate in an extraordinarily volatile period for "Exploration."


1. The Advisory Committee on the Future of the U.S. Space Program

2. Cohen, A., et. al., Report of the 90-Day Study on Human Exploration of the Moon and Mars, National Aeronautics and Space Administration, Washington, DC, November, 1989.

3. Space Transfer Concepts and Analyses for Exploration Missions, NASA Contract NAS8-37857, Boeing Defense and Space Group Advanced Civil Space Systems, Document No. D615-10054, May, 1992.

4. Stafford, T., et al, America at the Threshold: Report of the Synthesis Group on America's Space Exploration Initiative, U.S. Government Printing Office, May 1991.

5. Cohen, A., et. al., Report of the 90-Day Study on Human Exploration of the Moon and Mars, National Aeronautics and Space Administration, Washington, DC, November, 1989.

6. First Lunar Outpost Study Space Transportation Segment Phase Two Report, Volume I, NASA Johnson Space Center, Houston, Texas, Juner, 1993.

7. Lineberry, E.C., and Soldner, J.K., Mission Profiles for Human Mars Missions, AIAA Paper No. 90-3794, AIAA Space Programs and Technologies Conference, Huntsville, AL, September 25-28, 1990.

8. Wilson, S.W., Fast Round-Trip Mars Trajectories, AIAA Paper No. 90-2934, AIAA/AAS Astrodynamics Conference, Portland, OR, August 20-22, 1990.

9. For an excellent discussion of this topical area, see Strategic Considerations for Support of Humans in Space and Moon/Mars Exploration Missions; Life Sciences Research and Technology Programs, NASA's Advisory Council's Aerospace Medicine Advisory Committee Report, June, 1992. See also, Townsend, L.W., Nealy, J.E., Wilson, J.W., Simonsen, L.C., Estimates of Galactic Cosmic Ray Shielding Requirements During Solar Minimum, NASA TM-4167, 1990; and Simonsen, L.C., Nealy, J.E., Townsend, L.W., Wilson, J.W., Radiation Exposure for Manned Mars Surface Missions, NASA TP-2979, 1990.

10. K. Joosten, B. Drake, D. Weaver, and J. Soldner, Mission Design Strategies for the Human Exploration of Mars, IAF-91-336, 41st Congress of the International Astronautical Federation, Montreal, Canada, Oct. 1991.

11. Space Transfer Concepts and Analysis for Exploration Missions, NASA Contract NAS8-37857, Boeing Defense & Space Group Advanced Civil Space Systems, March 1991.

12. Soldner, J.K., and Joosten, B.K., Mars Trajectory Options for the Space Exploration Initiative, AAS Paper No. 91-438, AAS/AIAA Astrodynamics Specialists Conference, Durango, CO, August 19-22, 1991.

13. R. Zubrin and D. Weaver, Practical Methods for Near-Term Piloted Mars Missions, AIAA 93-2089, AIAA/SAE/ASME/ASEE 29th Joint Propulsion Conference and Exhibit, June, 1993.

14. S. K. Borowski, et al, Nuclear Thermal Rocket/Vehicle Design Options for Future NASA Missions to the Moon and Mars, AIAA-93-4170, AIAA Space Programs and Technologies Conference and Exhibit, Huntsville, AL, September, 1993.

15. Campbell, Paul D. and Moore, Nathan, Integration of a Plant Growth System into a Mars Surface Habitat, ASCE Space 94 Conference and Exposition, Albuquerque, New Mexico, February 26 - March 3, 1994.

* Planetary Projects, New Initiatives Office, Member AIAA.

Copyright 1993 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner.