David B. Weaver and Michael B. Duke,*
NASA Lyndon B. Johnson Space Center, Houston, Texas
Barney B. Roberts**
Futron Corporation, Houston, Texas
A year-long NASA-wide study effort has led to the development of an innovative strategy for the human exploration of Mars. A design reference mission (DRM) is advanced that significantly reduces the perceived high costs, complex infrastructure and long schedules associated with previous Mars scenarios. The strategy enhances the mission return, improves the safety of the crew, and reduces or eliminates many of the obstacles associated with conventional strategies. In particular, the Mars DRM supports crews which 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 emphasizes 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 production of the necessary water and life support gases for the crew's entire surface stay.
By emphasizing a robust suite of surface capabilities and high-leverage technologies, abort-to-the-Mars-surface emerges as an alternative to the traditional trajectory abort options. The DRM, consistent with this risk-mitigation philosophy, employs a single "common" habitat design for use by the crews 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 development, 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. Continuing missions to Mars can be conducted with only three Saturn VII launches every Mars opportunity (every 26 months).
Mars has long captured the imaginations of scientists, engineers, and explorers (1). However, conventional approaches to human expeditions have presented formidable engineering and fiscal challenges. Conventional plans for these missions (2,3,4) have included many or all of the characteristics presented in Table 1.
Table 1 - Conventional Mars Exploration 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)(5). 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 an integrated programmatic view.
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 (6).
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, in order 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:
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
The goal of the the Mars DRM is:
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:
The reference mission adopted for this study (1) defines 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) limits 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) provides 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) provides a flexible implementation strategy. Mars missions are complex, so that multiple pathways to the desired objectives have considerable value in insuring mission success; (5) balances 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 assessing the 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), another option exists in this reference mission, namely, abort to Mars' surface. This allows the mission designer 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 groundrule that the study would examine three human missions to Mars. Each mission returns to the site of the initial mission, with crews for 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. Mars Exploration Reference Mission Overview 2007 Launch Opportunity Cargo Missions
Figure 2. Mars Exploration Reference Mission Overview 2009 Launch Opportunity Cargo & Human Missions
Figure 3. Mars Design Reference Mission Timeline
|Flight 1: Cargo||Flight 2: Cargo||Flight 3: Cargo|
|Mars Orbit Payload
||Mars Orbit Payload
||Mars Orbit Payload
|Space Transportation Vehicles
||Space Transportation Vehicles
||Space Transportation Vehicles
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.
|Flight 4: Cargo||Flight 5: Cargo||Flight 6: First Crew|
|Mars Orbit Payload
||Mars Orbit Payload
||Mars Orbit Payload
|Space Transportation Vehicles
||Space Transportation Vehicles
||Space Transportation Vehicles
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.
More complete reports on the details of the Design Reference Mission can be found in (7) and an upcoming NASA TM. The analysis includes the surface science capability, habitation, life support systems, surface mobility and EVA systems, power, production of in-situ propellant for Mars ascent, and other elements of the surface systems, an analysis of nuclear thermal rocket systems, space vehicle designs, trajectories, and heavy lift launch vehicle requirements. A preliminary costing of the reference mission indicates costs substantially lower than those indicated in previous mission concepts. The risks are not known to be either more or less than other "flags and footprints" scenarios for the first mission; but, overall, the design reference mission should have lower risks due to redundancy, multiple opportunities to emplace needed infrastructure elements, and buildup of surface capability that could support people on Mars for extended durations, if that became necessary.
1. Report of The Advisory Committee on the Future of the U.S. Space Program, Washington, D. C., 1992.
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. First Lunar Outpost Study Space Transportation Segment Phase Two Report, Volume I, NASA Johnson Space Center, Houston, Texas, Juner, 1993.
6. Duke, M. and Budden, N. A., Results, Proceedings and Analysis of the Mars Exploration Workshop. JSC-26001 . NASA Johnson Space Center, Houston, Texas 1992.
7. Weaver, D. and Duke, M., Mars Exploration Strategies: A Reference Program and Comparison of Alternative Archi-tectures. AIAA 93-4212. AIAA Space Programs and Technology Conference, September 21-23, 1993.
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.