The crewed portion of the mission would depart in a long-duration habitat from a LEO staging point using NTP on a 6-month transfer to Mars orbit. The crewed system would be propulsively captured into Mars orbit and rendezvous with the predeployed vehicle. The crew of six would then transfer to the predeployed vehicle that contains the surface habitat, which would transfer them to the planet’s surface. The EDL system would land the habitat close to the predeployed assets, and the crew would then be able to conduct mobile scientific exploration of Mars. The Mars surface mission would last for about 500 days, and the crew would then board the ascent stage and return to the deep-space habitat and propulsion system that remained in Mars orbit. The surface assets would continue autonomous missions and data collection for possible use by future Mars exploration crews. On crew transfer to the deep-space habitat, which would have been in standby mode, the crew would jettison the ascent vehicle and return to Earth on another 6-month transfer and a direct Earth entry using the Orion vehicle.40

The crew and cargo portions of the mission would be sufficiently massive to require advanced propulsion stages, currently modeled using NTP, for the transfer to Mars orbit from a LEO staging point. Because NTP may not be feasible because of technical, financial, and/or political factors, NASA is still evaluating other advanced in-space propulsion options. The Mars DRA 5.0 study suggests that an NTP-based system would result in the lowest life cycle cost and mission risk, assuming a sustained campaign of many Mars missions. Other propulsion options, such as SEP and advanced cryogenic chemical propulsion, may have lower cost if only a few missions are planned. The Mars surface elements would require a highly reliable power source that is capable of generating a total of 30 kW or more. Mars DRA 5.0 concluded that a fission reactor would have about one-third the mass of a solar-based surface power system and that deploying large solar arrays robotically in the high wind and dust environment of Mars would be very challenging.

4.2.3 Potential Pathways

Now that the various stepping-stone destinations and associated mission concepts have been highlighted, potential pathways, all ending with a Mars surface mission, can be defined. As described above, each of the three pathways is a series of human spaceflight missions to various destinations. The first pathway (ARM-to-Mars) is essentially the current administration’s proposed U.S. human spaceflight program. The Moon-to-Mars pathway makes use of the Moon as a testing and development destination to mature Mars-oriented technology, while revisiting the lunar environment for more in-depth scientific study than was possible during Apollo. This pathway is also consistent with the goals of the United States’ traditional international space partners and the ISECG, of which NASA is a member.41 Finally, the Enhanced Exploration pathway essentially exhausts the potentially feasible classes of destination through a Mars surface landing, and it allows exploration of essentially all destinations that humans can explore, given the current understanding of physiology and technology.

The three pathways are used to compare and illustrate the challenges of sending humans as far as the surface of Mars. Although the specific destinations on the journey to Mars are few and each requires development and demonstration of hardware elements of various categories (such as propulsion systems, power systems, and habitation systems) as well as research related to human health issues, NASA, in concert with other international and domestic organizations, could further define, mature, and analyze a broad range of detailed conceptual pathways to Mars.

Table 4.1 and Figure 4.2 define and illustrate the three representative pathways to the horizon goal of a human mission to the Mars surface. Table 4.1 defines the specific DRMs of each pathway, while Figure 4.2 illustrates each of the stepping-stone destinations and the three pathways to the Mars surface. The first pathway, ARM-to-Mars, leverages the initial demonstration of the SLS and Orion systems in cislunar space via the ARM and then proceeds directly to activity in the Mars vicinity by focusing on exploring the moons of Mars, followed by a Mars landing.

The second pathway, Moon-to-Mars, first focuses on missions in the lunar vicinity and surface to demonstrate longer-duration in-space habitats and complex propellant staging in lunar orbit. These missions would also develop


40 The DRA 5.0 architecture outlined here is for the long-surface-duration, so-called conjunction mission. Given the orbital mechanics of Earth and Mars, a second, short-stay “opposition” class mission is also possible. This alternate mission has a transit time to Mars of about 200 days, a surface stay of about 60 days, and a return transit of 400 days.

41 ISECG, The Global Exploration Roadmap, 2013.

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