A number of different approaches are described to reach the Moon First and Flexible Path to Mars destinations. However, all of these approaches require a trend to a roughly $3B/year increase in the NASA Human Spaceflight budget. Such a large increase may be difficult to reach, and even more difficult to maintain over many years. Given this difficulty, I would suggest a third series of destinations, the "Flexible Path to the Moon".
The Flexible Path to the Moon is just a new name, not a new idea. The concept can be found in different forms in some of the documents linked here.
Let's suppose we cannot find the budget needed to implement either one of the Augustine Committee approaches on anything like a timely schedule without unacceptable sacrifices in other NASA areas. Instead of ignoring this problem and plowing ahead anyway on one of the two Augustine paths with an expensive heavy lift development effort that cannot be afforded while developing payloads for the vehicle, using the ISS to its fullest, doing needed technology development, and other important jobs, the Flexible Path to the Moon removes or at least postpones the heavy lift development and its expenses. It also delays or sacrifices the more ambitious Flexible Path destinations, such as NEOs and Mars orbit. These destinations can be addressed at a later time when the Flexible Path to the Moon has developed sufficiently that they are in reach. In compensation, the Flexible Path to the Moon focuses on easier and more near-term destinations like LEO, GEO, lunar orbit, and Earth-Moon Lagrange points, and lingers longer at these destinations to make the most of them. These destinations are used for a variety of purposes. Among other things, infrastructure at these destinations is developed to ultimately allow a cost-effective lunar surface astronaut program.
The Flexible Path to the Moon consists of three phases:
- Establish a Foothold - The first phase includes developing commercial and international partnerships for the full Flexible Path to the Moon, beginning a long-term program to maintain and fully use the ISS through at least 2020, developing U.S. commercial crew and cargo services to support the ISS, establishing a vigorous technology development program, and starting an ambitious robotic lunar precursor effort for science, resource scouting, engineering tests, and more with NASA, commercial, and international participation. This phase could also include robotic precursors to destinations that are beyond the scope of the Flexible Path to the Moon, like Near Earth Objects and Mars Moons. Given the fairly large number of robotic missions envisioned here in support of the Flexible Path to the Moon, it is likely that there would be fewer outer planets robot missions.
- Go Beyond LEO - The second phase builds on the first, which would largely continue in an operational mode while the second phase begins development. This phase includes astronaut missions to GEO, Earth-Moon Lagrange points, and lunar orbit using the same class of rockets used in the first phase. Astrophysics, Earth Observation, and Heliophysics observatories could be assembled by astronauts and perhaps robotic helpers in appropriate locations as described in this NASA Spaceflight article, but, given cost constraints, the observatories would most likely be considerably smaller than the ones described there. Satellite servicing capabilities, such as satellite refueling, instrument upgrades, and component replacement could be demonstrated at various locations. Lunar orbit could be used for lunar observations and telerobotics, perhaps to demonstrate telerobotics capabilities for mission beyond the scope of the Flexible Path to the Moon. Space transportation from LEO to and from the more distant destinations would be designed to be affordable and most likely reusable. Reusable space infrastructure would be developed to support these efforts, such as satellite assembly and servicing nodes, lunar orbit space stations, and fuel depots. Spacecraft refueling would become operational. Space infrastructure in LEO, such as commercial LEO space stations to team with the ISS efforts, would be encouraged. It should be noted that the destinations described here are similar to those in the early "Flexible Path to Mars". However, in the Flexible Path to the Moon, we are spending much more time at these destinations. Phase 2 could involve many missions over many years. If the budget does not allow all of this to be done at once, it is possible that there would be multiple sub-phases within Phase 2.
- Return to the Moon - This phase builds on, and maintains, the infrastructure and capabilities developed in the earlier phases. Using commercial access to space, refueling, reusable spacecraft, and space infrastructure nodes in lunar orbit and/or at Earth-Moon Lagrange points, astronauts would already be close to reaching the lunar surface. Lunar robotics would have prepared the way for productive work at the surface. As the budget allows, this phase adds a lunar lander that could be reusable. It also incrementally adds lunar surface capabilities as needed. These capabilities might include surface mobility, a base, or ISRU facilities, depending on the results of the many earlier lunar robotics efforts.
This Flexible Path to the Moon allows us to achieve great synergy between human missions beyond LEO and NASA science, including ISS Science, Earth Observation, Heliophysics, Astrophysics, and Planetary Science. It provides many opportunities for commercial and international participation. Given that most of our existing science, security, and commercial satellite infrastructure is in the locations described here, this approach provides many ways to achieve the original Vision for Space Exploration goals of science, security, and economic benefits.
I would suggest that the Flexible Path to the Moon is more affordable that either the Moon First or Flexible Path to Mars scenarios envisioned by the Augustine Committee. If one is skeptical that the sort of reusable infrastructure not launched by HLVs described here is cheaper than the HLV-based Moon-First approach, then simply discard or postpone phase 3 above. At least you will still be able to accomplish some useful work with the first 2 phases described above, and still get closer to the Moon and Mars, than if an HLV-based Moon-First or Flexible Path to Mars approach is used without the budget to make them possible.