The United States and its international partners need to build a strategic framework for human space flight that recognizes the political, economic, and technical constraints facing all spacefaring nations. A primary challenge to creating a practical and sustainable program of human space exploration is not a lack of ambitious goals but the difficulties in organizing a practical sequence of projects. The paper proposes a synthesis of major architectural elements, destinations, and national policy objectives into a sustained effort of human space exploration beyond low Earth orbit (LEO). The United States is building the Space Launch System and Orion spacecraft and considering an Asteroid Retrieval Mission (ARM). International consensus through the International Space Exploration Coordination Group (ISECG) has coalesced around cis-lunar operations as the next logical step beyond the International Space Station. Private sector advocates have identified unique planetary alignment opportunities in 2018 and 2021 for a human round-trip mission to the vicinity of Mars. A sequence of affordable human space exploration missions could begin with Space Launch System (SLS)/Orion flights tests to cis-lunar space, followed by a human (or robotic) mission to Mars taking advantage of the 2021 planetary alignment and the SLS, and then a series of lunar missions in the mid-2020s. The development of a lunar lander can be delayed to avoid overloading exploration budgets. The international community would have cooperative opportunities in the vicinity of the Moon as the International Space Station ends its operational life. An asteroid retrieval mission could be added as funds and interest allowed, but primary attention would be on lunar operations and building the capabilities necessary for human missions to Mars in the 2030s.
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