In the summer of 1970, NASA released a technical memorandum titled “Space Flight Evolution” that outlined an ambitious vision for the future of the American space program. The document, authored by Georg von Tiesenhausen and Terry H. Sharpe of the Advanced Systems Analysis Office at NASA’s Marshall Space Flight Center, presented concepts and objectives for an evolutionary space program that would take humanity from the early Skylab missions of the 1970s to a manned landing on Mars by the end of the 20th century. While many of the specific missions and hardware described were notional and in early study phases at the time, the memorandum provides a fascinating glimpse into NASA’s long-term strategic thinking at the dawn of the Space Shuttle era.
Approach and Early Period
The memorandum begins by describing NASA’s overall approach to space flight evolution. It emphasizes the importance of utilizing current Apollo-era hardware like the Saturn V rocket, Apollo Command/Service Module, and Lunar Module to their fullest extent in the early 1970s before transitioning to next-generation reusable systems later in the decade. The early period would focus on extending the duration of manned missions in low Earth orbit and on the lunar surface to establish the physiological and psychological effects of long-duration spaceflight.
The centerpiece of this early period would be the Skylab program – a series of manned space stations derived from Apollo hardware that would remain in Earth orbit for up to two years. Conducting a wide variety of scientific experiments, engineering tests, and biomedical studies, the Skylab missions would provide the data and operational experience needed to develop larger, more permanent space stations in the late 1970s and beyond.
Lunar exploration would also continue during this early period, with extended Apollo missions allowing astronauts to spend up to 3 days on the surface collecting samples, making observations, and deploying scientific experiments. The experience gained would inform future lunar base planning.
Unmanned exploration of the solar system would proceed in parallel, with Mars receiving particular emphasis. Orbiters and landers would map the Martian surface and study its environment to enable eventual human missions.
Enabling New Systems
To expand humanity’s spacefaring capabilities and drive down costs, NASA envisioned the development of several key new systems in the mid-to-late 1970s:
Space Shuttle
The Space Shuttle would be a fully reusable two-stage vehicle capable of routinely and economically transporting crew and cargo to and from low Earth orbit. Utilizing aircraft-like design principles and operations, the Shuttle would feature long-life subsystems, minimal ground support requirements, and rapid turnaround times between flights. It would serve as the “workhorse” for all manned missions, delivering crews, supplies, and equipment to space stations and enabling the launch and servicing of satellites and space probes.
Space Station Module
A new multi-purpose space station module, capable of supporting 6-12 crew for up to a decade in orbit, would be developed as a building block for permanent manned outposts. Featuring closed-loop life support, artificial gravity, and spacious work and living areas, the module would enable long-duration space missions and serve as an orbital laboratory for conducting research across a range of scientific and technical disciplines. Variants of the module could also be used for stations in lunar orbit, on the lunar surface, and eventually for manned flights to Mars.
Space Tug
The Space Tug would be a versatile propulsion vehicle used for a variety of in-space logistics and transportation functions. Comprised of a reusable engine module, crew module, and cargo pods, the Tug could be used to transfer payloads between the Space Shuttle and higher orbits, ferry crews to and from the Moon, launch robotic spacecraft to the outer solar system, and support the assembly and maintenance of large space structures. Different configurations of the Tug’s modular components would be tailored to specific mission requirements.
Intermediate Period
The 1980s would see a significant expansion of permanent human presence in space, enabled by the maturing of the Shuttle, Station Module, and Tug programs.
A large permanently-occupied space station would be assembled in low Earth orbit, gradually growing to support up to 12 crew members. The station would serve as a multi-purpose research facility and a transportation node for missions to higher orbits and the Moon. Closed-loop life support and artificial gravity capabilities would be demonstrated.
In lunar orbit, a station composed of several Station Modules would enable continuous exploration of the Moon using Tugs for transportation to and from the surface. Crew would be rotated to the station using the Shuttle and a new Nuclear Shuttle (described below). On the surface, a permanent lunar base would be established to support in-depth geological exploration, astronomical observations, and research into the utilization of lunar resources. Drilling equipment and pressurized roving vehicles would extend the range of surface operations.
Nuclear Shuttle
To support the lunar base and other high-energy missions, a reusable Nuclear Shuttle would be introduced in the mid-1980s. Utilizing a nuclear thermal rocket engine, the Shuttle would greatly reduce the cost of transportation between low Earth orbit and cislunar space. It would deliver crews, Station Modules, Tugs, and bulk cargo to lunar orbit for transfer to the surface. The Nuclear Shuttle would be essential for the economical operation of the lunar base and would also serve as the workhorse for initial manned missions to Mars.
Manned Mars Missions
The memorandum envisions the first manned landings on Mars occurring in the late 1980s, utilizing the hardware and expertise developed through the Shuttle, Station, Tug, and Nuclear Shuttle programs.
Two Mars ships, each consisting of several Station Modules and Nuclear Shuttle propulsion stages, would depart Earth orbit when the planets are favorably aligned. The ships would swing by Venus to gather scientific data before arriving at Mars several months later.
Upon reaching Mars orbit, a portion of the crew would descend to the surface in a purpose-built Mars Excursion Module that would utilize aerodynamic braking and terminal propulsion to land. The crew would spend 40-60 days exploring the Martian surface, conducting experiments, and collecting samples, while the remainder of the expedition remained in Mars orbit.
Prior to the crew landing, robotic probes would be dispatched from the orbiting ships to gather samples and return them to the mothership for analysis, minimizing the risk of back-contamination. A small pressurized rover would be used to extend the range of exploration around the landing site.
After completing surface operations, the Mars Excursion Module would ascend to orbit and rendezvous with the waiting Mars ships. The crew would then depart for a direct return to Earth, arriving home some three years after initial departure.
Late Period
The 1990s and beyond would see a continued expansion of human activities in space, building on the accomplishments of earlier decades.
The Earth-orbiting space station would evolve into a permanently occupied space base, perhaps constructed of dozens of Station Modules and supporting 50-100 specialists working on long-duration scientific, commercial, and technical projects. Materials processing and manufacturing under weightless conditions could enable the production of unique products for use on Earth. A permanent medical research facility would study the long-term effects of spaceflight on human health.
In lunar orbit, a large station would support ongoing surface operations and serve as a staging point for expanded exploration and development. If lunar ice deposits or other valuable resources were discovered, the base could evolve into a permanently occupied outpost engaged in mining and processing activities.
On Mars, additional landing missions would explore different regions of the planet, possibly leading to the establishment of a permanent base if resources like subsurface water were located. Robotic exploration of the outer solar system would also continue, perhaps using Nuclear Shuttle-derived propulsion stages to enable more ambitious missions to the moons of Jupiter and beyond.
Summary
The 1970 “Space Flight Evolution” memorandum provides a compelling long-term vision for NASA’s human spaceflight program, informed by the budgetary and political realities of the post-Apollo era. By leveraging existing hardware in the near-term and developing a small family of new reusable systems for use across a range of missions, NASA hoped to dramatically reduce the costs of spaceflight and enable a step-by-step expansion of human presence into the solar system.
While many of the specific mission proposals and timelines in the memorandum did not come to pass, the overarching themes of commonality, reusability, and evolutionary development have stood the test of time. The Space Shuttle, International Space Station, and commercial crew and cargo programs all trace their lineage to the concepts first laid out in this document half a century ago. As NASA now prepares to return humans to the Moon and eventually send them on to Mars, the ideas presented in “Space Flight Evolution” continue to resonate and inform the agency’s long-term planning.
Though the landscape of space exploration has changed dramatically in the decades since the memorandum was written, with new international and commercial players emerging and robotic exploration taking on increased importance, the fundamental challenge remains the same: to develop affordable, sustainable systems that can enable a permanent human presence beyond low Earth orbit. By learning from the past and leveraging the remarkable achievements of the Apollo era, NASA and its partners can build a future in space that realizes the ambitious dreams of visionaries like von Tiesenhausen and Sharpe.
Note: If anyone has a color PDF of this report please let me know using the contact form!
