The Space Review

Gemini/L2 concepts
Left: The Gemini 6/7 mission demonstrating on-orbit rendezvous in December 1965 (credit: NASA). Right: Concept for a long-duration habitation system at Earth-Moon L2 (credit: Boeing/Roskosmos).

NASA’s Gemini Program: a “stepping stone” to Mars?

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In early 1961, the nascent Mercury Program was making slow, steady progress toward demonstrating key capabilities necessary for humans to survive for short periods in the hostile environment of near-Earth space. A few months later, America’s new goal for human spaceflight was to land for the first time on another world, a challenge vastly more ambitious than the original intention of Mercury and its conservative successor missions. This new goal, NASA senior managers quickly recognized, would require building upon the successful first steps of Mercury with a program to demonstrate enabling new capabilities required to travel to the Moon.

Is the “stepping stone” Gemini Program of the 1960s a useful analogy in planning for the first human mission to Mars?

The 1960s: the Gemini Program as a necessary “stepping stone” to the Moon

The story has been told often and well.

President Kennedy’s goal was unambiguous as declared in May 1961: a deadline for a human mission to land on another world and return safely by the end of the decade. Major—some would say “breakthrough”—progress had already been made in the capabilities necessary for human operations in low Earth orbit (LEO). However, the goal of safely placing the first human on a far-distant world within a definite time would be the most ambitious technological, engineering, and management challenge during peacetime in the nation’s history. Humans would travel farther from Earth by far than had thought possible just a few years earlier when the Mercury Program was established during the Eisenhower Administration. Consequently, very different technological capabilities from those envisioned by the original Mercury planners would need to be demonstrated. Almost immediately, the architects of the Moon program recognized that the existing plans to follow Mercury were an insufficient enabling “bridge” to Apollo. Landing humans for the first time on another world had become a national priority. Therefore, technology investments, precursors, and intermediate steps must be clearly focused on achieving this goal.

For about two years, until the last Mercury flight occurred in May 1963, NASA was simultaneously assessing and, then, managing three major human space flight programs.

That intermediate “bridge” program from Mercury to Apollo evolved from existing plans to become NASA’s Gemini Program, which could be an analogy in preparation for human exploration of Mars. However, this analogy contrasts with many recent concepts for future human operations beyond LEO. Such concepts propose to achieve goals that are not unambiguously enabling for human exploration of Mars: for example, assembling large telescopes or exploring the lunar surface. Furthermore, most architectures for human Mars exploration, in turn, barely mention “stepping stone” missions or requirements for precursor activities beyond LEO.

Although there had been significant thinking about post-Mercury programs before Apollo was chartered in May 1961, extensive new design work began after the president’s announcement regarding the capabilities beyond Mercury would be necessary to enable Apollo. The Gemini Program was formally announced less than a year later, in early January 1962. Thus, for about two years, until the last Mercury flight occurred in May 1963, NASA was simultaneously assessing and, then, managing three major human space flight programs: the established Mercury Program, the Apollo Program to achieve President Kennedy’s vision, and the rapidly evolving Gemini Program to build on the former to make the latter possible.

NASA management recognized in 1961 that it was not possible to make the leap from the impressive accomplishments of Mercury to the surpassing goals of Apollo without a significant “bridge” demonstration program. However, a human landing on the Moon within a decade had become an overriding national goal. Gemini was to demonstrate that long-duration—for its time—spaceflight was possible: about an order of magnitude longer than had been achieved by its predecessor program. This required new systems for environmental life support and onboard power storage and management systems. Gemini famously proved American extravehicular activity was feasible. This would be necessary on the lunar surface, of course, although also as a backup in the event of failed docking during some stages of the Apollo missions. On-orbit rendezvous and docking was also a major Gemini milestone. This challenging capability required precision control of spacecraft orientation, as well as maneuver of a docked Gemini-Agena combination using the target vehicle’s propulsion system. Finally, the program perfected methods for re-entering Earth’s atmosphere and improved targeting for landing.

The 2020s: a “stepping stone” before travel to Mars?

The Gemini Program “stepping stone” as a 1960s analog for modern-day preparation for human travel to Mars contrasts with the majority of published concepts for next-step, long-duration human operations beyond LEO, including those advocated by this author.

Pre-Mars intermediate destinations have been widely proposed, however, these concepts to date almost always focus on a destination for the next “stepping stone.”

Decades of detailed concepts for a human mission to Mars would fill a very long bookshelf. No concept has been formally chartered by the world’s space agencies as the definitive design for the first human mission to Mars, and travel to the Red Planet is by no means the unanimous overriding goal for the world’s major spacefaring nations. That said, remarkably few existing Mars program concepts also include detailed requirements-driven and enabling precursor, demonstration, or “stepping stone” missions to complement ISS that would be comparable to the role played by the successful 1960s Gemini Program. The 1986 Report of the National Commission on Space, aka “The Paine Report,” is a notable exception in this regard, although is outside the consideration here of recent Mars architectures and concepts for operations in the Earth-Moon system.

It is the case that This contrasts with an approach that begins by identifying capabilities required for travel to Mars that would build upon the ISS program, followed by detailed precursor spacecraft design, and only subsequently identifying additional desirable capabilities and locations. For most recent proposed intermediate destinations in advance of travel to Mars, including some worked on by this author, concept studies focus on human operations at the venue, usually including how such operations could in addition be considered one day to support human capabilities in reaching Mars. This contrasts with Gemini fifty years ago, where requirements for a major step in human expansion into space determined the eventual design characteristics of the intermediate step(s) between Mercury and Apollo. Gemini-like assessments—detailed design work on candidate “bridge” missions between the successful ISS program and travel to Mars based specifically (or solely) on explicit requirements developed for the Mars mission(s) and no other criteria— have been far less frequent in recent years.

An obvious example of candidate “stepping stone” concepts that focus on destinations to achieve attractive science and human space flight goals has been the interest over the past several years in Earth-Moon libration-point human operations (e.g.,,, and “Human operations beyond LEO by the end of the decade: An affordable near-term stepping stone”, The Space Review, January 10, 2011). These candidate operations sites highlight assembly of large scientific facilities, telerobotic exploration of the lunar surface, and maintenance of depot systems, among other goals. Often included in the list of desirable goals for such a habitation system is enabling subsequent Mars voyages. However, retiring risk and demonstrating key capabilities in advance of very long-duration voyages is very rarely the stated sole or even highest priority for such a facility. This is exactly the reverse of the role that the Gemini Program played in retiring risk for Apollo. Because successfully landing Americans on another world in the 1960s was the nation’s overriding goal in space, the “stepping stone” between Mercury and Apollo was focused unambiguously on demonstrating capabilities necessary for the priority in the 1960s for human spaceflight. Additional goals for the Gemini “stepping stone” were decidedly secondary.

The alternative Gemini-like process, that of assessing in detail “stepping stone” precursor concepts focused unambiguously on retiring risk for an eventual human mission to Mars with all other applications decidedly of secondary importance, appears absent for now.

If a human mission to the Red Planet within two decades during the mid-2030s “window” in Earth-Mars relative orientation becomes an overriding goal for human spaceflight among spacefaring nations, any unrelated operation may be a costly and time-consuming delay. Of course, it is possible that a Gemini Program-like risk-reduction mission may turn out, upon detailed assessment, not to be necessary. Enabling capability development may be achievable using ISS alone, in parallel with terrestrial investments and demonstrations. In this case, the habitation and other systems necessary to demonstrate readiness for Mars travel may turn out to be the vehicles themselves that will carry out the mission. That is, the Mars-bound, post-ISS “stepping stone” may be deployed, assembled, and successfully demonstrated somewhere in the Earth-Moon system with no intermediary mission before departing on its challenging voyage. Although NASA’s 1960s-era engineers concluded that a separate capability- and requirements-driven development program was required between Mercury and Apollo, detailed engineering design of a future Mars mission may conclude that no such “bridge” in fact is necessary.

In conclusion, concepts for future human operations beyond LEO are widely reported, usually focusing on attractive human spaceflight and science priorities for exploration of the Moon or near Earth asteroids, or perhaps assembling very large space observatories. The alternative Gemini-like process, that of assessing in detail “stepping stone” precursor concepts focused unambiguously on retiring risk for an eventual human mission to Mars with all other applications decidedly of secondary importance, appears absent for now.



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