American Scientist

GEMINI: Stepping Stone to the Moon, the Untold Story. Jeffrey Kluger. 304 pp. St. Martin’s Press, 2025. $32.

In his new book, Gemini: Stepping Stone to the Moon, Jeffrey Kluger—perhaps best known as the coauthor of Apollo 13, the book that served as the basis for the movie by the same name—provides a thorough history of NASA’s Gemini program. Gemini bridged the Mercury and Apollo programs, launching a total of 12 missions between April 1964 and November 1966; 10 of these missions carried a two-astronaut crew. NASA used Gemini missions to test the skills and equipment needed for a successful moon landing and, while often overlooked, the program was essential to America’s eventual mission to the moon.

Kluger argues that Gemini had

. . . an outsized and often unappreciated impact on geopolitics, technology, and the fundamental science of space travel itself. It was the Gemini, certainly, that gave the U.S. the cosmic edge over the Soviet Union in the original space race, contributing to the cascading series of economic, engineering, and political victories that helped bring the original Cold War to a peaceful end, with the West ascendant and the former Soviet Union consigned to history.

The rest of the book catalogues the accomplishments of the Gemini program, in contrast to the lack of progress by the Soviets in the space race, and the effects this had on American society.

In May 1961, John F. Kennedy voiced a commitment by the United States to put a man on the moon and return him safely to Earth. That endeavor became known as the Apollo program. But when Kennedy made that speech, only one American had been into space (Alan Shepard) as part of the Mercury program, which ran from 1958 to May 1963. By the end of Mercury, six Americans had been into space and four had orbited the Earth, with the longest flight lasting a little more than 34 hours. But it was going to take three days to get to the moon, not to mention the trip home. In addition, Apollo required a much bigger, much more powerful rocket, the Saturn V. To meet Kennedy’s goal by the end of the decade, NASA needed to develop a number of new technologies and to practice important skills. Waiting until the Saturn V was ready to fly to test all those new elements practically guaranteed missing that deadline. Kluger explains that the Gemini program, using the U.S. Air Force’s Titan rocket, was the proving ground for those skills and technologies. Those skills—rendezvous and docking, spacewalking, and long-duration flight—together remain the basis of human spaceflight today, as astronauts have constructed and occupied the International Space Station as we plan for returning to the moon and then going to Mars.

Kluger’s retelling of the Gemini program’s history is not entirely new, especially to space historians. However, he pairs official histories of the Gemini missions along with interviews he conducted to highlight just how important this program was to the overall success of the space race. During much of Mercury, the United States’ space efforts lagged behind the Soviet Union’s. Kluger’s narrative emphasizes just how many challenges remained at the end of Mercury that the Gemini astronauts, NASA engineers, and contractors had to overcome. Kluger goes so far as to claim that “without Gemini, men would never have walked on the moon.”

Throughout the text, Kluger elaborates on the challenges that the Gemini program faced before its first flight. He brings in the voices of engineers working on the program, highlighting its complexity and all that it needed to achieve. For example, he details some of the issues with the development of the Titan rocket, particularly a phenomenon known as pogo. This occurs when the pressure in the fuel tanks or fuel lines fluctuates to the point that the vehicle bounces around like a pogo stick. Pogo added significant g-forces that the astronauts would have experienced, to the point that they could have been injured or incapacitated. There were also concerns about the escape system, should there be an emergency during or just after launch. Kluger illustrates how the engineers worked to solve such problems, and the pressure felt by everyone in the Gemini program. He writes, “If Gemini was indeed about to fall on its face, it meant NASA would, too, and if NASA failed, it meant that in the eyes of the world, America would be losing the space race.”

But the long list of challenges that NASA faced in putting a man on the moon were solved because of Gemini.

• Facebook
• Twitter

The second half of the book provides detailed accounts of the 10 crewed Gemini missions that took place between March 1965 and November 1966, with just over two months between each launch. These stories are well-documented in NASA flight journals that are available to the general public, but Kluger brings them together in a more accessible narrative that emphasizes how significant this program was to the ultimate success of Apollo.

What NASA accomplished over the course of the Gemini program in many ways showed that the United States was pulling ahead in the space race, which Kluger documents by providing a side-by-side comparison to the Soviet activity during the same period. He notes that for the entire 603 days that Gemini had been flying, the Soviets did not launch a single person into space. More importantly, NASA used the Gemini program to develop all the essential skills for going to the Moon while engineers built and tested the new Saturn V rocket, the Command Module, and the Lunar Module that would make a Moon landing possible.

While other books highlight the technical aspects of the program, Gemini emphasizes the idea that the Gemini program was a key turning point in the space race, not just a stopgap program to keep America flying before Apollo was ready. The importance of this program to the success of Apollo and the end of the space race gets overlooked by the general public because it was not sexy, patriotic, or triumphant in the ways that Mercury and Apollo were. But the long list of challenges that NASA faced in putting a man on the moon were solved because of Gemini. The glory belongs to those lesser-known engineers and astronauts who made the program possible, and Jeffrey Kluger deserves credit for bringing this program’s significance to the general public’s attention.