Hyperbole shrouds few topics in history so much as the human encounter with outer space. Astronauts are heroic pioneers, their missions testament to humanity’s “hunger to explore the universe and discover its truths,” as Ronald Reagan put it in 1986. At the dawn of the American space program 25 years before that, John F. Kennedy had gone so far as to declare exploration of the heavens no less than “the key to our future on Earth.” Through the rhetorical haze, it can be hard to see just how contingent and contentious the whole endeavor to send Americans into space really was, especially at its inception in the late 1950s and early 1960s. Jeff Shesol’s “Mercury Rising” highlights this fragility in a refreshing narrative that captures the sometimes dispiriting realities of America’s debut in space. To be sure, Shesol’s account of Project Mercury, the United States mission to put a man into orbit around the Earth, exudes admiration for the technical achievements that led to success in February 1962. Echoing Tom Wolfe’s classic “The Right Stuff,” Shesol acknowledges the sheer courage of the astronauts who risked their lives. But he strikes a distinctly ambivalent tone through his captivating portraits of two fallible men whose intertwined struggles and doubts tell the story that unfolded out of public view.One is Kennedy, the president who inherited an underfunded and underperforming space program when he entered the Oval Office in January 1961. By June of that year, Kennedy had transformed the effort, urging Congress to spend vast funds and proposing to land a man on the moon by the end of the decade. Space exploration, Kennedy intoned, was a “great new American enterprise” that would affirm the nation’s global leadership.But Kennedy’s grand words hid private worries. Affirming other studies revealing Kennedy’s essential cautiousness, Shesol underscores his doubts that the United States could catch up with the Soviet Union, which had surged ahead in the space race by launching the first satellite — Sputnik — into orbit in 1957 and then sending the first human into space four years later. Shesol also details Kennedy’s persistent misgivings about the cost of space exploration and skepticism about the tangible benefits that it would bring.In the end, Shesol argues, Kennedy embraced a dramatically expanded space program not out of genuine conviction of its value so much as a desire to bolster national prestige at a time when many Americans believed the Soviets held the upper hand in the Cold War. In fact, the lagging American effort posed no serious dangers; there was hardly any connection between Moscow’s successes in space and its military capabilities. But Kennedy, Shesol suggests, understood the damaging symbolism of Soviet astronauts, fresh off successful flights, parading triumphantly through Red Square while the American space program “churned in neutral.”Presidential decision-making lies squarely in Shesol’s analytical wheelhouse. An expert on presidential oratory and a onetime White House speechwriter for Bill Clinton, Shesol is the author of well-regarded histories of Franklin Roosevelt’s 1937 plan to expand the Supreme Court and Lyndon Johnson’s bitter relationship with Bobby Kennedy. Yet “Mercury Rising” is at least as successful when it departs the White House and zeros in on the other, less familiar man at the center of the story, John Glenn.Shesol dutifully relates the arc of Glenn’s life, from his humble origins in small-town Ohio to his 24-year career in the United States Senate. Most of the book, though, narrates the ups and downs of Glenn’s struggles to become the first American to circle the Earth, an ambition that culminated in the triumphant flight of his Friendship 7 capsule on Feb. 20, 1962. The feat, which came after 10 postponements due to technical glitches or bad weather, stirred huge jubilation and, Shesol stresses, gave the country “its swagger back.”Glenn’s early experiences made him an ideal candidate for this cathartic role. The son of hard-working parents, he oozed the virtues of middle-class America. He taught Sunday school, married his childhood sweetheart and raised kids. He enlisted in 1942 and, as a Marine fighter pilot, flew dozens of combat missions in the Pacific during World War II and in Korea. In 1957, he gained national fame for piloting a supersonic jet from coast to coast in record time. Faith, patriotism and futuristic possibility hung heavily around this “everyman superman,” in Shesol’s memorable phrase, at just the moment when the newly established National Aeronautics and Space Administration selected him as one of the seven men who’d be trained for the nation’s first manned spaceflights.Still, securing a leading role in Project Mercury was, Shesol writes, hardly assured. Just as Kennedy understood the power of symbols, Glenn knew that image counted as much as substance in the decisions about which men got the plum assignments. To burnish his chances, he cultivated journalists and fine-tuned the persona of the “aw-shucks, homespun hero” that NASA wanted him to be, Shesol explains.As with Kennedy, Glenn’s public face hid messier realities. Lengthy passages describe Glenn’s exasperation with other Mercury astronauts, including his worries that off-hours carousing in the beach communities near Cape Canaveral would tarnish the whole program. Exasperation escalated to fury when NASA chose Alan Shepard for the first manned launch in 1961. Shepard’s brief “suborbital” mission was nothing like the nearly five-hour flight that Glenn would accomplish a year later, but NASA’s decision gave Shepard, not Glenn, the distinction of being the first American in space.Shesol dwells too on Glenn’s resentment about NASA’s insistence on controlling the Mercury flights from the ground, making the astronauts more guinea pigs along for the ride than true pilots. Glenn’s anger at NASA management spiked especially, Shesol writes, after his flight when he learned that ground controllers had decided against informing him of a potentially catastrophic malfunction in one of the spacecraft’s systems.“Mercury Rising” relates such details, not to mention the blow-by-blow of Glenn’s three orbits around the Earth, with verve, revealing Shesol’s extraordinary talent as a storyteller. The only downside is that Shesol rarely breaks from his rollicking narrative to lay out the larger context or to engage the big questions his story poses.How did Project Mercury contribute to the 1969 moon landing or later space achievements? What lessons did NASA learn from the way it chose or trained astronauts in the early days? In the geopolitical realm, how did Glenn’s flight and other accomplishments in space during the 1960s alter the ebb and flow of the Cold War? The book offers no clear guidance, even though overall judgments about Kennedy’s and Glenn’s roles at the dawn of America’s space age depend on answers.Neither does Shesol explicitly engage the question that runs through “Mercury Rising” more powerfully than any other: Was the effort to launch humans into space, at its heart, more an enormously expensive public-relations exercise than a consequential scientific undertaking, as both Kennedy and Glenn seem to have suspected?Recognizing that the answer might be yes does not necessarily mean that the whole endeavor lacked serious purpose. As Shesol makes clear, Americans had good reason to believe that their nation was losing its edge in the late 1950s and to respond rapturously to Glenn’s flight. Yet Shesol’s story raises inescapable questions about whether space exploration is quite what its enthusiasts have often claimed.