Nathalia Holt wasn’t planning to write a book. While pregnant, the Boston resident researched the name “Eleanor” and found a scientist born in 1932 who discovered more than 800 asteroids and comets while working at NASA’s Jet Propulsion Laboratory in Pasadena: Eleanor Francis Helin. Holt named her daughter Eleanor Francis, and subsequently unearthed photos of other women employed by JPL from the 1940s onward as math “computers,” integral to launching America’s space program.

Holt, a 2009 USC graduate with a Ph.D. in molecular biology, resolved to uncover their stories. What began as fascination with an unexplored pocket of history became “Rise of the Rocket Girls: The Women Who Propelled Us, From Missiles to the Moon to Mars,” which she’ll discuss Tuesday at Vroman’s Bookstore.

JPL possessed archived pictures of some — but not all their names. Intrigued by these mysterious strangers who’d worked on vital projects then vanished, Holt doggedly made phone calls “everywhere,” she says in an interview, for instance calling over 30 Barbara Paulsons in Iowa before finally reaching the JPL retiree. One woman called another, and “it all kind of snowballed from there.”

The stories in “Rocket Girls” stretch from 1939 almost through the present day, and simultaneously illuminate American space exploration and women’s evolving social status. At a time when less than a quarter of American women were working outside their homes, JPL’s computers earned rare professional equality via their math skills and developed rich, supportive friendships through jobs they loved.

“The long days they spent together weren’t enough,” Holt writes. “The women were constantly planning evening parties where they would chat about their lives and JPL.”

Yet mathematically gifted women faced a hellish conundrum: mid-20th century society stigmatized single or childless women, and jobs were not held for those who took maternity leave. In 1960 the highly respected Paulson was abruptly fired from her supervisor job for “insurance purposes” because of her pregnancy. She was later hired for a different position by Helen Ling, whose own efforts to raise a family while working were downright herculean. Several marriages crumbled beneath the stress. Holt, who interviewed many of the women and some families, says most of their children had no idea “how essential” their mothers had been “or even that they were working at mission control during many of those late nights.”

In those early decades, the male engineers only trusted numbers crunched by the women computers because mechanical computers were prone to weird glitches — and the consequences of mistakenly plotted trajectories and acceleration speeds were deadly. “Rocket Girls” shows them all feverishly racing the clock to advance NASA’s space program, as Russia’s Sputnik successes unnerve Americans already shocked by McCarthyism, the Cuban Missile Crisis and President Kennedy’s assassination. Holt’s well-paced accounts of the stop-and-start progress of the Explorer, Mariner, Mars Pathfinder and, later, Voyager probe missions are grounded in relatable personal details. Computers get calloused fingers from calculating formulas on heavy graph paper with pencils for hours each workday; lab members toss snowballs when a freak storm blankets JPL with a foot of snow in 1949; men sensibly wear short sleeves and pants at windy airfields in the 1940s and ’50s while women must dress in stockings and heels; the accomplished Paulson doesn’t feel “brave” enough to wear a pantsuit in the late ’60s; the women discuss birth control with relief.

Behind them hover renowned scientists: Richard Feynman, Carl Sagan, onetime Nazi (and Saturn 5 inventor) Wernher von Braun and Hsue-Shen Tsien, a native of China and victim of racism and 1950s Red Scare hysteria. In one heartrending passage, the beloved JPL co-founder is unfairly deported back to China, where he eventually becomes known as the “father of Chinese rocketry.” Holt, who researched his FBI file, says, “It’s so unfair. It’s hard not to wonder what he might have contributed to JPL and to space exploration.”

Helen Ling, who was also born in China and survived Japanese bombings during World War II to earn a full-ride scholarship to the University of Notre Dame, also encountered discrimination, but, Holt says, “never felt slighted by colleagues or her boss because of her heritage.” (Ling retired from JPL in 1994, a year after Paulson.) The exceptionally well-educated Janez Lawson — long the only African-American woman on JPL’s team — opines, “Scientists are less prejudiced.” Their experiences contrast sharply with those of counterparts at Langley in Virginia, recently depicted in writer-director Theodore Melfi’s film “Hidden Figures.” According to Holt, the women at JPL weren’t aware of other math computers around the country because little was shared between labs. “It’s shocking to them that this other group of women was [discriminated against], especially because they were treated so well at JPL.”

Did they ever discuss social justice issues, or speak up about moral qualms over military projects? Not much. Holt notes there “certainly was discussion about Japanese internment during World War II,” and says she heard “over and over again how happy they were when they left military designs behind and went into space exploration.” She finds it incredible that later, “even before the Civil Rights Act was passed, they were making big changes in their workplace in terms of their rights as female employees.”

What disturbs them now is the decline in female computer-science graduates: from 37 percent in 1984 to 18 percent today, per the National Science Foundation.

“They remember what it was like before 1970, before engineering schools admitted women, and Helen was essentially [hiring] women with fashion degrees and mathematics and encouraging them to go to night school in engineering,” Holt says. “A lot of women ended up becoming computer scientists and engineers at JPL thanks to Helen’s guidance.” She’s heartened by actions taken at engineering schools like Harvey Mudd College in Claremont that are boosting the number of women graduates.

“I really was just interested in learning about these women,” she says. “I’m still finding them and would like to document their stories. Even if they’re not in this book, I’m sure they’ll be valuable to history.” 

Nathalia Holt discusses “Rise of the Rocket Girls” at Vroman’s Bookstore, 695 E. Colorado Blvd, Pasadena, 7 p.m. Tuesday, Feb. 21. Info: (626) 449-5320. Vroman’s will provide books when Holt appears at Sierra Madre Elementary School, 141 W. Highland Ave., Sierra Madre, 7 p.m. Saturday, Feb. 25.,