The Mercury 13 is a phrase that was coined by Hollywood producer James Cross to describe the 13 American women who underwent some of the same physiological screening tests in the 1960s as the United States' original Mercury Seven astronauts. The women were not part of NASA's astronaut program, and never flew in space.

Dr. William R. Lovelace II helped develop the tests for NASA's male astronauts and became curious to know how women would do taking the same tests. In 1960, Lovelace invited Geraldyn "Jerrie" Cobb to undergo the same rigorous challenges as the men.

Cobb, already an accomplished pilot, became the first American woman to pass all three phases of testing. The results were announced at a conference in Stockholm, Sweden. Lovelace and Cobb recruited more women to take the tests, financed by the world-renowned aviatrix Jacqueline Cochran.

Women responded after hearing about the opportunity through friends. All of the candidates were accomplished pilots. Some of them may have been recruited through the Ninety-Nines, a women pilot's organization.

This group of women that Jerrie Cobb called the Fellow Lady Astronaut Trainees (FLATs) accepted the challenge to be tested for a research program.

Since doctors didn't know what stresses astronauts would experience in space, tests ranged from the typical X-ray and general body physicals to the atypical, in which the women had to swallow a rubber tube so their stomach acids could be tested. Doctors tested the reflexes in the ulnar nerve of the women's forearms using electric shock. To induce vertigo, ice water was shot into their ears, freezing the inner ear so doctors could time how quickly they recovered. The women were pushed to exhaustion using specially weighted stationary bicycles to test their respiration. They subjected themselves to many more invasive and uncomfortable tests.

In the end, thirteen women passed the same physical examinations that the Lovelace Foundation had developed for NASA’s astronaut selection process (although the original number of male candidates was much larger, fewer men passed the tests). Those thirteen women were:

• Myrtle Cagle
• Jerrie Cobb
• Jan Dietrich
• Marion Dietrich
• Wally Funk
• Janey Hart
• Jean Hixson
• Gene Nora Stumbough
• Irene Leverton
• Bernice Steadman
• Sarah Ratley
• Jerri Truhill
• Rhea Woltman

A few women took additional tests. Jerrie Cobb, Rhea Hurrle, and Wally Funk went to Oklahoma City, Oklahoma for an isolation tank test and psychological evaluations. Because of other family and job commitments, not all of the women were asked to take these tests, however. Instead, the group prepared to gather in Pensacola, Florida at the Naval School of Aviation Medicine to undergo advanced aeromedical examinations using military equipment and jet aircraft. Two of the women quit their jobs in order to be able to attend. A few days before they were to report, however, the women received telegrams abruptly canceling the Pensacola testing. Without an official NASA request to run the tests, the Navy would not allow the use of their facilities for an unofficial project.

Jerrie Cobb immediately flew to Washington, D.C. to try to have the testing program resumed. She and Jane Hart wrote to President John F. Kennedy and visited Vice President Lyndon B. Johnson. Finally, on the 17th and 18th of July 1962, Representative Victor Anfuso (R-NY) convened public hearings before a special Subcommittee of the House Committee on Science and Astronautics. Significantly, the hearings investigated gender discrimination two full years before the Civil Rights Act of 1964 made that illegal, making these hearings a marker of how ideas about women's rights permeated political discourse even before they were enshrined in law. Cobb and Hart testified about the benefits of Lovelace's private project. Jacqueline Cochran talked about her concerns that setting up a special program to train a woman astronaut could hurt the space program. NASA representatives George Low and Astronauts John Glenn and Scott Carpenter testified that the women could not qualify as astronaut candidates. They claimed that NASA required all astronauts to be graduates of military jet test piloting programs and have engineering degrees, even though John Glenn himself did not have an engineering degree when he was selected. In 1962, women were still barred from Air Force training schools, so no American women could become test pilots of military jets. Despite the fact that several of the Mercury 13 had been employed as civilian test pilots, at least one had jet time, one was an engineer, and most had considerably more flying time than the male astronaut candidates, NASA refused to consider granting an equivalency in light of experience as they had for Glenn . Although some members of the Subcommittee were sympathetic to the women's arguments, no action resulted.

Lovelace's privately-funded women's testing project received renewed media attention when Soviet cosmonaut Valentina Tereshkova became the first woman in space in 1963. In response, Clare Boothe Luce published an article in Life criticizing NASA and American decision makers. By including photographs of all thirteen Lovelace finalists, the names of all thirteen women became public for the first time. (Significant media coverage had already spotlighted some of the participants, however.)

Although both Cobb and Cochran made separate appeals for years afterward to restart a women's astronaut testing project, the U.S. civil space agency did not select any female astronaut candidates until the 1978 class of Space Shuttle astronauts. Astronaut Sally Ride became the first American woman in space in 1983 on STS-7, and Eileen Collins was the first woman to pilot the Space Shuttle during STS-63 in 1995. Collins also became the first woman to command a Space Shuttle mission during STS-93 in 1999. In 2005, she commanded NASA's return to flight mission, STS-114. At Collins' invitation, eight of the eleven surviving Lovelace finalists attended her first launch and she has flown mementos for almost all of them.

In May 2007, the eight surviving members of the group were awarded honorary doctorates by the University of Wisconsin-Oshkosh.