Her website, Sally Ride Science has a short Memorial to her.
Ride died at her home in the San Diego suburb of La Jolla, said Terry McEntee, a spokeswoman for her company, Sally Ride Science. She was a private person and the details of her illness were kept to just a few people, she said.
Ride rode into space on the space shuttle Challenger in 1983 when she was 32. After her flight, more than 42 other American women flew in space, NASA said.
"Sally was a national hero and a powerful role model. She inspired generations of young girls to reach for the stars," President Barack Obama said in a statement.
NASA Administrator Charles Bolden, a former astronaut, said Ride "broke barriers with grace and professionalism -- and literally changed the face of America's space program."
"The nation has lost one of its finest leaders, teachers and explorers," he said in a statement.
Ride was a physicist, writer of five science books for children and president of her own company. She had also been a professor of physics at the University of California in San Diego.
She was selected as an astronaut candidate in 1978, the same year she earned her doctorate in physics from Stanford University. She beat out five women to be the first American female in space. Her first flight came two decades after the Soviets sent a woman into space.
"On launch day, there was so much excitement and so much happening around us in crew quarters, even on the way to the launch pad," Ride recalled in a NASA interview for the 25th anniversary of her flight in 2008. "I didn't really think about it that much at the time -- but I came to appreciate what an honor it was to be selected to be the first to get a chance to go into space."
Ride flew in space twice, both times on Challenger in 1983 and in 1984, logging 343 hours in space. A third flight was cancelled when Challenger exploded in 1986. She was on the commission investigating that accident and later served on the panel for the 2003 Columbia shuttle accident, the only person on both boards.
She also was on the president's committee of science advisers.
The 20th anniversary of her first flight also coincided with the loss of Columbia, a bittersweet time for Ride, who discussed it in a 2003 interview with The Associated Press. She acknowledged it was depressing to spend the anniversary investigating the accident, which killed seven astronauts.
"But in another sense, it's rewarding because it's an opportunity to be part of the solution and part of the changes that will occur and will make the program better," she said.
Later in the interview, she focused on science education and talked about "being a role model and being very visible."
"She was very smart," said former astronaut Norman Thagard, who was on Ride's first flight. "We did have a good time."
It was all work on that first flight, except for a first-in-space sprint around the inside of the shuttle, Thagard recalled by phone on Monday. He didn't know who won.
One of Ride's last legacies was allowing middle school students to take their own pictures of the moon using cameras aboard NASA's twin Grail spacecraft in a project spearheaded by her company.
"Sally literally could have done anything with her life. She decided to devote her life to education and to inspiring young people. To me, that's such a powerful thing. It's extraordinarily admirable," said Maria Zuber, a professor at the Massachusetts Institute of Technology who heads the Grail mission.
Ride's office said she is survived by Tam O'Shaughnessy, her partner of 27 years; her mother, Joyce; her sister, Bear, a niece and a nephew.
While some commented that what she did was nothing remarkable, the Soviets did it decades before. One must remember that the Soviet female Cosmonaut Valentina Vladimirovna Tereshkova was ordered to do nothing during the flight but sit back and watch the view.
Dr. Ride was not on board the Shuttle to be an fixture. Dr. Ride was a physicist and on board to accomplish scientific inquiry. In other words, she worked as a scientist during both of her missions. (STS-7, STS-41-G).
Yesterday Sally Ride was stuck here on Earth, and now she is soaring among the stars that she loved.