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Kelly says his twin brother, Scott Kelly, who just returned to Earth after 340 days in space, was temporarily 2 inches taller. NASA is studying the pair to explore what spaceflight does to the body.

During his 340 days aboard the International Space Station, the astronaut documented his time there with hundreds of photos. Kelly says the perspective makes him feel like an environmentalist.

Early Air Force experiments helped pave the way for the space program. Joseph Kittinger, who jumped from a balloon 103,000 feet up, talks about his experience.

After NPR reported Bob Ebeling's story on the anniversary of the Challenger explosion, hundreds of people responded. Ebeling, now 89, says those letters "helped bring my worrisome mind to ease."