NASA's shuttle program, set to make its final flight later this week, has resulted in the death of 14 astronauts. But it could have been a lot worse. The agency’s fleet of 100-ton orbiters faced numerous imminent threats for more than 30 years, both on the Earth and traveling at 28,000 kilometers per hour a slice above its surface. Some are well-documented, but many drew little public attention, let alone scrutiny. 

Dangers ranged from faulty equipment to orbital debris to human error—but the bottom line is that space travelers faced risks far greater than the space agency likely cared to publicize. The International Space Station (ISS), which orbital crews traveling via the shuttle (as well as Russian Soyuz spacecraft) spent the past 13 years constructing, will continue to face such issues long after Atlantis launches and closes the books on NASA's shuttle program.

If anyone can speak to the tensest moments in the history of the two historically linked wonders of engineering, it would be former shuttle chief Wayne Hale and astronaut Leroy Chiao.

Hale worked on the space shuttle’s propulsion systems in the late 1970s and eventually oversaw 41 missions as a Mission Control flight director. He later stepped up as manager of the space shuttle program, a position he held until he retired a year ago.

Chiao, meanwhile, rode three space shuttle flights under Hale’s watch, lived in space for a total of 229 days and ventured outside of the space station six times within the confines of a bulky space suit for carefully choreographed spacewalks.

"It’s hard to pick the closest calls," Hale says. "There were surprises every day because the space shuttle and space station are such extraordinarily delicate and complex systems. Everything may look very calm and unexciting on TV, but let me tell you, my heart raced every time a mission was going on and I was in the control center. You had to be ready for anything and everything."

With NASA’s last functioning orbiter preparing to part ways with the ISS and the final frontier forever, Scientific American chatted with Hale and Chiao about their experiences from the ground and in orbit. We stroll here through some of the closest calls in the space shuttles’ and ISS’s intertwined careers.

» View a Slide Show of the Top 10 Most Dangerous Moments in Shuttle History