SACHA PFEIFFER, HOST:
What goes up must come down, so Boeing's Starliner capsule returns to Earth tomorrow after a short stay at the International Space Station. It's a test mission before NASA gives the company the go ahead to launch astronauts later this year. And the final step of a safe return comes down to parachutes. From member station WMFE in Orlando, Brendan Byrne reports these are complex systems that are challenging to design and worrisome to engineers.
BRENDAN BYRNE, BYLINE: Falling back to Earth from space is no easy feat. Before its return, a spacecraft is traveling at orbital speed, something like 17,000 miles per hour or 25 times the speed of sound. The vehicle has to punch through the atmosphere at just the right angle and slow to a crawl before touching back down on the planet.
CHRIS SEMBROSKI: So you're hurtling back through the atmosphere. And you're seeing, well, particularly from my point of view, through my toes, I was able to see the flashes of pink and yellow and white and see the sparks fly by.
BYRNE: Chris Sembroski flew in SpaceX's Crew Dragon capsule last summer. The density of Earth's atmosphere takes care of a lot of that slowing, but creates an incredible amount of energy and G-forces. The atmosphere can only do so much. About 6 miles up is when the intricate parachute sequence begins, with pre-parachutes known as drogues, says Pedro Llanos, a professor of spaceflight operations at Embry-Riddle Aeronautical University.
PEDRO LLANOS: These two drop chutes slow down the capsule significantly, going from almost supersonic speeds to just about .2, .3 mach.
BYRNE: That's still not slow enough, so another set of massive parachutes deploys next, letting the capsule float down at just 15 miles per hour.
LLANOS: But a lot of astronauts have referred to this being one of the most difficult technologies.
BYRNE: Parachute technology has been around since the dawn of human spaceflight, but it's still an engineering challenge. The technology is complex. The atmospheric conditions of deployment are constantly changing. And Llanos says modeling parachutes using computers has been difficult.
LLANOS: It is rare that if you do test after test of these parachute technologies, you will encounter the same conditions, you will perform at the same level.
BYRNE: Both Boeing and SpaceX are working with NASA to transport astronauts to space and back. While they land in two different places - Boeing's Starliner touches down on land, SpaceX's Dragon splashes down at sea - the parachute technology is similar. And both companies have run into challenges. During the return of SpaceX's capsule with astronauts on board, one of the parachutes took an extra 75 seconds to deploy. Two months later, an uncrewed mission had a similar problem. NASA's Steve Stich says an investigation afterwards found no major issues.
STEVE STICH: So far, we don't see anything that looks strange in any of the imagery or off nominal.
BYRNE: Still, those hiccups are top of mind for the astronauts inside the capsule making the journey back through the atmosphere, says spaceflyer Chris Sembroski.
SEMBROSKI: For me, it was just a point where, you know, these parachutes are either going to work or they're not. But at this point, you just put your hands in the engineers and the technicians. And you just accept that risk.
BYRNE: All eyes will be on Boeing's Starliner return, including the astronauts who will soon fly in the vehicle, waiting patiently for the parachutes to inflate and the spacecraft to make its graceful fall from space safely. For NPR News, I'm Brendan Byrne in Orlando. Transcript provided by NPR, Copyright NPR.