updated 2/20/2012 2:25:05 PM ET 2012-02-20T19:25:05

When John Glenn launched on the United States' first orbital spaceflight 50 years ago today (Feb. 20), NASA scientists weren't sure where he'd come down — or if he'd even survive the trip.

Before Glenn completed three laps of Earth on Feb. 20, 1962, no American had spent more than 15 minutes in space. So NASA top brass and medical personnel had a laundry list of worries, from where Glenn's spacecraft would touch down to whether or not the astronaut's eyes would function properly in microgravity.

"There were a lot of unknowns in the early days of spaceflight," former astronaut Scott Carpenter, who completed an orbital mission of his own in May 1962, said Friday (Feb. 17) at a NASA event commemorating Glenn's flight. "We were considered guilty of being unable to fly in space and required to prove our innocence, counter to the American custom."

Medical concerns

By the time of Glenn's flight, space-race rival the Soviet Union had already launched two manned orbital missions. Cosmonaut Yuri Gagarin circled the Earth once in April 1961, and his countryman Gherman Titov orbited our planet 17 times in August of that year, staying aloft for more than 24 hours.

Both Gagarin and Titov returned to Earth safely after their flights. But NASA had no experience with human spaceflight beyond the 15-minute suborbital jaunts of Alan Shepard and Gus Grissom in 1961, so agency personnel worried about how Glenn's body would hold up during nearly five hours in space.

Doctors were particularly concerned about how prolonged exposure to microgravity would affect Glenn's vision. [ Photos: John Glenn's Space Legacy ]

"Some of the ophthamologists were literally concerned at that time that your eyes might change shape and your vision might change enough you couldn't even see the instrument panel enough to make an emergency re-entry if you had to," Glenn said during Friday's festivities.

"They were enough concerned about it, we actually put a little miniaturized eye chart at the top of the instrument panel," he added. "And that's still in Friendship 7, up in the Smithsonian [National Air and Space Museum]."

NASA also worried that spaceflight might cause fluid to move around randomly in Glenn's inner ear, perhaps resulting in nausea and vertigo. And doctors weren't even sure if the astronaut would be able to swallow properly in microgravity, Glenn said.

Concern and curiosity about digestive functions persisted through Carpenter's flight, three months later. Doctors weren't convinced astronauts would be able to metabolize food on orbit, so they had Carpenter perform a little experiment. [ Biggest Revelations of the Space Age ]

"I was given some radioactive food — pap — in a toothpaste tube," Carpenter said. "And I was told to eat that on the first orbit."

History has shown, of course, that the human body can perform basic functions in space. Astronauts routinely live on the International Space Station for six-month stints, though they must exercise assiduously to minimize the effects of microgravity, which include muscle wasting and decreased bone density.

A dicey re-entry

Glenn's flight plan called for Friendship 7 to splash down in the Atlantic Ocean several hundred miles southeast of Florida. But NASA wasn't entirely sure that would work out, so Glenn prepared for the possibility of landing among "primitive" aboriginal peoples in backcountry Australia, Papua New Guinea or southern Africa.

"You land, and the side blows off, and out steps this thing in a silver suit. You're going to be either like the god-king or dead pretty quick," Glenn said. "So I wanted a message for these people."

So linguists at the Library of Congress translated a basic message of peace and friendship for Glenn, arming him with a few phrases in various aboriginal languages should the need arise.

Friendship 7 eventually did re-enter roughly where it was supposed to, dropping into the Atlantic just 40 miles (67 kilometers) short of the planned landing zone. But the capsule's return to Earth was a bumpy and somewhat harrowing one.

As Glenn prepared to re-enter the atmosphere, mission managers told him that Friendship 7's protective heat shield may have come loose. This was bad news; if the heat shield came off, the capsule would almost certainly burn up.

Strapped to the outside of Friendship 7 was a package of small retro-rockets, which were designed to help slow the capsule's re-entry. Glenn was told not to jettison the rockets after firing them, in the hopes that the straps would help hold the heat shield on.

During re-entry, "there were flaming chunks of the retro-pack burning off and coming back by the window," Glenn said. "I didn't know for sure whether it was the retro-pack or the heat shield, but there wasn't anything I could do about it either way, except just keep trying to work and keep the spacecraft on its actual best attitude coming back in."

Though everything worked out in the end, Glenn's success was far from assured or pre-ordained. In fact, launching him into orbit was something of a game of Russian roulette, experts say. The odds of success were comparable, anyway.

"His odds of not surviving this was about one in six," former astronaut Steve Lindsey, who flew with Glenn on the space shuttle Discovery's STS-95 mission in 1998, said in a recent NASA video. "So it was an extremely high-risk, unknown effort that they were going into, having never done it before."

You can follow senior writer Mike Wall on Twitter:@michaeldwall. Follow for the latest in space science and exploration news on Twitter@Spacedotcomand onFacebook.

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Photos: John Glenn through the years

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  1. John Glenn's ascent

    John Herschel Glenn Jr. flew into the history books on Feb. 20, 1962, when he became the first American to go into orbit. But his trajectory to greatness was set years earlier, as a Marine Corps pilot. Glenn flew 59 combat missions in World War II and 90 combat missions in the Korean War. Here we see Glenn climbing out of the cockpit of his F-8-UI Crusader jet at Brooklyn's Floyd Bennett Field in 1957, after making the first nonstop, supersonic flight from Los Angeles to New York. The flight was called "Project Bullet" because Glenn traveled faster than a bullet. The 3-hour, 23-minute trip set a transcontinental speed record, and put Glenn on the radar screen for selection as an astronaut a couple of years later. (Anthony Camerano / AP) Back to slideshow navigation
  2. Getting a checkup

    Glenn and six other military fliers were selected in 1959 to become the first American astronauts as part of Project Mercury. NASA put the "Mercury 7" through a grueling series of medical and psychological tests. Glenn is seen here being outfitted with a biosensor during astronaut training activities at Cape Canaveral, Fla., in 1961. (NASA) Back to slideshow navigation
  3. The Mercury 7

    The seven Mercury astronauts pose for a photo in their spacesuits in 1962. Front row, left to right, are Wally Schirra, Deke Slayton, John Glenn and Scott Carpenter. Back row, from left, are Alan Shepard, Gus Grissom and Gordon Cooper. As of 2012, only Glenn and Carpenter are still alive. (NASA) Back to slideshow navigation
  4. Running man

    By 1962, astronauts Alan Shepard and Gus Grissom had flown on suborbital Mercury missions. Those flights set the stage for John Glenn's orbital odyssey. Glenn took a no-nonsense, straight-arrow approach to his physical training program, including frequent runs like this one in 1962. (NASA) Back to slideshow navigation
  5. Godspeed, John Glenn

    John Glenn's orbital flight was initially set for launch in January 1962, but postponements pushed it back to Feb. 20. Glenn's Friendship 7 capsule was launched atop an Atlas rocket from Cape Canaveral as an estimated 60 million people watched via live television. Fellow astronaut Scott Carpenter called out to him over a radio link from the launch pad's blockhouse: "Godspeed, John Glenn!" (AFP - Getty Images) Back to slideshow navigation
  6. Safe landing

    During Friendship 7's three-orbit flight, NASA controllers saw indications that the capsule's heat shield had come loose - which could be a fatal flaw for re-entry. They told Glenn not to jettison the craft's retro-rocket pack, as an added precaution. Glenn ended up making a safe splashdown in the Atlantic Ocean, and it turned out that the apparent heat-shield problem was merely due to a sensor failure. Here you see Glenn's capsule attached to a retrieval cable hanging down from a helicopter. The capsule released green dye into the water to help searchers find it from the air. (Rex Features via AP Images) Back to slideshow navigation
  7. Triumph in Florida

    President John F. Kennedy, astronaut John Glenn and Gen. Leighton I. Davis, commander of the Air Force Missile Test Center, ride together in the back seat during a 1962 parade in Cocoa Beach, Fla., to celebrate America's first human orbital spaceflight. (NASA) Back to slideshow navigation
  8. Family time

    Astronaut John Glenn, far right, accompanies his family onto an Air Force plane in Key West, Fla., on Feb. 26, 1962, en route to a series of celebrations after his spaceflight. From left are his wife, Annie, their daughter Lyn and their son David. John and Annie were childhood playmates and high-school sweethearts while growing up in New Concord, Ohio. They were married in 1943, just after John Glenn received his commission in the Marine Corps. (Harold Valentine / AP) Back to slideshow navigation
  9. Ticker-tape parade

    Astronaut John Glenn, riding in the car seen at left, gets a huge New York welcome on March 1, 1962, during a ticker-tape parade along Broadway on the way to a City Hall ceremony. (AP) Back to slideshow navigation
  10. Skiing with Jackie

    Astronaut John Glenn and First Lady Jacqueline Kennedy water-ski on a tandem tow on Massachusetts' Lewis Bay, near Hyannis Port, on July 22, 1962. Glenn and his family were the weekend house guests of Jackie's brother-in-law, Attorney General Robert F. Kennedy. (AP) Back to slideshow navigation
  11. Running for office

    NASA made it clear that John Glenn would not be going back into space anytime soon - he was just too important as a national hero. Glenn left the astronaut corps in 1964 and decided to get into politics. At a 1969 news conference in Columbus, Ohio, he announced his candidacy for the Democratic nomination for U.S. senator. He lost out to fellow Democrat Howard Metzenbaum in the 1970 primary, but in 1974, Glenn prevailed and finally entered the Senate. (Gene Herrick / AP) Back to slideshow navigation
  12. Senatorial chat

    Sen. John Glenn confers with a fellow Democrat on the Senate Foreign Relations Committee, Joe Biden, during a 1981 hearing on the sale of an AWACS radar plane to Saudi Arabia. Glenn would serve in the Senate until 1999, while Biden would go on to become vice president in 2009. (Bob Daugherty / AP) Back to slideshow navigation
  13. Back in a spacesuit

    Thirty-six years after John Glenn's first orbital flight, NASA decided to give him another space shot on the shuttle Discovery in 1998, during his final months as a U.S. senator. Here you see Glenn checking the communications system on his headgear, prior to bailout training at Johnson Space Center in Texas on April 12, 1998. Jean Alexander, a NASA suit expert, waits to help him with his helmet. (NASA via Getty Images) Back to slideshow navigation
  14. Try a bite, Mr. President

    President Bill Clinton gets a helping of a space shuttle meal from senator/astronaut John Glenn while shuttle commander Curt Brown looks on, during a tour of the space shuttle mockup at NASA's Johnson Space Center in Texas on April 14, 1998. (Pool via AFP - Getty Images) Back to slideshow navigation
  15. Zero-G and I feel fine

    Senator-astronaut John Glenn works on the shuttle Discovery's Advanced Organic Separation experiment after the STS-95 mission's launch on Oct. 29, 1998. Glenn was accompanied on the nine-day research flight by four other U.S. astronauts, a Spaniard and a Japanese spaceflier. The trip made Glenn the oldest human to fly in space, at the age of 77. (NASA) Back to slideshow navigation
  16. Back on parade

    Senator-astronaut John Glenn and his wife Annie, along with other members of the shuttle Discovery's crew, parade up Broadway's "Canyon of Heroes" on Nov. 16, 1998. This was the second time Glenn received a ticker-tape parade in New York. The first one came after his history-making 1962 orbital flight. (Timothy A. Clary / AFP - Getty Images) Back to slideshow navigation
  17. Medal-winner

    At the age of 90, retired senator-astronaut John Glenn shows off his Congressional Gold Medal in the Capitol Rotunda on Nov. 16, 2011, flanked on the left by Senate Majority Leader Harry Reid, D-Nev., and on the right by Senate Minority Leader Mitch McConnell, R-Ky. Glenn received the medal for helping to "pave the way for the first lunar landing" in 1969. (Evan Vucci / AP) Back to slideshow navigation
  18. Elder statesman

    This Jan. 25 photo shows retired Sen. John Glenn at his office in Columbus, Ohio. It's been 50 years since his milestone spaceflight in a Mercury capsule, and almost 14 years since his space shuttle flight and retirement from the Senate. "I've been very fortunate to have a lot of great experiences in my life, and I'm thankful for them," he says. (Jay LaPrete / AP) Back to slideshow navigation
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Video: John Glenn’s 1962 Friendship 7 flight


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