The number is bleak and every astronaut who flies into space knows it: There is a one in 75 chance they will not return home.
Mike Fincke, who will set a U.S. record for time in space, 380 days, during STS 134 Endeavour's mission scheduled launch Monday, said the risk became more real to him when he became a father.
"I know it is risky, but I have chosen to do this because of what we learn," he said. "Think of everything you have that makes life easier -- smart phones, wifi, that is the result of microelectronics developed for the space program. Whole worlds are opened up to my children because of the Internet."
The memory of the Columbia accident in 2003 is still vivid for them -- the seven astronauts were colleagues, and friends. If it were not for the Columbia accident the space shuttle program would probably not be ending this year.
Greg Johnson, Endeavour's pilot, is cool, competent with a devilish sense of humor and a calm acceptance of the danger ahead.
"I feel the risk, and I compare launching on the space shuttle a little like going into combat," he said. "Any sane astronaut will feel the fear, or concern just prior to liftoff. If they don't admit they are lying to you."
Johnson is a veteran fighter pilot; he flew 38 combat missions for the Air Force during Operation Desert Storm. He remembers sitting in Endeavour's cockpit before his first flight in 2008 waiting for the engines to ignite.
"I had a sense of wow, I don't have control over this, and I was hoping all the engineers and everybody that was on the team making this mission happen did their homework when they were in college," he said.
The Columbia accident refreshed memories for Americans who had perhaps forgotten about the Challenger accident. Yes, space travel is dangerous, but the danger is something astronauts are willing to accept.
Eileen Collins commanded the first flight after the Columbia accident and she told ABC news before her flight: "I like to refer back to history to explorers who travel across the ocean to places where we've never been before just because it is our human nature to explore. We are taking certain risks yes, we fly into space, we think we understand the risk -- we hope we understand the risk -- but the benefit that we gain is definitely worth it, so we explore."
The explorers flying on Endeavour are led by Commander Mark Kelly, flying on his fourth mission. Kelly was thrust into the spotlight after the assassination attempt on his wife, Rep. Gabrielle Giffords, earlier this year.
He considered withdrawing from the mission to stay by his wife's side, but ultimately chose to command Endeavour's last flight, which is expected to be a complicated 16-day mission to deliver and install the $2 billion Alpha Magnetic Spectrometer, a science platform to prove or disprove the Big Bang Theory of the formation of our universe.
Greg Johnson said what we will learn from the Alpha Magnetic Spectrometer makes him willing to accept the risk of spaceflight.
"The excitement and the anticipation to just go do it, far outweigh that little bit concern of the unknowns," he said. "I want to come back and tell the story to my kids and my wife."