NASA Says Shuttle Risk Overstated; Yet Some Risk Unavoidable

NASA's official overall probability risk assessment number (PRAN) for complete loss of life and vehicle for the Space Shuttle remains at the widely quoted 1/100. Theoretically, there's a one percent chance of catastrophe. While the agency stresses that Space Shuttle flights are extremely dangerous missions—sending humans into space always is—officials worry that the risk has been overstated. According to NASA, in practice, it's not as bad as it sounds.

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"This one in one-hundred is the probability risk assessment number given," NASA space operations spokesman Allard Beutel tells PM. "It's the chance of the possibility of loosing the space shuttle and crew." It's not, however, the odds of losing the crew on any given mission. "It doesn't mean that if you have a hundred launches, you're going to have an accident," says Beutel.

Karl Sigman, a Columbia University engineering professor, explains the difference: "If you flip a coin, you have a 50 percent chance of getting heads," he tells PM. "But on your next flip, you're not going to get tails simply because the other option already occurred." For odds to make sense, says Sigman, it must be considered over a large number of tries. The larger the number flips, the more accurate the coin flip probability becomes. According to Sigman, the 1/100 chance of a catastrophe would only apply after a large number of launches. It's called the Law of Large Numbers: The more frequent an event, the more likely you are to see the pattern.

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There are thousands of data points that go into calculating the PRAN and there are PRANs calculated for a multitude of different shuttle components. And, while 1/100 sounds extraordinarily dangerous, NASA has launched with worse: A 1993 study of the Shuttle put the PRA number at 1/90. After the Challenger accident, the assessment hit its all-time riskiest 1988, when PRAN for loss of vehicle and crew was assessed at 1/78. Safety improvements have improved probability of catastrophe, but only based on the data available at the time. In 2003, Space Shuttle Columbia launched with a 1/254 PRAN; only after the orbiter broke up during re-entry did engineers realistically factor the risk of foam damage into their PRAN studies. Accordingly, last year, Discovery launched with a 1/123 PRAN, and the huge amount data gathered from that mission has further refined that estimate to the current 1/100—which is, oddly enough, equivalent to the annual voluntary risk associated with a career as a professional stuntman, according to 2001 study.

"Does that mean that it's actually getting riskier?" asks Beutel. "I say the answer's probably 'no.' We understand so much more, better than we ever have before." Although the odds of complete disaster are growing more pessimistic (pre-Challenger the PRAN was thought to be 1 in 7000), the overall likelihood of having a problem lessens because more is known about the functions of each of the more the two-and-a-half million space shuttle parts.

PM contributor and four-time shuttle astronaut Thomas D. Jones, who is currently in Cape Canaveral to cover the launch for Fox News, is confident that Discovery's launch will mark the safest shuttle mission to date. "They've driven the risk of foam down to a level that I'm comfortable with," he says. "This is going to be a safer flight than the one last summer, and the flights before Columbia. But is there some catastrophic problem that hasn't presented itself yet in the 114 launches to date? That's the big unknown, but NASA's confident that Discovery is ready for flight."

"We made sure when we did the calculations we made the biggest chunk of foam come off at the worst time and hit us in the worst possible place," says Beutel. "I don't want to downplay the numbers at all because there's no question that what we do is high risk. The question is, are we taking those high risks responsibly? Based on every engineering assessment they've done, all the information, all the dissention and different engineering opinions, if our managers thought it was likely that we would have foam come off that could critically damage the shuttle, we wouldn't be trying to launch."

Nonetheless, Apollo astronaut Buzz Aldrin, tells PM that the safety risk of a particular mission should be considered in the context of the opportunities that mission presents. "We had better than a 99% chance of survival," Aldrin says. "And the gain was enormous. With the Space Shuttle, the gain is not as important."

So what, exactly are the odds of catastrophe? Columbia professor Sigman calculates that with a 1 in 100 PRAN for a complete loss of life and vehicle factored over the remaining 16 planned shuttle missions, there's a 15 percent chance that a crew won't return to Earth before the Space Shuttle program retires—safer, for example, than climbing Mt. Everest.

But, says astronaut Jones, odds are not much comfort to a Space Shuttle crew. "When I was flying," Jones tells PM, "they said the chance of catastrophe was one in two-hundred and fifty. Did that make me feel better? It certainly didn't make my wife feel any better. But we're in the realm of spaceflight, and that's a risk we're willing to take."—Erin Scottberg
LIVE COVERAGE OF THE LAUNCH OF SPACE SHUTTLE DISCOVERY

WEATHER WATCH: According to Tom Jones, the risk of a scrub due to thunderstorm activity remains at 60%. If bad weather prevents Discovery from launching tomorrow, additional attempts will be made Sunday, Tuesday or Wednesday. Monday is designated a rest-day for those involved with the mission.

Notes:

In fact, the Shuttle's real-world track record provides a bleaker assessment: The number of shuttle catastrophes (2) over the number of shuttle launches (114), places the statistical, historical catastrophe odds around one in 57, or 1.7%. This number is misleading, say NASA officials, because it doesn't take into account lessons learned, and safety measures implemented to prevent similar accidents in the future.

Apollo illustrates another way in which PRAN assessments can deviate sharply from reality: According to NASA documents, early Apollo program estimates of mission success were about 2%, or 1/50. However, six out of the seven Apollo missions landed on the moon, making the actual success probability 86%.