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" An hour after sunrise on December 7, 1996, my crewmates and I stepped haltingly onto the smooth concrete of Kennedy Space Center&amp;#039;s Runway 33. Before us, Space Shuttle Columbia stood solidly on her landing gear, at rest, after serving as our home for nearly 18 days in orbit\u2014the longest shuttle mission ever.  The five of us wandered slowly, almost reverently, under our ship, her black-tiled belly still warm in spots from the plasma of re-entry, as we paid our respects to the orbiter that had brought us safely to Earth. We inspected Columbia&amp;#039;s thermal protection system tiles carefully, assessing the damage from the various scuffs and bruises of spaceflight. The worst was a 3-inch-long, \u00BE-inch wide, and \u00BD-inch deep tile gouge just aft of where the hydrogen fuel line from the external tank (ET) once entered Columbia&amp;#039;s aft end. Engineers chalked that up to a piece of ice flung back from the propellant line during launch. But overall, there was surprisingly little damage: a total of only 93 debris hits overall\u2014and just 9 impacts measuring an inch or larger in diameter\u2014after our 7-million-mile flight. The damage on this eightieth shuttle mission had been minor, much lower than average, in fact.  Seven years later, Columbia and her STS-107 crew would encounter much worse. A 1.6-pound chunk of foam from the shuttle&amp;#039;s ET struck the front edge of her left wing at some 500 mph, fatally damaging the orbiter. Then, last July, the nation watched as yet another chunk of foam broke free, this time a one-pound piece from the external tank&amp;#039;s 37-foot-long PAL ramp, narrowly missing the orbiter&amp;#039;s wing. With Discovery barely into her first day in orbit, NASA effectively grounded the shuttle fleet until the foam problem could be licked. Whatever relief Discovery&amp;#039;s crew felt at having literally dodged a bullet was crushed by the clear evidence that NASA&amp;#039;s attempts to solve the foam loss problem for good had failed. Now, after nearly a year of troubleshooting and repeated delays, the question remains: Is the shuttle safe? Back to the Drawing Board According to NASA, the answer is yes. Since last year&amp;#039;s launch, engineers have made the most significant changes in the External Tank&amp;#039;s aerodynamic design since its first flight in 1981. The ET is covered with a 1-inch-thick foam layer that has two functions: It keeps the super-cold propellants inside from boiling, and prevents ice from forming on the tank while it&amp;#039;s sitting on the launch pad. (This insulation is so effective that even though the liquid oxygen and liquid hydrogen are stored at -297\u00BA F and - 423\u00BA F, respectively, the tank surface feels only slightly cool to the touch.) But the insulation has been consistently troublesome: Ppieces of the foam (and small chunks of ice) have always torn loose in flight. To minimize the risk of foam breaking free, NASA removed 34 pounds of foam in the form of two thick &amp;quot;ramps,&amp;quot; or wedges, originally designed to protect pressurization lines and wiring from wind loads during flight. In addition, six cameras have been added to the solid rocket boosters to monitor the ET for sources of debris.  Last week, Shuttle managers met in Florida to review those efforts. Shuttle program manager Wayne Hale promised additional changes, on future flights, to the &amp;quot;ice/frost ramps,&amp;quot; the foam-covered brackets that carry those electrical and pressurization lines. He emphasized in his May 31st briefing that if foam pieces do fall from those areas, they will be too small (about 3 ounces) to seriously damage the tiles.  After last week&amp;#039;s review, NASA declared that the risk of damage to Discovery from ET foam loss was small enough to clear the orbiter for her scheduled July 1 launch of STS-121. In other words, although some foam will come off the tank, the debris will be too small to cause critical damage. So, NASA concludes, the shuttle is again safe to launch. Discovery&amp;#039;s 7-member crew will dock with the Space Station to deliver much-needed supplies, make exterior repairs, and test spacewalking techniques for repairing shuttle thermal tiles. The larger purpose of the test flight is to show that NASA has successfully figured out how to keep the ET from damaging the orbiter&amp;#039;s tiles in the first place. How Safe Is Safe Enough? The Space Shuttle was oversold as a reliable craft with an almost airliner-like level of safety\u2014and even after Challenger, we had perhaps fooled ourselves as to the complexities and dangers of human spaceflight. Columbia harshly reminded us that no flight is routine, and we will never make spaceflight perfectly safe. The shuttle is, by its very design, an amazing yet vulnerable machine. &amp;quot;It is a risky vehicle to fly, and nobody should mistake that,&amp;quot; Hale said during the meeting. The chance of a catastrophic failure on each flight, from sources such as engine failures, orbital debris impacts and reentry hazards, is about one in a hundred. My big worry during each of my four launches was that one of the three main engines would suffer an &amp;quot;uncontained&amp;quot; failure during the eight-and-a-half-minute climb to orbit\u2014violently tearing itself apart and blasting shrapnel into the remaining engines and nearby systems. Moonwalker John Young used to tell us new astronauts how that would &amp;quot;blow the back end off&amp;quot; the orbiter\u2014we wouldn&amp;#039;t be coming back. By comparison, tile damage from foam strikes seemed like a mere nuisance. I should have known better. But for me, it was simply not possible to worry about everything that could go wrong. If I did, I would never muster the courage to strap into that machine. I trusted the managers and engineers\u2014my colleagues and friends\u2014to hammer down those risks to the lowest possible level. Yes, humans were and are fallible, but I knew they were doing their best to minimize the danger. Once we&amp;#039;ve done that, it&amp;#039;s necessary to fly the mission\u2014or get out of the human spaceflight business entirely.  We call this notion &amp;quot;acceptable risk.&amp;quot; As an astronaut, the bottom line for me was that the benefits of a flight had to equal or exceed the hazards I faced. It&amp;#039;s what I truthfully told my family before saying good-bye on each mission. And it&amp;#039;s the case now as we critically examine this upcoming launch, and the shuttle&amp;#039;s future. Flying the shuttle is risky. But if the test flight of STS-121 goes well, and if we are prudent about reducing those risks during the final three years of the shuttle&amp;#039;s service, we will reap a final, valuable payoff from our investment: a completed Space Station, and a shorter, smoother transition to a safer, more reliable spacecraft. That new Crew Exploration Vehicle should be many times safer than the shuttle. But in 2006 we have only the shuttle. We should not commit it to flight lightly, or without full awareness of the risk. But I&amp;#039;m convinced we risk more of our future in not flying than in forging ahead with our eyes open, determined to learn from each attempt we make at exploration.\u2014Tom Jones Thomas D. Jones was a mission specialist astronaut on four shuttle flights. His latest book is Sky Walking: An Astronaut&amp;#039;s Memoir, from Smithsonian Books\u2014Collins. Reach him via AstronautTomJones.com. Earlier: NASA Alters Its Course To The Moon. Again. Is the Program Heading For Trouble? All Dressed Up And Ready To Go: Space Shuttle Discovery arrives at Kennedy Space Center&amp;#039;s Pad 39B last week. NASA/KSC Missing Pieces: The view from Discovery of the empty external tank, a small white gouge on the PAL ramp where a piece of foam broke away during last summer&amp;#039;s launch. NASA/JSC Retooled: External tank ET-119, redesigned for July&amp;#039;s launch of Discovery and STS-121. NASA/KSC Safe Landing: The crew of STS-80 stands in front of Space Shuttle Columbia on the Kennedy Space Center runway after the longest shuttle mission in history. From left: Mission Specialist (and PM contributor) Thomas D. Jones; Commander Kenneth D. Cockrell; Mission Specialists Story Musgrave and Tamara E. Jernigan; and Pilot Kent V. Rominger. NASA/KSC "^^<http://www.w3.org/2001/XMLSchema#string>
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