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" The official launch countdown for Saturday&amp;#039;s launch of Space Shuttle Discovery and STS-121 began today at 5 p.m. EDT, with the launch clock set at T-43 hours. But for many at NASA, the countdown has been running since last July, when a piece of foam fell off Discovery&amp;#039;s external tank (ET) during launch. NASA managers quickly grounded the entire fleet, including Discovery, barely into her first day in orbit.  Since then, engineers have been struggling with how to best re-engineer the Space Shuttle&amp;#039;s ET to avoid the type of catastrophic impact damage doomed Columbia. The 15-story tall, bright orange fuel tank is covered by an inch-thick layer of polyurethane-like foam, which insulates the shuttle&amp;#039;s 535,000 gallons of cryogenic oxygen and hydrogen fuel, and prevents ice formation on tank exterior. Minute tank expansion can crack the foam insulation, allowing air into the cracks and causing pieces to fall off. When Discovery launched, a chunk of foam fell off the protuberance air load (PAL) ramp, a wedge shaped section of foam that serves as aerodynamic protection for fuel lines that feed the orbiter.  The end result of nearly a year of testing is the most significant re-design of the ET in its first flight in 1981. The PAL ramp is gone, and with it, 34 pounds of problematic foam insulation. Earlier this month on this page, astronaut Thomas D. Jones wrote that foam damage was not a major concern during his shuttle missions:  &amp;quot;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.&amp;quot; Indeed, the Space Shuttle is the most complex machine ever built, and, Jones continues, &amp;quot;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.&amp;quot;  But with the time ticking away from the launch clock, Saturday&amp;#039;s launch might be facing a bigger problem than safety concerns: According to NASA, the launch weather officer reported today that &amp;quot;there is a 60-percent chance that weather may prevent launch on Saturday.&amp;quot; Forecasts for the following two days are the same.\u2014Benjamin Chertoff; Additional reporting: Andrew Nusca UPDATE: Today&amp;#039;s launch weather forecast has not improved since yesterday&amp;#039;s prediction of a 60% chance of a scrub due to thunderstorm activity in central Florida. Posted: 6/29/2006, 11:23 EDT -BC Earlier: NASA&amp;#039;s &amp;quot;Go&amp;quot; for Launch, But Is the Space Shuttle?, The Great Space Shuttle Debate: Is the Space Shuttle Safe? A Special Report. Go Watch: The mission clock in Cape Canaveral, the official timepiece of the Space Shuttle.  "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/space/a736/3246971/>