Feb. 1, 2003 If 40% of all the 747s that ever flew or all the luxury liners that ever sailed crashed or sank and killed everyone aboard, chances are you'd take them permanently out of service. Not so for the shuttles. Five have flown, and two have now crashed, the second going down in 2003 after 22 years of use. Like an army that does a bang-up job of fighting the last war, NASA made sure that no flawed O-ring would ever again destroy a ship the way Challenger was but neglected to address another problem: the way hardened insulating foam falling from the external tank could damage the orbiter's heat-resistant tiles and cause the hull to burn through on re-entry. That's what claimed Columbia, requiring the names of seven more astronauts to be etched into NASA's memorial wall. And still the shuttles fly though only until 2010, when the woebegone ships are, at last, set to be put in permanent drydock.