LIVE NOW: Watch coverage of today's national memorial honoring the seven fallen Columbia astronauts. news_spaceshuttle_columbia_montagebush_rts.jpg home Columbia Memorial superpass.gif http://abcnews.go.com/sections/us/videofiles/video/streamer6_video_r1.ram 10443788364930 Live ABCNEWS lock_icon.gif superpass.gif lock_icon.gif lock_icon.gif big_headline