The shuttle Discovery's crew might have dodged a bullet when a piece of foam debris broke away from an aerodynamic ramp on the side of the ship's external fuel tank during launch Tuesday. Had the foam broken away earlier, when the shuttle was deeper in Earth's atmosphere, the chunk could have hit the orbiter with potentially catastrophic results, engineers said today.

A piece of the PAL ramp breaks away from Discovery's fuel tank. Credit: NASA

James Hallock, a member of the Columbia Accident Investigation Board, told CBS News today he agreed with NASA's decision to put shuttle flights on hold while engineers figure out what caused the external tank to shed foam and what might be needed to fix it.

"I personally think they made the right decision because obviously, they now can see they are getting these large pieces of foam coming off," Hallock said. "The small ones they'll probably never cure, but the large pieces are obviously the ones you really worry about."

Columbia was brought down by a piece of foam the size of a small suitcase that weighed an estimated 1.67 pounds. At the moment it broke free 81.7 seconds after launch, the foam and the shuttle were traveling at roughly 1,568 mph. A mere 0.161 seconds later, the low-density foam had slowed to about 1,022 mph. That's when Columbia ran into it, at a relative velocity of some 545 mph.

It was the foam's remarkably rapid deceleration in the airstream that gave it the high relative velocity and that was dependent on the thickness of the atmosphere at the time of separation.

During Discovery's climb to space, a few seconds after the shuttle's twin boosters separated, a large piece of foam measuring a yard across and weighing 0.9 pounds broke away from a so-called protuberance air load - PAL - ramp on the side of the external fuel tank. The ramp is there to smooth the airflow over a cable tray and two pressurization lines as the tank goes supersonic.

The debris tumbled away in the thin air below the shuttle's right wing and quickly disappeared from view. It did not hit the shuttle.

"The piece in question here was pretty close to the same size to the one that brought down Columbia," Hallock said. "The good news at this point is it occurred at such high altitude that it really didn't slow down much, you could see it on the camera, there weren't enough molecules to really slow it down so if that did hit something, it probably wouldn't do too bad.

"But if it had fallen off down around the 81 seconds that we had at Columbia, or anywhere around that time, there would be enough air molecules that that piece of foam would slow down rapidly, which would then make a very large difference in speed between it and whatever it hit."

Likewise, had the foam that doomed Columbia come off "30 seconds earlier or later, it may not have even hit the shuttle itself," Hallock said. "You need to be low enough into the atmosphere that there's enough molecules to slow it down enough to cause a problem. In Columbia, it slowed down so the difference was like 500 miles an hour. If you take a basketball, because that was what its size was, and throw it at 500 mph, you do get a sense that could do some damage. In this case, it did occur at higher altitudes, but it still says the problem is there."

John Shannon, a senior manager at the Johnson Space Center in Houston, said it didn't matter how close the foam that fell off Discovery's tank came to hitting the orbiter because engineers don't yet understand the failure mechanism. As such, it represents a potentially catastrophic threat.

"The systems engineering and integration guys are off and running what they call a debris transport analysis and they're trying to understand anywhere the PAL ramp could come off, or even at a specific point at any time during the ascent, what would that mean?" Shannon explained. "It's bad. I'll tell you that right now, it's bad. You could hit the orbiter and so that's why you've got to go fix it."

Depending on where - and equally important, when - the debris came off, "there's a large cone (of trajectories) that comes out that, depending on different times of release, because you've got different aerodynamics going around the vehicle as you go through different Mach numbers, or a different place on that PAL ramp, it could go in many different areas."

He said NASA currently does not have anything on the drawing board to replace the tank's PAL ramps.

"Originally what we thought is, let's just take the PAL ramp off because it's a big piece of manually poured foam, let's get rid of it, you don't need it," Shannon said. "And we ran some tests in the wind tunnel and the aero guys did a lot of computational fluid dynamics stuff and they said you can't do it. It's protecting these cable trays and those press (pressurization) lines back there as you go supersonic. So you've got to have something that keeps the air loads off of that area."

Additional coverage for subscribers:
AUDIO: LISTEN TO TODAY'S MISSION STATUS BRIEFING MP3 FILE
VIDEO: BEHIND THE SCENES IN MISSION CONTROL FOR DOCKING PLAY
VIDEO: COMMANDER COLLINS GUIDES DISCOVERY TO DOCKING PLAY
VIDEO: DISCOVERY'S BACKFLIP AS SEEN FROM STATION PLAY
VIDEO: STATION CAMERAS SEE SHUTTLE'S APPROACH FROM BELOW PLAY
VIDEO: SHUTTLE PULLS IN FRONT OF STATION FOR DOCKING PLAY

VIDEO: CREW'S CAMCORDER VIDEO OF JETTISONED FUEL TANK PLAY

VIDEO: NASA GROUNDS SHUTTLE PROGRAM DIALUP
  BROADBAND VERSION: PART 1 & PART 2
AUDIO: LISTEN TO PROGRAM NEWS CONFERENCE FOR IPOD
VIDEO: WEDNESDAY MISSION STATUS BRIEFING DIAL-UP | BROADBAND
VIDEO: SHUTTLE FUEL TANK HITS BIRD AT LIFTOFF PLAY

VIDEO: AMAZING WB-57 AERIAL LAUNCH VIDEO NORTH | SOUTH PLANE
VIDEO: BEHIND THE SCENES IN MISSION CONTROL AT LAUNCH PLAY
VIDEO: OFFICIALS DESCRIBE DEBRIS EVENTS DIAL-UP | BROADBAND
AUDIO: LISTEN TO THE DEBRIS DESCRIPTION FOR IPOD

VIDEO: LAUNCH OF DISCOVERY! SHORTER | LONGER
VIDEO: FOOTAGE OF OBJECT BREAKING FREE FROM TANK PLAY
VIDEO: TANK-MOUNTED CAMERA SHOWS ENTIRE LAUNCH SMALL | LARGE
VIDEO: ONBOARD CAMERA VIEW OF TANK SEPARATION PLAY
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