Even if no obvious signs of foam shedding or damage show up in the initial imagery or wing leading edge data, CAIB recommendation 3.4-3 calls for NASA to "provide a capability to obtain and downlink high-resolution images of the underside of the orbiter wing leading edge and forward section of both wings' thermal protection system."

Computer animation shows the OBSS surveying the shuttle's wing leading edge. Credit: NASA TV

The Discovery astronauts are going to spend their second day in space carefully inspecting the leading edges of both wings, along with the RCC nose cap of the shuttle, using a new 50-foot-long boom on the starboard side of the payload bay known as the orbiter boom sensor system, or OBSS.

Thomas will unlimber the shuttle's 50-foot-long robot arm and lock it onto the OBSS. Thomas, Kelly and Camarda then will spend the entire day maneuvering the boom back and forth along the leading edges of both wings, using an OBSS television camera and one or more laser sensors to inspect every square inch of the RCC panels. The astronauts also will use the boom to inspect the nose cap.

Maximum speed of the boom survey: 2 inches per second.

"If you think about the laser, the way we're recording the data is similar to recording video," Hill said. "Imagine standing on the side of a soccer field watching one of your kids play soccer with a camcorder. And you're panning that camera real fast so you can watch him run down the field. But when you play it back at home, you can't make out hide nor hair because everything is blurred. That's the problem we've got. We've got a translation constraint. If we move too fast, we blur the image, which directly affects the resolution and we can't see the small stuff we're looking for."

As of this writing, it's not yet clear whether the boom will be equipped with one or two laser sensors. That's because engineers are still debating what constitutes entry critical damage to the wing leading edges.

"A year ago, we thought - actually until about six or seven months ago - we thought entry critical damage required a penetration of the RCC, not just coating damage or even small damage to the substrate on the outside," Hill said. "More recent arc jet testing has us worried that coating damage alone, if it's large enough and if we had internal damage - delamination - between the layers, that the combination of those two could be entry critical."

In the early years of the shuttle program, Hill said, tests indicated the leading edge RCC panels could tolerate penetrations a quarter of an inch across. But that testing was with a clean hole punched in the panel, which is what one would expect with a hypervelocity impact in space. But during launch, impact velocities would be much lower and any resulting penetrations would be more ragged.

Engineers then began wondering if lower-velocity impacts might be entry critical.

"What we found out in more recent arc jet tests, in the last couple or three months - this is the thing that's key - if (RCC coating is intact), our conventional wisdom is good, we don't care, internal delamination's not an issue.

"But if the coating is gone and underneath that coating you're delaminated, then picture the RCC itself from a side view like a cross section. Now you've got this bubble or this void in between layers. What you've done is, you've significantly reduced the density of this RCC that's exposed to the heat load. So it burns faster.

"So now instead of being this more solid material that's hard to light, kind of like if you take a piece of hard wood like oak and you hold a match to that oak, it won't light typically. But if you shave off some splinters of that oak, you can get them to flash. Damned if that's not what we found in a handful of RCC runs for uncoated RCC."

For the tests, engineers deliberately damaged an RCC panel by pushing on it with a metal cylinder. After confirming the panel developed delamination as a result, "they put that bad boy under the arc jet and it burned like there was no tomorrow. The whole area that covered the delamination burned off like a fuse."

Engineers are debating whether coating damage, coupled with delamination, represents a credible damage scenario.

"If it's credible for us to take an impact that has enough energy to cause a delamination and takes off the coating, then that does not have to be very big to be catastrophic," Hill said. "From an RCC damage perspective, that looks like a penetration. So now the question is, do we believe that testing? Have we done enough of those tests to be sure that is an entry critical damage form? And then, is it credible for us to take an impact that could cause that kind of damage? Those are the questions before the orbiter project office."

How the debate plays out could play a role in Discovery's eventual launch date. One OBSS laser sensor, known as the laser dynamic range imager, or LDRI, can support a May 14 launch date. But it does not have the resolution to detect the sort of coating damage engineers are assessing.

Another sensor under development, known as the laser camera system, or LCS, can detect such damage but more time is needed to ready it for flight. If the LCS is required, launch could slip a week or two.

"I'm holding out hope that what we will conclude is yes, if we take that kind of damage it's something we're worried about," Hill said. "But with our increased knowledge of the transport model, there's only very few parts of the leading edge where we're really at risk of taking an impact that can do that."

Flight Day 2 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/15/05
   Sun  06:11 AM...00...14...00...STS crew wakeup
   Sun  07:41 AM...00...15...30...NC2 rendezvous rocket firing
   Sun  08:41 AM...00...16...30...Robot arm checkout (if necessary)
   Sun  09:26 AM...00...17...15...Thomas unberths the OBSS
   Sun  10:16 AM...00...18...05...OBSS RCC survey begins
   Sun  12:36 PM...00...20...25...Docking ring extension
   Sun  01:01 PM...00...20...50...Crew meals begin
   Sun  02:06 PM...00...21...55...OBSS survey resumes
   Sun  02:06 PM...00...21...55...Middeck equipment prepped for transfer
   Sun  02:41 PM...00...22...30...Robinson and Noguchi checkout their spacesuits
   Sun  04:36 PM...01...00...25...The OBSS is reberthed
   Sun  04:41 PM...01...00...30...Public Affairs event (audio only)
   Sun  05:21 PM...01...01...10...NPC rendezvous rocket firing
   Sun  06:51 PM...01...02...40...NC3 rendezvous rocket firing
   Sun  08:11 PM...01...04...00...Crew sleep begins

PREVIEW REPORT PART 6 --->