This close-up view of damaged tile on the underside of the Endeavour was photographed during a focused inspection of the shuttle's heat shield while docked with the International Space Station. (NASA/Associated Press)

It is only nine centimetres at its longest point and just 2.5 centimetres deep, but the gouge at the bottom of the space shuttle Endeavour has attracted a world of attention since a camera attached to the Canadian-made robotic arm first spied the damage on Saturday.

The damage occurred when a piece of foam insulation fell off the external fuel tank during liftoff, bounced off a strut holding the tank to the orbiter and slammed into the ceramic heat tiles on the orbiter's belly.

NASA has downplayed the damage to heat tiles, saying it posed no threat to crew safety or the shuttle's mission to the International Space Station and back. But the space agency is taking few chances, particularly after the loss of the space shuttle Columbia and its crew in 2003. Columbia disintegrated upon re-entry because of a hole in the left wing created by a piece of foam from the fuel tank. That hole allowed super-hot gases to enter the wing and melt it from the inside.

So what's different this time around? Here's a look at the materials and processes at work in keeping the space shuttle crew safe:

How do heat tiles work?

Space shuttles have different types of surface tiles to protect the shuttle's aluminum skin from exposure to both the intense heat of liftoff and re-entry and the cold of space. Generally, the sections more likely to be exposed to intense heat upon re-entry — the nose cone and the forward edges of the wings — have a greater resistance to heat and are able to withstand temperatures of greater than 1,260 C.

The bulk of these tiles — including the ones damaged during Endeavour's liftoff — aren't exposed to the hottest temperatures during liftoff, but are still capable of withstanding temperatures up to 1,260 C.

These tiles, known as High-Temperature Reusable Surface Insulation tiles, are made from a silicon-foam fibre that is heated until it is a rigid, lightweight ceramic material. The bulk of the protection, however, comes from the black glossy coating painted onto the foam — a mixture of powdered tetrasilicide and borosilicate glass. This coating protects the tile from over 80 per cent of the heat exposure.

But because the aluminum skin of the shuttle expands and contracts in response to heat and stress, the rigid tiles can't be attached directly to the metal surface because of the danger that they could break apart. So the tiles are attached with a silicon-rubber glue to a felt pad, which is then attached to the aluminum.

The damage to the tiles on Endeavour is small, but deep enough to have exposed the felt underlayer.

How is the situation different from Columbia's?

One big difference to Endeavour's tile damage and Columbia's is knowledge: when the falling foam hit Columbia's wing, NASA engineers didn't know where it had hit and weren't able to properly simulate how much heating would occur during re-entry.

The inspection of the damage by the Canadarm on Sunday, for example, allowed NASA to pinpoint exactly where the damage was, an advantage the Columbia crew didn't have.

The location of the gouge — near the rear of the shuttle — is also less worrisome for NASA than if it had occurred on the cone or the wing. In 1988, the space shuttle Discovery appeared to have suffered similar damage during its flight and had to have six tiles replaced.

NASA mission management team chairman John Shannon said the lessons learned from Columbia have been put in place.

"We have really prepared for exactly this case, this is exactly what we've prepared to do," he told reporters on Monday.

Canadian astronaut Chris Hadfield was dismissive of the gouge's potential for harm and said NASA will likely be able to handle the problem without much trouble.

"They know that there is a tiny little divot at the bottom of the space ship like we've seen ten thousand time before," Hadfield told CBC News.

How could the gouge be fixed?

Should NASA decide to repair the damage, the space agency will likely call on either Canadian astronaut Dave Williams or his American colleague Rick Mastracchio to plug the hole during a fourth spacewalk. To get there, the astronaut would have to travel on the end of the shuttle's Canadarm and its extension boom. Once there, the astronaut would apply a similar protective paint and then add a caulk-like filler for added protection.

The strategy is not without its own peril, however, as one wrong move of the robotic arm or the astronaut could cause a collision.

"Any time you put an astronaut on a boom, you have the risk that you might hit the tile and cause more damage," said Canadian astronaut Steve MacLean, who flew on the space shuttle Atlantis's mission to the International Space Station in 2006 and has experience operating the station's own robotic arm, called the Canadarm2.

So, can NASA just leave it alone?

Given the risks, and the shuttle crew's tight schedule, NASA might decide to wait until the shuttle lands safely on Earth. But the space agency is considering fixing the damage in case the tiny gouge becomes a much larger fix after shuttle re-entry. At issue is whether Endeavour would need extensive repairs, something that might further push back NASA's tight schedule to complete the space station before the shuttle program winds down in 2010.

Endeavour Commander Scott Kelly said the issue of future repairs, and not crew safety, is the real issue NASA engineers face.

"If the gouge will damage the orbiter to where it can't be used again, I would vote to repair it," said Kelly on NASA TV. "But I think the jury is out on that right now."

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