MacLean's first shuttle flight was delayed by the 1986 Challenger disaster and his second by the 2003 loss of Columbia. He believes NASA has addressed the major known risks of flying the shuttle.

"We had eight major known risks that came out of the Challenger accident and we have fixed those," he said in an interview with CBS News. "The brakes are better, we have nose wheel steering, we have a drag chute, we have better engines with the new turbopumps. We have a whole different abort boundary sequence going up the East Coast now with five Canadian sites. That abort sequence is so much safer than it was prior to Challenger. I was impressed with what NASA was able to do in two years and eight months after the Challenger accident with respect to improving the safety of the vehicle by an order of magnitude.

"After the Columbia accident, we had a problem that was plaguing us for a while and we haven't solved it yet. The foam is not understood. Do we understand the foam problem? Yes. Do we understand the interaction of the foam? No. Can we fix it? I don't think we can. But can we minimize it? Yes, we have. We have minimized that problem, foam will not bite us on future missions. We may have some issues with foam, but it won't bite us the way it did when Columbia happened.

"So I think we've minimized all known risks on the vehicle. I looked at it as seriously as anybody given that I'm flying on it. It's something that you really do need to look at because I wouldn't go if I thought it wasn't safe. I have decided, just like I did after Challenger, it's worth it. I'm looking very much forward to doing it."

That doesn't mean he turns a blind eye to the risks posed by riding on a space shuttle, regardless of post-Columbia safety upgrades and improvements.

"People ask, what's the hardest thing in spaceflight, the most difficult thing?" MacLean reflected in an interview. "It isn't walking out to the launch pad. It's saying goodbye to your kids, that's what's hard, and to your wife. That's very hard to do. It's something I wasn't prepared for on my first flight and I didn't realize until I had to do it that that was very difficult."

When Columbia returned to Earth on Feb. 1, 2003, no one knew the ship's left wing had suffered catastrophic impact damage during launch 16 days earlier. Long-range tracking cameras showed a chunk of foam debris from the shuttle's external tank breaking away from the tank's left bipod ramp 81.7 seconds after liftoff, but they did not show where the foam hit.

From the perspective of the only camera with a good view, the foam disappeared under the left wing followed by a shower of debris an instant later. Clearly, the foam hit the wing. But where? Engineers ultimately concluded it probably hit on the underside of the wing and, at most, damaged a few of the heat shield tiles in the area. No one believed the damage was catastrophic. But lacking good camera views, no one really knew.

As it turned out, the 1.67-pound chunk of foam hit the left wing's leading edge at a relative velocity of nearly 550 mph, blasting a 6- to 10-inch hole in the reinforced carbon carbon insulation. During re-entry, super-heated plasma burned its way inside, melting the left wing from the inside out and triggering a catastrophic structural failure.

NASA eliminated the bipod ramp foam that was the actual cause of Columbia's demise and made other improvements intended to minimize foam shedding in general. But when Discovery took off on the first post-Columbia mission in July 2005, engineers were surprised to see a large one-pound chunk of foam insulation fall away from a so-called protuberance air-load ramp on the shuttle's external fuel tank. The manually applied foam ramps were in place to provide an aerodynamic barrier of sorts to smooth the flow of supersonic air over two pressurization lines and a long cable tray. After months of computer analysis and wind tunnel testing in the wake of Discovery's 2005 flight, engineers were able to prove the ramps were not needed, clearing the way for Discovery's launch July 4 with a tank that did not have any PAL ramps.

The largest foam buildups still in place on the tank are the ice-frost ramps. Shuttle program manager Wayne Hale said earlier this month engineers are working on two redesign options, one of which will fly on the first 2007 mission. But until then, Jett's crew and the crew of the planned December mission will have to rely on the old design. Based on the performance of Discovery's tank last month, they can rest relatively easy. The tank performed well and no large pieces of foam fell off. But NASA managers want one more test flight before relaxing at least some of the post-Columbia safety constraints.

Like NASA's self-imposed requirement for two problem-free daylight launchings.

To reach the international space station, the shuttle must launch into the plane of the lab's orbit during periods when the angle between that plane and the sun provides acceptable temperatures. Those requirements, plus the need to launch in daylight and to ensure external tank separation in daylight for photo documentation, severely limit when a shuttle can fly.

For Atlantis, all of those conditions are met between Aug. 27 and Sept. 13. But the Russians are scheduled to launch the station's next full-time crew - Expedition 14 commander Mike Lopez-Alegria and cosmonaut Mikhail Tyurin - and space tourist Anousheh Ansari in September and flight rules require at least one full day between when a shuttle undocks and when a Soyuz arrives.

If the Russians delay their launch past Sept. 18, the station's returning crew - Expedition 13 commander Pavel Vinogradov, flight engineer Jeff Williams and X-Prize sponsor Ansari, would face a dead-of-night landing in Kazakhstan. The Russians are using new management for the Soyuz recovery and want to make sure landing occurs within a few hours of dawn at the worst.

As a result, Atlantis must get off the ground by Sept. 7 or the flight will be delayed to a short two-day window in late October. Only one lighted launch opportunity is available between then and February. Hoping for the best, NASA's launch team plans to make up to seven launch attempts between Aug. 27 and Sept. 7 if the weather or some other problem prevents an on-time takeoff.

As with Discovery's flight last month, NASA will utilize an upgraded tracking camera network at the Kennedy Space Center and Cape Canaveral Air Force Station to make sure any foam impact damage is seen as soon as possible.

The ability to quickly spot such damage, giving engineers time to assess the consequences and possible repair options, was a large part of Griffin's justification for proceeding with flight even though the tank still features ice-frost ramps that are considered an unacceptable risk.

At launch complex 39B, some 38 16mm cameras are mounted on the launch pad itself with three short-range camera sites around the pad perimeter featuring two 35mm cameras and one high definition TV camera each. Another 11 medium-range camera sites are positioned around the pad between one and six miles away, each one equipped with a 35mm camera and all but one equipped with an HDTV camera. Another 11 long-range camera sites are located between four and 40 miles of the pad. All long-range sites include 35mm cameras, two have 70mm cameras and 10 are equipped with HDTV.

From liftoff through the first 30 seconds of flight, objects an inch wide or larger can be seen. Between 30 seconds and one minute, the resolution drops to objects three inches in diameter or larger and from one minute to 90 seconds, it drops to objects eight inches or larger. Between 90 seconds and booster separation two minutes after liftoff, ground-based tracking cameras can detect objects 15 inches across and pinpoint an impact site to within five feet.

One WB-57 jet will be cruising 60,000 feet up to photograph the shuttle using infrared sensors and HDTV cameras attached to powerful 11-inch Celestron telescopes. The WB-57 will acquire most of its imagery in the minute leading up to solid-fuel booster separation.

Finally, a radar system is in place featuring one ground-based C-band and two ship-based Doppler X-band instruments to look for debris coming off the external tank.

For Atlantis' flight, eight cameras mounted on the shuttle, its tank and twin boosters will provide close-up views of the external tank and the orbiter's belly during ascent.

Another camera mounted high up on the external tank looking down on the underside of the space shuttle will beam back live television views throughout the eight-and-a-half-minute climb to orbit.

As with Discovery's flight last month, Atlantis is equipped with four other cameras, two near the top of each booster and two mounted near the back end of the powerful rockets. Each booster also carries a camera focused on a region of the tank known for losing small, popcorn-like pieces of foam.

Imagery from the six booster cams will be available after the spent rockets are recovered and towed back to Port Canaveral a few days after launch.

In addition, a digital camera mounted in a cavity where a propellant line enters the belly of the orbiter will photograph the tank as it separates in space.

As if all that wasn't enough, an X-band marine radar seven-tenths of a mile from the pad will be on the lookout for vultures and other large birds. During Discovery's launch in 2005, a large vulture struck the external tank a few seconds after liftoff, rammed by the shuttle at some 70 mph. If any large birds are seen prior to Atlantis' launch Sunday, the countdown can be halted briefly if necessary.

In short, if any impact damage occurs from any source, shuttle engineers expect to see it. NASA managers hope the radar systems, still somewhat experimental, eventually will provide enough resolution to permit acceptable debris coverage for night launches. But in the meantime, good lighting is required.

Once in orbit, Jett and company will take over the inspection work, photographing the external tank as it tumbles away using a digital still camera and a movie camera. Depending on what sort of motions, or "tip-off rates," are imparted at separation, lighting may be marginal.

Data collected by the wing leading edge impact sensors also will be downlinked to mission control for detailed analysis. Located on each wing's forward spar behind every reinforced carbon carbon panel, the 132 accelerometers will provide data telling flight controllers whether anything struck the leading edges during launch.

On the second day of the mission, the astronauts will spend six-and-a-half hours using Atlantis' robot arm and the 50-foot OBSS extension to inspect the wing leading edge panels and the shuttle's reinforced carbon carbon nose cap.

A laser sensor on the end of the boom is capable of spotting any wing leading edge damage that could pose a threat to the shuttle. The astronauts will start with the starboard, or right-side, wing leading edge, making six passes up and down the wing to cover all the angles. After scanning the nose cap, they will move on to the port wing and repeat the procedure.

A high-resolution camera is mounted on the end of the OBSS to take close-up photographs of any potential damage sites. No such "focused inspection" is planned, but if any sites of interest are identified, the camera is capable of resolving features as small as .08 inches across.

"That has been a major role for Canada, the inspection of the vehicle," MacLean said in a NASA interview. "The shuttle arm is basically going to pick up this 50-foot-long telephone pole that's going to act like a dental mirror, and then work under the wings of the shuttle and see if there's any damage.

"Dan Burbank and Chris Ferguson and myself are the three operators of the inspection system," MacLean said. "Initially Dan uses the arm, picks up the boom, and then we start the scanning. We rotate a little through our positions and it takes the entire day to scan the port wing and to scan the starboard wing and to look at the nose cap as well. It's not a pun, but it is a focused day in order to make sure that we did not sustain any damage through the launch sequence."

As it turns out, one of the most effective ways to look for signs of damage occurs during final approach to the space station. At a distance of about 600 feet directly below the lab complex, Jett will guide Atlantis through a slow end-over-end flip known as a rotational pitch maneuver, or RPM. The maneuver will take about nine minutes to complete - three quarters of a degree per second - allowing station commander Vinogradov and Williams to photograph the shuttle's belly using digital cameras equipped with 400mm and 800mm telephoto lenses.

It was during an identical flip during Discovery's flight last year that controllers spotted two so-called gap fillers protruding above the tile on the shuttle's belly, prompting an impromptu spacewalk repair later in the mission. Since then, thousands of gap fillers have been removed and replaced.

Good imagery, radar and impact sensor data form one leg of Griffin's three-pronged justification for flight. The other two components are the crew's ability to repair minor damage and, in a worst-case scenario, the capability of the space station to support a combined crew of nine until a rescue flight can be mounted.

It would not be easy. The Russian Elektron oxygen generator has a history of malfunctions, there is only one toilet on board and supplies would be tight. Worse, the only way down in a major emergency would be a single three-seat Soyuz capsule.

But NASA and the Russians have pre-positioned additional supplies for just such a contingency, including food, water and lithium hydroxide to scrub carbon dioxide from the air. Even if the Elektron failed the day Atlantis took off, the station has enough alternative oxygen generation capability to support the combined nine-member crew for about 76 days.

NASA managers are increasingly optimistic they will never have to invoke the "safe haven" option. Along with removing all large concentrations of foam on the external fuel tank, engineers have been working on techniques for repairing damage to a shuttle's heat shield tiles and even the critical wing leading edge panels.

Tests carried out during a spacewalk last month aboard Discovery show small cracks in the reinforced carbon carbon material may be repairable with a material known as NOAX. Two repaired cracks subjected to arc jet tests showed the material held up well under actual re-entry level temperatures.

"These first two samples ... have passed with flying colors, very intense re-entry profiles," Hale said. "So we are demonstrating good progress toward having good repair capability should we have damage to the thermal protection system."

That doesn't mean NASA will ever be able to formally certify a repair technique. Certification means a process has been subjected to enough tests to prove it will work under specified conditions. "I doubt that we will ever have a certified TPS repair capability in the way that NASA likes to define certification," Hale said. "We're going to have some repair techniques, we're going to gain increasing confidence on the types of damage that they would be effective to repair, but I doubt that in the life of the shuttle program we will be able to achieve what we would normally call a certified capability."

For his part, Jett believes NASA has done the best it reasonably can to minimize the threat posed by falling foam insulation. He believes it's time for NASA to get on with space station assembly.

"Since the Columbia accident, the two missions since then have been focused a lot on the return to flight objectives, primarily fixing the external tank and some of the other things recommended by the CAIB (Columbia Accident Investigation Board)," Jett said in a NASA interview.

"Now those missions have been very valuable to station as well - logistically, making repairs for the station, adding a third crew member. But really, 115 is the return to the assembly sequence, and I think that's significant. We have a mandate to finish the station by 2010 and retire the shuttle. So we need to shift from the return-to-flight mode back to a more operational assembly sequence, where we're flying hopefully four to five times a year and, completing the assembly fairly quickly."

PREVIEW REPORT PART 3 --->