The attachment of the truss segments give vitality to the ISS in the years to come, Hedin said.

STS-110

For complete launch to landing coverage and the most up-to-date news about this assembly mission to the International Space Station click here.

"Our experience to date would give us confidence that we'll be able to continue to press forward without any showstoppers. From the technical perspective, it's been phenomenal," Hedin said.

Hedin saluted both U.S. and Russian station engineering and safety teams.

"The Russians question the way we think. We question the way they think. Frankly, I think we've come up with approaches that are collectively better. We've worked through things and satisfied each other's logic path, and we're all the better for it," Hedin said.

Stable point of safety

Even if the space shuttle fleet was grounded, the ISS program has reached a stable point in terms of crew safety, said Richard Blomberg, just retired chair of the Aerospace Safety Advisory Panel (ASAP), an independent advisory group to NASA.

Hardware is now in orbit to assure crew safety. The necessary life support gear, panels of solar cells for electricity, batteries for energy storage, and gyroscopes for keeping the ISS stable - all the components to sustain human occupants in the ISS -- are up and operating.

"From a safety standpoint... at any stage from now on, the ISS crewmembers are safe, even if they don't get this next launch package up," Blomberg said. "They are able to maintain the safety of the station, they just couldn't accomplish as much science as they would if they continued building," he said.

"What you are doing is adding capability from here on out," Blomberg said, who is president of Dunlap and Associates, Inc., of Stamford, Conn., and a consultant in human factors and systems analysis.

In the grand scheme of things, the coming together of ISS has been a safely executed story, Blomberg said.

"From an engineering standpoint, their achievements have been magnificent. Redundancy schemes are good, and where problems have arisen, the backup systems have worked properly. They've done good things with safety in terms of space walks, and not taking risks when they don't have to," Blomberg said.

"It's one large, elegant piece of engineering. And it has been going together relatively flawlessly," Blomberg said.

Putting engineering success stories aside, people zipping through the vacuum of space in pressurized modules remains a dangerous occupation.

An omnipresent danger is a run-in between the station and a chunk of space debris. Toxic air wafting through the complex, an out-of-control science experiment or an onboard fire all are issues high on any ISS program worry list.

As a result, NASA has begun anew looking at an old idea. Studies are underway to see if ISS crewmembers, rather than bailing out in an expensive rescue vehicle, can move to a "safe haven" situation within the outpost itself. That locale could sustain a crew for a measured period of time.

"They are at an embryonic stage of looking at this," Blomberg said. "It will have to pass through all the sanity checks. There's nothing wrong with doing a little out-of-the-box thinking on this."

Blomberg said the safe haven idea could be combined with other risk-mitigating factors. However, bringing back a sick or injured crewmember in a hurry is a situation whereby the safe haven doesn't fit the bill.

The question, he said, is straightforward: For the money that's available, what is the best way to manage risk?

Enter Enterprise

One idea percolating within NASA is use of Spacehab's Enterprise module.

Spacehab and its Russian partner Rocket Space Corporation (RSC) Energia joined forces in late 1999 to fabricate the $100 million Enterprise space station module to accommodate commercial activities, even offer live-in room for space tourists.

Outfitted with life support essentials, Enterprise could feed into the safe haven idea to some extent, said John M. "Mike" Lounge, Spacehab senior vice president.

"I understand these days that ISS crewmembers might be willing to stay up there without a lifeboat. But that's not the issue. You can probably get comfortable with the probability of failure on the station that says you wouldn't have to have a full-time lifeboat there. But what you can never get comfortable with is the loss of the ability to launch a rescue," Lounge told SPACE.com.

Lounge said that, in his estimation, assured crew rescue means two things: protecting against some major catastrophe on the station or a launch system failure that knocks out the normal means of getting crewmembers home.

"So I think it's still a real tough issue to deal with," Lounge said.

Presently, the ISS and its three-person crew -- with a Soyuz return capsule at the ready -- is a stable situation, Lounge said.

"We're confident we can provide a system that does the life support necessary for three more people. We can package Enterprise with crew return capability. Technically, I don't think we have any issues. I think it's a matter of NASA and the partners settling down on what station they want to finish with. We're still very much, I think, one of the options for going forward. I'm ready to get on with it," Lounge said.