The tragic loss of Columbia and its crew will bring about a series of tough decisions by NASA, the White House, and the American public.

Both the future direction of human space travel and overall operation of the International Space Station must be addressed in short order.

Harrison Schmitt, Apollo 17 moonwalker and former Senator from New Mexico, said that getting a shuttle back into space is critical. "You can no longer stand-down indefinitely. There is commitment to the International Space Station effort and the people stationed there," he told SPACE.com.

"You have to have enough confidence in the rest of the fleet to keep goingotherwise you don't have a fleet," Schmitt said.

The shuttle's unique lift capability, as well as its use to periodically re-boost the massive International Space Station are key to the vitality of the effort, Schmitt said.

One worry, however, is finding a generic problem that may have cropped up in the space shuttle orbiter fleet. Rather than a short span of time required to find and fix trouble, and getting a shuttle back into space, discovering trouble common to all space planes would force a major slowdown.

Although Schmitt was doubtful this will be the case, if found to be true, "you'd have to have a crash program" to get the fleet airborne again, he said.

The loss of Columbia, leaving NASA with just three vehicles, is likely to have ripple effects in moving forward with an Orbital Space Plane. "I suspect this tragedy will add new impetus to the Orbital Space Planemore than likely changing its direction to become not just a rescue vehicle, but also a vehicle for access to space," Schmitt said.

Late last year, the White House gave NASA a go-ahead to move toward a smaller, highly maneuverable space plane. In the process, mega-dollar plans to replace the space shuttle with a new generation reusable launch vehicle were downgraded.

"With this having happened, and now down to three vehicles," Schmitt continued, "I suspect it will add new impetus to this Orbital Space Plane effort. More than likely it'll change the direction somewhatso it becomes not just a rescue vehicle but also a vehicle for access."

Following the loss of Challenger in January 1986 -- over 17 years ago -- the space plane fleet was grounded for more than two-and-a-half years.

Schmitt recalls both the technical and political finger pointing associated with that long delay.

"I think everybody needs to understand that a full-fledged accident investigation is now underway. They are not going into the delay mode apparently that they did after Challenger," Schmitt said. "They delayed a week thenand the whole process became politicized. They [NASA] want an objective investigation, technical as well as a board investigation, but no big full-fledged commission to do this. And hopefully they can stick with that."

Schmitt, one of twelve people to have walked on the Moon, said the public must come to grips with a clear fact.

"Everybody onboard Columbia was a volunteer. They believed whole-heartedly in what they were doingand that risk is part of the job. It has been and always will be when you are doing great things," he concluded.

Keeping space shuttles on the ground means hard choices ahead for those building and running the International Space Station (ISS), said Marcia Smith, space policy analyst for the Congressional Research Service - a study arm of the U.S. Congress.

"Of course there is always a way for the station crew to get home. So they are in no danger. They can come back on the Russian-supplied Soyuz if they need to," Smith said. You can occupy a space station without a shuttle, she said.

But Smith said that NASA and the ISS partners are going to have to sit down and figure out what their strategy is going to be. "You can take people back and forth. And using the Russian Progress re-supply vehicle you can take an amount of cargo upbut you cant bring anything back," she noted.

What the shuttle contributed to the program, Smith said, was the vehicle's "down mass" abilities. Also, there are numerous hardware items -- such as the yet-to-fly add-on modules of both Europe and Japan -- major segments being prepared for a shuttle lift to the ISS.

"So I think that space station assembly is going to be dramatically impacted by this. That equipment, with few exceptions, has to go up on the shuttle, Smith told SPACE.com.

One option may be to operate the station in a "steady state" status, Smith said.

The ISS can remain functioning, utilizing only what capabilities are now onboard. Some modest resupply of the ISS can be accomplished through the automated Russian Progress vehicle. "But it will certainly limit what can be done," Smith added.

Building a replacement orbiter -- as was the case in the post-Challenger aftermath -- may not be possible. In the late 1980s, available backup hardware, called "structural spares," made the difference in building a new space plane.

"Back thenthey [NASA and industry contractors] were able to do it for less cost than they expected and within a relatively short period of time," Smith said.

"I'm not aware of any structural spares that are available now. So building a replacement would be more of a problem. So the question now is what can NASA do with a three orbiter fleet instead of a four orbiter fleetand how is that going to effect its long-term plans?," Smith concluded.

"This is all very, very sad," said Phillip Clark, a noted space analyst in the United Kingdom, in learning of the horrific accident that downed Columbia and its crew.

Clark speculated that, in theory, the ISS operations should be able to continue while relying on crew rotations provided by Russian Soyuz and Progress ships. That's much the way Russia's Mir space station survived, he said.

But in looking to Russia there's one catch.

"In the old days the Russians seemed to have an almost never-ending production line of Soyuz spacecraft," Clark said. "But now they are funded for just one spacecraft every six months. It would mean no more "taxi" missions for Soyuz, with the resident crew members for ISS trained to fly Soyuz spacecraft."

Clark said while crews can continue to fly and supplies can be sent to ISS, it will mean that the optimum crew rotation will have to shift from 4 months to 6 months.

Lastly, Clark said that Columbia is the one space plane that does not figure in the ISS program. As the first operational orbiter, it is too heavy to carry useful payload to the space station's altitude. That's why Atlantis, Discovery and Endeavour are all assigned the "heavy lifting" assignments to the orbiting complex.

"Where there will be problems," Clark said, "are missions like the occasional Hubble servicing or other non-ISS science missions. ISS shuttles will have to be diverted to these. I would expect that -- apart from Hubble missions -- other non-ISS flights might simply get scrapped," he explained.