CAPE CANAVERAL, Fla. -- NASA's space shuttle launch calendar will remain up in the air until after the Fourth of July weekend, program officials said Thursday.

Three shuttle missions remain on NASA's calendar for 2002 -- two International Space Station (ISS) assembly flights and one science research mission. But when they fly and in what order is anyone's guess right now.

NASA engineering images show an example of a crack in the liner of propellant line inside shuttle Atlantis.

"There's still a lot of questions we haven't been able to answer yet," Kennedy Space Center Director Roy Bridges told SPACE.com on Thursday.

The schedule is in disarray due to the recent finding of several tiny cracks on the surface of parts located inside the main propulsion system plumbing of Atlantis and Discovery.

Shuttle program engineers are trying to determine why those cracks formed, if they could grow larger and what risk -- if any -- do they pose to safely launching, Bridges said.

Engineers also are considering repair options.

Inspections must be performed on Columbia and Endeavour as well so investigators can better determine the scope of the problem and the potential affect any resulting action might have on the shuttle launch schedule.

For now, the only thing NASA managers know is that Columbia's targeted July 19 launch on a 16-day science mission is on hold for a few weeks. During that time the spaceplane's main engines to be removed, the suspect parts looked at and the engines re-installed.

Workers on Thursday were removing the heat shields that surround Columbia's trio of Rocketdyne powerplants. That will allow technicians access to begin disconnecting the engines themselves.

Once the engines are free, inspectors will be able to look inside the propulsion system plumbing that is within the shuttle's rear engine compartment -- not the engines themselves.

They will be looking at the surface of metal liners that are welded within the pipes that carry liquid hydrogen and liquid oxygen from the external tank to feed the main engines.

The liners direct the flow of propellant past accordion-shaped bellows, which are built into the plumbing at various locations to allow the pipes to be flexible as they contract and expand when exposed to different temperatures.

Of the dozen liners inspected in each of Atlantis and Discovery, one liner in each orbiter had three cracks measuring one-tenth to three-tenths of an inch (2.5 to 7.6 millimeters).

Endeavour's liners will be inspected once the shuttle returns to Florida from Edwards Air Force Base in California, where it landed June 19 after a two-week mission to the space station.

Bad weather between coasts has prevented the shuttle from beginning its ferry flight home atop a NASA 747 jumbo jet, said NASA spokesman George Diller.

Columbia's liners should be inspected by the end of next week, and then NASA managers may be able to begin making decisions on what to do next.

Unlike the rest of the fleet, Columbia's liners are made of stainless steel; the liners in the remaining three spaceplanes are made of inconel, a metal alloy.

However, if Columbia's liners do not have cracks on them, that does not necessarily mean the shuttle will be cleared to fly right away, officials said.

If Columbia's parts have cracks, the shuttle fleet could remain grounded for several more weeks while NASA figures out what to do next.

Once the space agency begins to reassemble its manifest for the rest of the year, it will have to come up with a schedule that takes into account a couple of non-negotiable items.

First, the Expedition Five crew now onboard the space station must be replaced by the Expedition Six crew at some point, preferably before flight engineer Peggy Whitson threatens to break the U.S. spaceflight endurance record of 196 days.

Astronauts Dan Bursch and Carl Walz set that mark during their recent ISS stay as members of the Expedition Four crew. Whitson wouldn't reach that point until Dec. 17.

Mission planners say they like to keep station visits to no more than about 180 days because of the way long-term spaceflights affects the body.

The Expedition Five crew is targeted to come home aboard Endeavour in October.

But due to scheduling conflicts with a Soyuz mission to ISS in late October, and the already late return of Endeavour to Florida, that shuttle mission was in danger of being bumped to November even before the problem with the cracks came up.

Second, the order of assembly missions to the space station cannot be changed. Atlantis must carry the new S1 truss segment up, followed by Endeavour with the P1 truss segment.

Those flights are now penciled in for August and October but are very likely to be delayed into the year's end.

That would have a domino effect on NASA's 2003 launch plans, pushing back later missions and spreading the work out over the calendar year.