CAPE CANAVERAL, Fla. - NASA's massive transport vehicle hauling the Atlantis orbiter towards a storm shelter stopped in its tracks Tuesday and once more headed back to its Florida launch pad in an unprecedented move.

"We are going back to the pad," NASA KSC spokesperson Bruce Buckingham said shortly before 3:00 p.m. EDT (1900 GMT). "We expect to be back there in about five hours."

The announcement came about five hours after top shuttle officials gave the word to move Atlantis into reinforced shelter inside NASA's 52-story Vehicle Assembly Building (VAB) - known as a rollback - to weather high winds from Tropical Storm Ernesto. Ernesto was expected to pass within 15 nautical miles (27 miles) of Atlantis' Pad 39B launch site sporting wind speeds in excess of 40 knots (46 miles per hour).

"It does not make sense to fool with Mother Nature," said Wayne Hale, NASA's shuttle program manager, in a Tuesday teleconference before Atlantis changed course. "You want to do what is safe and we decided it would be most prudent to go back to the barn and wait it out there."

But Atlantis, which has been delayed from a planned Aug. 27 launch to the International Space Station (ISS), was midway though its 4.2-mile trek to the VAB when its crawler transport stopped, then slowly began reversing direction on course for the launch pad.

"Note even in Apollo did we do that," NASA spokesperson Bill Johnson said of Atlantis' rollback reversal.

A NASA press briefing to discuss Atlantis' return to the launch pad is slated to begin here at KSC by 4:30 p.m. EDT (2030 GMT).

Earlier today, Atlantis' STS-115 crew - commanded by veteran shuttle astronaut Brent Jett - left KSC and returned to NASA's Johnson Space Center (JSC) in Houston, Texas for additional training. But a JSC spokesperson told SPACE.com that the six STS-115 astronauts had not been taken out of quarantine, in which they have only limited access to family and friends to avoid sickness before launch.

Atlantis has been plagued by weather-related delays for its planned STS-115 space shot, a mission to resume ISS construction by delivering new solar arrays and truss segments to the ISS.

NASA's window to launch Atlantis extends through Sept. 7, when the space agency has said it would stand down to avoid launch and landing conflicts with ISS-bound Russian Soyuz spacecraft next month. If the agency fails to launch Atlantis by Sept. 7, the next firm opportunity to do so opens in late October.

Atlantis' launch pad return was prompted by updated forecasts for Tropical Storm Ernesto, which weakened as it entered the Florida Straits Tuesday morning.

"The storm has weakened and we will not exceed our wind requirements," Buckingham said.

NASA launch director Michael Leinbach said Monday that it would take at least eight days "give or take", once Atlantis returns to the launch pad, to again prime Atlantis for liftoff. That means the shuttle could make another launch attempt within its current window, if things go NASA's way.

Leinbach, NASA shuttle weather officer Kathy Winters and other mission managers have kept close tabs on Tropical Storm Ernesto.

When NBC reporter Jay Barbree asked Leinbach what he would do if, in fact, shuttle officials were able to stop Atlantis' rollback mid-stride and return to the launch pad, he answered with a mix of humor and cynicism.

"If that scenario played out, I'd buy you dinner, I'd buy Kathy dinner," Leinbach said. "That's so extraordinarily rare, that's way out there."

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