File image of Atlantis sitting on the launch pad. Photo: NASA-KSC

It began in secret sometime Monday but NASA says the three-day countdown to Thursday's launch of space shuttle Atlantis is underway at the Kennedy Space Center.

"I'm pleased to be here this morning to report that the STS-110 launch countdown has been begun," NASA Test Director Jeff Spaulding announced to the world at 9 a.m. EST today.

"Things are proceeding very well. The shuttle Atlantis is in excellent shape," Spaulding reported.

Atlantis is loaded with the 27,000 pound S0 truss structure that will delivered to the space station during the 11-day flight. The S0 truss is the centerpiece of a cross truss that eventually will stretch the length of a football field, supporting four huge sets of solar arrays.

The Atlantis flight marks the first time in the 21-year history of the space shuttle program that NASA is trying to keep secret the launch time for a civilian mission.

NASA isn't saying exactly when launch will occur, only that will happen sometime between 2 and 6 p.m. EST (1900-2300 GMT). The precise launch time to be announced officially 24 hours in advance as part of the agency's new security plan.

But NASA-provided tracking elements of the space station's orbit can be used with readily available home computer software to show Atlantis will take off within a few minutes of 5:16 p.m. EST (2216 GMT) Thursday.

Atlantis' seven astronauts -- commander Mike Bloomfield, pilot Steve Frick, flight engineer Ellen Ochoa and spacewalkers Jerry Ross, Steve Smith, Lee Morin and Rex Walheim -- arrived at Kennedy Space Center from their homes in Houston on Monday at about 12 noon EST.

The arrival lacked the usual fanfare as NASA barred the press from attending the event and the agency did not provide its usual live TV coverage.

"We're really happy to be down here to the Cape," said Bloomfield, speaking to an agency video camera set up at the runway to record the arrival for replay three hours later. "A trip into space always begins down here at the Kennedy Space Center and there's a great group of people down here that build spaceships that allow people to go into space. We're very fortunate to be part of that team."

The seven astronauts gather to make a taped statement to the public after their arrival at Kennedy Space Center for launch. Photo: NASA-KSC

The weather forecast for Thursday now calls for a 70 percent chance of acceptable conditions. Meteorologists will be keeping a close eye on low cloud ceilings and rain as possible constraints to liftoff.

"A cold front will be moving through Florida on launch day. The front will be located in the northern Florida peninsula in the morning and moving through central Florida during the late morning. By launch time, the front will be located south in the vicinity of Lake Okeechobee. Expect thunderstorms and showers ahead of the front intermittently affecting KSC throughout the day. A threat of showers and low cloud ceilings will also continue behind the front," said Kathy Winters, the shuttle weather officer.

The launch time conditions are predicted to include scattered stratcumulus clouds at 2,000 feet with 4/8ths and occasional 5/8ths sky coverage and tops at 5,000 feet, altoscattered cumulus clouds at 10,000 feet with 3/8ths sky coverage and tops at 13,000 feet and scattered cirrus clouds at 25,000 feet with 3/8ths sky coverage and tops at 27,000 feet, visibility of 7 miles, north-northwesterly winds from 340 degrees at 12 peaking to 17 knots, temperature of 74 degrees F and relative humidity of 79 percent.

Should the launch slip to Friday for some reason, the weather improves to a 80 percent chance of meeting the launch rules. The concerns are low cloud ceilings and crosswinds.

Saturday's forecast is worse with a 60 percent chance of acceptable weather due to low cloud ceilings, rain and crosswinds.