The shuttle Endeavour's countdown began ticking late Monday night for launch Thursday on a space station crew rotation and robot arm repair mission. Under a post Sept. 11 security policy, the shuttle's exact launch time will not be released until Wednesday. Until then, NASA will only say liftoff is scheduled sometime between 4 p.m. and 8 p.m.

There are no technical problems at pad 39A, but forecasters say there's a 60 percent chance of a launch delay because of thundershowers within 20 nautical miles during the shuttle's actual five-minute launch window. Should launch slip to Friday, the outlook calls for a 70 percent chance of a delay due to the same weather system. Saturday's odds also are 70 percent "go." Conditions are expected to improve early next week.

NASA test director Steve Altemus said the launch team's strategy going into the STS-111 campaign is to make two back-to-back attempts Thursday and Friday, if necessary, and then to stand down for two days to top of the shuttle's on-board supplies of liquid oxygen and hydrogen. Two more launch attempts then could be made Monday and Tuesday, giving the crew four chances in six days to get off the ground.

Endeavour's crew, meanwhile, flew to the Kennedy Space Center Monday to make final preparations for launch, weather permitting.

"Things are becoming more and more real every day and I'm looking forward to the flight," said rookie French flier Philippe Perrin. "I'm sure it's going to be extremely exciting."

Joining Perrin for the 110th shuttle mission is commander Kenneth Cockrell, pilot Paul Lockhart, Franklin Chang-Diaz and the next full-time space station crew, Expedition 5 commander Valeri Korzun, Peggy Whitson and Sergei Treschev.

Mir-veteran Korzun and his two rookie crewmates will replace Expedition 4 commander Yuri Onufrienko, Daniel Bursch and Carl Walz, who were launched to the station Dec. 5. They will return to Earth aboard Endeavour on June 11, completing a 187-day 20-or-so-hour flight just a few hours shy of the 188.4-day U.S. endurance record set by Shannon Lucid during a flight aboard the Mir space station in 1996.

The Expedition 4 crew originally planned to return to Earth early this month, but Endeavour's mission was delayed several weeks when NASA managers decided to add a third spacewalk to replace a faulty joint in the space station's $600 million Canadarm2 space crane.

The shuttle crew also will deliver critical supplies, fresh food, clothing, scientific equipment and a sophisticated mounting platform that will allow a compact rail car on the lab's unfinished solar power truss to carry Canadarm2 to various worksites.