Engineers started the shuttle Endeavour's countdown Sunday evening and with good weather expected, officials said today that so far, there are no technical problems at pad 39A that would prevent an on-time liftoff Wednesday.

"At this point, we are on schedule and we are tracking no significant issues," said NASA Test Director Jeff Spaulding.

Shuttle weather officer Kathy Winters said she still expects a 70 percent chance of good weather at launch time Wednesday with the only concern being a chance for thunderstorms or electrically active anvil clouds to blow into the launch area. The forecast remains 70 percent "go" Thursday and improves to 80 percent Friday should launch be delayed.

Liftoff is targeted for 6:36:39 p.m. Wednesday, the middle of the shuttle's 10-minute launch window and roughly the moment Earth's rotation will carry the launch pad into the plane of the space station's orbit. If Endeavour takes off between 6:36:39 p.m. and 6:41:39 p.m., commander Scott Kelly will be able to catch up with the station for a docking on Friday, flight day three (NASA considers launch day flight day one).

But if some problem crops up Wednesday, mission managers can extend the launch window an additional three minutes and 10 seconds. A launch between the end of the flight-day-three window and 6:44:49 p.m. would result in a space station linkup on Saturday - flight day four.

NASA normally passes up FD-4 launch opportunities because the extra day required to complete the rendezvous is not available for the docked phase of the mission. But Endeavour, fresh out of a major overhaul and making its first flight since the 2003 Columbia disaster, is equipped with a new station-to-shuttle power transfer system that will allow the orbiter to plug into the lab's solar power grid.

As such, Endeavour's crew could get to the station a day late and still accomplish all of the mission objectives by utilizing station power to stretch out on-board supplies and get an additional docked day.

This afternoon, engineers plan to clear the pad and remotely load liquid oxygen and hydrogen aboard the shuttle to power the ship's three electricity producing fuel cells. The goal is to get 11 days of liquid oxygen aboard and five days of liquid hydrogen, permitting four launch attempts in five days. If Endeavour is not off the ground by then, the launch team will stand down for 48 hours and reload the fuel cell system.

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