Despite threatening skies, the shuttle Endeavour's countdown continues to tick smoothly toward a sky-lighting pre-dawn launch Thursday.
CBS News Space Consultant Bill Harwood reports that forecasters are monitoring threatening weather in Florida and at all three of the shuttle's overseas emergency landing sites.
Hoping for the best, technicians plan to begin pumping a half-million gallons of supercold liquid oxygen and liquid hydrogen fuel into Endeavour's external tank beginning around 7 p.m.EST Wednesday, setting the stage for a launch attempt at 3:59 a.m.
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If all goes as planned, Endeavour's six-astronaut crew will start putting the new international space outpost together while in orbit following a predawn launch Thursday.
The exact launch time will not be set until shortly before take-off, based on final radar tracking of the Russian space station module Zarya with which the shuttle crew plans to rendezvous Saturday. The latest estimate from flight controllers at the Johnson Space Center in Houston shows launch at 3:58:56 a.m. EST.
"We'll continue to count down and get our final launch window update about an hour and 30 minutes prior to launch," said NASA test director Steve Altemus.
While launch preparations are going smoothly, forecasters say the weather is not being particularly cooperative. To catch up with the Zarya module, Endeavour's crew only has about five minutes to get off the ground. Forecasters are predicting scattered-to-broken clouds at 4,000 feet and showers within 20 nautical miles. That adds up to a 60 percent chance the flight will be delayed.
The local forecast is pretty much the same for Friday, improving to a 60 percent "go" forecast on Saturday.
Should one of the shuttle's three liquid-fueled main engines fail between two minutes 17 seconds after launch and four minutes 21 seconds, comander Robert Cabana and pilot Rick Sturckow would be forced to attempt a landing at one of two emergency runways in Spain or one in Africa. At least one of those sites must be acceptable for Cabana and company to be cleared for launch.
At Zaragoza, Spain, a broken deck of clouds at 3,000 feet is expected Thursday, along with winds at 16 knots gusting to 24 - well beyond shuttle safety limits. At Moron, Spain, scattered clouds and fog is expected, and at Ben Guerir, Morocco, a broken deck of clouds is predicted at 3,000 feet.
All three landing sites are expected to be no-go Friday, while Zaragoza alone will be a go on Saturday.
"It looks like we've got a lot of weather issues to contend with," said shuttle weather officer Ed Priselac.
The last time a space shuttle flight was delayed by weather was mission STS-94, which blasted off July 1, 1997. Launch originally was scheduled for 2:37 p.m., but it was moved up 47 minutes, to 1:50 p.m., due to threatening weather. The actual liftoff was delayed 12 minutes because of rain showers north of the pad.
The last time a space shuttle flight was postponed 24 hours or more by weather was mission STS-80, which blasted off at 2:55:47 p.m. on Nov. 19, 1996. The flight was delayed from Nov. 15 to Nov. 19 because of conflicts with another launch and because of a dismal weather forecast.
NASA has five days to launch Endeavour before standing down while a new Mars probe is launched atop a Delta rocket from the nearby Cape Canaveral Air Station on Dec. 10.
Endeavour's crew is to mate the American-built Unity module in Endeavour's payload bay to the unmanned Russian-built component that was launched 11 days ago by a Russian Proton rocket. It is the first step in constructing an international station in space.
The United States and Russia are playing the largest roles of 16 nations involved in the space station project, but Russia's recent financial crisis has caused delays in the effort.
The first crew to live aboard the station is not expected until January 2000, a year and a half behind schedule, because of Russian delays in building the module that will be the initial living quarters.