After more than a week of storm delays, NASA may be getting its best shot weather-wise for launching space shuttle Atlantis, officials said Monday.

There was only a 20 percent chance that weather would prohibit the shuttle from blasting off at 12:28 p.m. Wednesday as planned, said Kathy Winters, shuttle weather officer.

Unlike last week's planned launch attempts, the window Wednesday opens before Florida's afternoon summer thunderstorms normally blow through, and a tropical depression brewing in the Atlantic wasn't expected to interfere.

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If Atlantis doesn't lift off on Wednesday, NASA will try again Thursday and possibly Friday.

"If you go back a week, it looked we were not going to be able to have a launch attempt in September," said Robbie Ashley, the mission's payload manager. "So we're very thankful to the shuttle folks for carving out these three days of launch attempts and giving us another shot."

Atlantis originally was scheduled to launch Aug. 27 on a mission to resume construction of the international space station, but that launch was delayed after a lightning strike at the launch pad.

The lightning didn't hit the spacecraft, but by the time the shuttle was cleared for launch, Tropical Storm Ernesto was approaching Florida.

NASA managers on Tuesday ordered the shuttle returned to its protective assembly building, then reversed course midway through the 10-hour journey when Ernesto's forecast was downgraded.

By sending the shuttle back to its launch pad immediately, NASA gained enough time to prepare for a launch this week.

A longer wait would have run up against the timetable for a Russian launch to the space station.

The Russian space agency plans a Sept. 18 launch of a Soyuz capsule with the first international space station-bound female space tourist, Anousheh Ansari, a Dallas-area entrepreneur.

Atlantis' six astronauts have the complicated job of restarting space station construction, which was stopped after the Columbia disaster in 2003.

Crew members, who have been training for four years, will deliver a 35,000-pound, $372 million addition to the half-built station. Their 11-day mission includes three spacewalks.

"Atlantis and her crew have been waiting for several years to complete this mission. Thanks to Ernesto they've had to wait a week longer or so," said NASA test director Jeff Spaulding. "But I'm pleased to announce that that wait is almost over and we'll be ready Wednesday."