Published: Jul 05, 2011 5:21 PM EDT
Updated: Jul 05, 2011 4:21 PM EDT

CAPE CANAVERAL, Fla. (AP) - NASA's last space shuttle launch this week may have to contend with lousy weather.

Shuttle weather officer Kathy Winters said Tuesday there's a 60 percent chance that rain or thunderstorms will delay Friday's planned liftoff of Atlantis. The weather improves with each passing day, as the launch time gets slightly earlier.

Friday's launch time is 11:26 a.m.

The countdown clocks will start ticking Tuesday afternoon. The four astronauts assigned to the 12-day flight arrived at Kennedy Space Center on Monday.

Atlantis will make one final supply run to the International Space Station before retiring. As many as 1 million spectators are expected to jam the Cape Canaveral area for the historic send-off.

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