NASA hoped the weather at Endeavour's landing site would finally cooperate and allow the space shuttle to touch down after two days of delays.
Clouds, high wind and rain have thwarted efforts to bring Endeavour -- the ride home for the international space station's former crew -- back to Earth. Wednesday's attempts were canceled while Thursday's opportunities were skipped altogether.
The forecast on Friday was slightly better for two landing attempts.
"It looks like we do have a fighting chance," said shuttle commander James Wetherbee.
If Friday's attempts fall through, Endeavour would touch down on Saturday in either Florida or the backup landing site at Edwards Air Force Base in California, said flight director Wayne Hale.
However, NASA prefers that shuttles land in their home port in Florida in order to save the expense and time of ferrying them from California.
Endeavour has enough fuel and supplies to stay in orbit until Sunday.
The returning space station Alpha crew of U.S. astronaut Peggy Whitson and Russian cosmonauts Valery Korzun and Sergei Treschev lived on the orbiting outpost for six months. Thursday was their 184th day in orbit.
The shuttle, which launched Nov. 23, left the space station on Monday. Endeavour's four-astronaut crew helped install a new girder and dropped off Alpha's sixth resident crew -- two Americans and one Russian -- who will remain aboard until March.