NASA's countdown clocks are ticking again for the next-to-last space shuttle launch.
The countdown began at 7 a.m. ET Friday for the second time for Endeavour's final mission. Launch is set for 8:56 a.m. ET Monday.
A launch attempt on April 29 was foiled by an electrical snag aboard Endeavour that turned up less than four hours before launch time. NASA says repairs have fixed the problem. This time around, forecasters predict a 70 percent chance of acceptable weather for launch, with crosswinds and cloud cover looming as potential concerns.
Commander Mark Kelly and his five crewmates returned to Kennedy Space Center on Thursday. This weekend they're being joined by their families, including Kelly's wife, U.S. Rep. Gabrielle Giffords, D-Ariz., who was shot in the head in January.
Giffords has been undergoing rehabilitation in Houston, and had to gain her doctors' clearance to attend last month's launch attempt. She returned to Texas for further treatment when liftoff was delayed, but her doctors say she's fit to travel back to Florida for the second attempt.
"She's doing great, and she's very much looking forward to the launch of Endeavour," Giffords' spokesman, C.J. Karamargin, told msnbc.com. He said Giffords has been in "constant contact" with her husband during the break.
NASA expected that Endeavour's April launch would draw up to 750,000 spectators, including President Barack Obama and his family. Even though the launch was delayed, the Obamas followed through with their planned visit to Kennedy Space Center, and the president had a 10-minute private meeting with Giffords.
Obama is not expected to return to Florida for Monday's launch attempt, but NASA still anticipates a big turnout. "We are expecting probably slightly less than before, just due to the timing," launch test director Jeff Spaulding said. He supposed that fewer people would be in attendance on a Monday morning as opposed to a Friday afternoon. The crowd "may be down closer to about 500,000 this time," he told reporters.
Endeavour will fly to the International Space Station and deliver a $2 billion science experiment as well as a storage platform. Four spacewalks are planned during the 16-day STS-134 mission.
If Endeavour launches on time, this mission will mark the first time ever that a Russian Soyuz spacecraft leaves the space station during a shuttle stopover. The Soyuz is due to carry three station crewmates back to Earth on May 23.
After this mission, only one other shuttle flight remains, by Atlantis in July. Endeavour is slated to be prepared for display at the California Science Center in Los Angeles.
The shuttle fleet is being retired to make way for a new generation of spacecraft capable of going beyond Earth orbit for deep-space exploration. To send astronauts to the space station, NASA will rely on Russian transports — and eventually, commercial spaceships that are currently under development.
This report includes information from msnbc.com and The Associated Press.