The final crew of the space shuttle Atlantis has boarded the spacecraft at Florida's Kennedy Space Center for the 135th and final flight of the 30-year-old space shuttle program.

As many as 1 million people are gathering to witness the historic lift-off, but the weather forecast remains questionable, with a 70 percent chance for clouds and thunderstorms at the U.S. spaceport by launch time . The bad weather could force officials with the U.S. space agency NASA to delay Atlantis's launch until Saturday or Sunday.

The shuttle's four-member crew will deliver supplies, spare parts and science experiments to the International Space Station during its 12-day mission.

The space shuttle has played a central role in the construction and operation of the orbital outpost.

NASA is ending the shuttle program to concentrate resources on deep-space exploration. The agency is working with several commercial U.S. aerospace companies to develop vehicles to replace the shuttles. Until then, Russia's Soyuz spacecraft will ferry U.S. astronauts to and from the ISS, while Russian, European and Japanese cargo rockets will continue their resupply and waste disposal missions to the station.