The U.S. space shuttle Atlantis lifted off as scheduled from the Kennedy Space Center in Florida Friday afternoon, in one of the last missions of the space shuttle fleet.

Twenty-five years after its first launch, the space shuttle Atlantis lifted off under sunny Florida skies on its final planned mission.

Six astronauts are on board Atlantis on this 12-day mission.  They will make three spacewalks and deliver a small Russian research module to the International Space Station.

NASA is retiring its shuttle fleet this year.  When the shuttle program ends, space station partners will rely on Russian and private space vehicles to ferry supplies and astronauts to and from the International Space Station.

The U.S. space agency's proposed budget calls for putting more money toward science, research and development, with an eye toward developing the next generation of space flight to explore deep space.

Michael Moses, the shuttle program's launch integration manager, spoke about the historic liftoff during a post-launch news briefing.  "Personally, it has not hit me yet that this was Atlantis and possibly the last flight. It just was another launch, so I'm wrapped up in the focus and the attention on that.  So probably later tonight and heading into the week, I'll probably start thinking about that," he said.

Only two shuttle missions remain.

Atlantis is NASA's fourth space-rated shuttle.  It was not named for the legendary lost and submerged city of Atlantis, but for a two-masted research ship that scientists used to study the ocean floor until 1966.