The final space shuttle mission means that the 30-year-old shuttle program is about to enter the history books alongside the famous Mercury, Gemini, and Apollo programs.
And as the end of the shuttle era looms, NASA leaders say they're about to build a new vehicle, one that will let astronauts go exploring deep into space. But some experts doubt that plan will ever get off the ground.
To understand the big question mark hanging over NASA's future, it helps to first turn the clock back to 2004 — the year after the space shuttle Columbia disaster.