"Trying for '09 would require us to assume too much risk, more than I think is appropriate for a flagship mission," said NASA Administrator Michael Griffin.
It's the second time in two years that NASA has been forced to postpone a Mars mission. Last year, the space agency delayed a planned 2011 launch of a $485 million Mars atmospheric probe by two years because of an unspecified conflict of interest in the selection process.
Since Mars and Earth only pass close enough to launch probes every 26 months, NASA will fly the Mars Science Lab in 2011.
Delivery delays and additional testing caused the project to fall several months behind schedule, NASA said.
The Mars Science Lab is billed as the most advanced spacecraft to probe the Martian surface. The size of a small sports utility vehicle, it will study whether the environment was habitable in Mars' early history and will carry high-tech instruments to analyze rocks and soil in greater detail than previous surface missions.