Repeated delays in the final flight of space shuttle Discovery have raised doubts over NASA's ability to launch an extra shuttle later this year as planned.

The most recent delay of the space shuttle Discovery's mission over issues with its fuel tank has raised some questions over NASA's ability to launch an extra shuttle later this year as planned.

NASA has three more shuttle trips planned before the retirement of three-orbiter fleet. The third mission was approved last year by the US Congress.

But, US Senator Bill Nelson said that issues with the Discovery's fuel tank did not bode well for the extra mission, called STS-135.

Speaking on the topic, Nelson said, "If worst came to worst and they felt like this tank was not safe, they would stand down and at that point would only fly two more shuttle flights instead of the three that are authorized."

Space shuttle Discovery is now expected to fly to the International Space Station on February 24. Originally, Discovery was scheduled to fly on November 5, but was delayed numerous times due to cracks in the fuel tank and necessary repairs.