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

NASA has two more shuttle trips planned before the aging three-orbiter fleet is retired. A third, final mission was approved last year by Congress in a NASA authorization bill signed by President Barack Obama.

Yet, funding for that bill has not been appropriated. Instead, the country is working under a continuing resolution – a stopgap measure put in place until lawmakers can agree on a formal budget – that freezes NASA and the rest of the government at 2010 funding levels.

Now some say the issues with the shuttle Discovery's fuel tank, which include numerous cracks discovered on some support beams, don't bode well for the extra mission, dubbed STS-135, said Florida's Senator Bill Nelson.

NASA engineers have been working to repair the cracks for the last several weeks.

"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," Nelson told Florida's WFTV.

If NASA decides Discovery's tank is unusable, there are no spares. There are only the two remaining tanks reserved for the other two missions in the queue. If Discovery's tank cannot be fixed, Nelson suggested that NASA could cancel the STS-135 mission and use the tank allocated for it on the upcoming launch of Discovery.

However, since the tank set aside for the possible STS-135 shuttle mission was manufactured and assembled from the same batch of material as Discovery's tank, it would probably suffer the same problems and need similar fixes.

"I expect it to have exactly the same issues that this tank had," said NASA's shuttle program manager John Shannon during a Tuesday (Jan. 11) press conference. "Of course, we're going to continue to do testing. I'm going to say it's likely we'll do the exact same repair" on the tank for the extra mission.

Discovery's upcoming STS-133 flight has been delayed repeatedlyfrom an initial launch target of November 2010. It is now slated to launch Feb. 24on its mission to deliver spare supplies and a robot helper to the International Space Station. After that, NASA is aiming to launch the shuttle Endeavour on its final voyage in mid-April to carry an astrophysics experiment to the space station.