WASHINGTON / NASA doesn't know yet where it will get the money, but on Thursday the space agency officially added another space shuttle launch to its schedule the final one for the fleet.

The space agency set a target launch date of June 28 for the shuttle Atlantis and started preparations for the 135th and last shuttle flight.

The four-member crew will take up supplies to the International Space Station, make one spacewalk, and return a faulty pump that has bedeviled engineers.

Now three missions remain before NASA retires its shuttle fleet this year.

The shuttle Discovery's last mission is slated for Feb 24, and Endeavour's is set for April.

Thursday's move allows different parts of the shuttle program to start work on Atlantis' 12-day flight, including astronaut training and mission planning, NASA spokesman Michael Curie said.

Atlantis was originally planned as an emergency-only rescue mission if needed for the Endeavour crew, and it will continue to serve as a backup shuttle for that mission, as "STS-335" But if Endeavour's flight proceeds normally, Atlantis will then be prepared for its own resupply mission, STS-135.

Last year, the Obama administration and Congress clashed over the future of the human space program and came up with a compromise that authorized one extra flight of the shuttle the Atlantis mission.

But Congress never gave NASA the few hundred million dollars needed for the extra flight.

That left NASA in a quandary about whether the flight was real or not.

The initial money is coming from the space shuttle program's regular budget, but that can't provide the big dollar amounts needed for a shuttle flight, Curie said.