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NASA has set a target launch date of June 28 for shuttle Atlantis, shown here last May before the launch of its previous mission.

NASA doesn't know yet where it will get the money, but the space agency on Thursday 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 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 and return a faulty pump that has bedevilled engineers.

Now three missions remain before NASA retires its shuttle fleet this year. Shuttle Discovery's last mission is slated for Feb. 24, Endeavour's in April.

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

Originally, Atlantis was planned as an emergency-only rescue mission if needed for the Endeavour crew.

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.

NASA 'optimistic' about funding

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

"We're optimistic that the funding will be there," Curie said, but he couldn't give details about where the money will come from.

NASA was under the gun to start preparations. Otherwise, the Atlantis mission wouldn't have been able to launch in late June, he said.

The final flight will be commanded by Christopher Ferguson, and includes Douglas Hurley, Sandra Magnus and Rex Walheim.

The extra flight means Mark Kelly — the husband of wounded U.S. Representative Gabrielle Giffords — will not command the final shuttle flight even if he stays on the Endeavour mission. With his wife's expected long rehabilitation, after she was shot along with several others outside a grocery store in Tucson, Ariz., last week, Kelly asked for a backup commander to be named in case he couldn't fly as scheduled in April.