NASA's final visit to the Hubble Space Telescope has been delayed at least a month, until the fall, because of extra time needed to build the shuttle fuel tanks needed for the flight and a potential rescue mission.

Atlantis and a crew of seven were supposed to fly to Hubble at the end of August, but now won't make the journey until the end of September or early October.

Shuttle program manager John Shannon said it's taken more time to incorporate all the post-Columbia design changes to the external fuel tanks than had been expected.

"It's a small price to pay to tell you the truth, four to five weeks, for all the improvements that we're getting on this tank," Shannon said Thursday.

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The fuel tank for the next shuttle launch is the first to be built from scratch with the design changes. That work delayed Discovery's flight to the international space station from April until May 31.

The mission to Hubble, orbiting 350 miles above Earth, is unique. Not only must Atlantis be ready, but another shuttle must be on the launch pad ready to rush to the rescue in case Atlantis suffers severe launch damage that might prevent a safe re-entry.

Unlike other shuttle crews, which travel to the space station, the astronauts on the Hubble mission would have nowhere to seek shelter in the event of a gaping hole in their ship's thermal shield.

In the case of a rescue, the Hubble astronauts would put on spacesuits and float out of their ship and into the other shuttle.

Columbia was destroyed and its seven astronauts killed during re-entry in 2003 because of a plate-size hole in the shuttle's left wing. A chunk of fuel-tank foam insulation broke off during liftoff and gashed it.

Because of the delay in the Hubble mission, NASA will have to settle for five shuttle flights this year, instead of six. Despite the setback, NASA still hopes to complete the space station and retire its shuttles in 2010, Shannon said.

As for Russia's trouble-plagued Soyuz re-entry on April 19, NASA's space station program manager, Mike Suffredini, said Thursday that the investigation into the mishap will determine whether the three astronauts were at any more risk than normal.

The Soyuz spacecraft descended much more steeply than usual and subjected the crew to considerably more gravity forces. It was the second time in a row that the capsule malfunctioned like this.

The crew included U.S. astronaut Peggy Whitson, who was ending a six-month space station stay, as well as a Russian and a South Korean who ended up in the hospital because of back and neck pain.

Russia hopes to complete its investigation by the end of May.

With U.S. astronaut Gregory Chamitoff scheduled to fly to the space station aboard Discovery and remain there for several months, NASA will have to decide before May 31 whether the Soyuz will serve as a safe lifeboat if there is an emergency.

Suffredini said it would be "pretty dramatic" for NASA to pull Chamitoff or anyone else off the space station.

"But we will do whatever is necessary based on the findings of the commission," he said.