NASA (search) returned the space shuttle Discovery to the launch pad Wednesday for the first mission since the Columbia disaster, after replacing the external fuel tank with a new model designed to prevent dangerous ice buildup.

Shuttle managers are aiming for a liftoff as early as July 13.

Discovery's four-mile trek from the assembly building to the launch pad aboard the giant shuttle transporter took more than 10 hours. The morning trip was halted several times because of overheated bearings in the 40-year-old old transporter, and the speed fell below the usual mile per hour.

"A little bit of deja vu — we've done this before, we're doing it again," vehicle manager Stephanie Stilson said as Discovery (search) crept to the pad.

The shuttle team was disheartened when it had to haul Discovery off the pad last month for more work, but understood it was the right thing to do, Stilson said.

"Today was not quite the excitement of the first time, but still, what a great thrill to be back on track, moving in the right direction, getting ready for launch, getting back out to the pad," she said.

Discovery was transported to the pad in April but removed May 26 after NASA determined that potentially deadly pieces of ice could form over an expansion joint on the external fuel tank after the super-chilled fuel was loaded. Managers decided to install a heater at the joint, located along the feed line for liquid oxygen.

Falling chunks of ice could be even more menacing than pieces of the fuel tank's insulating foam, which was responsible for Columbia's (search) destruction during re-entry and the deaths of seven astronauts 21/2 years ago. Because of their focus on keeping big chunks of foam from coming off the tank during liftoff, engineers did not realize the ice threat until April.

NASA replaced Discovery's fuel tank with one intended for Atlantis on the second post-Columbia flight, because it was quicker to add a heater to the expansion joint on that second tank. Discovery also got the two booster rockets intended for Atlantis.

Stilson said Discovery should now be the safest shuttle to ever fly, with all the other safety modifications made in the wake of the Columbia catastrophe.

NASA is still assessing the potential danger of ice buildup on brackets on the tank. Options for reducing that risk include using infrared lamps at the pad or putting bags over the brackets.

Discovery's 12-day mission to the international space station is considered a test flight because of all the changes.