The planned launch next week of the space shuttle Discovery has been delayed to mid-December to give more time for engineers to evaluate repairs made to the orbiter’s external fuel tank, NASA said on Wednesday.

NASA has encountered numerous delays over the last month in trying to get Discovery off the ground for its final voyage before the shuttle fleet is retired.

The main culprit has been several cracks found on brackets on the external tank. The fear is the cracks could cause insulation foam to break off and damage the shuttle during lift-off.

Broken foam damaged the shuttle Columbia in 2003, causing it to disintegrate while re-entering the atmosphere and killed all seven astronauts aboard.

“Managers decided the analysis and tests required to launch Discovery safely are not complete. The work will continue through next week,” NASA said in a statement.

In scrapping the December 3-6 launch window, NASA set a new one that begins December 17.

The cracks were found on aluminium ribs that support an empty space inside the external fuel tank that separates the liquid oxygen tank at the top and the liquid hydrogen tank at the bottom.

The Discovery mission is to deliver the last major U.S. contribution to the International Space Station (ISS) - an extra room - along with supplies, including a human-like robot known as Robonaut 2, the first-such robot ever sent to space.

Discovery is the oldest active shuttle in the fleet. After the shuttles are retired, astronauts will travel to and from the ISS on Russian spacecraft.

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