Commander Jim Halsell during training last fall. Photo: NASA

NASA will delay shuttle Atlantis' upcoming launch to the International Space Station until April 24 to allow the ship's commander more time to recover from a sprained ankle.

The postponement was ordered because veteran commander Jim Halsell could not finish routine training in time to launch on April 18. The extra week will allow Halsell to complete practice flights in T-38 jets and the Shuttle Training Aircraft.

Halsell twisted his ankle when climbing out of a simulator at Johnson Space Center in Houston on March 15.

Doctors are not so much concerned about Halsell's "moderate" sprain, but rather the chance of another accident making the injury worse.

"The most dangerous thing about a sprained ankle is reinjuring it, so we're going very cautiously, very conservatively," Halsell told reporters earlier this week.

NASA said it would evaluate Halsell's condition next week before allowing him to resume jet training.

Atlantis will carry a seven-person international crew into space. The astronauts are scheduled to complete a practice countdown and emergency training at Kennedy Space Center on April 6 and 7.

If Atlantis is cleared for liftoff on April 24, an approximate 10-minute launch window will open at about 2015 GMT (4:15 p.m. EDT). The 10-day shuttle flight is aimed at replacing broken and suspect equipment aboard the 16-month old station. One-ton of supplies also will be delivered to the outpost for use by future residents.

The last shuttle launch delayed by an astronaut's health was in February 1990 when Atlantis commander John Creighton had a bad cold.