A file image of space shuttle Atlantis sitting atop a 76-wheel transporter inside its processing hangar. Photo: NASA

NASA officials announced Thursday that the next space shuttle mission to the International Space Station would be pushed back to no sooner than April 17, confirming what had been expected for some time.

The delay amounts to four days and allows additional time to finish readying Atlantis for its first spaceflight in 2 1/2 years. A firm launch date will be authorized during a senior management review on April 4.

If the veteran spaceship is cleared to start its 10-day mission on April 17, an approximate 10-minute launch window will be available opening at 2302:49 GMT (7:02:49 p.m. EDT). The shuttle would spend the next two days catching up with the space station before docking at 1557 GMT (11:57 a.m. EDT) on April 19.

Atlantis was scheduled to be transported from its processing hangar to Kennedy Space Center's Vehicle Assembly Building on Thursday but last-minute work was running behind due to minor problems, an agency spokesman said.

The rollover was reset for Thursday evening but bad weather put the quarter-mile move on hold. Workers were hoping weather conditions would improve for an overnight transfer.

Once inside the 52-story VAB, Atlantis will be mated with an external fuel tank and twin solid rocket boosters atop a mobile launching platform.

Officials also decided Thursday to replace a suspect main engine on the shuttle inside the VAB. The three-day job probably will start Saturday, delaying rollout to launch pad 39A until March 24.

The replacement was ordered earlier this week after officials became worried that metal seals installed inside the engine's high-pressure fuel turbopump were defective. Documents indicate there is a high probability two segments of the nickel-alloy seals should have been thrown away and never used in a shuttle engine. A paperwork snafu is blamed for the parts accidentally being used.

Each turbopump contains two tip seals amounting for six segments per engine. The seals run the circumference of the pump housing, about 18-thousandths-of-an-inch away from each of the two turbine blade wheels. The seals prevent airflow above the turbine blades to direct super-hot gases into the turbine machinery. The turbine shaft spins at 37,000 rpm.

The tip seal problem first surfaced following shuttle Discovery's launch in December. Post-flight inspections revealed a seal segment's top surface peeled back and was rubbed by the turbine blades. That segment should have been scrapped too.

The International Space Station as seen during previous shuttle flight. Photo: NASA

Atlantis' will carry six Americans and one Russian into Earth orbit to perform maintenance on the 16-month old International Space Station. Also on the agenda is one spacewalk and transferring one ton of supplies into the station.

The repair mission was ordered to extend the life of Russian-built Zarya module until the end of 2000 while NASA waits for next station segment to be launched -- the Russian Zvezda service module sometime this summer.

Zarya -- the first station element launched -- must continue to work and keep the station boosted to a safe altitude until Zvezda arrives. Once attached, Zvezda's engines will be used to raise the station's orbit.

Veteran shuttle commander Jim Halsell will lead the STS-101 mission, also known as ISS flight 2A.2a. The other crewmembers include pilot Scott Horowitz and mission specialists Mary Ellen Weber, Jeff Williams, Jim Voss, Susan Helms and Russian cosmonaut Yuri Usachev.

The astronauts hope to bring Atlantis home to Kennedy Space Center on April 27. Touchdown on the three-mile-long shuttle runway is planned for 1935 GMT (3:35 p.m. EDT) after 9 days, 20 hours and 32 minutes in space.

Voss, Helms and Usachev will later fly again as the second long-duration residents aboard ISS.