Beginning with the next space shuttle flight in two weeks, NASA is keeping its launch times secret until 24 hours in advance to guard against possible terrorist attack.

It is the first time in more than a decade that the space agency is refusing to give out a shuttle launch time well in advance. "NASA is choosing to be extra careful," Kennedy Space Center spokesman Bruce Buckingham said Monday.

Under the new policy, approved late last week by top NASA officials, the space agency will give out four-hour launch periods until about 24 hours before liftoff, when the precise time will be announced. Once NASA releases the exact launch time, everything about the flight — including the landing time — will be made public.

The next launch, by Atlantis on April 4, will occur sometime between 2 p.m. and 6 p.m., Buckingham noted. It is a space station assembly mission.

Even though the seven astronauts assigned to the flight were at the launch site Monday, going through the customary practice countdown, NASA did not announce their arrival in advance for security reasons.

"What we're trying to do is protect the credibility of this agency with the public and the press, and also with the security measures that this nation has put into place at the highest government level," NASA spokesman Kyle Herring said from the agency's Washington headquarters.

NASA, as a civilian agency, needs and wants to put out as much information to the public as possible, Herring said. "But we also want to protect the national assets, that is the hardware, the crew and the personnel that work for this agency," he said.

The launch times of the December and March space shuttle missions were widely known before Sept. 11, and so there was no attempt to make them secret following the terrorist attacks, Herring said. Both launches were conducted amid unprecedented security, which is expected to continue.

Herring said the new policy took a while to develop and will remain in effect for subsequent missions, "with the caveat that it will be reviewed on a flight-by-flight basis."

Still to be decided: whether and how NASA will confirm the start of the launch countdown. The countdown clocks in public view will not start ticking until 24 hours in advance, Herring said.

Even tougher rules were in effect for the seven space shuttle flights that carried classified Defense Department satellites from 1985 through 1990. In those cases, the launch time was not announced until nine minutes before liftoff and a news blackout was imposed on virtually the entire flight.