CAPE CANAVERAL, Fla. (AP) _ They'll talk at length about the metal balls they plan to eject from the space shuttle Discovery next month and the special camera that they'll use to pinpoint ground targets.

But don't expect the five astronauts assigned to NASA's next shuttle flight to discuss their primary payload, a Defense Department satellite.

It's classified. Top-secret. Hush-hush. And that's that.

''I've dealt with classified material all my military career, and it's very easy to tell folks that, 'I'm sorry. That's classified. I can't discuss it,' '' said shuttle pilot Robert Cabana, a Marine colonel.

For mission commander and Navy captain David Walker, what's hard is making sure he doesn't let something secret slip while discussing the unclassified portions of the flight.

Everything about the weeklong mission, except the satellite, is open to public perusal. NASA focused its attention on the flight after Columbia landed safely at Kennedy Space Center on Sunday after a satellite delivery and research mission, none of it secret.

On the seven previous shuttle missions to deploy classified military satellites, virtually everything about the flight was secret. Launch times were announced just nine minutes before liftoff, and communications between Mission Control and orbiting astronauts were blacked out to the public.

''This is a better way to operate,'' Walker said, referring to the upcoming mission. ''We don't have to have a ton of classified books. We don't have to have a bunch of special procedures. We don't have to spend money on a very expensive security communications system. That saves taxpayers money, and that's good.''

Lt. Col. James McLeroy of the Air Force Space Systems Division said the Defense Department had been spending up to $50 million a year to maintain complete classified capability for shuttle missions and that was expected to climb to $70 million a year.

Given the decreased number of military cargoes targeted for the shuttle, the expense just wasn't worth it, McLeroy said.

The five astronauts, all military men, are to deploy the satellite six hours after liftoff and then spend the rest of the flight conducting nearly a dozen military experiments.

They will release six metal balls at the behest of space debris researchers. By tracking the spheres, scientists hope to improve their ability to detect small pieces of space junk.

Other studies include testing a camera that will provide the longitude and latitude of ground targets, and a laser receiver intended to pick up laser signals sent from the ground. Both systems could benefit battlefield commanders in time of war, military officials said.

One experiment that has been omitted from the flight involves the shuttle's automatic landing system. Walker and Cabana were supposed to use the system at the end of the flight, but NASA officials recently canceled the test and instructed the pilots to manually land the shuttle as usual.

Walker and Cabana had spent days practicing with the autoland system, which has never been tested in flight.

They're disappointed but also relieved.

''We worked hard on it,'' Walker said. ''On the other hand, no pilot that I know of prefers to let a machine land the vehicle instead of himself.''

Officials said the autoland system needs more work and that there is no hurry to test it.

NASA plans to send Discovery on the year's eighth and final shuttle flight around Dec. 2, a few weeks later than planned because of mechanical problems.