The countdown began early Friday for a Monday launch of space shuttle Discovery on a flight its commander says will demonstrate "we're a space-faring nation again."

The launch will be the third for the shuttle program since the Challenger disaster more than three years ago and the first of seven flights scheduled this year as NASA moves toward its target of safe, routine, once-a-month missions by 1992."The countdown started right on time," said NASA spokeswoman Lisa Malone. The ambitious 1989 schedule is getting a late start, with Discovery's flight delayed nearly four weeks so faulty or suspect engine parts could be replaced.

The countdown clock began ticking at 12:01 a.m. after test conductor Jerry Crute issued the traditional call to stations that summoned members of the launch team to their posts. The initial task was to electrically activate the spaceship's systems.

Liftoff is set for 6:07 a.m. MST Monday.

The five astronauts for the five-day mission were to arrive Friday afternoon from their training base in Houston to make final flight preparations.

The commander, Navy Capt. Michael L. Coats, said in a recent interview that the first two post-Challenger missions, flown successfully last September and December, "were important to show that we could fly the space shuttle again after the catastrophe.

"The next step," he said, "is to prove that we can do it on a regular basis, that we're back in the business of space, to show we're a space-faring nation again.

"It was very frustrating for us in the space program the last three years to watch the Russians set flight time records and do all the things in space with their shuttle while we've been grounded," Coats said.

Flying with Coats will be Air Force Col. John E. Blaha, the pilot; Marine Cols. James F. Buchli and Robert C. Springer and Dr. James M. Bagian, a medical doctor.

Bagian and Springer are to release a $100 million Tracking and Data Relay Satellite six hours after liftoff.