Engineers carefully wired a satellite booster in the shuttle Discovery's cargo bay with small explosives Saturday and were on track to begin the countdown to blastoff Thursday on the first post-Challenger flight, officials said.

"Right now, everything's going according to plan and it looks like we'll be able to pick up the countdown on time," said NASA spokeswoman Lisa Malone.The countdown is scheduled to begin at 12:01 a.m. EDT Monday - one minute past midnight Sunday - leading up to blastoff at 9:59 a.m. Thursday, weather permitting and assuming no problems crop up.

If engineers do run into trouble, however, they will have more time than usual to correct it. An additional 24 hours or so of "hold" time was built into the countdown to give engineers time to take care of any unexpected problems.

As for the weather, Air Force meteorologists said hurricane Helene was not expected to threaten launch. The long-range outlook for launch time Thursday called for scattered clouds, winds up to 15 knots and "widely scattered showers."

NASA implemented tough new weather guidelines in the wake of the Challenger disaster, and Discovery will not be launched through rain or if lightning is within 10 nautical miles of launch pad 39B.

Discovery's crew - commander Frederick Hauck, 47; co-pilot Richard Covey, 42; John "Mike" Lounge, 42; David Hilmers, 38; and George "Pinky" Nelson, 38 - took the weekend off to spend time with their families.

The astronauts, all veterans of at least one previous shuttle flight, plan to put on bulky new spacesuits Monday and practice launch procedures in a shuttle simulator at the Johnson Space Center in Houston before flying to the Kennedy Space Center later in the day for final preparations.

The primary goal of the 26th shuttle mission is the launch of a $100 million NASA communications satellite identical to one destroyed in the Challenger disaster that will be fired into its proper orbit 22,300 miles above the equator by a $45 million two-stage "inertial upper stage" _ IUS _ booster.

At the Kennedy Space Center Saturday, Discovery's launch pad was closed to normal work so engineers could install small explosive charges on the IUS rocket that will be used to separate the two stages after the first stage has fired. The devices also will be used to kick the satellite away from the spent second stage.

"They're cartridges," said NASA spokesman George Diller, adding that similar devices already were in place on the satellite itself. "They cut bolts and do stuff like that."

Because such ordnance installation is considered a hazardous operation, launch pad 39B was closed to all but 58 engineers and technicians involved in the work. Normal work was scheduled to resume about 10 p.m.

With Discovery's launching less than a week away, activity at the Kennedy Space Center is reaching a fever pitch.

Less than 600 journalists and photographers covered Challenger's launching, but as of Saturday, nearly 5,000 had requested accreditation to cover the 26th shuttle launch and the NASA press site was bustling with activity as television networks, newspapers, wire services and other news organizations made final preparations for blastoff.

"We're just slightly under 5,000," said NASA's spokesman Richard Young. "And we're still getting letters."

Access to the press site for launch, however, will be limited to 1,800 or so as part of a NASA-Air Force plan to limit the risk of injury or death in the event of a launch catastrophe.

Journalists not allowed to remain at the press site for blastoff will be forced to witness Discovery's launch from a causeway about 3 miles farther away. After the launch, however, they will be allowed to return to the press site.

VIPs will be able to watch blastoff from a special site about the same distance from the launch pad as the causeway. Among the celebrities expected to attend the launch are actress Daryl Hannah, actor John Travolta and singer John Denver.