The $100 million satellite to be deployed Thursday from shuttle Discovery will nearly double the time ground controllers can talk to astronauts as they orbit Earth.

Aside from proving shuttles can fly again, deploying the Tracking and Data Relay Satellite is Discovery's main mission. One is already in orbit, but a nearly identical one was destroyed in the Challenger explosion Jan. 28, 1986.The satellite, once it reaches it final orbit of 22,300 miles, will help ground controllers monitor shuttles and unmanned spacecraft. Coverage provided by the satellite is more continuous than the space agency's worldwide network of ground-based communications tracking systems.

Deploying the satellite is important to NASA because the one in orbit has had minor breakdowns and is operating at about 98 percent capacity, said Jim Elliott, a spokesman at the agency's Goddard Space Flight Center in Greenbelt, Md. That satellite can monitor shuttles about 45 percent of each orbit.

A satellite relays signals and data between a spacecraft or another satellite and a ground terminal at White Sands Missile Range, N.M., which then routes it to the appropriate NASA facility.

The deployment was scheduled to begin about six hours after liftoff, although there were five later times it could be done if needed.

A spring-loaded ejection device first pushes the satellite and an Inertial Upper Stage booster rocket from Discovery's cargo bay, and the satellite waltzes away at 2.7 mph.

About an hour later, the rocket's first stage fires. The second stage ignites 121/2 hours after launch and puts the satellite in its final orbit.

The satellite and booster and its carrying cradle weigh about 45,000 pounds, making it the heaviest payload to be carried aboard a shuttle, National Aeronautics and Space Administration officials said. The satellite itself weighs about 5,000 pounds and has two 50-pound antennas that look like giant umbrellas.

The satellite will be placed in orbit over the Pacific Ocean near the Gilbert Islands. Its orbit keeps it always hovering over the same part of the Earth.

On Tuesday, NASA had a brief scare when a booster rocket being readied for a shuttle flight next February was found to have a damaged seal.

There was concern that the booster rocket aboard Discovery might have the same flaw, but engineers worked overnight and determined from tests that even with such a defect, the seal would perform properly.