Two astronauts, in a sequence of space acrobatics, will try to jump-start a lifeless Navy communications satellite loaded with rocket fuel during the August flight of the space shuttle, the astronauts said yesterday.

They said they didn't think their mission was any more dangerous than the two other rescue missions by astronauts in the last two years, but there is a small chance that the satellite is "armed," or in the position in which it is ready to fire its engines. The danger to the astronauts is great if the engines ignite accidentally while they are working.

"Sure, this satellite is full of fuel, but it was full of fuel when it was put into orbit last April," said James D. Van Hoften, who helped rescue a smaller satellite more than a year ago.

"My biggest concern is that this satellite is already armed, that the arming switch on the satellite has already been thrown," Dr. William F. Fisher said.

The arming device sets in motion a sequence of events that normally cannot be stopped, leading to firing the satellite's engines. NASA officials believe that the arming switch was never turned on. But if it has been, "that means we back out of there and that's the end of the mission. That means we don't understand what happened up there in the first place," Fisher said.

What happened in April was that the drum-shaped satellite called Syncom -- which is insured for about $84 million -- was deployed from a shuttle cargo bay, cold and dead. An arming switch apparently never came on. The satellite was supposed to fly into geostationary orbit 22,300 miles above Earth and handle radio communications for the U.S. Navy for the next 10 years.

To hear Van Hoften and Fisher tell it at a news conference at the Johnson Space Center in Houston, the hardest part of their upcoming rescue mission is the awkwardness, not the danger.

The way the two astronauts describe their task, they will leave Discovery's cargo bay on the seventh day of their mission, which is to begin from Cape Canaveral on Aug. 24.

Van Hoften will stand in foot restraints at the end of the shuttle's 50-foot-long mechanical arm as Discovery flies in orbit 35 feet below the Syncom satellite.

Van Hoften's first task is to fix a capture bar onto the side of the satellite, then hold himself in place while forcing the satellite to come to a standstill from its current once-a-minute spin.

Fisher, standing in temporary foot restraints fixed to the side of the shuttle cargo bay, will then put two plugs on either side of the arming switch to prevent it from accidentally turning itself on.

Standing on opposite sides of the huge, cylindrical satellite, the two astronauts will then bypass the satellite's electronics so that flight directors on the ground can begin to command it from Earth. Flight directors give them a 50-50 chance of succeeding.