The rockets of the space shuttle Discovery lit up the gray Florida sky this morning, sending Utah Sen. Jake Garn and six other crew members on a five-day mission that had been delayed nearly two months.

Less than a minute away from a sixth postponement, the 100-ton spaceliner raced away from Earth at 8:59 a.m. into a sky so cloud-filled that mission directors had waited almost an hour for it to clear.

Following the liftoff, the 16th in the shuttle program, Discovery's crew deployed a $40 million Canadian communications satellite and began medical tests on each other to study what causes body fluids to shift and the heart to change speeds in weightlessness.

"I've got three data tapes on the echocardiograph," physician-astronaut Margaret Rhea Seddon called down to Mission Control in Houston, "and all three hearts look beautiful." Garn and astronauts Jeffrey A. Hoffman and Charles D. Walker will be given echocardiograms by Seddon each day in space.

The echocardiograph, a relatively new instrument that uses high-frequency sound waves to picture blood flow in and out of the heart, is one of the devices being used for the first time on this flight to probe how the body deals with weightlessness. The primary guinea pig on this flight, Discovery's fourth, is Garn, 52, the first politician in space.

The Utah Republican is wearing a waist belt with two stethoscopic microphones to record the sounds his stomach and intestines make during digestion. His head and chest are wired to record the electrical signals from his brain and heart, and other instruments will measure the way his bones grow and shrink in zero-gravity.

The senator also has been asked to record his impressions if he feels sick and has been told to take a non-aspirin painkiller, acetaminophen, to test its strength against headaches in space.

The experience of rocketing into orbit left the senator almost speechless today. After more than nine hours in orbit, the only time his voice was heard was when he answered a call from Mission Control: "Houston, this is PS2." Garn is Payload Specialist No. 2 on the mission.

The senator's wife, Kathleen, was anything but speechless as she watched him riding away from Earth on a 500-foot-long sheet of flame.

"The kids screamed, I cried," she told the Associated Press. "It was frightening. Even though I had seen it on TV, I didn't realize there was going to be so much fire. I expected it to go much faster. I thought, 'Oh my gosh, it's going to stall out.' I looked at Michelle Bobko wife of commander Karol J. Bobko and she looked okay, so I knew it was all right."

A heavy layer of clouds at 15,000 feet that covered almost all of central Florida delayed the liftoff until the last minute of the second launch "window" available today. Chief astronaut John Young repeatedly flew a training aircraft through the cloud layer and reported that his windshield was covered with water.

"The moisture and the density of that upper cloud layer was a real concern to us," launch operations director Robert Sieck said at the Kennedy Space Center here. "Those water drops hitting John Young's windshield could be striking the leading edges of the wings and the nose cap of Discovery at such high speeds they could cause serious damage to the shuttle. We decided to wait things out to see if that moisture would burn off with the sun."

Less than two minutes before a sixth postponement would have been necessary, Young reported that the clouds had begun to thin. Sieck then gave the green light, and Discovery's engines ignited 55 seconds before the launch would have been called off.

"There was an involuntary cheer when the 'go' was given for launch," Sieck said. "We really wanted this one because it seemed like we'd been on a long dry spell until this morning."

Less than half an hour after the "go" sign was given, the crew was in orbit. "We're getting great television from you guys," astronaut Robert C. Springer radioed from Earth. Answered pilot Donald E. Williams: "You might believe it looks better out the window." Said Springer: "Looks like you're having fun." "Only a little," replied astronaut S. David Griggs.

Discovery, which blasted off today on the fourth anniversary of the first shuttle flight, is scheduled to land here early Wednesday on its 79th orbit.