he changeable weather permitting, two astronauts stand ready to rocket the space shuttle Columbia into Earth orbit tomorrow on its second voyage in space.

Joe Henry Engle, 49, and Richard H. Truly, 46, were scheduled to fly the winged spacecraft away from the Kennedy Space Center at 7:30 a.m. EST on the first leg of a five-day flight that will take them around the world 84 times. If Engle and Truly get off, they will be the first astronauts to reuse a spacecraft that's already been flown in space.

The three major television networks will telecast periodic live reports of significant developments throughout the week and weekend. Live coverage of the landing, planned for 11:40 a.m. Monday, Nov. 9, at Edwards Air Force Base, Calif., will begin at 10 a.m. on ABC and 10:30 a.m. on CBS and NBC.

"The astronauts are in excellent health, both physical and mental," Dr. Charles K. LaPinta, space agency flight surgeon, said at a pre-launch briefing this afternoon. "They're ready to go."

If Engle and Truly had been scheduled to fly today, they would have flown but not on time.

Early this morning, the cloud cover exceeded the maximum allowable cover at the launch site. When the clouds began to clear, the winds picked up until the crosswinds were heavier than mission rules permit.

Most of the liftoff weather restrictions are intended to prevent a mishap if the astronauts must abort their flight and bring Columbia back to an emergency landing on the three-mile-long runway at the Kennedy Space Center.

"We want a clear approach path and a clear, dry runway," orbital flight test director Donald K. (Deke) Slayton said. "We also have crosswind limitations on a landing, and the winds at times today were more than we could tolerate."

Though mission rules often are bent, they call for skies that are no more than half-covered by clouds and have a visibility of at least seven miles. No rain can be falling on the landing runway, though a green light is given for liftoff if the runway is dry and rain is falling five miles away but moving out to sea. There are tail and crosswind limitations of 10 knots at the landing site.

"The problem the last couple of days is strong crosswinds," Slayton said. "If that's our only problem on Wednesday morning, I'm sure we'll talk about that and decide whether to go or not just before liftoff."

A threat of rain forecast for tonight and early tomorrow morning also could delay the launch. One of the last major countdown tasks before liftoff is to load three million gallons of supercold liquid hydrogen and oxygen aboard the huge external fuel tank that Columbia rides piggyback into space.

If it rains while technicians are loading the supercold fuel, there is a chance that excess ice will build up on the outside of the tank as the rain freezes. The weight of the ice is excess baggage at liftoff. The ice also poses a threat to the heat-resistant tiles that cover the shuttle fuselage; it might tear loose at liftoff and smash against them.

Columbia's second flight will do more than extend the shuttle's time in space from two days to five. On their second day in orbit, Engle and Truly will deploy and operate for the first time a 50-foot-long robot arm placed in the 65-foot-long shuttle cargo bay. The arm will be used on future flights to deploy new satellites in space and bring old satellites home for repair between missions.

Engle and Truly will spend more than 90 percent of their five days in space flying upside down. This will aim five instruments in the cargo bay directly down at Earth to study the terrain, measure pollutants in the atmosphere and map differences in ocean colors that could indicate concentrations of plankton and schools of fish.

The most important of the five instruments is a radar that looks straight down and to the sides and reads terrain elevations and depressions so precisely it could measure a mountain's height to within 20 feet. The radar also penetrates clouds, rain, fog and vegetation and is expected to be a valuable instrument from a geological and military standpoint in the years ahead.