Discovery Liftoff Set, Barring Cold Spell : Space: It marks the last time a NASA shuttle will carry a military payload in orbit. Analysts say the unnamed cargo is likely a Lacrosse spy satellite.

Barring interference from a late autumn cold snap, the manned space shuttle Discovery is set to lift off early Wednesday, carrying the last major military payload that will be placed in orbit by the National Aeronautics and Space Administration’s shuttle fleet.

As is customary, NASA and Air Force officials refused to say what the Defense Department has stowed in the spacecraft’s cargo bay. But the payload’s announced weight of 23,215 pounds and its launch in a sharp northeasterly direction from the Kennedy Space Center in Florida suggest that Discovery will be hauling a Lacrosse all-weather spy satellite, analysts said.

As the final phase of launch preparations began Monday, NASA officials were keeping a wary eye on the weather. Forecasters said there is a 60% chance of an overnight chill that could postpone the liftoff until Thursday.

Cold weather launches have been tightly restricted since the 1986 explosion of the space shuttle Challenger, a disaster attributed to the effect of freezing temperatures on a seal in the vehicle’s solid rocket motor. Launches are now prohibited if temperatures dip below 47 degrees in winds of 5 m.p.h. or less for 30 minutes after fueling. The combination of lower temperatures and wind speeds encourages the formation of frost or ice.

Although forecasters said that the restrictions could be violated before dawn Wednesday, NASA officials planned to begin fueling Discovery’s 153-foot-tall external tank with liquid hydrogen and liquid oxygen at 7:39 p.m. (PST) Tuesday. Liftoff is now scheduled for 3:59 a.m. (PST) Wednesday, with the spacecraft set to return to Florida on Dec. 9.

The shuttle, once the sole launcher for large spy satellites, such as the Lacrosse and Keyhole series, has played a crucial role in modernizing the U.S. space intelligence network. Future launches will be carried out by unmanned boosters, while the shuttle is restricted to more complex scientific, maintenance and construction missions requiring the skills of astronauts.

After deploying the secret Pentagon cargo on the first day in orbit, Discovery’s five-man crew will turn to six days of scientific and engineering experiments designed to gain knowledge for future space station operations.

The flight will be commanded by U.S. Navy Capt. David M. Walker, a veteran of two previous missions. His crew is made up of Marine Corps Col. Robert D. Cabana, the pilot, and three mission specialists: Air Force Col. Guion S. Bluford Jr., Army Lt. Col. James S. Voss and Army Lt. Col. Michael Richard Clifford.

The flight marks the 10th major shuttle launch for the Defense Department, a NASA-Pentagon collaboration that Administrator Daniel S. Goldin has called “a wonderful and remarkable demonstration of interagency cooperation.”

Through the first several military flights, shuttle activities were kept secret from launch to landing. But security precautions have since been relaxed and the flight of Discovery will be conducted as a routine NASA mission after the military payload is deployed.

With the apparent launch of another Lacrosse, the Defense Department would have about 10 so-called imaging satellites--Keyhole and Lacrosse--in orbit, compared to only two or three at the height of the Cold War, analysts said.

Advanced versions of the Keyhole satellite are said to be able to photograph objects on Earth that are only inches in size, while Lacrosse can penetrate thick cloud cover to record radar images of objects only a few feet in size.

John Pike, a space policy analyst for the Federation of American Scientists, speculated that the instrument aboard Discovery is an addition to the surveillance network rather than a replacement for a satellite that has ceased to function.

“My guess is that they are launching them chiefly because they are in the pipeline,” he said. “They were contracted for in the ‘evil empire days’ (before the demise of the Soviet Union), probably completed before the Berlin Wall came down and now are taking up floor space. They are like last year’s cars that have to be moved off the lot.

“It is more a reflection of just how long-range and inflexible the planning process is than it is a particular requirement of national security,” he said.

As for future shuttle flights, NASA announced last week that it will carry out practice “spacewalks” to prepare for construction of Space Station Freedom, now expected to begin in three years.

Astronauts Greg Harbaugh and Mario Runco Jr. will perform a five-hour walk from the space shuttle Endeavour while it orbits the Earth sometime in January. No exact launch date has been set.