The maiden commercial flight of the space shuttle Columbia is under such tight weather and launch constraints that it may not be able to carry two $50-million communications satellites into orbit on time Thursday morning.

"Cloud-cover conditions were unacceptable for launch this morning," launch test conductor Stanley Gross said at the Kennedy Space Center here, where the shuttle leaves the Earth. "This Thursday, it obviously has to be better than it was today or we won't get off on time."

Columbia astronauts Vance Brand, Robert Overmyer, William Lenoir and Joseph Allen are due to rocket into space at 7:19 a.m. EST on Thursday carrying a pair of almost identical satellites owned by Satellite Business Systems (SBS) of McLean, Va., and Teleset of Canada that the astronauts will drop off in orbit.

Thursday's flight is the first for any of the 64 commercial and military satellites that the space agency has contracted to take into orbit on the shuttle through September, 1987. It also will be the first time astronauts will haul commercial cargo into orbit on the shuttle.

The main reason the flight would not have gone off today is that the "launch window" for this flight is narrow. It opens at 7:19 a.m. Thursday and closes 40 minutes later. The clouds that covered the Kennedy Space Center at 7:19 a.m. today still covered it at 7:59, though they disappeared two hours later.

After four test flights, shuttle astronauts are still not ready to take off and land at any time of day or night and in any kind of weather. They must lift off in the morning so there is still daylight at alternate landing sites in Spain, Africa and California in case they must abort the flight.

Spain and Africa are alternate landing sites if the astronauts do not have engine power to reach orbit. Edwards Air Force Base in California is an alternate if they reach what flight directors call a "minimum orbit" of about 100 miles, which triggers what is called a "once-around abort" that would bring them down in California near the end of their first revolution of the earth.

Another reason for the restrictive 40-minute window is that the SBS and Telesat satellites must be deployed in daylight over specific regions of the Earth so that their sensors can line up with the sun at precise angles.

While today's cloud cover would have prevented an on-time launch, flight directors expect a steady improvement in the weather. Said Gross: "The weather people tell us we might be getting some rain on Thursday but that the cloud cover is expected to go away, and it's the cloud cover that's the main thing."

Two small problems occurred in the countdown today: a radar altimeter aboard Columbia failed an electrical test and had to be replaced, and what was described as a "creeping" leak was found in a helium tank that pressurizes fuel. Gross said the leak was so small that launch directors decided to ignore it.