The crew of the shuttle Atlantis, the first astronauts to spend Thanksgiving in orbit in 12 years, deployed a powerful communications satellite yesterday and prepared for spacewalks today and Sunday to practice construction techniques that will be used to build space stations in the 1990s.

The RCA Satcom K2 satellite, the third launched by Atlantis this trip, is the most powerful domestic communications satellite ever built. It is also the first ever launched from the shuttle without insurance. RCA said it did not insure its $50 million satellite because seven satellite failures in the last two years had pushed premiums to more than $10 million per launch.

"Three for three," Jim Weatherbee in mission control told the astronauts. "Now we can look forward to some ground-breaking activities with your new construction techniques tomorrow."

Once the cargo bay was cleared, the crew began the tedious tasks of getting ready for two marathon spacewalks by astronauts Jerry L. Ross, an Air Force major, and Sherwood C. Spring, an Air Force lieutenant colonel. Ross and Spring are to spend 12 hours outside the shuttle practicing and repracticing techniques to erect metal structures that crews will use to fabricate the first orbiting manned space station in 1992.

Ross and Spring are scheduled to enter the cargo bay at 5:28 p.m. today for the first of two six-hour spacewalks.

Their tasks are simple but not easy. They have been told to construct a 45-foot-high aluminum tower and a 400-pound aluminum pyramid, then do it again and again to see which construction procedures are quickest and safest in weightlessness.

Previous astronaut crews have stripped, retrieved and repaired machinery in space, but no crew has built anything from scratch before. The only experience Ross and Spring have had in space construction has been undertaken while wearing spacesuits in underwater tanks, which is the closest astronauts can come on Earth to simulating any kind of work in weightlessness.

Ross and Spring are to be alone in the cargo bay, but will be helped from the shuttle cockpit by Atlantis commander Brewster H. Shaw Jr., pilot Bryan D. O'Connor and mission specialist Mary L. Cleave.

Cleve, the only woman in the crew, will work the shuttle's 50-foot-long robot arm to assist the spacewalkers. Also in the crew are payload specialists Charles D. Walker and Rudolfo Neri Vela, the first Mexican to fly in space.

Ross and Spring are to move first to a 15-foot-wide platform that holds the aluminum beams, struts and joints they are to join and take apart. No tools will be used by either astronaut. The structures will be "snapped" together at the joints, the way children create things with Tinkertoys.

The astronauts are to stand at the bottom and top of the platform, their feet held down by restraints while they put together 10 identical bays that rise to a height of 45 feet. Each time a bay is built, it is slid upward along three vertical guiderails, leaving room at the bottom for assembly of the next bay. Ross and Spring then are to break down the tower.

The task will be filmed. At the end of the three-hour task, Ross and Spring move at once to the job of erecting a 400-pound frame that, when completed, will look like an inverted pyramid.

This time, the two spacewalkers are to assemble and disassemble the structure over and over again to see how fast they can learn to complete the task. This time, one of the astronauts will be held down only by a tether attached to a slide wire to see if he can work better floating freely.

On Sunday, starting at 4:55 p.m., Ross and Spring are to don their spacesuits and go into the cargo bay to try out some other space-construction techniques. They are to erect the same tower and pyramid, but one astronaut will have his feet in a pair of restraints at the end of the shuttle's mechanical arm, which will be moved around the cargo bay by Cleave.

The second time out, Ross and Spring are to practice repairing a "broken" bay in the 10-bay tower and will move the 45-foot tower around the cargo bay to see how future astronaut crews might best assemble the pieces of a space station.