A 23,000-pound rocket and spacecraft assembly named Pegasus 2 is expected to fall out of orbit and break up and scatter as much as 1,600 pounds of metal over the earth in the next three weeks.

Acting on information received from the North American Air Defense Command, the National Aeronautics and Space Administration said Wednesday that Pegasus 2, which it launched May 25, 1965, should reenter, burn and break up in the earth's atmosphere on or about Nov. 5.

Like Skylab before it, Pegasus could fall on any part of the earth as it circles the globe.

Far lighter than the 78-ton Skylab, Pegasus' orbit also covers a lot less of the earth than Skylab's did. Orbiting the earth at an inclination of 31.8 degrees, Pegasus 2 never flies farther north than Florida nor farther south than northern Argentina.

Still, Pegasus 2 manages to fly over almost all of Florida and parts of Texas, Louisiana, Mississippi and Alabama. It also flies over most of Africa, South America and almost all of Australia, where pieces of Skylab debris fell last August.

The Pegasus 2 "assembly" as the space agency calls it, is 70 feet long and weighs 23,000 pounds. Attached to the spacecraft is the empty upper stage of the Saturn 1 rocket that put Pegasus into orbit.

The space agency said that an estimated 1,600 pounds of Pegasus and its upper-stage rocket may fall all the way to earth after it breaks up in the atmosphere. It said that most of the pieces striking the earth will weigh no more than 10 pounds but that several will weigh as much as 200 pounds apiece.

Pegasus 2 is one of the three identical spacecraft the space agency put into orbit in 1965 to study the impacts of micrometeoroids on spacecraft. The two other Pegasus craft have already broken up in the atmosphere, one in 1969 over the Pacific Ocean and the other in 1978 over Africa. No surviving pieces of either caused any damage or hit anybody, the space agency said.