The third stage of Sputnik I's rocket may have plunged to its death on its 879th circuit Saturday night, announced Fred L. Whipple, Director of the Smithsonian Astrophysical Observatory.

Whipple bases his belief in the rocket's death on the rapid shortening of its orbit in the last few days. He thinks this orbital decrease resulted from the rocket's being drawn down into the denser atmosphere which would hasten its end.

However, Moscow radio, broadcasting future schedules for the rocket, says it made its 886th circuit yesterday and would be visible today. Soviet scientists have predicted that the rocket's death would be sometime in the near future.

Red China's Nanking Observatory predicted that it would be destroyed within 10 days, and the Peiping radio said the rocket woul pass over parts of China today and tomorrow.

The last definite observation of the rocket was received by the radar telescope in Palo Alto, California early Saturday evening. At about the same time, a Smithsonian moonwatch team in Los Altos saw a bright object in the sky, which Whipple said, "could well have been the rocket starting to glow as it neared its end."