The oldest operating spacecraft marked a quarter-century in orbit Sunday.

Pioneer 6, a 140-pound cylindrical probe about a yard long and a yard wide, was rocketed into space atop a Delta rocket 25 years ago. But retired managers of the National Aeronautics and Space Administration Pioneer program won't celebrate the anniversary until Monday when they plan to tune the probe's signal from the Ames Research Center in Mountain View.The probe was designed to relay to Earth the first detailed measurements from the other side of the sun of magnetic fields, cosmic rays and solar wind particles.

The journey was expected to last six months. But the signal has never stopped, despite the failure of three of six instruments and the flagging efficiency of the solar cells.

Engineers at NASA and TRW Inc. of Redondo Beach, which built the probe, estimate that the probe has traveled 15.4 billion miles during 29 orbits around the sun.