The Pioneer spacecraft is the mechanical resemblance of the old man in the song who, when he went to bed, parked an eye in a glass, his teeth in a jar and his hair on a peg, but still enjoyed what he had left of life. Pioneer 8, which celebrated its 20th anniversary yesterday, is down to the last of its eight instruments. It continues to beep back to Earth information about the planet's magnetic field.

The 140-pound Pioneer 8 has orbited the sun every 388 days since December 13, 1967. It was designed to last six months, and was intended to study solar flares and other phenomena of the atmospheres surrounding the Earth and sun.

It was one of four spacecraft made to research the phenomena; they found, confirmed and studied the "magnetic tail" that extends behind the Earth in its orbit.

The instrument now operating was switched off for 13 years, from 1971 to 1984. It came out of hibernation immediately when called on, put back in operation just as the craft was about to pass through the Earth's "magnetic tail."

Pioneer 8 has transmitted about 11 billion bytes of information to Earth over the past two decades. Another of the Pioneers, No. 6, will celebrate its 22nd anniversary in space next Wednesday. It is the oldest functioning spacecraft.