The North and south magnetic poles of Uranus may be in the process of flipping, undergoing the kind of reversal that has taken place on Earth at least nine times in the last 3.5 million years.

Evidence suggesting a flip-flop in progress came from observations carried out by the Voyager 2 spacecraft as it flew by Uranus last January and is published in this week's issue of Science.

Astrophysicists are astonished at the possibility that they may be witnessing a phenomenon never before observed by humans. Earth's last polar reversal occurred about 730,000 years ago.

Support for the theory comes from Voyager's observations that the Uranian magnetic field is strangely tilted and that its axis passes not through the planet's center, as on other planets, but through a point about one-third of the way out from the center.

The offset in the magnetic axis suggests that the magnetic field is not caused by phenomena in its central core, as is the case with Earth, but by the flow patterns of electrically charged particles in the superheated, 6,000-mile-deep ocean that envelops Uranus.

Geologists know that Earth's magnetic field has reversed several times because of the field's effect on the cooling of rock that forms as molten magma wells up along the mid-ocean ridges to form new sea floor. While the rock is molten, its atoms are free to move, aligning their own magnetic fields with Earth's field, like a compass needle.

Once the rock cools and hardens, the atoms are frozen in place, all their little north poles pointed -- because opposite poles attract -- toward Earth's South Pole. Once Earth's polarity has reversed, newly hardened rock freezes its atomic poles in the opposite direction.

As the sea floor spreads to both sides of the mid-ocean ridge, the result is broad stripes of reversed or normal polarities lying parallel to the ridge.

NASA's Voyager scientists emphasized that a magnetic reversal on Uranus is not the only possible explanation for what Voyager saw. Another theory is that the odd field was the indirect result of an Earth-sized object smashing into Uranus long ago and tipping it on its side.