The Voyager 2 spacecraft, rapidly approaching a pass within 51,000 miles of Uranus' cloudtops on Friday, has detected signs that the seventh planet from the sun has a magnet- ic field, an almost certain indica- tion that it has a molten core much like the Earth, scientists said to- day.

Voyager 2, less than 700,000 miles away from its encounter with Uranus and closing at midday, is scheduled to buzz the planet and one of its nearby moons during a six-hour period beginning Friday about 10 a.m. eastern time. Today Voyager continuously poured back to Earth photos that grew more dramatic and clear by the hour.

Scientists announced today that in addition to photos, Voyager 2, beginning Jan. 19, had started to pick up radio signals that are the tell-tale signs that Uranus possesses an unexpected magnetic field.

The radio signals have been streaming from the planet in spiraling bursts of low-frequency electrons. The signals took most Voyager scientists by surprise because the spacecraft did not pick them up until a few days ago.

"We don't understand why we didn't see these signals a month ago," Voyager Project Scientist Edward C. Stone said today at the Jet Propulsion Laboratory where Voyager's historic meeting with Uranus is directed. "I think the reason we missed them is that the radio bursts are riding out from the planet on top of emissions so weak we weren't able to see the forest for the trees."

A magnetic field means Uranus has an internal heat source, a surprise. Earth observations of the distant planet have never revealed any more heat leaking from Uranus than it receives from the sun. Even Neptune, almost a billion miles farther from the sun, shows signs of an internal heat source.

Like the Earth, most planets have radioactive molten cores that discharge heat. The rotation of the hot cores along with the planets has a dynamo effect, producing magnetic fields. Venus is the only known exception.

Photographs released today show more distinct features, rotating rapidly counterclockwise through the planet's clouds. Scientists plotting the movement of these features say they believe the features are being driven by winds of at least 200 miles an hour in the planet's upper atmosphere.

"We have a dynamic weather system on Uranus, somewhat like the turbulent systems we saw on Jupiter and Saturn," Andrew Inger- soll of the California Institute of Technology said today. "This is almost as unexpected as the magnetic field . . . . "

New photographs of the planet's five large moons and nine known rings show new features and suggest that the number of known rings may soon be increased from nine to at least 12.

The moon Miranda has subtle yellow streaks across its surface while Ariel appears spattered with bright spots that scientists are at a loss to explain. The moons Titania and Oberon show a different streak pattern and Oberon displays one distinct feature that could be a huge crater left by a recent meteorite impact.

One photograph of the south polar region of Uranus, which is facing the Earth and the sun, shows an orange oval inside a yellow oval surrounded by regions whose color was closer to aquamarine.

"The south pole still looks like a big eyeball in space that's changing its shape and color each day we get closer to the planet," Bradford A. Smith of the University of Arizona said. "We still believe the colors reflect the methane haze that covers most of the planet and gets its color from ultraviolet radiation from the sun."

Voyager 2 makes its historic closest approach to Uranus at 12:58:51 p.m. EST on Friday when it is to pass 51,000 miles above its cloud tops and 18,000 from Miranda, the smallest of its five large moons.

Early in the morning, the spacecraft will photograph Oberon and take pictures of Umbriel and Titania that should reveal surface features no more than five miles across.

The spacecraft will next search the planet's dense atmosphere for signs of aurorae like the Earth's "northern lights" that are now likely to appear, assuming Uranus has a magnetic field.