Uranus, which is the seventh planet from the sun and is almost 1.9 billion miles from it, appears to have no magnetic field, which would make it only the second known planet in the solar system with none. Venus is the other.
"We see no radio emissions from Uranus that would tell us it has a magnetic field, and we're less than 46 million miles from the planet" with the Voyager 2 spacecraft, Dr. Michael L. Kaiser of the Goddard Space Flight Center told reporters yesterday. Kaiser appeared at a news conference to discuss Voyager's planned flight past Uranus Jan. 24. "We still pick up Jupiter's radio noise, Saturn's radio noise and even the sun's radio noise but we're not hearing any radio events at Uranus," he said.
Kaiser said the trademark of a planet's magnetic field is its radio noise, generated when protons and electrons pour off the sun and collide with the planet's magnetosphere.
This collision triggers the "northern lights," a 1 billion-watt radio signal from Earth, a 100 billion-watt signal from Saturn and a signal so loud from Jupiter that it dwarfs every other radio signal in the solar system.
"You don't need much to form a magnetosphere that would generate noise out at Uranus' distance," Kaiser said, "so we figure we're either dealing with a planet that has no magnetic field at all or is so bizarre we don't even know what to look for."
A missing magnetic field suggests that Uranus has no internal heat source, no radioactive core such as that which makes the rotating Earth behave like a dynamo and no internal heat source such as those that give the rapidly rotating Jupiter and Saturn a strong magnetic field.
"You need an internal heat source to drive a magnetic field," Kaiser said. "A planet's rotation is not enough by itself to create one." Voyager 2, which has been in space for eight years and has passed Jupiter and Saturn, is expected to fly within 51,000 miles of the cloud tops of Uranus Jan. 24.
In August 1989, Voyager 2 is expected to encounter Neptune, but its flight will not take it near Pluto, the last known planet from the sun. The craft's path will take it out of the solar system.
"Although we don't know what we will find at Uranus, we do know it will be different than anything we've seen so far," said Burton I. Edelson, associate administrator of the National Aeronautics and Space Administration.