THE PLANET Jupiter is 12 times the size of the Earth, has a gravity so strong is supports no fewer than 14 moons and an atmosphere so violent its 250 mile-an-hour winds feed no fewer than 10 visible hurricanes on any given day. The planet's distinctive feature is a permanent hurricane called the Great Red Spot that covers an area as large as the Earth.
So powerful is Jupiter's magnetic field that it has trapped enough cosmic radiation to destroy every living thing on Earth. This radiation is trapped inside Jupiter's magnetosphere, which itself is the biggest "thing" in the solar system. If Jupiter's magnetosphere were shining instead of being an invisible force, it would appear to us on Earth as big as the sun even though it's five times farther away.
Jupiter is so big it's still shrinking from the explosion that created the planet 4.6 billion years ago, producing in its collapse today as much heat as it gets from the sun. Spinning on its own axis once every 10 hours, Jupiter rotates faster than any other planet, generating its own radio noise, producing a planetary "wind" like the solar wind and causing such striking electrical storms they can be heard by radio telescopes on Earth. No matter how it's sized up, Jupiter is the most compelling planet in the solar system.
Drawing nearer every day to this compelling giant is an 1,800-pound spacecraft named Voyager, whose cameras and instruments promise the deepest single probe of a planet in history. Less than 30 million miles away right now, Voyager will reach Jupiter on March 5 and whip so close by the planet that it will have a full view of its moon Io out of the other side. By the time Voyager passes beyond Jupiter, it will have gathered more information about a single planet than all the planetary spacecraft that have flown before it.
Voyager's statistics are almost as impressive as Jupiter's. Three times bigger than the two Pioneers that flew by Jupiter before it, Voyager carries four times as much instrumentation as Pioneer. More important, Voyager will gather information and send it back to Earth at 100 times the rate Pioneer did and 10 times better than any planetary spacecraft that's ever flown.
So technologically superior is Voyager that it heard Jupiter's radio noise three months ago, when it was almost 100 million miles away. One Voyager instrument felt the stir of the Jovian magnetic field, another saw evidence of the chemicals in its atmosphere. Voyager took its first picture of Jupiter last month, when the planet was 52 million miles away. The picture was better than anything Pioneer took except for the few frames it got in the last 24 hours of its flight past Jupiter.
By the end of January, Voyager will have taken hundreds of zoom pictures of five different slices of the planet covering its entire 360 degrees. Every two hours, the cameras will see another fifth of the planet so that by the time Jupiter makes a 10-hour revolution the entire planet will have been photographed and a new sequence begun. This way, and change in the planet's atmosphere will show up on Voyager film. 18 Different Regions
CHANGE IS more common on Jupiter than anywhere in the solar system. The face of the planet is striped by 18 different regions, divided into white-colored zones and darkhued belts. No two zones are alike, no two belts are alike. The white zones are the highest and coldest, like clouds on Earth except they're made of ammonia instead of water ice. The dark belts are thought to be heavier chemicals like sulfur and phosphorus seeking a lower level than the ammonia clouds.
Both the zones and the belts drift, come together and often dissipate into nothing. At times, they're polka-dotted over with white or dark-colored whirls, which are little more than hurricanes the size of North America swirling ablut at the tops of the clouds. Winds at the edges of the belts and zones blow at 250 miles an hour, usually in the opposite direction.
Why is that? One theory is that Jupiter's internal heat is enough to supply warmth to its poles, meaning there need be no mechanism to transport heat from the planet's equator to its poles. That being so, the equatorial heat moves laterally across the planet, setting an example for the rest of Jupiter and dividing the planet into its striped belts and zones.
About the only thing that's stable on Jupiter is the Great Red Spot, but even it shrinks and grows and strays from its usual place in the South Tropical Zone. But it's been there like a sore thumb for more than 300 years, ever since it was first, observed in telescopes.
"The Great Red Spot may be nothing more than a huge whirl embedded in the top of the zone like a huge ball bearing," said Voyager project scientist Ed Stone. "But the point is... what's the energy source that keeps it going? Where does the energy come from? Why doesn't it dissipate itself?"
Most of February, the cameras on Voyager will take a lapsed time sequence to show Jupiter's rapid rotation. Some scientists think the planet keeps its stripes because Jupiter's rapid spin won't let the stripes change. The ammonia at the tops of the zones tries to move north or south and it's deflected back in by the planet's spin. Same thing with the darker chemicals in the belts.
"If we can image the planet enough times, we should see material flowing from the center of the zones toward the edges," Ston said. "That would confirm the idea that there is this strange convective pattern going on." Erosion by Radiation
WHEN VOYAGER reaches Jupiter in March, it will sail between the planet and its largest moon Io and therein will lie a whole new story.
While only a moon, Io may be the strangest object in the solar system. Io is the only one of Jupiter's 14 moons to have a solid surface. but what a solid surface! In recent years, telescopes have seen clouds of soldium, sulfur and potassium sputter off Io's surface. A cloud of hydrogen wraps around it like some nebulous cloak. For awhilo, all of these sputtering clouds remained a mystery -- until scientists realized that Io is in the midst of the most intense radiation that Jupiter's magnetic field has trapped.
"These things are literally being mined off the surface of Io by the radiation," Stone said. "If you do some simple calculations, you figure that meters of Io's surface must have been eroded away by this process."
Stranger still is the "flux tube" that connects Io with the tops of Jupiter's clouds more than 170,000 miles away. As Io moves through the strongest part of the Jovian magnetic field, it literally generates an electric potential the same way electricity is generated in a car's engine. The size of this potential is believed to be an incredible 400,000 volts.
Making things even more interesting is the fact that there are billions of charged cosmic ray particles trapped by the magnetic field in the regions around Io. What's the result? A current of electricity called the "fluxtube" that carries 1 million amperes under a pressure of 400,000 volts from Io all the way down to Jupiter. There is nothing like it anywhere else in the solar system. Is the flux tube the source of the mysterious whistles one hears coming from Jupiter? Could it be the source of all the radio noise that Jupiter makes?
Questions just as curious remain about the Jovian clouds, abundant and trubulent mixes of ammonia, hydrogen and methane which may still comprise the oldest, most unchanged atmosphere in the solar system. Shat chemicals darken the belts? Are there exotic streaks in the clouds? Does the temperature change deep in the clouds? Voyager instruments studying the cloud tops and probing miles below will almost surely have some of the answers.
There's another question. Why is the Great Red Spot red? Is it really spinning on top of the clouds like a giant ball bearing? Just before Voyager flies by the planet, it will aim its zoom lens at the Red Spot. So close will it be that it will take 81 pictures to cover the whole spot. In scientific history, no planetary feature will have ever been so thoroughly studied.
In it search for the answers, Voyager faces some fierce obstacles in the month and a half it has in deep space before its encounter with Jupiter. The big hurdle is Jupiter's radiation field, enough perhaps to burn out Voyager's instruments and render them useless. Voyager scientists are taking that gamble.
More pedestrian is the possibility of too much rain in Spain, the site of one of three huge antennas on Earth thatwill record all of Voyager's signals. During a test last month, a cloudburstin Spain washed out almost all ofVoyager's signal. It seems that waterabsorbs most of the frequency used bythe key transmitter aboard the spacecraft.