Jupiter must feel that it is under attack. A little over a year ago, a barrage of fragments from Comet Shoemaker-levy 9 crashed into Jupiter's southern hemisphere, vaporizing clouds, creating giant plumes and generally making waves in the planet's atmosphere. Now, despite technical problems, the Galileo spacecraft is preparing for a second assault, with a 750-pound "probe" poised to penetrate Jupiter's atmosphere on Thursday and a Jupiter-orbiting companion module to watch as it happens. Why Jupiter?

The giant planet brought all this upon itself. It is not unusual for Jupiter to push an asteroid in the direction of Earth, or to fling one out of the solar system altogether. Last year's comet had been orbiting the sun, as most comets do, until the massive planet captured it. As a result, Shoemaker-Levy 9 was actually in orbit around Jupiter when gravitational stresses broke the comet into a string of pieces that subsequently crashed into the planet's atmosphere.

Galileo's journey is more deliberate. Its mission is to discover secrets about the earliest history of the solar system, including Earth. The spacecraft has been traveling for six years toward the cold, dark outer reaches of space. In the sky over Jupiter, the sun gives less light than a 50-watt light bulb at arm's length. In these remote parts, it is common to find chunks of rock and ice that froze when the solar system formed, and they have not thawed since. The spacecraft's trip to Jupiter is a voyage back in time.

Jupiter is an enormous ball of gas, so massive that very little escapes the planet's gravitational pull. There is no solid surface to react chemically with the atmosphere. So protected is it from outside influences that the composition of Jupiter's atmosphere hardly has changed since the birth of the solar system, 4.6 billion years ago.

In contrast, the shallow atmosphere of Earth has been altered by selective escape of the original gases to space, bombardment of meteors and comets, chemical reactions with surface rocks and volcanic activity. Almost every clue about the materials from which our planet formed, and the processes that turned a primordial cloud of gas and dust into a sun, nine planets and numerous moons, asteroids and comets, has been erased. But the atmosphere of Jupiter is a nearly pristine sample of that primordial cloud.

Among the gases Jupiter has retained are hydrogen and deuterium. Hydrogen, the lightest element known, also is the most abundant material in the universe. It was the main constituent of the early solar system, and makes up 90 percent of Jupiter's atmosphere today.

Earth began with some hydrogen, too, and deuterium, a heavier form of hydrogen. Most of these gases escaped from Earth's atmosphere, but more of the hydrogen was lost than the deuterium. Today's Earth atmosphere is enriched in deuterium. In Jupiter's atmosphere, the relative amounts of hydrogen and deuterium are the same as what it had in the beginning. Knowing this, we can figure out how much gas escaped from Earth. Such cosmic numerology applies to other gases as well. Each tells a part of the history of our atmosphere. On Thursday, the Galileo probe will go in, grab a sample of Jupiter's atmosphere and analyze it. The results will be radioed up to the orbiter and relayed back to Earth. The data will give scientists the keys to a deeper understanding of how Earth's atmosphere developed into what it is today.

Our view of the evolution of the primordial gas cloud itself is deeply connected to another part of the Galileo orbiter's itinerary: Jupiter's moons. In 1610, the Italian scientist Galileo, for whom the NASA spacecraft is named, observed Jupiter with his newly invented telescope. He discovered four bright moons, now called the "Galilean Satellites," orbiting the planet. In Galileo's time, it was hotly contested whether the planets all orbit the sun, or whether they move in complex patterns around Earth. The image of moons orbiting Jupiter, like four tiny pearls, provided a metaphor for the heliocentric picture of the solar system. So powerful was this image that it threatened the authorities of the day. They kept Galileo under house arrest for the last eight years of his life for supporting the view that the sun is the center of the solar system instead of Earth.

In some ways, the moons of Jupiter offer a literal model for the solar system's formation as well. They are believed to have come together from gas and dust that was orbiting a much-hotter early Jupiter. The planets formed from the primordial cloud of gas and dust that orbited the sun.

As the primordial cloud cooled, clumps of material formed. Grains of dust containing heavier elements, such as iron, collided, and stuck together. Ices condensed from the vapors of water, ammonia and methane, contributing lighter elements to the grains. These processes continued, and the clumps grew larger, eventually becoming the denizens of the solar system we know today. But the sun, in the center of all this, was hot and grains nearer the sun accumulated less ice than the grains farther away. As a result, Earth, which formed closer to the sun from these grains, is made of denser material than Mars, which formed farther away.

The properties of the Galilean Satellites follow a pattern much like that for the planets themselves: the two inner ones are made primarily of rocky materials, and the other two are larger, less dense and presumable contain more ice. What might we learn about the so called "planetary accretion" process from these moons? For this, we must wait for the new data.

While in the area, Galileo also will observe atmospheric storms, clouds and dust; photograph aurora, and detect lightning in Jupiter's atmosphere. It will measure Jupiter's magnetic field, the strongest of any planet, and study heat that emanates from the planet's interior. The four Galilean moons, many smaller ones and some rings around Jupiter will be imaged. Some of these objectives may be compromised by recent problems with the on-board tape recorder. But if all goes well, we still may get a glimpse at our own earliest moments.

Copyright 1995 by R. Kahn. All rights reserved. Used here by permission.

This article first appeared in the Denver Post on Sunday, December 3, 1995.