More than any comet before, 1999 S4 LINEAR has revealed its nature, its very insides and has given researchers precious clues that reflect on a long, cold history. While some of the details are speculative, the scientists who have studied LINEAR helped SPACE.com put together this icy adventure.

This is the life of a comet:

Along the outskirts of a single spiral arm of the Milky Way Galaxy, roughly 4.6 billion years ago, a cloud of gas and dust began to gather and draw inward. The loose amalgam of material began to rotate and the collective force of gravity forced a collapse. A star, our Sun, was born.

The final days This sequence of images shows the comet before and during its breakup, which began in July 2000. The larger images have been enhanced to highlight features. The bright triangle is an ion tale. A plume develops and goes away, likely a jet of material rotating with the comet. The small inset images are at the same scale as the larger images, but show just the inner third of the view and are not enhanced. Click to enlarge

Around the rotating Sun, a dense fog of remaining debris swirled into an ever-flattening disk. Small bits of ice and dust collected into frozen clumps. The scene was violent, collisions rampant. Clumps sometimes stuck together and formed fluffy boulders.

Planets were created, and one became a giant, with a rocky core and a gaseous exterior. It came to be called Jupiter.

In the shadow of the giant, other boulders roamed. Some were the size of football fields. They were brittle -- less like rock really, more like Styrofoam -- and loaded with water ice but overloaded with dust.

Some of these primordial boulders met up with others, and sometimes their frozen, sticky exteriors joined. In time, a group of 16 or more had become a single object, some 2,500 to 3,300 feet (750 to 1,000 meters) across and carrying about 3.6 million tons (3.3 billion kilograms) of water.

This newly formed comet had many cousins, all under the influence of Jupiter's mighty gravity. Many were destined to die soon, flung like rocks from a slingshot toward the Sun. On the way, a handful hit a small rocky planet, third from the Sun, bringing precious water and organic chemicals to an otherwise barren world. Others were slung outward, beyond the reach of the new Sun's gravity into interstellar space. Lost forever.

But this one comet lived on. It was cast outward, but it managed to join up with a cast of other comets inhabiting a vast but sparsely populated halo of comets surrounding the Sun that reached a fifth of the way to the next nearest star. It was a place called the Oort Cloud.

A new path had been set for the comet, a distant orbit that might or might not allow a return to the inner solar system.

Then, billions of years later, something jostled the comet's trajectory -- possibly the gravity of a passing star. A new course was set, back toward the Sun.

The comet returned, zooming ever more quickly on a course that would take it around the Sun and then send it back out to the Oort Cloud. But something was wrong. The Sun's energy was converting the comet's ice directly into gas, bypassing the liquid phase entirely. Sublimation, it was called. It had happened to other comets and they had dealt with it by simply giving up a little of their exteriors.

But as the precious ice burned off into a glowing halo around this comet, the original boulders began to loosen. First one broke off, then another, releasing vast amounts of ice that had not seen space for 4.6 billion years. Over a period of a few days, the whole comet was in pieces.

The original boulders continued on, mini-comets in their own right.

These primordial building blocks of a once-large comet now travel amidst a sea of their own destruction -- as bits no larger than sand grains and pebbles -- or even as boulders the size of a bus.

Just as it has gathered itself together, this comet has come undone.

Next page: New Model of Comet Formation