The human race will reach a half-billion miles across space and touch the colossus of the solar system next week when NASA's Galileo space probe parachutes into the atmosphere of Jupiter.

The probe's 75-minute descent through the gas giant's upper layers on Thursday will be the first time a man-made object will touch any planet beyond Mars, if all goes as planned.The Jupiter mission could unlock secrets of the solar system's creation, astronomer Toby Owen of the University of Hawaii said during a briefing at the National Aeronautic and Space Administration's Ames Research Center south of San Francisco.

"Jupiter is a giant ruin left over from events we hardly understand," he said. "It's like opening a tube that has been sealed for 41/2 billion years."

Unlike Earth, Jupiter is still made of the original gas and dust that congealed to form the sun and planets. And unlike the sun, which has transformed that material through nuclear explosion, the mix on Jupiter has undergone relatively little change.

Richard Young of Ames, which designed the probe, said the exciting part of the mission is not what scientists expect to find, but what they don't expect.

"We know we're going to get big surprises," he said.

Humans likely owe their very existence to the gas giant, which is 300 times more massive than Earth and 1,300 times larger, making up two-thirds of the solar system's mass outside the sun.

In the 4.5 billion years since the solar system congealed, Jupiter has swept up many of the comets and other space debris that might otherwise have bombarded Earth and made life impossible for complex organisms.

One mystery the probe may solve is why Jupiter radiates twice as much energy as it receives from the sun.

While most of its mass is gas or liquid, astronomers theorize Jupiter has a rocky core made of similar material to Earth, but five or 10 times larger.

Around that core a swirling mass of fluid hydrogen extends to half the planet's 85,000-mile diameter. The compressed gas acts like a metal, conducting electricity and possibly forming the planet's intense magnetic field.

Above that layer, scientists believe, is a "slurry" of liquid and gaseous hydrogen and helium, and above that, a gaseous atmosphere of the same materials, with traces of ammonia and other chemicals. That atmosphere has been whipped into Jupiter's characteristic colored bands by the rapid rotation of the planet - its "day" is only 10 hours.