PASADENA, Calif. -- In the first penetrating exploration of Jupiter, an instrument-laden capsule from the Galileo spacecraft plunged yesterday into the roiling atmosphere of the largest planet in the solar system and took the measure of its temperatures, density and other-worldly chemistry.
Last night, the Galileo spacecraft fired its rocket to slow down and entered a wide-ranging orbit of Jupiter. It is the first spacecraft to orbit one of the giant outer planets for a long-term campaign of photography and detailed observations.
But the most exciting moment was the probe into Jupiter's gaseous interior.
Traveling 106,000 mph, the 746-pound capsule streaked into the fringes of the planet's mostly hydrogen atmosphere, the friction of its passage producing a fiery glow as bright as the sun.
The capsule breached ammonia crystal clouds and, further slowed by parachutes, dropped deeper into the ever-denser layers of reddish-brown clouds.
All the way, it was buffeted by hurricane-force winds and, very likely, storms of rain and crackling lightning.
The capsule's radio transmitted data to scientists who hope to find clues to Jupiter's history and the origin of the solar system. The exact duration of the transmission will not be known until next week, but it was expected to be about 75 minutes.
Eventually, the tremendous pressure and heat crushed and vaporized the visitor from Earth, its carefully assembled components disappearing without a trace in the depths.
This was the first spacecraft encounter with a distant planet since Voyager 2 flew by Neptune in 1989. Never before had a scientific probe peered beneath the skin of one of the mysterious outer planets, and from preliminary indications, mission officials at the National Aeronautics and Space Administration's Jet Propulsion Laboratory here proclaimed the deliberately suicidal operation a success.
"Fantastic!" said Dr. Torrence V. Johnson, the chief project scientist, as the first confirmation of the probe's entry was received at the laboratory. "This is really neat."
The probe's atmospheric descent began shortly after Galileo made its closest approach to the planet, which was more than 130,000 miles away. The entry maneuver was reported as beginning at 5:56 p.m. Eastern time.
Actually, it had occurred 52 minutes earlier, which is the time it takes radio signals to travel the nearly half-billion miles between Jupiter and Earth. The times for all mission events are given as the time the signal reached Earth.
During the maneuver, Galileo acted as the relay station, receiving the radioed data from the probe and storing it in its computer memory bank and on a tape recorder for transmission later to Earth.
It was not immediately known how deep into Jupiter's atmosphere the probe was able to operate and send scientific data. It could have been as deep as 400 miles. Jupiter has a radius of about 44,000 miles at its equator.
Unlike the inner planets, Jupiter is mostly a sphere of gases, primarily hydrogen and helium. It has no solid surface in the sense that Earth and Mars do.
But under the pressures at great depths, Jupiter's hydrogen behaves like a liquid metal, and some scientists think there may be a solid core of rock and metal.
Yesterday was a culmination and a beginning for the $1.3 billion Galileo mission, one of the most ambitious in the more than three decades of planetary exploration.
Galileo's arrival at Jupiter and the plunge of its detached capsule into the atmosphere were moments of triumphant relief. It had been six long, trying years and 2.3 billion miles since Galileo was sent on its way by a space shuttle in October 1989.
"On this mission, it's really been the perils of Pauline," said NASA Administrator Daniel S. Goldin.
The mission was delayed three years by the explosion of the space shuttle Challenger in 1986. In flight, Galileo's 16-foot-wide umbrellalike antenna failed to unfurl, severely reducing its ability to communicate scientific data. A tape recorder malfunctioned and must be operated gingerly.
But with clever changes in software transmitted to Galileo's computer, which engineers likened to brain surgery, scientists expect that the mission should still be able to accomplish 70 percent of its objectives.
A smaller antenna is handling the data transmissions, though at a much slower rate. That means fewer pictures and none before next summer.
For the next few months, Galileo should be slowly relaying the probe's findings to Earth. Confirmation that the probe's data have been safely stored on the main spacecraft should occur Sunday.
The first quick-look analysis of the data is expected to be reported Dec. 19 at the Ames Research Center in Mountain View, Calif., where the probe was developed.
If all goes well, the 2 1/2 -ton Galileo spacecraft is expected to operate in orbit for at least two years. It is expected to make 11 circuits of the planet, swinging by for close looks at three large moons -- four times by Ganymede, three times each by Callisto and Europa. The first flyby of Ganymede, a moon as large as the planet Mercury, should come next summer.