A half-billion miles from Earth, a tiny spacecraft is racing toward a historic and fiery rendezvous with the planet Jupiter, now far beyond the sun in the daytime sky.
About 5 p.m. Thursday, a 747-pound, bullet-shaped capsule packed with seven scientific instruments, a radio and a parachute will plunge into Jupiter's atmosphere. Accelerated by gravity, it will reach speeds faster than any man-made object has ever flown.
Back on Earth, scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif., will be waiting at computer screens for the first signal that the probe has survived and begun its study of the jovian atmosphere.
Traveling from Jupiter at the speed of light, the radio message would take 52 minutes to reach Earth. It is expected at 6:16 p.m. EST. It will be the most critical moment of Galileo's six-year, $1.3 billion mission.
Although astronomers have been peering at Jupiter for centuries, they have struggled to understand its
three-dimensional processes by examining the planet in just two dimensions. Even with today's high-tech devices, no one has ever seen beneath the planet's colorful, banded cloud tops.
Galileo's atmospheric probe, if it survives its meteoric encounter, should change all that. It would be the first spacecraft to penetrate the atmosphere of any of the solar system's giant outer planets.
Jupiter is composed almost entirely of hydrogen and helium, most of it held in a dense liquid state by extreme pressures. The 40,000-mile-thick liquid layer is surrounded by an outer blanket of gas about 650 miles thick.
Galileo's probe will explore only the outermost 120 miles of that ,, gaseous layer. The deeper layers are too hot for any man-made craft. But even the outer regions are mostly unknown to science.
"Some things have been predicted by models, but there are certainly going to be surprises and interesting discoveries that come along. It's tremendously exciting," said Dr. Paul R. Mahaffy.
Dr. Mahaffy, a scientist at the Goddard Space Flight Center in Greenbelt, helped to develop the probe's Neutral Mass Spectrometer, which will analyze Jupiter's atmospheric chemistry.
Data on the composition and structure of Jupiter's atmosphere may allow scientists to look back in time, to the beginnings of the solar system.
The inner solar system and its small rocky planets (including Earth) are believed to have been altered and scoured by heat and radiation from the early sun.
Jupiter, however, thanks to its size and distance from the sun, "has had the opportunity to stay pretty much the way it was when it was originally formed," said Jim Erickson, a deputy manager for the science and sequence office at the Jet Propulsion Laboratory. "Examining what it's made out of will tell us a good deal about what the original solar system looked like" and how it formed.
Work on Galileo began in 1977. It was built as a two-part spacecraft, consisting of the atmospheric probe and an orbiter.
The 2 1/2 -ton orbiter, after relaying the probe's initial report to Earth, is to fire its rocket for 49 minutes. That should send it into an orbit that will circle the giant planet 11 times in two years. It will fly close encounters with four jovian moons, including Io, with its active volcanoes. Scientists hope to get perhaps 2,000 photos and other data on those moons, on Jupiter itself, its faint ring and its powerful magnetic field.
The atmospheric probe's mission will be far shorter, lasting 60 to 75 minutes after it enters Jupiter's atmosphere.
Miniaturized detectors weighing 66 pounds will send data on atmospheric temperature, pressure, chemistry and structure as it descends.
A key goal is to measure the relative amounts of helium and hydrogen. If they differ much from those in the sun, scientists will have to rethink their ideas about how the planets formed within the primordial solar system.
Other instruments will measure the amounts of sunlight shining down, and heat rising up from the planet's deep interior; detect and report any lightning in the roiling cloud layers, and measure the planet's powerful radiation.
The probe will sample only the top 120 miles of one column of the jovian atmosphere. But "once you have a good base line, you can begin to make better educated guesses about other areas," said Dr. Erickson.
A bumpy ride
It has been a bumpy ride for Galileo. Nearly ready in 1986, it was put into storage after the fatal Challenger disaster grounded the shuttle fleet.
On Oct. 18, 1989, a refurbished Galileo was carried aboard the shuttle Atlantis and rocketed toward Jupiter.
Its six-year journey has followed an expanding spiral trajectory. It passed Venus once and Earth twice, gathering the gravitational energy needed to fling itself toward Jupiter.
Galileo also passed and photographed two asteroids -- Gaspra and Ida -- and recorded the 1994 assault on Jupiter by the comet Shoemaker-Levy 9.
Galileo and its atmospheric probe have flown through Jupiter's powerful radiation belts and the most intense interplanetary dust storm ever measured.
There have been casualties. Early in the mission, the orbiter's high-gain antenna, designed for high-speed data transmission to Earth, failed to fully unfurl. That forced scientists to rely on a slower, low-gain antenna, reducing by a factor of 1,000 the data they will be able to retrieve.
"So, you make compromises in terms of redundancy. I might want to look at the Red Spot 25 times. Now maybe I'll settle for one. Somebody will pay a huge penalty in science," said Dr. Darrell F. Stroebel of Johns Hopkins University. While not part of the Galileo team, he is an expert on Jupiter and is eager to see the data.
JPL engineers had another scare Oct. 11. The orbiter's data recorder jammed after Galileo photographed Jupiter and its moons from a distance of 22 million miles. It has since been unstuck, but the segment of tape on which the Jupiter photo was recorded is now inaccessible.
Scientists seem confident about the fixes they have made. "Now we're stretching to think of other things we may have missed," said Dr. Erickson. "That's when you can't sleep until you go downstairs and write them down."
Into the atmosphere
The atmospheric probe separated from the orbiter July 13, and began a 51-million-mile free fall toward Jupiter. The probe is now silent. A timer should awaken its instruments six hours before beginning the plunge into Jupiter's atmosphere.
After being slowed by friction from 106,000 mph to 100 mph in barely four minutes, the Galileo probe will have to endure deceleration forces equal to 300 times the force of Earth's gravity.
The friction will broil the capsule at 28,000 degrees -- akin to flying through a nuclear fireball. Its heat shields, up to 6 inches thick, will dissipate the heat by melting and shedding 208 of their 335 pounds.
At 250 mph, the probe will pop its 8-foot Dacron parachute and jettison the heat shields. It will then spend the next hour or more drifting down through turbulent winds, rain and hot gases.
The probe will descend, but it will never land. Jupiter is believed to have a rocky core equal in size to 10 Earths buried deep inside the planet's impenetrable blanket of liquid hydrogen. The core is believed to make up 4 percent of Jupiter's mass.
Scientists say even the outermost gas layers Galileo can explore remain largely a mystery.
"We've only been probing the atmosphere by the remote sensing of infrared and ultraviolet radiation," said Goddard's Dr. Mahaffy. "If you're looking for things that don't produce signals in those spectral regions, you need in-situ sensing. You need to go there."
Temperatures will vary widely on the way down, with winds of 200 mph or more, lightning, and heavy rain at the base of the water cloud layer.
Beneath the water clouds, some 50 miles below the cloud tops, the probe may break into a uniform region of dense, hot gases.
As it falls, the probe's descriptions of the environment will be radioed back to the orbiter 130,000 miles above. The orbiter will record data on a computer chip and on tape for relay to Earth. The chip sample will be played Dec. 10. The full taped report will follow from January to March.
The probe will continue sending its findings until its batteries quit or its instruments are destroyed. Engineers say that is likely at 120 miles or more beneath the cloud tops, where pressures rise to 28 times sea-level pressure on Earth, and temperatures climb to 360 degrees.
Dr. Erickson suspects the spacecraft has been over-designed and will last "maybe a little longer." Less than two hours after entry, however, the parachute will melt. By 10 hours, the heat will have vaporized the craft's aluminum and titanium components.
"We don't have any materials that will survive," he said.
* Size: Diameter is 88,729 miles. It could hold 1,400 Earths. Its mass exceeds all eight other planets combined.
* Distance: The fifth planet, 483.6 million miles from the sun.
* Orbit: Circles sun once every 11.86 years.
* Rotation: Jupiter spins on its axis once every 9 hours and 56 minutes.
* Composition: 88 percent hydrogen, 11 percent helium, with traces of methane, ammonia and water. It is believed to have a small, rocky core.
* Moons: 16, and one faint ring.
Instruments on Galileo's atmospheric probe.
* Neutral Mass Spectrometer (Goddard Space Flight Center) -- Measures chemical components of Jupiter's atmosphere at various altitudes.
* Helium Abundance Detector (Bonn University) -- Determines relative abundances of hydrogen and helium, for insights into planetary evolution.
* Atmosphere Structure Instrument (San Jose State University) -- Measures the density, pressure and temperature structure of the atmosphere.
* Nephelometer (San Jose State University) -- Determines location of cloud layers and the characteristics of cloud particles.
* Net Flux Radiometer (University of Wisconsin) -- Measures heat and light radiated by Jupiter and the sun at various levels.
* Lightning and Radio Emissions Detector (Bell Labs and the Max Planck Institute) -- Measures electromagnetic and radio emissions, and light flashes from lightning, for clues to nature and size of cloud turbulence.
* Energetic Particle Instrument (University of Keil) -- Measures electrons, protons and other particles in Jupiter's magnetic field.