About five years from now, a tough little egg-shaped probe will plummet through the primordial atmosphere of Jupiter, signaling its discoveries back to Earth for perhaps an hour before it is consumed in heat and corrosion.
Its messages may shed light on the original ingredients of the solar system.
Scientists will learn more about Jupiter in the hurtling probe's last five hours than in all previous recorded history, says Nick Vojvodich, deputy manager of the project from the National Aeronautics and Space Administration's Ames Research Center in California.
Space engineers at Hughes Aircraft Company here have just finished building the five-foot-wide Galileo Probe. It will mark NASA's first attempt to penetrate the atmosphere of one of the giant outer planets.
What scientists expect from the mission is a glimpse at an early chapter in the history of the solar system. Because the planet is so gigantic (a thousand times the size of Earth) and so distant from the sun, its atmosphere is assumed to still contain the primordial elements present at the formation of the solar system.
The Galileo Probe is scheduled to go up in the space shuttle in May 1986. From the height of the shuttle, a Centaur launch vehicle will fire the probe and its orbiter toward Jupiter. The 750 million-mile trip will take a little over two years. When they arrive, the probe and orbiter will part ways. The orbiter will spend the next 20 months making close passes at Jupiter's moons - Io, Ganymede, Europa, and Callisto - and snapping pictures. Then it will return to Earth.
The probe, in turn, will spend 150 days angling toward Jupiter before making its short and fiery descent. It will face extremes of heat and pressure greater than any other spacecraft has faced. More than half of the probe's 742 pounds is in its heat shield, the largest NASA has every used. Half of it will be eroded away in the entry.
Galileo will hit the Jovian atmosphere at well over 100,000 miles per hour and slow down in seconds to a few thousand m.p.h. In two minutes or so it will be slowed to the speed of sound (2,000 m.p.h. on Jupiter), and a parachute 12 feet across will release, dragging the probe to about 100 m.p.h.
The heat shield and parachute have to work, notes project manager Joel Sperans of NASA's Ames center. Each represents a ''single point of failure'' that could not be corrected.
On the way down, Galileo will be searching out what kind of gases it travels through, how dense they are, and what the energy balance is. For some reason, Jupiter radiates twice the energy it receives from the sun. Galileo will also probe the composition of Jupiter's colorful clouds.
About an hour after the probe has penetrated the atmosphere, engineers expect it to be so deteriorated that it will cease transmitting. It will probably survive another hour unheard before its fall is over.
The Galileo project will have cost $650 million at liftoff in 1986. During the following four years it will cost another $200 million. It is the last planetary probe expedition NASA currently has planned.