ONCE around Venus, twice past Earth, and on to Jupiter: That's the six-year voyage that lies ahead for what Galileo mission manager Clayne Yeates calls ``the Rolls-Royce of spacecraft.'' If successful, this should be one of the most scientifically productive planetary missions yet flown. It should return new information on Venus, Earth, and the moon. It includes the first close surveys of two asteroids. At Jupiter, it involves detailed studies that should shed light on the evolution of the entire solar system.
The stunning flyby glimpses of the Pioneer and Voyager surveys have tantalized scientists. They have felt like cruise passengers who wistfully scan exotic islands their ship passes by. Now they look forward to a 22-month exploration of that intriguing territory.
``Galileo will give us more than just snapshots in passing,'' says planet scientist Edward Stone of the California Institute of Technology. ``Being there and observing over a period of nearly two years will allow us to sort out the dynamics of Jupiter and its moons,'' he adds.
The roundabout journey of the $1.5 billion robot explorer reflects a need for inflight ``refueling.'' The space shuttle Atlantis placed Galileo in a parking orbit Wednesday. The booster that then kicked it out of orbit wasn't powerful enough to send it directly to Jupiter. So the spacecraft is to pick up extra energy by the same gravity-assist technique that enabled Voyager 2 to move from Jupiter to Saturn to Uranus and finally to Neptune.
Galileo should pass within 9,300 miles of Venus next February. Like the Biblical David slinging a stone at Goliath, Venusian gravity will whip Galileo around the planet and sling it back toward Earth.
Other encounters of the 2.5 billion-mile voyage include:
December 1990 - Earth flyby at 600 miles distance.
October 1991 - Asteroid Gaspra flyby (600 miles).
December 1992 - second pass by Earth (185 miles).
August 1993 - fly by Asteroid Ida (600 miles).
December 1995 - reach Jupiter 10 years behind original schedule.
Galileo will sample the solar-system space environment as it moves through it. It will scan Venus's cloud patterns, looking for lightning.
It will likewise scan Earth and its atmosphere from a deep-space perspective that no Earth-orbiting satellite can achieve. Among other things, it is equipped to analyze the makeup of the surface of the far side of the moon and to look for ice in the moon's north-pole region. By taking time-lapse pictures, the craft should enable scientists to put together a ``movie'' showing the moon orbiting Earth.
Its close pass by two asteroids should yield detailed data on their surface composition, shapes, and masses.
The spacecraft includes a 2,509-pound orbiter, a 739-pound atmospheric probe, and about 2,400 pounds of rocket propellant. The probe will separate from the main spacecraft five months before Jupiter arrival. As Galileo swings into orbit around the planet, the probe will fall into its atmosphere to give the first detailed report on its physical characteristics.
This massive planet - the largest in the solar system - has a composition close to that of the sun. Scientists think that knowing it thoroughly will reveal much about the evolution of the solar system. The four largest of its 16 moons, which are Galileo's target, were first seen by Galileo Galilei in 1610. Orbiting among them for 22 months, coming as close as 125 miles, Galileo's namesake spacecraft will make them as familiar to people on Earth as is their own moon.