The Galileo spacecraft will pass one of the most critical milestones on its long journey to Jupiter late tonight when it loops around Venus and uses that planet's gravity to fling itself back toward Earth.
The sophisticated robot will make a series of observations as it passes 10,000 miles above the dense clouds of Venus. But scientists at the Jet Propulsion Laboratory in Pasadena will have to wait until October to find out what Galileo learned. The results of the experiments will be recorded on a four-track tape recorder aboard the craft, but it will take eight months for Galileo to get close enough to Earth for its limited communications system to relay the data to ground controllers.
The observations of Venus are a bonus; the real reason for flying past the planet is to change the spacecraft's course. Galileo was launched from the space shuttle last October, but it was not possible for it to fly directly to far-off Jupiter. Instead, Galileo was forced to fall toward the sun, picking up speed along the way, and then whip around Venus to be propelled back toward Earth.
Galileo will pass close by the Earth twice--this December and again in 1992--and use the Earth's gravity to pump up its speed enough to eventually reach Jupiter.
Although officials believe the mission is safe, Galileo scientists will anxiously await the completion of each phase of the complex operation. "There are lots of white-knuckle times," said Torrence Johnson, project scientist.
Finally, in December of 1995, Galileo will reach Jupiter, a planet so large that, had it been a little bigger, it might have ignited and become a star. Galileo will spend two years roaming through the Jovian system, passing breathtakingly close to each of its moons, and even sending a probe into Jupiter's dense atmosphere.
By the time it reaches Jupiter, Galileo will have traveled about 2 1/2 billion miles.