A six-year, 2.3 billion-mile odyssey will culminate Thursday when the Galileo spacecraft swings into orbit around Jupiter. "The accuracy of this is really amazing," says TIME's Leon Jaroff. "Scientists were able to make incredibly precise calculations to place Galileo in exactly the right place, using Venus and the Earth in a 'crack-the-whip' maneuver to boost the probe's velocity to give it sufficient speed to make it all the way to Jupiter." Also Thursday, a smaller probe released from Galileo 147 days earlier will enter Jupiter's atmosphere. "There won't be any dramatic pictures, just a data stream," notes Jaroff, "but this will help clear up a lot of speculation about the composition of the Jovian atmosphere. Scientists have been able to make a lot of inferences about the planet, but this would be the first time they've been able to sample the atmosphere. They expect to find a high water content, and some lightning, at the higher levels in the hour before the probe descends so far that Jupiter's extreme heat and pressure destroy it."