NASA's Jet Propulsion Laboratory had originally planned to launch Galileo from the space shuttle with a large liquid-fueled Centaur rocket. While passing Venus, Galileo will look for flashes of light in the clouds and certain electromagnetic energy which would be evidence of lightning discharges. Galileo will also make the first close approaches to two asteroids, Gaspra and Ida. Galileo will orbit Jupiter for at least two years, giving scientists the first long-term look at an outer planet, providing perhaps the best new information about Jupiter since its moons were discovered by astronomer Galileo Galilei four centuries ago. Using gravity assists to alter its orbit, the vehicle will also study Jupiter's moons, including the mysterious volcanic Io.
Scientists believe Jupiter is a dense ball of gases, mostly hydrogen and helium, with some ammonia.
That gaseous composition, they believe, has remained largely unchanged since the formation of the universe. And because of its 16 moons, Jupiter resembles a miniature version of our solar system. We really don't know what the clouds are made of, at what altitudes they exist, and how dense they are. The only way to really measure these clouds is to be physically there. To survey the atmosphere, Galileo will drop an instrument probe into the Jupiter atmosphere.