Basic Questions About the Solar System: The Need for Probes

Probes are an essential element in the scientific study of planets with atmospheres. In-situ measurements provide the most accurate determination of composition, winds, temperatures, clouds, and radiative fluxes. They address fundamental NASA objectives concerning volatile compounds, climate, and the origin of life. Probes also deliver landers and aerobots that help in the study of planetary surfaces. This talk focuses on Venus, Titan, and the giant planets. I review the basic science questions and discuss the recommended missions. I stress the need for a balanced program that includes an array of missions that increase in size by factors of two. Gaps in this array lead to failures and cancellations that are harmful to the program and to scientific exploration.