Michael Meltzer - The Cassini-Huygens Visit to Saturn

Only when we have flown missions to every part of the solar system will we have the vital statistics of all its components and be able to turn back the pages of the book of cosmology to the origins of our own world, and perhaps of the universe itself.

Arthur C. Clarke

Soon after NASA was formed, its scientists began to envision what outer solar system explorations, still many years in the future, might look like. In 1959, JPL scientist Ray Newburn, Jr. developed several mission concepts for investigating the portion of our solar system beyond Mars. He foresaw deep space flights that would pass only through interplanetary space, making observations and measurements as they went. Flybys, on the other hand, would have brief encounters with planets. Although the data collected would be restricted by limits on time, flyby missions could study several different planets during one trip. Flyby data might be compared to observations taken through the windows of a tourist bus: brief, but varied. Orbiter missions would involve long-term trajectories around target planets and permit in-depth data collection. Finally, planetary entry and lander missions would penetrate planetary atmospheres, enabling observations that could not be obtained by missions flying above the atmosphere.

In 1965, Caltech graduate student Gary Flandro made a calculation that proved important for the feasibility of missions to Saturn and other outer planets. Flandro used the work of Michael Minovitch, a graduate student from University of California Los Angeles, who had been investigating a spacecraft mission strategy called gravity-assist, in which a planets gravitational field is used to modify a crafts trajectory. Minovitchs and Flandros work demonstrated that one spacecraft could use gravity assists to visit Jupiter, Saturn, Uranus and Neptune on one mission, if the planets were aligned just so. Gravity assists allow far more to be accomplished in a mission with a given amount of fuel and launch energy, because such maneuvers can greatly augment a spacecrafts velocity and kinetic energy and increase the amount of mass that can be flown.

Serious planning for specific outer solar system missions began only a little more than a decade after Newburns conceptualizations. By the early 1970s, with Mars and Venus already being explored by flyby and orbiter spacecraft, many astronomers identified outer solar system targets for exploration. In 1972, NASA released a Space Vehicle Design Criteria Monograph that envisioned the environment that a spacecraft would encounter at Saturn, including aspects of its gravity field, charged particles, ring particles, and atmospheric structure and composition.