The planetary scientists constitute an almost fully distinct community from the astronomical community. This in part reflects the different type of physical problems encountered with planets; geology, meteorology, and chemistry play a big role in planetary physics, but no role in the study of stars and galaxies. To the extent that there is an overlap, it occurs in the study of the giant gaseous planets and in the study of planetary magnetospheres.
Methods of studying the planets also divide the planetary scientist from the astronomer. Astronomers can only observe their subjects through the radiation that reaches Earth, but planetary scientists have the luxury of studying their subjects up close with scientific spacecraft. They can directly measure the magnetic field, the particle composition, and the particle momentum distribution of a planet's magnetosphere; they can directly measure the composition and temperature of a planet's atmosphere; they can directly measure the distribution of mass in a planet, and they can directly observe the geology of a planet's surface. This great advantage of direct measurement over the indirect measurements available from Earth makes planetary study principally a space-based profession.
The Cassini spacecraft is now orbiting after a seven-year trip to Saturn. It is the fourth spacecraft to visit Saturn—the last visit was on August 25, 1981, when the Voyager 2 spacecraft flew by the planet. Cassini is the first spacecraft to enter into orbit around Saturn. It is studying the the structure of Saturn's rings, magnetosphere, and moons, and the composition and structure of Saturn's atmosphere. Cassini carries the Huygens probe, which was dropped onto Titan, Saturn's largest moon, to measure the structure and composition of Titan's atmosphere and surface. (continue)