Introduction to special issue

Diving within Saturn's rings

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Science  05 Oct 2018:
Vol. 362, Issue 6410, pp. 44-45
DOI: 10.1126/science.aav4175

A diagram of Cassini's final orbits (blue curves), showing the last ring-grazing orbit and the 22 Grand Finale orbits.

IMAGE: NASA JET PROPULSION LABORATORY

Launched in 1997, the Cassini spacecraft arrived at Saturn in 2004 and spent the following 12 years orbiting far outside the planet's rings. Among the mission's many discoveries were potentially habitable environments on Saturn's moons, including a subsurface ocean of water on Enceladus.

To avoid accidental contamination of the moons with any Earth bacteria that might have hitched a ride on the spacecraft, mission planners decided that when Cassini ran out of fuel it would be deliberately crashed into Saturn. As fuel reserves ran low, the spacecraft embarked on a series of maneuvers that took it through previously unexplored regions and ended with its destruction.

The first phase was a series of 20 “ring-grazing” orbits in which Cassini passed near the outer edge of the rings. Then the trajectory was shifted to fly through the gap between the planet and the inner edge of the rings, a phase dubbed the “Grand Finale.” Cassini flew through this region 22 times, in various orientations designed to optimize the scientific measurements made by its numerous instruments. Finally, in 2017 Cassini dove into the planet itself at 35 km s−1, leading to the spacecraft's fiery disintegration in the upper atmosphere.

The papers in this section present results from those final phases of the mission. They include studies of Saturn's puzzling magnetic field, an isolated inner radiation belt, auroral radio emission, the mass and composition of solid grains falling from the rings toward the planet, and the composition of Saturn's atmosphere. Many more results will follow from Cassini's last period of exploration in the Saturn system.

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