Vision and Voyages for Planetary Science in the - Solar System ...

Cassini Solstice Mission
ADVANCING STUDIES OF THE GIANT PLANETS: 2013-2022
Previously Recommended Missions
The Cassini spacecraft has returned an unprecedented volume of data from the Saturn system. It
completed its main mission in 2008, returning nearly 2 terabytes of data on the planet, magnetosphere,
rings and satellites. The mission has also completed its first extended mission, ending in mid-2010.
During this time, many advances were made in our understanding of Saturn, including a new value for the
most basic of quantities—its deep internal rotation rate. In addition, detailed observations showed the
existence of a warm polar vortex, detailed 5-micron cloud structure, long-lived storms, and the presence
of equatorial wind and temperature changes.
In the Solstice Mission, Cassini will continue its operations until a planned atmospheric entry in
2017. The value of this data set cannot be overestimated. The extended time base of observations is
critical for understanding several aspects of Saturn’s atmosphere, including the much longer and larger
seasonal variations (as compared with Jupiter) as well as its long-period equatorial oscillation. The
Solstice Mission results will provide many insights into the dynamics and circulation on this planet, as
well as understanding of polar vortex formation, ring shadowing effects, and other atmospheric
phenomena; it will also greatly extend the time baseline for the study of variable features in the rings,
such as spokes, propellers, and noncircular ring edges, while permitting radio occultation probes of ring
structure at many different incidence angles. In addition, the planned end-of-life scenario to place the
craft into a Juno-like orbit (to constrain the internal mass distribution and higher order magnetic field
components) adds an economic mini-mission that will allow comparison of the internal structure of
Jupiter and Saturn.
Europa Geophysical Orbiter
The Europa Geophysical Orbiter recommended in the 2003 planetary decadal survey is now
being studied in the context of a proposed joint NASA-ESA Europa Jupiter System Mission (EJSM).
This cooperative venture combines a NASA-provided Jupiter Europa Orbiter (JEO) with an ESAprovided
Ganymede orbiter. There is an extended period of time during Jupiter approach that is suitable
for low-phase-angle observations of the jovian atmosphere and for Jupiter system observations that will
enable time-domain science, including fluid dynamics studies. After Jupiter orbit insertion, there is a
further 2- to 3-year period that could be dedicated to Jupiter system observations before each spacecraft
achieves its final satellite orbit. With the available extended time and with jovian-atmosphere-specific
instrumentation, these observations could provide significant insights into several remaining questions
and poorly understood atmospheric phenomena, such as aurora and polar haze structure and interactions,
wave-induced dynamical processes, and coupling across atmospheric boundary layers. Although the
2010 Science Definition Team Report expanded the mission science objectives to include some valuable
Jupiter and ring science, the focus and priority remains Europa (see Chapter 8). The huge gaps in our
knowledge of the Uranus and Neptune systems, combined with the narrower advances in Jupiter science,
together put JEO at a lower priority for giant planet science than a mission to an ice giant.
Juno
The Juno mission was selected for the second of New Frontiers’ launch opportunity. Due to
launch in 2011 and arrive at Jupiter in 2016, Juno will study the planet’s deep interior structure,
abundance and distribution of water, and polar magnetic environment. Combined with results from the
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
7-34