Only two outer planet missions are presently active and in a mission operations mode: Cassini and New Horizons. The Cassini orbital tour of Saturn, its preliminary ring-related discoveries, and its planned future observations were discussed in detail in Chapter 10. The New Horizons spacecraft was launched on January 19, 2006, and passes close to Jupiter on February 28,2007 . Specific plans for Jupiter observations will likely include a measure of the dust environment near Jupiter, along with five other ring-related studies: (a) ring phase curve observations; (b) high-resolution ring imaging; (c) search for new inner satellites; (d) ring-plane crossing observations; and
(e) studies of quadrant asymmetry (like those seen in Saturn's A ring). The primary goal of the New Horizons mission is to study Pluto and its moons and to study from close range at least one Kuiper Belt object beyond Pluto.
One other outer planets mission is presently in preparation: the Juno Mission will be a Jupiter Polar Orbiter, and is presently scheduled for a launch in 2011. Several others are being considered by the National Aeronautics and Space Administration . Three additional Jupiter missions are currently under study. These include a Europa Geophysical Explorer, with a possible launch no earlier than 2015; a Jupiter Flyby with Probes, with a launch around 2020; and a Europa Astrobiology Orbiter, with a launch about 2035 or later. Of these four Jupiter missions, Juno (the Jupiter Polar Orbiter) offers the greatest opportunity for good ring system data.
Following on the heels of the Cassini/Huygens Mission, a possible Titan Explorer with Orbiter is being considered. The launch date would be no earlier than 2025, and a Titan orbiter spacecraft is not well positioned for Saturn ring studies because of its distance from Saturn and its likely confinement near the equatorial plane of Saturn.
No missions are presently under study for Uranus. A Neptune Triton Orbiter, with unspecified launch date, is presently being considered.
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