The end of the Cassini mission finally came last week. After nearly twenty years in space, and more than thirteen years at Saturn – nearly half a Saturnian year – the Cassini spacecraft entered Saturn’s atmosphere on FRI 15 SEP 2017, and signal was lost at about 11:55 UTC.
Cassini’s scientific legacy is immense. It has discovered a liquid water ocean beneath Enceladus’s surface, and it has found evidence for another one beneath Titan’s surface. The Huygens probe peered beneath Titan’s haze for the first time and found liquid methane flowing into lakes and seas, carving canyons into ice. The water vapor from plumes on Enceladus escapes into orbit around Saturn, filling its magnetosphere with oxygen that has been liberated by sunlight. The hexagonal storm at Saturn’s north pole, first observed by the Voyager probes, remains today, and as it moved into sunlight during spring, it changed colour from blue to yellow-orange.
Galileo and Cassini have both proven that the outer solar system is far more exciting and unusual than we first thought. So where in the outer solar system should we go next?
Juno arrived in the Jovian system last year and has been in polar orbit – the first spacecraft to do so – since then. Its scientific mission concentrates on Jupiter itself rather than its moons. Its mission objectives include determining the mass of Jupiter’s core, mapping its gravitational and magnetic fields, and observing its aurorae.
The moons of Jupiter are also fascinating, but nobody is currently looking at them up close. ESA is due to launch the Jupiter Icy Moon Explorer in five years; it will study Europa, Ganymede, and Callisto. NASA is proposing its own mission to Europa.
Io, the closest of the Galilean satellites to Jupiter, experiences strong tides from Jupiter and the other moons, heating its interior and driving the most intense geologic activity anywhere in the solar system. Io is begging for a lander, but at that distance from Jupiter, the radiation is fierce, and any spacecraft would require heavy shielding.
There are no return missions to Saturn currently planned. There are proposals for missions that would enter orbit around Titan and drop a hot air balloon or return samples from Enceladus.
The complex chemistry on Titan and the water at Enceladus have made them the most popular destinations, but what else is interesting at Saturn? Iapetus has a dramatic colour shift: its leading hemisphere is made up of very dark material left behind when water ice sublimates. The water then solidifies on the surface of the trailing hemisphere, making that face bright. Ice cliffs on Dione, a possible ring around Rhea, and the gigantic impact craters that dominate the surfaces of Tethys and Mimas also warrant exploration.
The Uranian system has been visited only once, by a Voyager 2 flyby in 1986. NASA considered sending Cassini from Saturn to Uranus as a mission extension, but this was dropped in favour of the equinox and solstice missions.
The most fascinating thing about the Uranian system is not its moons, its rings, or even the planet itself. It’s the planet’s axis. Tilted at about 98° from the ecliptic, Uranus rotates on its side. Its poles receive more sunlight, averaged over the Uranian year, than its equator. The northern hemisphere is on its way toward summer solstice; a mission that launches within the next few years could reach Uranus in time for the solstice in 2028.
Miranda, the innermost of Uranus’s five large moons, is a bizarre mix of terrains: some old, some young, some flat, some layered, some deep canyons. It is believed to have formed once, smashed apart by an immense impact, and then reformed. There is evidence of geologic activity on Titania and Ariel. However, we just don’t know enough about these moons just yet.
As with Uranus, Voyager 2 is responsible for humans’ only visit to the Neptunian system, a flyby in 1989.
Voyager 2 observed a storm system called the Great Dark Spot. Images from the Hubble Space Telescope five years later showed that it had disappeared, but further images since then have shown similar systems reforming. But we have no images from the last few years; HST rarely gets the opportunity to point at objects in the solar system.
Voyager 2 passed within about 40,000 km of Triton, Neptune’s largest moon. It has a retrograde orbit around Neptune, moving opposite to Neptune’s rotation. This suggests that it did not form around Neptune, and was captured instead. Voyager 2 observed a thin atmosphere with clouds that appear to be made of nitrogen ice. Although Triton is by far the largest, Neptune has thirteen other known moons of which very little is known.
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