|A view impossible from Earth: a fat crescent Saturn throws its shadow across its rings; its rings return the favor. Image credit: NASA.|
Nearly half a century later, Dutch astronomer Christian Huygens revealed the true nature of Saturn's ears. He wrote in 1655 that the Sun's sixth planet "is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic." Giovanni Cassini observed in 1675 that Saturn's ring is made up of several concentric rings separated by gaps. The most prominent of the gaps, separating the inner B and outer A rings, became known as the Cassini Division.
In 1859, James Clerk Maxwell demonstrated that the rings could not be solid structures; rather, they consist of myriad particles, each orbiting Saturn independently like a tiny moon. James Keeler confirmed Maxwell's theory through telescopic observations in 1895.
Spacecraft exploration of Saturn began with the Pioneer 11 flyby on 1 September 1979. The 259-kilogram robot explorer left Earth on 6 April 1973 and received a gravity-assist boost from Jupiter on 4 December 1974. By passing through the plane of the rings 21,000 kilometers from Saturn, Pioneer 11 acted as a pathfinder for the Voyager 1 and Voyager 2 Saturn flybys.
Voyager 1 flew past the planet a little more than a year later, on 12 November 1980, revealing that Saturn's rings consist of a multitude of ringlets, gaps, and small "shepherd" moons. It also confirmed that the bright B ring is marked by strange ephemeral "spokes." The gaps and ringlets are the result of gravitational interactions with Saturn’s many moons; the spokes, on the other hand, remain mysterious. Voyager 1's twin, Voyager 2, flew past Saturn on 26 August 1981, en route to Uranus and Neptune.
|Voyager 2 sweeps past Saturn in this NASA painting.|
On 1 July 2008, NASA granted Cassini a 27-month mission extension called the Cassini Equinox Mission. Scientists then proposed that the space agency extend Cassini's mission of exploration until 15 September 2017 at a cost of $60 million per year. This would enable observation of seasonal phenomena in the Saturn system — such as anticipated increased ring spoke activity — over half a Saturnian year. NASA announced approval of the extension, dubbed the Cassini Solstice Mission, in February 2010.
|Artist's impression of Cassini's arrival in Saturn orbit, 1 July 2004. Image credit: NASA.|
As one might expect, Cassini has many science priorities besides study of Saturn's rings: to cite just two examples, the Cassini Solstice Mission includes 11 flybys of enigmatic Enceladus, and the primary objective of the 2017 ring-plane crossings is to examine Saturn's magnetosphere. In fact, Cassini planners generally steer clear of the rings because to approach too closely would place Cassini at risk from collision with ring particles.
If JPL engineers Robert Abelson and Thomas Spilker had their way, the next mission to Saturn after Cassini would focus on the rings exclusively. Spilker first proposed the Saturn Ring Observer (SRO) mission concept in 2000. A paper written with Abelson and presented at the 2006 Space Technology and Applications International Forum (STAIF) in Albuquerque, New Mexico in February 2006 fleshed out the conceptual mission.
NASA's Planetary Science Decadal Survey Giant Planets Panel requested a detailed study of the SRO mission concept, which a team under Spilker's direction performed in April 2010. The study focused on new propulsion and power technology. A future post will describe the 2010 SRO study.
|In the ring plane: Saturn's largest satellite, cloudy Titan, orbits between Cassini and Saturn's nearly edge-on rings. Image credit: NASA.|
SRO would launch atop a next-generation heavy-lift rocket capable of placing about 28,000 kilograms on course for Venus. For the first 11 years of its mission — the cruise phase — SRO would consist of a 4648-kilogram lifting-body aeroshell surrounding a 12,227-kilogram cruise stage and the 1823-kilogram orbiter.
Upon arrival at Saturn, SRO would dive through the planet's cloudy atmosphere, reducing its speed by 28 kilometers per second in 15 minutes and allowing the planet's gravity to capture it into a 61,000-by-110,000-kilometer orbit tilted slightly relative to Saturn's equator and the plane of its rings. Its work completed, the aeroshell would separate, exposing the cruise stage and orbiter to space for the first time.
Two hours after aerobraking, the four chemical propulsion rocket motors on the cruise stage would fire for two hours, circularizing SRO's orbit at an altitude of 110,000 kilometers. This would place it near the middle of the B ring. The cruise stage, its propellants exhausted, would then detach, and the orbiter would deploy its eight science instruments and two-meter-wide steerable high-gain radio antenna.
Abelson and Spilker explained that SRO's 129-kilogram instrument suite would be tailored to study "centimeter-scale ring particle interactions," the shepherd moons, the "ring atmosphere," and the electromagnetic environment of the ring system. Data returned from SRO would have application not only to the study to Saturn's rings, they explained, but also to understanding of other planetary ring systems and to protoplanetary disks around other stars.
|Intricate rings, intricate shadows. Image credit: NASA.|
Abelson and Spilker also considered a power system comprising four Sterling Radioisotope Generator units. These would produce less waste heat — handy during aerobraking, when the power system would be unable to radiate heat into space — but would also include turbines that might vibrate and interfere with SRO's sensitive science instruments.
The most novel element of Abelson and Spilker's proposed SRO mission would be the orbiter's intricate maneuvers near Saturn's rings. In its initial circular orbit, the orbiter would circle Saturn once every 10 hours, keeping pace with and studying nearby ring particles but remaining just outside the ring "surface."
Every 2.5 hours, as its slightly inclined orbit about Saturn brought it to one kilometer from the ring surface, it would point its engines toward the ring and fire them for about two seconds. This would move the orbiter an additional 0.4 kilometers away from the ring and would shift the point at which its orbit intersected the ring surface one-quarter of the way around the planet. Other hops would be triggered automatically if the orbiter detected a ring particle or spoke on a collision course.
About once per week, the SRO orbiter would maneuver outward slightly from the planet. Fifty such maneuvers over one Earth year would take it past the Cassini Division to the middle of the A ring, where it would orbit Saturn at a distance of 128,000 kilometers once every 13 hours with hops every 3.25 hours.
Soon after, Abelson and Spilker calculated, the orbiter's propellant supply would become depleted. In all likelihood, the mission would end the first time the SRO intersected the A ring a few hours later, and the spacecraft's battered wreckage would become a permanent (though insignificantly small) part of Saturn's ancient rings.
"A conceptual Saturn Ring Observer mission using standard radioisotope power systems," T. Spilker and R. Abelson, 2006 Space Technology and Applications International Forum, Albuquerque, New Mexico, 12-16 February 2006.
"Saturn Ring Observer Mission Concept: Closer Than We Thought," T. Spilker, et al., abstract #P23B-1634, American Geophysical Union 2010 Fall Meeting, San Francisco, California, 13-17 December 2010.
Peeling Away the Layers of Mars (1966)
On the Moons of Mighty Jupiter (1970)
Touring Titan by Blimp & Buoy (1983)
The Seventh Planet: A Gravity-Assist Tour of the Uranian System (2003)