This wide angle field-of-view artist's rendering shows the surface of Pandora, one of the shepherds of the F ring, a thin wispy band of material just on the outside of the main rings of Saturn. The F ring is brightly shown on the left hand side of the picture, and Prometheus, the companion shepherd can be seen farther on down the ring on the inner side. (Image P-46505AC)
Saturn's small moons help support the ring system and are little understood, since most of them were discovered when the Voyager spacecraft was looking at something else. We hope Cassini will be able to study these satellites closely during the tour to help determine where they came from and what their role is in Saturn's environment. Unfortunately, close flybys of these bodies are difficult as they often orbit near dangerous regions of the ring system. These regions contain particles (rocks, ice and dust) that are big enough to seriously hurt the spacecraft, and are abundant enough to make the risk of a collision too high for us to fly near them.
The tour is the focus of Cassini's science goals. These goals require radically different observations of Saturn and its system from a wide variety of perspectives, so mission planners have to work hard to meet all the goals. Cassini must have a good trajectory that will take it around to all of the targets of interest to science; it must waste as little time, fuel and other resources as possible; and it must be able to collect and return a wealth of data every single day to fit everything the scientists want to do into four years. After four years, all of the science goals must have been met.
Of all the satellites orbiting Saturn, Titan is the largest by far, with a diameter of 5150 kilometers (3200 miles). It is the only satellite massive enough to provide any appreciable gravity assist. We can use Titan just like we use Venus, Earth, and Jupiter to get us to Saturn. With repeated flybys of Titan, Cassini can travel around to any portion of the system -- but it takes a lot of flybys, which means it takes time. So we must use our gravity assist "resource" wisely.
This artist's rendering is rendered near the center of Herschel crater, which occupies a large portion of Mimas' leading hemisphere. The near ice formations comprise the central mountains of the crater, with the crater walls visible in the distance. (Image only available electronically)
Titan is also interesting from a scientific standpoint. Multiple flybys of Titan will allow us to study it with radio waves; observe its atmosphere and its dynamics; map its surface at a number of wavelengths of light; study its gravity field; and learn about its magnetic field and the particle environment around it. The other satellites are important too: we'll plan at least five close flybys of the icy satellites as well as those of Titan.
Much of the tour phase will be very busy; we have to perform a lot of propulsive maneuvers, conduct science observations, and point back to Earth every day to transmit data we've collected (that's our operations concept). We also have to keep calibrating and maintaining the spacecraft instruments and other components like we did in cruise, since the tour is the time we need everything to work at its best. Not everything will be on at once, but instruments and subsystems will turn on and off (or be in what we call "sleep" mode, where they're quiet and not using a lot of power) throughout a day or few days, and we have to manage power and the flow of information carefully over time. Most of this is done with operational modes.
This artist's rendering shows the notable bright surface of icy Enceladus. In the foreground, an ice geyser can be seen projecting a jet of vapor into space. Enceladus is considered by some as the source of the E ring (which can be very faintly seen along Saturn's equatorial plane); icy geysers may be responsible for sustaining the E ring's supply of micrometer-sized particles. (Image P-46505BC)
During most of the tour, the reaction wheels will be used to control the spacecraft. These wheels will be used since they can keep the spacecraft very stable and don't use up propellant. This last issue is important since, unlike during cruise, we'll want to point the spacecraft at many different things all over the sky, even during a single day. Some of the time, like during close Titan flybys, we'll use the thrusters to control the spacecraft instead. This is useful mainly when we need faster turn rates and the science needs don't include ultra-stable motion.
The figure to the right is a sample timeline of some major activities planned for a tour. The actual tour may be different, but this timeline is representative of how events will take place. The timeline shows the major maneuvers (small arrows pointing up), Titan flybys ("T" triangles pointing down) and the other flybys of the icy satellites (circles), as well as those periods when the Sun gets between the Earth and Saturn ("c" triangles pointing up). These last events interfere with communication, since the radio waves have to pass through the Sun's corona, or outer atmosphere, to get to and from the spacecraft and the ground, and are called superior conjunctions. (Image only available electronically)
The tour, in general terms, contains about 60 orbits with various orientations; orbital periods ranging from over a hundred days to less than ten; Saturn closest approaches (or periapses) from under three Saturn radii (about 180,000 kilometers or 110,000 miles) to more than seven (420,000 kilometers or 260,000 miles); and over thirty close flybys of Titan. Orbital inclinations range from near zero to over sixty degrees (ninety degrees would be above Saturn's north and south pole) and provide good opportunities for ring imaging, occultations of Saturn and the ring system by the Earth and the Sun, and observations of a wide range of activity in Saturn's magnetic field.
When describing orbits and orbit characteristics, there are some common terms mission planners use to convey what an orbit is like. The orbit period is the amount of time it takes to complete a single orbit; the larger the orbit, the longer it takes to complete it. An orbit's inclination measures how far it is tilted out of the equator of the planet. Orbits with high inclinations will fly above a wide range of latitudes on the surface of the planet, whereas low inclination orbits will stick to the equator. For a low-flying weather satellite to take a close-up picture of a storm over Rekyavik, Iceland, for example (which has a latitude of 64 degrees), it must have a high enough inclination (actually, it needs an inclination of at least 64 degrees) to fly over the storm. A satellite with a low inclination would only be able to take close-up pictures of weather in central Africa, South America, India, and other countries near the equator. The orbit's periapsis is its point of closest approach to the planet, whereas its apoapsis is the farthest point in the orbit.
One of the most exciting features of Tethys (and of the whole Saturnian system as well) is Ithaca Chasma, a huge trench which extends from near the north pole down almost all the way to the south pole. It's average width is 100 kilometers (60 miles) and is 4-5 kilometers (2-3 miles) deep. This artist's rendering is drawn from the lip of the large chasm looking into it, with Saturn in the background. (Image P-46506BC)
Since Titan is the only satellite massive enough to provide any appreciable gravity assist, the tour orbits must always include a "free return" to Titan, that is, return to Titan without requiring large maneuvers. Though small propulsive maneuvers are included to allow some flexibility, the number and size of these maneuvers must be minimized. Fortunately, Titan gravity assists provide very effective orbit control, and the large number of flybys during the tour result in extensive Titan coverage.
One of the primary features of the tour is the sequence of high inclination orbits. These are included to allow investigation of the near-polar Saturn atmosphere and magnetic field, particularly some interesting radiation in the kilometer-sized wavelengths. Such high inclinations can be achieved using Titan gravity assists, but must be done in small steps to ensure that the spacecraft maintains a "free return" to Titan each time. In other words, raising inclination to high levels takes a great deal of time.
This narrow angle field-of-view artist's rendering is drawn from the floor of Aeneas crater, showing Dione's icy surface and the irregular features inside the crater viewed from space. Saturn, Titan, and the Cassini spacecraft are all visible in the sky. (Image only available electronically)
Occultations happen when the Earth, the Sun, or stars cross behind Saturn, Titan, or the rings as viewed from the spacecraft. Scientists can use radio waves or image these occultations to determine the structure and composition of the material being occulted. Since the Earth and Sun are outside of Saturn's equator during the tour, Saturn occultations can be provided only from inclined orbits.
There will be roughly thirty to forty close Titan flybys during the tour, all of which fly by the satellite at a speed of from five to six kilometers per second (11,000 to 13,500 miles per hour!). Closest approaches (also called "Titan periapses") range from 950 kilometers (590 miles) to 16,200 kilometers (10,100 miles). The geometry of each flyby must be chosen primarily to produce the desired effect on the spacecraft orbit, rather than to optimize flyby science. However, since other science objectives require a wide range of orbital inclinations and orientations, the Titan flybys should produce fairly complete coverage of the surface.
Rhea is a densely cratered satellite, and this narrow angle field-of-view artist's rendering shows two of the most prominent, Izanagi (the larger) and Izanami (the smaller), which partially overlap. These craters are well into the southern hemisphere. Saturn is seen on the horizon and a small meteor is seen striking the surface inside the Izanagi crater. (Image P-46506AC)
Very low titan flyby altitudes tend to favor RADAR imaging (which needs to be low to get high resolution imaging) as well as Titan atmospheric science (which needs a high enough density of the atmosphere for good data) and radio science (which needs occultations and close flybys through Titan's gravity field). Other flyby science, navigation and spacecraft safety considerations, however, tend to favor higher flyby altitudes. For example, remote sensing science, which favors the use of reaction wheels to stabilize the spacecraft, would have trouble at low altitudes because the motion of Titan is too fast for the reaction wheels to keep up with. This dichotomy of science needs is further complicated by the incomplete knowledge of Titan's atmosphere. If the flyby altitudes are chosen too conservatively, RADAR, atmospheric and radio science would suffer, whereas a much less conservative approach might endanger the spacecraft to excessive "wind" from Titan's atmosphere, should it prove to be denser than currently thought.
Tour designers will solve this problem by attempting to make the tour robust to altitude changes of 100 kilometers (60 miles) in either direction (up or down), which should be more than enough to compensate for a wide range of Titan atmospheric characteristics. The early Titan flybys can be used to determine what flyby altitudes are reasonable, and the subsequent flyby altitudes can then be "tweaked" to compensate. Recent studies show that flybys as low as 800 to 1000 kilometers (500 to 630 miles) could be acceptable, after several flybys at higher altitudes have allowed refinement of the Titan atmospheric model.
Hyperion is one of the smaller of Saturn's main satellites,
is irregular in shape, about 400 by 250 by 240 kilometers (250
by 160 by 150 miles), and is noted for its odd scarp system. Scarps
like that shown in this narrow angle field-of-view artist's rendering
are long cliff-like features, and may be as much as 30 kilometers
(20 miles) above the main surface level of the satellite. (Image
only available electronically)
The tour includes several close icy satellite flybys (among Mimas, Enceladus, Dione, Rhea, and Iapetus). Flybys of Mimas and Iapetus tend to be more difficult since Mimas is so close in and Iapetus so far out; most orbits will occupy the space in between. Icy satellite flybys will be planned, if possible, to occur far enough from a Titan flyby so the supporting maneuvers can be planned, the recorder emptied of previous data, and the ground system rested for a new and busy sequence.
Since the spacecraft must always be on a Titan return trajectory, and the icy satellites are too small to provide appreciable gravity assist, propellant must usually be expended to allow inclusion of targeted icy satellite flybys. In addition, the required transfer frequently delays other orbital goals, interfering with acquiring other types of science. For these reasons, the number of close icy satellite flybys is limited, and the times of their occurence must be chosen to minimize the propellant required to get to the flyby as well as the delay and impact to other orbital science. Combining icy satellite flybys with other tour objectives has been a key goal of tour design.
In this extremely narrow angle field-of-view artist's rendering Iapetus, with its notable dark surface, occupies the foreground with a dimly lit crescent Saturn low in the sky. Iapetus' surface shows some strange wavy ice formations, some low scoured hills, and mountains in the background. The Sun as well as three other satellites are also visible. (Image only available electronically)
The Saturn tour concludes after four years at Saturn in July of 2008. Nothing in the tour design precludes an exciting extended mission, which could consist of any number of possible orbits addressing different science objectives. Any extended mission is likely to be successful if sufficient propellant remains for attitude control and propulsive maneuvers.
