Many exciting options are available for an extended mission. One approach for the extended mission is to repeat many of the observations taken during the nominal mission with different geometries and instruments. Perhaps Cassini will discover new moons of Saturn, or a strange magnetic field anomaly that deserves further study. The Voyager spacecraft, for example, are still operating and collecting data on the outer solar system and the solar wind twenty years after launch. Pioneer 11, launched in April 1973, passed within 21,000 kilometers (13,000 miles) of Saturn in 1979 and is only now running out of operating power. We expect that Cassini will have the same success in extended lifetime as the Voyager and Pioneer spacecraft. In fact, if attitude control were the only concern, Cassini should be able to survive up to 200 years before losing its stable attitude!
However, there are all sorts of more outlandish opportunities for Cassini after its nominal mission ends. Cassini could go into orbit around Saturn's largest moon Titan, or even use its gravity assist to escape Saturn altogether! It may even be possible for Cassini to go to another planet or visit an asteroid (though it's likely that this would take many years to do). However, the Saturn system is interesting enough -- and may even have some surprises left for us -- that Cassini will probably still have lots of useful measurements left to take.
What exactly would Cassini do during an extended mission? There are lots of interesting things we could do in and around Saturn after four years. First of all, the way Saturn and its rings change with time is very important scientifically. Studying Saturn for a longer period of time, in and of itself, could be of immense value. Four years may seem like a long time to study something, but imagine how little we'd understand just the Earth's weather - storms, El Niño, global warming, the ozone hole, or merely weather forecasting - if we only studied it over four years. Most of our knowledge of the Earth comes from study and experience gained over much longer periods.
Here are some specifics for Cassini's extended mission:
Escape Saturn's Gravity. It is possible to escape Saturn by using Titan's gravity to change the orbit of the spacecraft. This process would require at least several flybys of Titan, and would likely take at least six months -- depending on where the spacecraft winds up at the end of the nominal mission. On the last Titan flyby, the spacecraft is in orbit around Saturn before the flyby, and then attains an escape trajectory leaving Saturn after the flyby. Where the spacecraft would go next is uncertain; but it's unlikely that Cassini could move very far from Saturn's orbit without later (probably MUCH later) flybys of Saturn or a great deal of leftover fuel.
Fly closer to Titan. During the tour, there are many flybys of Titan during which we'll be investigating Titan's unique atmosphere. The spacecraft cannot get too close, however, or it will pick up too much drag in the "wind" of Titan's atmosphere. Significantly lower Titan flyby altitudes might still be an option for an extended mission to examine the atmospheric characteristics at lower altitudes.
In fact, it may even be possible to go into orbit around Titan! "Aerobraking," or using a body's atmosphere to slow a spacecraft down, has been tested with other spacecraft and is planned for at least one JPL mission in the future. Aerobraking and/or spacecraft maneuvers could be sufficient to place Cassini in orbit around Titan, allowing the spacecraft to study Titan very closely over a long period of time.
Fly Closer to Saturn. After the 4 year tour, the s/c will most likely be in a near polar orbit which flies over latitudes approaching +/- 80 deg. One extended mission option is to further reduce the closest approach distance to Saturn, allowing the spacecraft to pass inside the G ring while skipping over the regions known to contain a high density of potentially damaging ring particles. This option also has the added advantage of increasing the maximum orbit inclination by a small amount. Traversing ring plane crossings inside the G ring may increase the hazard of ring particle impacts but such increased risk may be more acceptable in an extended mission.
More Flybys. Next to more observation time in general, this is one of the most obvious advantages of an extended mission. Additional close flybys of Titan could improve the RADAR mapping coverage, add data on atmospheric dymanics and composition, and map Titan's gravity field more accurately. If interesting features on Titan were detected during the nominal mission, the Titan flyby ground tracks during the extended mission could be designed to cover these interesting areas.
Additional close flybys of the icy satellites are an obvious choice for an extended mission regardless of other extended mission objectives. These flybys would be a great boon, since we can only get close (under several thousand kilometers at closest approach) to a handful of the icy satellites during the tour. Additional flybys of Titan also offer gravity-assist resources that trajectory designers can use to move the orbit to different and interesting regions of the Saturnian system.
Investigate Saturn's Rings more Closely. During the tour, Saturn's inner rings are observed from afar, since many ring particles pose a threat to the safety of the spacecraft. After completing our nominal tour, however, the project might be inclined to take more risk to get some really good firsthand information about ring composition. In fact, there are some ring-related events -- flybys of moons near or through part of the ring system, for example -- that happen in the few years following the end of Cassini's nominal mission. Observing a small moon plow through a ring may be interesting enough in itself to merit continued operations!
Alter Orbit Inclination. There are several science objectives on reaching different inclinations, or orbital tilt. This is important to fully map Saturn's magnetic field in three dimensions, and is very useful for high resolution observations of the polar regions. Altering inclination, however, takes gravity assists from Titan that we have to split among many other science objectives. During an extended mission, we'd have more time to get higher or lower in inclination than we can afford to during the nominal tour (since we have to meet all science objectives equally during the tour).
Rotate the Orbit. During the tour, different orbital geometries will offer a wide variety of "phase angle" geometry for Saturn and its satellites. The phase angle at which a body is viewed is a measure of what angle the Sun strikes the surface. A full moon, for example, has a phase angle of near zero degrees, which is best for color and surface composition analysis. When the moon is half full, it's phase angle is near ninety degrees and shadows of highlands and lowlands are easily discernible for altitude mapping. A new moon would have a phase angle of near 180 degrees, and is good for Sun occultations, when the Sun passes behind the planet (typically used to provide information on the body's atmosphere). Rotating the orbit in an extended mission would give the scientists the opportunity to observe the Saturnian system at phase angles not available (or briefly so) during the nominal tour.
Many of these scenarios take many Titan flybys and a good deal of time to accomplish. Also, extended mission options which result in greater risk to the spacecraft may become more attractive in an extended mission in which the science objectives have already been achieved, especially if these options provide opportunities to enhance what Cassini has already discovered. The type of extended mission will depend greatly on the amount of propellant left onboard, the spacecraft health, and the available funding to support continued operations here on Earth.
