During all of Cruise, the Cassini scientists have been busy planning what science they'll do when they get near Saturn. About six months before Saturn arrival, their plans go into action. And 19 days before arrival, there's a flyby of Saturn's most distant satellite Phoebe, the only close encounter (around 50,000 kilometers) that's possible during the tour. During the rest of the approach period, observations are tested, pictures of Saturn (though it's still far away) are snapped, and all the kinks are worked out for the big day: the Saturn Orbit Insertion Maneuver.
Of all the moons in the solar system, Phoebe is certainly one of the most curious. Its inclined, retrgrade (it rotates around Saturn the opposite way from the other satellites), chaotic orbit suggests that the moon may be a captured object, perhaps an old comet or asteroid that wandered close to Saturn in ages long past. Determining Phoebe's origin is an important scientific goal, and the closer we can get the better. Since this encounter is the only possible close encounter (Phoebe is just too far away from Saturn for later flybys), this opportunity is unique. Mission planners have even done some juggling of the arrival date, propellant budget and the maneuver after Saturn Orbit Insertion to make this opportunity a good one. This juggling has been deemed acceptable to get good flyby geometry. The science community is expecting great things to come from this flyby of Phoebe. (Image only available electronically.)
Next to launch, SOI is probably the single most important event for the spacecraft. It must align itself in the proper direction and fire the main engines for over an hour to make sure the spacecraft goes into orbit around Saturn. Without the SOI burn, the spacecraft would merely fly by Saturn like it did Venus, Earth and Jupiter without spending very much time investigating the system. So everything must be planned very carefully. Like an Olympic diver, every distraction from the purpose at hand must be eliminated, and success depends on good preparation and a keen focus. Fortunately, we have almost seven years to plan it right and make sure everything works.
This is an image of Cassini during the Saturn Orbit Insertion burn, just after the main engine has begun firing. The spacecraft is moving out of the plane of the page and to the right (and, therefore, firing to reduce the spacecraft velocity with respect to Saturn, so it can be captured by Saturn's gravity) and has just crossed the ring plane.(Image P-46507AC)
One of the many things mission planners will do to ensure that SOI goes off without a hitch is turn on both command and attitude control computers. If there should be a problem with one, we can then switch to the backup unit rapidly. Remember, radio waves take more than eighty minutes to reach the Earth at this distance; by the time we learned of a problem, it'd be too late to fix it. For this reason, the SOI sequence is designed to run on its own -- and the spacecraft will have all of its resources like backup computers standing by if they are needed.
Upon reaching Saturn, Cassini will swing close to the planet - at an altitude only one-sixth the diameter of Saturn itself - to begin the first orbit of some five dozen orbits during the rest of its four year mission. The maneuver itself will take about 100 minutes (a very long one) and will begin after the spacecaft crosses Saturn's rings (in a "gap," of course) and end before it crosses back through the ring plane. The burn will provide a change in velocity of more than 600 meters per second (2000 feet per second, or 1300 miles per hour!) and slow the spacecraft down enough to be captured by Saturn's gravity in a 5 month long orbit. During the burn itself, fields, particles and waves instruments will be operating as this is the closest the orbiter will ever get to Saturn. After the burn, the remote sensing instruments will be able to take advantage of this unique science opportunity.
