Iapetus Coming Up
Cassini’s first and only targeted flyby of Iapetus is coming up on September 10 with a closest approach of a little over 1600 km. Iapetus is the second largest of Saturn’s moons, and it is also quite distant from Saturn. The distance by itself is not what makes it tricky for Cassini to get there, rather it is the inclination of Iapetus’s orbit relative to Saturn’s equatorial plane. A reasonably large orbital inclination, combined with the large size of its orbit, means that Cassini needs to get to a difficult spot (high above the equatorial plane at a large distance from Saturn) unless it encounters Iapetus at one of its two orbital nodes (places where its orbit crosses the equatorial plane). In the case of this particular targeted flyby Cassini is in fact fairly high above Saturn’s equatorial plane (about 600,000 km) when it flies by Iapetus (which will be about 3.2 million km from Saturn at the time). In the two-year extended mission of Cassini (still not formally approved by NASA, by the way), there simply was not enough time to put an Iapetus flyby into the tour because large orbits are slow and therefore eat up a lot of the available time. This is an unfortunate consequence of planning only two years instead of three.
At any rate, we do have an excellent flyby coming up with some spectacular “photo op” images planned along with detailed scientific observations. Iapetus is notable for the large ridge extending at least one-third of the way around the planet on the equator and its bimodal distribution of colors: roughly half the moon is quite bright (like Saturn’s other icy moons) and the other half is among the darkest surfaces in the solar system. There will be a special session of the Division of Planetary Sciences meeting in Orlando in October devoted to Iapetus (the session was named “The Yin-Yang Body” by yours truly to reflect this unusual color distribution).