Iapetus Flyby

Todd J. Barber, Cassini lead propulsion engineer

It's 5 am--do you know where your spacecraft is? Many of us on the Cassini flight team were unceremoniously disturbed from slumber on 9/11 with the first spacecraft safing event in over four years. Spacecraft safing is a protective mode initiated by the spacecraft autonomously when things just aren't quite right. We had quite a streak going without a safing entry--I liken it to one's personal streak between vomiting events, a la "Seinfeld"! Just as with regurgitation, though, safing is here to help us. At first glance, it appears that perhaps an errant cosmic ray tripped the solid-state power switch on the prime traveling wave tube amplifier (TWTA-B), causing a temporary loss of downlink before the swap to TWTA-A. The most beguiling aspect of this safing is that the flight team practiced recovering from exactly this event should it ever occur on the spacecraft! I thought one purpose of simulating anomalies on the ground was to preclude ever seeing them in flight, but I guess I've been proven wrong yet again.

At first blush, the timing of this safing couldn't appear bleaker--immediately after the only close Iapetus flyby of the Cassini mission! Despite Murphy's best attempts, however, I'm happy to report that very little Iapetus science data were lost, and Cassini's closest approach images are now mere hours from revealing themselves before our anxious (if bloodshot) eyes. It has been a long campaign of planning, Labor-Day-weekend propulsive maneuvers, and anticipation, but Iapetus' moment in the spotlight is imminent. Almost lost in the Iapetus fervor were wonderfully successful encounters with Titan and Rhea, the two largest satellites in the Saturnian system. The Titan-35 flyby included a smorgasbord of infra-red measurements of methane abundance in the Titan troposphere, temperature profiling, ultraviolet observations of a stellar occultation, and high-resolution imaging in visible wavelengths. As important and exciting as each Titan flyby is, Rhea perhaps stole the show during this orbital pass of Saturn. The closest approach to this icy orb was a mere 5737 kilometers (3565 miles), and last-minute fine changes to spacecraft pointing enabled a bulls-eye view of a perplexing icy crater. This very busy period only serves to remind us of the rich and endless scientific bounty that is the Saturnian system.