Headlines suggest (as do their articles) that as a gravitational result of Cassini's final flyby of Titan, it's fate is sealed and it's orbit is destined to take it into Saturn's atmosphere on September 15, 2017, with 22 complete orbits happening first.
- Phys.org: Cassini completes final—and fateful—Titan flyby
- Space.com: No Turning Back: Titan Flyby Assures Cassini's Crash Into Saturn
What phenomenon is causing this clockwork-like certainty? At Earth, passive drag-induced de-orbit and re-entry is hard to predict in part because the upper atmosphere is constantly changing due to the influence of the sun and solar wind. Saturn's atmospheric scale height is almost 60km which means that the atmospheric pressure increases much more gradually with decreasing altitude than that it would in Earth's atmosphere, but I don't know if that has anything to do with it or not.
Is it repeated atmospheric drag during each periapsis that will result in the systematic orbit-lowering that will bring Cassini into Saturn's atmosphere on the 23rd periapsis after the Titan flyby, or is it some other force that is causing this to happen?
Here's a plot of data extracted from the JPL Horizons database. Altitude above Titan (thin green line) and Saturn (equatorial radius assumed) (thick blue line) as a function of time relative to an arbitrary
2017-sep-15 00:00:00 utc (
JD=2458011.5) the day of the entry into Saturn's atmosphere. Data were downloaded and plotted at 10 minute intervals. The Titan swingby is indicated by the thin green line reaching down to 1000 km on the left, the planned incineration is the thick blue line approaching 30 km near 2017-sep-15. It looks like the simulation was run longer just to see what would happen.