The above image is of a feature that’s only seen on two objects in our Solar System – sunlight reflected off the surface of large bodies of liquid on a surface. Though Earth and Titan are so unalike, they’re akin in some striking ways, with predominantly nitrogen atmospheres, seas on the surface, and rain-storms.
In Part 1 I mentioned the difficulty of re-entry into Titan’s atmosphere from a Parabolic Transfer Orbit from Earth, but didn’t quantify it. Here’s the basic data for comparison:
Firstly, re-entry conditions from the “Easy Elliptical Orbit” which halves the trip-time from the basic Hohmann Co-Tangential Trajectory.
Secondly, re-entry conditions for the Parabolic Transfer Orbit, which shaves another 6-7 months off the trip-time.
The real problem is the comparative energy ratios of the two. The Parabolic Transfer Orbit means almost double the re-entry energy, thus double the experienced gee-forces during re-entry. Just like braking to a halt in a car, the deceleration force required increases with the square of the re-entry velocity. Or a much longer deceleration trajectory, which isn’t really an option in the atmosphere of a moon or planet.
Or is it? Can the atmospheric brakes be applied further out from a moon or planet? Or can the braking force be increased without overly straining the re-entry thermal protection system?
NASA’s Innovative Advanced Concepts (NIAC) office has seen several clever ideas for doing both. One concept that I think will be explored is using a magnetically trapped plasma to interact with the neutral atoms of the atmosphere, exchanging kinetic energy with them over a much wider area than the bow-shock of a re-entering rocket. This keeps the hot stuff away from the vehicle’s solid surface, increases the effective braking area and means the braking is spread out more, instead of rising to a fiery crescendo. There are limits to what the system can do, but as the above speeds are close to Earth re-entry speeds, which Plasma Braking has been studied for, I think we’re safely within its margin.