The last couple of weeks I've been doing feasibility studies concerning
Titan aerocapture. Titan is a moon of Saturn that's bigger than Mercury
and Pluto. Titan (mean radius 2575.0 km) is a bit smaller than Mars (mean
radius of 3389.715 km). Due to it's lower density, Titan has significantly
less mass than Mars, viz. Gravitational parameter for Mars is 4.282838233e13
meter^3/sec^2 while for Titan it is 8.9782e12 meter^3/sec^2. However
Titan has a dense atmosphere with a surface pressure of 1.46 bars
compared to a Martian surface pressure of 6.36 millibars. This combination
of low gravity with a dense atmosphere makes Titan the best world in the
Solar System for aerocapture. For example, aerocapture into Titan's
atmosphere at 13 km/sec is no problem at all (ignoring heat load) with a low
lift blunt body like the Mars Pathfinder / Viking aeroshell. To aerocapure
into the thin Martian atmosphere at 13 km/sec is technically impossible.
From the standpoint of planetary science, Titan is an extremely interesting
world. It's atmosphere is essentially primordial at 95% nitrogen, 3% methane
and 2% argon. That 3% methane is what makes Titan interesting in the context
of pre-biology. Supposably the planet is covered with all sorts of complex
hydrocarbons and amino acid precursors. Unfortunately that 3% methane
also makes modelling aerocapture very tricky. The shock layer in front
of an entering aeroshell can get over 5000 deg.K. The methane will first
dissociate into hydrogen gas and graphite dust. That phase transition
from gas to dust breaks the thermodynamics routine in my code which means
I'll have to rewrite it for Titan. However the story gets better. If the
shocklayer gas gets hot enough, the graphite dust will recombine with
atomic nitrogen forming CN (cyanide) gas. CN is a very strong radiation
emitter. We get some CN production when we do Martian entry but the
atomic oxygen in the Martian atmosphere tends to eat up the CN. However
because there is almost no oxygen in Titan's atmosphere we get lots of CN
in the shocklayer. Nobody knows for sure how much radiation will be
produced by the CN. Some experts believe it will be insignificant while
others think it will dominate. There is currently an atmospheric probe
called "Huygen" on the way to Titan attached to the Cassini spacecraft.
Huygen was built by the European Space Agency (ESA) and is a very nice
piece of aerospace engineering. The French wisely designed Huygen for the
worst case entry condition which assumed strong CN radiation. The other
day we were speculating whether or not the aeroshell on Huygen was
instrumented, i.e. if it had thermocouples, a radiometer, etc. Somebody
said they had read there was indeed a radiometer on Huygen. I got
on the web and found there is indeed a radiometer on Huygen but it is
within the aeroshell and not active during atmospheric entry. Normally
it's a big deal to setup a radiometer to observe the shocklayer because
it requires installing a window in the main heat shield (a conservative
design mandates no penetrations in the heat shield). However the French
have this funny design for the Huygen aeroshell where they use tiles bound
upon a back structure. Standard American practice is to cast the heat
shield as a single body. We use tiles only for low heat reusable thermal
protection systems such as on the Space Shuttle. Because the French
used a tiled design on Huygen's heatshield, they could have easily
installed a window for a radiometer. An optical fiber could have directed
shock layer radiation from the heat shield's window to the probe's existing
on-board radiometer. Discovering this lost opportunity was a real
disappointment for me.

Gary