mission to Mars (GEMS had been renamed InSight, as it
turned out a previous mission had been called GEMS) to fly.
We were devastated.
We had hopes that we might get the chance to
propose the mission again, but NASA suspended the ASRG
program in 2013. Perhaps the mission could be done with
an RTG — there would be more waste heat than would be
ideal, but that could be managed with bigger heat pipes.
When NASA announced a solicitation for new Discovery
concepts in February 2014, it revealed that the pace of
certain steps in the Plutonium fuel production process (the
material has to be sealed in special iridium alloy capsules for
safety in the event of a launch accident) meant there would
not be enough to allow an RTG to be ready for a Discovery
mission this time around (with missions selected for Phase
A in 2015, a launch might happen in 2020, and one might
arrive at Titan in 2026/2027).
So, that was that. We were dead in the water, so to
speak. Even if in another couple of years NASA was ready
with a radioisotope power source — whether ASRG or RTG
— launching in the early-mid 2020s wouldn’t allow arrival
until almost 2030, when it would be well after the Titan
northern fall equinox (Figure 8). The Sun and the Earth
would then be well south of the equator, and thus below
the horizon as seen from Titan’s arctic seas. The efficient
direct-to-Earth communication plan would be impossible.
One could imagine a capsule in the dark, communicating
via a relay spacecraft, but that won’t fit in the Discovery
program’s cost box. So, exploring Titan’s seas has become
more of an exercise in imagination for now.
Freed of the constraints of near-term implementation
details, it is easy to consider not just free-floating capsules
but boats, hovercrafts, or submarines. It is just such far-reaching ideas that NASA’s Institute for Advanced Concepts
(NIAC) likes to explore, and last year they funded a team
led by Steve Oleson at Glenn Research Center in Cleveland,
OH to pursue how a submarine might look. The team
included myself to define the scientific goals and
environment, and experts from Penn State ARL to consider
the hydrodynamics of propulsion and buoyancy control.
What could a submarine do? For a start, we could go
wherever we wanted to go scientifically. For example, after
landing safely in the middle of Kraken, we could sail
northeast to inspect the eastern shoreline and profile the
depth, then turn northwards to ‘sniff’ whether liquid
emerging through a labyrinth of channels from Ligeia Mare
might be more methane-rich than the main body of Kraken
(much like the Black Sea draining into the much more salty
Then, we could cruise around the western margin of
Kraken, where the wind patterns may lead to different
amounts of wave action and thus different shoreline
morphology (beaches, cliffs, etc.). A big motivation for
inspecting shorelines and the seabed is that there is
evidence (seen in Cassini data) of evaporites: bright once-dissolved material precipitated onto what is now land when
more extensive seas dried up to their present extent. Such
evaporites formed a ‘bathtub ring’ on Lake Mead and Lake
Powell due to a recent drop in water level. Yet, like those
two bodies — which have an irregular shape, filling canyons
and valleys that were once in the open air — the shape of
some of Titan’s shorelines suggest the sea level has been
rising. So, there is an interesting climate and sea-level
history that we are only just starting to see hints of in
Cassini data that a submarine could explore in detail.
42 SERVO 06.2015
Figure 8. Twilight on Kraken. As Titan's long seasons march on, the Sun
and Earth get too low over Titan's northern seas to offer useful line-of-sight for communication after about 2027; missions without relay satellites
will have to wait until around 2040 for the geometry to improve again.
Figure 7. A beefy sonar transducer, covered in frost at left as it
is still cold from a test in liquid nitrogen. The conical section
covered in frost helps couple the acoustic energy into the liquid,
while a heavy steel back-end helps reflect the energy from the
piezoelectric ceramic (gray band in middle) forwards. This sensor
configuration goes by the German name 'Tonpilz' ("sound
mushroom") and actually used the same piezoelectric ceramic as
the Huygens penetrometer (see previous article).