Aerospace and Electronic Systems - August 2018 - 20

Proposed Landing on Europa

BURST MODE
Times may arise, such as when there is movement on the landscape, that require a series of images that are taken only fractions
of a second apart. We have referred to this rapid acquisition of images as burst mode. Assuming high-definition imagers with 1,280
× 1,280 pixels and 16 bits/pixel, each image will need 3.277 MB
of storage. If 128 GB of nonvolatile memory are available, then
the lander could store slightly more than 39,000 images in succession. Consequently, at a frame rate of 30 fps, about 22 min of
video could be stored; if the frame rate reduces to 5 fps, slightly
more than 2 h of video could be stored. At 1 fps, nearly 11 h of
video images could be stored. Regardless, transmitting this many
images to Earth will take a long time, 237 days at 50 kb/s, which
is not practical.
A burst mode for wide-bandwidth seismic-acoustic data could
collect very high-frequency acoustic data, even to 20 kHz. Sampling at 50 kilosamples/s (satisfying the Nyquist criterion with
some margin) at 24 bits/sample will generate 1.2 Mb/s or 150 kB/s
of storage.

DUAL-MODE SENSING OF SEISMIC EVENTS
No one knows what type of acoustic phenomena may exist on Europa. Detecting movement and acoustics from very low, subhertz
frequencies to 20 kHz would be important. The current seismometers reviewed for this article are limited: they have an upper frequency cutoff of about 100 Hz, which falls far short of 20 kHz [9],
[10]. Each sensor would need two transducers to cover the entire
band from 0.001 Hz to 20 kHz: one would be a seismometer to
cover from 0.001 to about 100 Hz, the other would be a microphone to cover from about 40 to 20 kHz. The good news is that
conversion of the physical phenomena to better than 20 bits at 50
kilosamples/s is easily done with the current technology in deltasigma analog-to-digital converters. Compact microphones having
response to 20 kHz are also available.

MAGNETOMETER COILS
We propose that only the coils of the magnetometers reside on the
surface. If the 2 m-long conductors to and from the coils cannot be
sized sufficiently, then the coils must be buried with the electronics or radiation-hard components must reside on the surface with
the coils.

SKY CRANE FUEL
Fuel for the sky crane is a concern. Two possibilities are readily
available: hydrazine/nitrogen tetroxide or LOX/LH2.
Hydrazine (N2H4), while convenient, simple, and controllable,
may not be the best choice if planetary contamination is to be considered. A combustion byproduct is ammonia (NH3), which is a
small molecule of interest on Europa; the sky crane's hydrazine
could pollute the local area around the lander on Europa.
The other choice would be LOX/LH2. This fuel requires the
maintenance of cryogenic conditions throughout the launch-and20

cruise phase of the mission as well as some way to pump the liquids into the combustion chamber. LOX/LH2 provides the highest
impulse, and its only byproduct is steam (H2O), but its use requires
more complex plumbing for the engine. Pumping could be either
mechanical, which adds complexity and lowers reliability, or a dry
nitrogen pressure source to push LOX/LH2 out of the tanks and
into the combustion chamber.

SINKING THE POD AND PROBES INTO THE ICE
Sinking the pod and probes into the ice could be problematic. The
titanium cases should drop in straight lines downward, with the
meltwater flowing around the cases and refreezing above the cases.
An important objective while accomplishing this goal of a straightline bore is to keep this operation simple and avoid complexity; not
using mechanical pumps or boring-drill bits would be a priority
for the design and operation. The original concept was to allow
Europa's gravity to draw the cases downward; this may or may
not work.
Several follow-up concepts developed following discussions at
MAS 2017. One concept was to have a simple guide channel keep
the cases aligned downward; Figure 9a provides an illustration.
Another concept was for motors to drive vertical rods attached to
the cases as the heaters melt the ice; Figure 9b illustrates a motor drive on a vertical rod attached to cases. Finally, Figure 9c illustrates a motor bore and pump integrated into a case to drive it
downward.
One reviewer has pointed out that sublimation of ice to water
vapor may cause the water to escape after melting, which would
mean that water might not refreeze above the pod as it burrows into
the ice. In Figure 9, we propose a disc to cap the borehole and thus
reduce the amount of water vapor that escapes, with most of the
water condensing on the cap and refreezing until the capsule has
burrowed far enough downward that the ice caps the borehole and
the water flows around the pod and probes and refreezes.
Only experimentation will reveal the most effective means to
bury the pod and probes.

ACTIVE SEISMIC OPERATIONS
The original concept was for passive sensing of seismic events.
Discussions at MAS 2017 raised the possibility of using active
seismic operations to explore the layering of the ice. The lander
could incorporate an electromechanical thumper to rap the ice
with a mechanical impulse. The SRG has enough power to drive
a power supply for the thumper, which could be a capacitor storage system that drives a linear motor. Active seismic exploration
would add weight and complexity to the lander; however, a more
detailed picture of the structure of the icy crust could be a most
valuable tradeoff.

AREAS NOT ADDRESSED
While we studied the feasibility of landing on Europa, we did not
address some areas. We did not feel that neglecting these areas in
our study would have a major impact on our findings. In addition,

IEEE A&E SYSTEMS MAGAZINE

AUGUST 2018

Aerospace and Electronic Systems - August 2018

Table of Contents for the Digital Edition of Aerospace and Electronic Systems - August 2018