South Pole Ice Discovery Explorer:



Introduction:



The Explorer lands in a mountain range (assuming Lunar Highland
terrain) and has no map to this range.  It has the positioning
cabability to orient itself in latitutude and longitude on the
lunar surface but no map to correlate  its position and to allow
for intelligent global path planning, although it is possible
that large craters could be identified with current images and
these images could give clues to pursue general directions.  It
has to explore promising craters which are likely (36% of the
surface area within 2.5 deg latitude of the South Pole is in
permanent darkness, see assumptions below)  near enough to it to
allow it to have time to reach the craters and explore them
within the first lunar day (14 earth days).  We want to descend
into the craters to take samples, but travel out of the craters
to run experiments and communicate these to earth.  



Some assumptions that are held throughout:



Lander/Rover Configuration:

The lander will become the rover.  This means we can't configure
a lander that deploys a rover.  We must strive to make dual use
of sensors for landing and the rest of the mission in order to
keep down overall development costs.  (This may not be true if
we can just copy another lander configuration completely.)



Percentage of Darkness:

Within 2.5 deg latititude (75 km) radius circle of the south
pole, at least 6361 km^2 is in darkness. (See "The Clementine
Bistatic Radar Experiment",Nozette,Lichtenberg,Spudis,...)  The
ideal area of this circle is 17,671 km^2, meaning roughly 36% of
the surface at 87.5 - 90 deg latitude is in darkness.



Apriori Map Assumption

We don't have a map with adequate resolution and will have to
make one during the landing and/or roving mission or we will
have to do without one if we don't have that ability.  (If we
could assume that we had good high resolution topological
information it would make the problem much easier.  Are any
missions scheduled before this mission that will map the south
pole?  Could we orbit the moon X times and map the south pole
before descending to the surface?  )





Thermal Assumptions:  It can be as low as 24 K and as high as
400 K (? - taken from 130 C as upper extreme during the day.)

Lowest possible temperature (being attributed to Mike Duke) is
24 K.  The papers cited above (under Crater Goals) state (< 102
K).

As stated by Stewart, the explorer should minimize the heat loss
through its bottom, due to the possibility of sublimating the
ice.



Communication Assumptions:  Communication can be spotty.

Initially I assumed that we could always communicate via the DSN
except when the rover is down in a crater.  The assumption being
that we can do what we need to on the near side of the moon.  I
now believe this is an incorrect assumption because of the
position of the earth in the "sky" of the Explorer.  The
position of the earth in the sky an object on the moon is
dependent upon the position of the observer on the moon.  The
moon does not change with respect to the earth, except for the
fact that it's axis are 1.6 degrees from being aligned.  This
means that when a vehicle on the surface of the moon is at the
moon's equater, the earth will be directly overhead.  The actual
angle of the earth above the horizon is equal to 90 degrees
minus the number of degrees above or below the lunar equator of
the observer.  So if an observer is at 60 degrees of lunar
latitude, the earth will appear to be 30 degrees above the
horizon.  (See MRD 1994 Course, Final Report.) In our case we
will be at a latitude of 85 - 90 degrees.  The earth will be
always be on the horizon.  This complicates the communication
issue by causing the earth to dissappear behind mountain ranges
in the distance or behind even fairly small hills that are close
to the rover. This either complicates mission planning by adding
a constraint that the planned path not take the explorer into an
area that is shadowed from communcation, or more likely it means
the rover needs more autonomous capability to get through these
areas or to reverse it's steps if it proceeds into a "shadowed"
communication area for too long of a time.  The  explorer needs
many of these capabilities anyway to descend into a crater. 
This issue also affects antenna placement; the communication
antenna must be placed on the vehicle where it will not be
shaded by parts of the vehicle in between the antenna and the
earth.   



Power Assumptions:

The battery (or other alternative) needs to have a BIG factor of
safety and not rely too heavily on solar power because the
explorer can be shaded from the sun due to several reasons. 
These reasons are exacerbated by the fact that the sun is also
always on the horizon.  It moves around the horizon as the lunar
day advances.  The explorer can be shaded from the sun for the
following reasons:

- It descended into a shadowed crater

- Lunar night

- It was just sitting on the surface, but the edge of the
shadowed area "creeped up on it"

- It went into a shadowed area, and the shadowed area got bigger
before the rover could take science measurements and get out,
meaning it had to stay in the shadowed area longer then it would
have if the shadowed remained stationary.



The solar panels will need to be vertical all the time because
the sun will be on the horizon.  Either than can move around the
vehicle or they are placed in a circle around the vehicle as
shown in the proposal.  This is true if the rover is on flat
ground, however, as the ascends and descends slopes, vertical
solar panels will not be optimal, but may be good enough?  



Mass Assumptions:

Explorer is X kg.



Crater Goals

We will look for a crater that has a small depth to diameter
ratio (< 0.2)  but still meets the requirements of having a
portion of the crater as a shaded area.  This will probably be a
more mature crater with shallower slopes.  This is typical of
craters within the lunar highlands according to Ingersoll,
Svitek and Murray in "Stability of Polar Frosts in Spherical
Bowl-Shaped Craters on the Moon, Mercury, and Mars", Icarus 100,
40-47, 1992.  They cite Pike, Geophysics Research Letter 1,
291-294, 1974 and Roddy et al., "Impact and Explosion Cratering,
pp 489-509.

Also Ingersoll, Svitek and Murray conclude, "As long as the
crater is deep enough to have shadows, the lowest temperatures
are for the shallowest craters-those with the smallest
depth/diameter ratio."   This happens because, according to
their model, the shadowed elements of the crater warm each
other, assuming that there is some light scattered into the
shadow and some fraction of that light is absorbed.  They also
cite temperatures as less than 102 K, which is different from
the 24K that we've talked about previously in class. With the
smaller depth to diameter ratio, less of a shaded area will
exist and therefore less total ice, but perhaps the crater will
be easier to descend into.     

OR  An alternative to this could be:

We will land in the Aiken basin and assume we can immediately
start looking for ice.  Perhaps close to the point where the
Clementine radar data indicated ice is the best starting point.
This appears to be within the spherical area surrounded by
87.5-90 degrees.  The actual pole is within the South Pole Aiken
basin and is estimated to be 5 - 8 km below the 1 km rim on the
near side of the moon.  PROBLEM with this strategy is that we
may have no communications.  We may be completely shaded from
the earth by the rim of the crater.  ( arctan(5/200) = 1.4 deg,
and we have no room for shading however with the earth being 90
degrees on the horizon) It is extremely risky to land in an area
where you have no communications and expect the Explorer to
accomplish things autonomously before moving to a communication
area.  

OR Best alternative:

Solution could be to land on the rim of the Aiken basin on the
near side of the moon and descend into the crater after system
check-out.  Leave a communications relay station on the rim of 
the Aiken basin on the near side of the moon.    



Scenario I:  Complicated Sensing



The explorer maps out the immediate area with a range finder as
the landing is made.  The map is used by the lander to choose a
landing spot.  The projected landing spot is 1 km from the South
Pole Aiken (SPA) basin rim on the near side of the moon.  The
explorer lands here in sunlight and in communication.  During
landing a high resolution map (~ 1m resol.) roughly 1500m x
1500m area with the rover roughly centered at this area.  This
map gives the necessary information to perform system check-out,
to proceed to the rim of the SPA, and to drop/build a
communications relay station on the rim.   The map is sent back
to mission control to provide a good topological map for the
beginning of the excursion.



The rover jettisons the landing gear and awaits instructions.The
topological data is analyzed at the mission control center and
the robot is sent a set of waypoints.  Initially the waypoints
will be only to test the operation of the locomotion subsystem
of the Explorer.  Scientific instruments will activate and an
engineering, calibration, and testing period will occur in the
sunlight.  Maps built by cameras on-board the explorer will be
compared to the map built while landing.

The landing should have been made so that the robot can
negotiate the required slopes.  (This could be an issue because
Apollo 16 landed in the Lunar Highlands and some of the slopes
would not have been traversible by the LRV, although they did
cover 36km with the LRV during the mission.) 

  

Once the testing period has been completed, a path (or
waypoints) will lead the robot in direction of the rim of the
SPA or toward a crater that was detected in the topological map
(what's the probability of this?), if not landing near the rim
of the SPA.  The robot uses its cameras and its laser range
finder to map the immediate area and traverse the set of
waypoints that have been sent to it.  The range maps are used
for obstacle detection and on-board positioning sensors indicate
possible tip over conditions.  The robot can be stopped by
mission control or if unsure of information can stop and request
a new direction or confirmation of its current direction.     



Approaching the rim of a crater:



If the crater being approached was not completely mapped out
during landing, which is unlikely given the extent of the map,
then the explorer must approach the crater and map the crater
with it's on-board sensors.  Approaching the rim of the crater
could be tricky due to the following:

-  Ejecta thrown out around the rim of the crater could result
in large boulders making it very difficult to drive to the rim
of the crater (Lunar Sourcebook - pg 32, Apollo 16 mission,
South Ray Crater,  the freshest large crater in the area, has a
high block concentration near the crater rim; the astronauts
noted that it would have been impossible to drive to the rim
because of the blocks.) 

- The soil tends to get softer as you approach the rim of a
crater.  Lunokhod 2 sunk up to 20 cm in the soil as it
approached the rim of craters.



Once the explorer reaches the rim of the crater, the mapping
process begins with a laser scanner.  (What dimensions on the
crater?? - Can the whole thing be mapped?)  The laser scanner
has a sufficient field-of-view to map even the area almost
directly underneath the rover, but probably can't be mounted in
a way that will allow it to "see" the area directly beneath it. 
This may lead to a section of the crater just under the rover
not being mapped.  This data is sent back to mission control and
a path is planned based on the visual assesment as well as this
topological data.  At this point the explorer may descend into
the crater if enough information was available to allow direct
entry into the crater, or possibly negotiate around the rim and
descend at a more gentle location, or decide that the crater
contains no permanent shadowed areas, or has little promise for
meeting the scientific goals, and will be given a path to
traverse around the crater toward another crater in the detailed
map, or just traversing in a general direction searching for
possible craters.

  

Descending into a crater:



If the initial crater is the SPA, then this should be a gradual
descent since the SPA has a diameter of 2500 km and a depth of
12km (See "The Clementine Bistatic....", Nozette,et.al. - can't
find SPA on the maps we have), although localized sharp slopes
due to cratering could exist.  According to the Nozette paper,
the explorer could reach icy areas by a 200 km traverse to the
south pole and that sunlight areas exist within 30 km of this
point.  This means power needs to be able to withstand possibly
a 230 km trek in only partial sunlight.  How long would this
take, can we travel this far?  In this scenario we should look
for craters that have shadowed areas as we move into the SPA.  



Descending into a generic crater:



A typical crater with a depth to diameter ratio of 0.2 may be
2000m to 4000m in diameter.  This gives an average slope of 22
deg (arctan(0.4)) for the rover to traverse.  For a factor of
safety, the explorer needs to withstand 30 - 45 deg. slopes (is
this possible?).  The explorer needs to be able to reverse
direction and climb up the slope if box canyon type situations
are encountered.  In general these can be avoided by the range
maps and the immersive video provided by the panospheric camera.
 



As the explorer is descending, small indentations can be
examined by a camera which can focus at close distances, perhaps
mounted on the drill or on the spectrometer.  These indentations
can be sampled with the drill to determine if ice exists there.
The IR camera will continuously be used to determine when
material changes occur on the soil.  





How fast will we move out of the 1500m x 1500m area?  

That's about a mile, at an average speed of 0.25 mph (what is
the speed of the rover?), in about 4 hours, the bigger question
may be what is the probability of finding interesting sites in
this area.



What is the range-finder?  A lidar seems like the logical
choice, they are small, lightweight, but will the infrared laser
sublimate the ice.  Can a lidar have a range of 2000m?  (The
papers so far don't really answer that question, they attempt to
answer the question of what the sumlimation rate is from
sunlight and the latitude to which the polar ice cap extends.   



Radar - can't be a conventional radar from an orbiting type
vehicle, they are lower frequency but use synthetic aperature
techniques to achieve high resolution, might require a new kind
of design for a radar that could map.  Also requires a big
aperature since the SAR technique can't be used ( or it possible
could be if the device is only used for landing)



Is a landing radar just an altimeter, meaning a 1 beam radar or
do they typically scan in 1 or 2 directions.  (I guess 1 beam.)



Will we ever be out of communication with the earth when we're
not down in crater?  How could we if we stay on the near side of
the moon for the whole mission?

 

Other issues still to be tracked down for the mission scenario
and ops are:

(From Stewart's list:)

Determine which part of the Aitken basin is within direct line
of site comm. with Earth



Determine what the maximum slope the robot will/should be able
to handle



Will active sensors or the robot's wheels sublimate the ice?