Project Leader Jim Martin stands with Mars Viking
Introduction
When Tess Mercer, the content administrator from Nanothinc,
asked me to pen a series of articles on Nanotechnology and Space Exploration,
I was enthusiastic. Nanothinc is a sophisticated site including a variety
of articles, links and resource information. A clearinghouse for nanotechnology.
Tess had seen my writing on an
to nanotechnology which proposes a top
down reductionist strategy. This article entertains that line of thinking
in the context of space travel.
Kinds of Space Exploration
I had intended to write a summary of miniaturized space exploration; In pedagogical
order; Earth, planets, stars. Due to a recent NASA announcement, I am discussing
planetary exploration, specifically Mars exploration, first. In article II,
I will return to a discussion of Earth missions drawing from my experience
with the GeoSphere and EOS projects.
After sketching a couple of "small" modifications to our "Mission to Planet
Earth" program I will tie this together with microsatellites.
A thriving area, which portends the democratization of space, microsatellites
will be covered in article III. My initial thoughts on this subject appear in
a web piece entitled Grapefruits to Orbit.
Free access to space is a critically decisive issue. Miniaturization is the
key to this. Space exploration, in case you didn't know, is currently held in
exquisite check by NASA a
fact that I take no joy in revealing. An important principle of nanotechnology
is that of exploiting replication. Building prototypes at any scale is expensive.
Building successive copies becomes exponentially less expensive. To exploit
this, nanotechnology missions to the inner and outer planets, beginning with
Mars is apropos. This is based on first hand experience gleaned in the design
of flight hardware at the Jet Propulsion Laboratory.
This week NASA announced it had uncovered evidence of microbiological fossils
on the red planet, a claim which demands attention. Future articles will discuss
missions of longer range and duration, specifically RTG & electric rocket motor versions,
whose cruising speeds are significant faster.
There is an interesting learning curve in space missions. This learning
can be seen clearly in the Ranger missions to the moon that preceded the
planetary exploration era. We crashed seven Ranger spacecraft into the moon
before we figured out how to get pictures back of the surface. This wasn't
because we were stupid. It is because we were new at it. With time we got
better. We successively refined our approach but always ran into the limitation;
One can't reach out and tweak a mechanism on a spacecraft when trouble arises,
Murphy on the other hand, is always available. Mars Observer found this
out when it pressurized it's hydrazine tanks prior to orbit insertion. I
found this out when testing parachute systems in the Mojave desert. Craters
happen. The era when JPL really worked as a team was the Voyager-Viking
era.
Project Leader Jim Martin stands with Mars Viking
The engineers and scientists of that era practiced, made mistakes, and went on to pursue excellence. This reached its peak in the flying of two missions of notable success, Viking A and B. These were completely equipped flying laboratories with seismometers, weather stations, soil sampling, microbiology testing and video panoramas of the Martian ground and sky; Capabilities not appearing on the current mission to Mars.
Spacecraft Size Reduction
The functional architecture of the Viking era system was more comprehensive
than the blind "rock-chipper" now nearing its time to fly. How
might we compare the full featured 1968 Viking
Platform with a more appropriate 21st century counterpart? For the first
few size reduction cycles it might be possible to miniaturize by scaling
the Viking blueprints, unfortunately they have been lost. How might we proceed
to evolve milli, micro and nano clones of a Viking class spacecraft? One
could make the Viking lab mobile to retain the advantages of a roving platform.
This could be accomplished using a legged spider system. How big would such
platforms be? A measuring stick is required.
Measuring Stick One: Off the Shelf Sensing
A microsatellite
is defined as a satellite with a mass of between 10 and 100 kilograms. By
comparison , the launch mass of the Viking platform was 3527 kg. including
a handy dandy communications orbiter. As a rough estimate let's look at
how off-the-shelf sensing technology has evolved since 1968. Compare the
one inch TV/VTR camera system of that time with it's modern 8mm counterpart.
The old studio camera/VTR had a mass of approximately 120 kilograms. An
8mm handheld camera performing identical tasks comes in at 500 grams. How
many generations of size reduction does this take? If in each size reduction
generation we divide size in two, the mass decreases
by a factor of eight.
This represents a series of mass reductions from 120 to 15 kg, 15 to 2 kg, and finally 2 kg to 0.25 kg. About three generations, give or take. Performing a similar size-reduction program on the original Viking Lander + Orbiter yields a "microViking" mass of 7.4 kg (16 pounds at Earth sea level). This falls into the "official" microsatellite classification cited above. The original Viking had a 3.5 meter heat shield. A "microViking" would have a heat shield of 0.44 meters in diameter or 17 inches. Using the microsatellite definition given above, three additional generations of size reduction would provide a nanosatellite class platform with a mass of 15 grams.
It would have a 6 cm heat shield, a little over two and a quarter inches
in diameter, about the size of a plum .
Some Things Don't Scale Like Others: RF Antenna
One could make a similar technology reduction argument by comparing a
mid 1960's IBM 360 series mainframe to it's modern Pentium® based rival.
Microelectronics progress, by itself, is not a good metric for size reduction.
There are a multiplicity of spacecraft functions that are not solely electronic
in nature. Even functions that are solely electronic in nature do not all
scale. Example: RF communication antennae. For a given bandwidth and separation
distance, a certain minimum power must be expended to make communication
happen. This required transmitter power does not depend on spacecraft size.
Fixing the minimum transmitter bandwidth at some useful value imposes the
following demands; The product of the areas of the two antennae must be
a constant:
The two antennae can be the same jumbo size. One can be enormous, the
other small (sounds like canned olives doesn't it?). It doesn't matter which
antenna is big and which one is small. For convenience in packaging we place
the small antenna on the spacecraft. Consider a 1 meter antenna, like the
ill-fated high gain parasol on the Galileo mission to Jupiter. This antenna
was to have transmitted high rate image data from Jupiter to the 70 meter
Deep Space Network (DSN) antenna at Goldstone. It jammed during opening.
We tried to fix it from the ground, but budget limitations had already limited
the designers to use a one way motor, one that could only open the
antennae, instead of the big budget item... a two way motor. Why would you
ever want to close an antenna anyway? We spent a great deal of time and
money literally hammering that antenna open via uplinked instructions. It
just would not open. JPL was so embarrassed they buried the designer in
cement beside the uprights in the Rosebowl across the freeway. But a parasol
folding antenna is still a good idea. Small in storage, big in use.
The projected area of a working 1 meter parasol is:
The area of the 70 meter Goldstone dish is:
For communication to happen the product of the areas must be at least:
If our spacecraft is the size of a plum, we have to build a much larger
Earth antenna to compensate. How big a radius would that Earth antenna require?
Now my friend says, "That is one big antenna." I agree. The
antenna is 588 miles across, assuming a 5 cm plum. An Earth antenna the
size of Texas. What does this teach? It teaches we need to keep the antenna
on the spacecraft big... even if we have to fold it like a parachute
.. or a parasol. Of course we should test it to make sure it works. We should
have tested the mirror on the space telescope too. Oh well. Everything fails
once in a while. Sigh. We need a fault tolerant approach to space exploration.
As a designer I always like to ask, "What would Nature do in this situation?"
Answer, call on the department of redundancy department, of course! Launch
more than one spacecraft simultaneously to increase the probability of success.
Launch lots of little spacecraft. Just like nature launches seed pods.
If we take the hint, we suddenly find this nasty little antenna problem
is solved. If we electronically gang the little antennas together, we can
use them as ONE large antenna, VLA
style. Imagine an unfolding ensemble of 10 cm parasols, stopping on the
daily sojourn to point, bloom and phone home:
Ensemble Mission
Redundancy has another value as well. The value obtained from flying more
than one spacecraft at once dates back to a lesson I learned as a freefall
photographer, to wit, falling by oneself is a lonely journey. It's more
fun to spend one's last seconds with at least one other creature falling
beside you, better yet one with a camera who can document the capers for
posterity. By the same token, ensemble planetary missions have much
more excitement and appeal. Even a mere tandem televised approach and entry
into any planetary atmosphere would be a thrilling experience. One spacecraft
photographs the other while flying in a staggered formation, the same one
invented by ducks. Very televisable, very enjoyable; This kind of videography
applies to planetary approach, and to the whole mission as well.
Sample Return
The third innovation is to incorporated sample return into our nanoViking
spacecraft. Following the recent NASA announcement, Mars sample return is
officially big business. This will continue unless the Martian meteorite
turns out to be "cold fusion". In any event, Martian bullion should
be valuable, perhaps valuable enough to base a currency standard on. The
return operation might be similar to that depicted in the memorable Apollo
lunar blastoff footage. The question is how big a weight penalty do we have
to pay? A little back of the envelope shows
that the best we can do with chemical propulsion is about 84 grams
returned from Mars for every 10,000 grams launched from Earth. To return
1 gram would require at least a 12 kg booster, if the only mission objective
was sample return. An ensemble could return Mars bullion from various locations.
Lessons in Miniaturization
One of the lessons of miniaturization is units cost less per copy than prototypes.
The cost benefits of miniaturization apply to spacecraft, much more
than to Earthly endeavors. The single controlling issue of space exploration
is mass times velocity squared. Payload mass is the volume knob on the cost
function of a space mission. As mass goes down, so does launch weight, complexity,
and cost. Mass producing spacecraft is one thing NASA has never been inclined
to do so far. Launching a single spacecraft, at half a billion a pop, and
then waiting for it to fail is too much to ask of our engineers, scientists
and project managers. The microsatellite movement did not wait for permission
to proceed. They just did it. Perhaps the nanoSat
movement should as well.
© 1996 L. Van Warren, All Rights Reserved.
lvwarren at wdv dot com