Al Globus, David Bailey, Jie Han, Richard Jaffe, Creon Levit, Ralph Merkle, and Deepak Srivastava
Published in The Journal of the British Interplanetary Society, volume 51, pp. 145-152, 1998.
Laboratories throughout the world are rapidly gaining atomically precise control over matter. As this control extends to an ever wider variety of materials, processes and devices, opportunities for applications relevant to NASA's missions will be created. This document surveys a number of future molecular nanotechnology capabilities of aerospace interest. Computer applications, launch vehicle improvements, and active materials appear to be of particular interest. We also list a number of applications for each of NASA's enterprises. If advanced molecular nanotechnology can be developed, almost all of NASA's endeavors will be radically improved. In particular, a sufficiently advanced molecular nanotechnology can arguably bring large scale space colonization within our grasp.
This document describes potential aerospace applications of molecular nanotechnology, defined as the thorough three-dimensional structural control of materials, processes and devices at the atomic scale. The inspiration for molecular nanotechnology comes from Richard P. Feynman's 1959 visionary talk at Caltech in which he said, "The problems of chemistry and biology can be greatly helped if our ability to see what we are doing, and to do things on an atomic level, is ultimately developed---a development which I think cannot be avoided." Indeed, scanning probe microscopes (SPMs) have already given us this ability in limited domains. See the IBM Almaden STM Gallery for some beautiful examples. Synthetic chemistry, biotechnology, "laser tweezers" and other developments are also bringing atomic precision to our endeavors.
[Drexler 92a], an expanded version of Drexler's MIT Ph.D. thesis, examines one vision of molecular nanotechnology in considerable technical detail. [Drexler 92a] proposes the development of programmable molecular assembler/replicators. These are atomically precise machines that can make and break chemical bonds using mechanosynthesis to produce a wide variety of products under software control, including copies of themselves. Interestingly, living cells exhibit many properties of assembler/replicators. Cells make a wide variety of products, including copies of themselves, and can be programmed with DNA. Replication is one approach to building large systems, such as human rated launch vehicles, from molecular machines manipulating matter one or a few atoms at a time. Note that biological replication is responsible for systems as large as redwood trees and whales.
Another approach to nanotechnology is supramolecular self-assembly, where molecular systems are designed to attract each other in a particular orientation to form larger systems. Hollow spheres large enough to be visible in a standard light microscope have been created this way using self-assembling lipids. There are many other examples and this field is rapidly advancing. Biological systems can do most of what molecular nanotechnology strives to accomplish -- atomically precise products, active materials, reproduction, etc. However, biological systems are extremely complex and molecular nanotechnology seeks simpler systems to understand, control and manufacture. Also, biological systems usually work at fairly mild temperature and pressure conditions in solution -- conditions that are not found in most aerospace environments.
Today, extremely precise atomic and molecular manipulation is common in many laboratories around the world and our abilities are rapidly approaching Feynman's dream. The implications for aerospace development are profound and ubiquitous. A number of applications are mentioned here and a few are described in some detail with references. From this sample of applications it should be clear that although molecular nanotechnology is a long term, high risk project, the payoff is potentially enormous -- vastly superior computers, aerospace transportation, sensors and other technologies; technologies that may enable large scale space exploration and colonization.
This document is organized into two sections. In the first, we examine three technologies -- computers, aerospace transportation, and active materials -- useful to nearly all NASA missions. In the second, we investigate some potential molecular nanotechnology payoffs for each area identified in NASA's strategic plan. Some of these applications are under investigation by nanotechnology researchers at NASA Ames. Some of the applications described below have relatively near-term potential and working prototypes may be realized within three to five years. This is certainly not true in other cases. Indeed, many of the possible applications of nanotechnology that we describe here are, at the present time, rather speculative and futuristic. However, each of these ideas have been examined at least cursorily by competent scientists, and as far as we know all of them are within the bounds of known physical laws. We are not suggesting that their achievement will be easy, cheap or near-term. Some may take decades to realize; some other ideas may be scrapped in the coming years as insuperable barriers are identified. But we feel that they are worth mentioning here as illustrations of the potential future impact of nanotechnology.
The applicability of manufacturing at an ever smaller scale is nowhere
more self-evident than in computer technology. Indeed, Moore's law
[Moore 75] (an observation not a physical law) says that computer chip
feature size decreases exponentially with time, a trend that predicts
atomically precise computers by about 2010-2015. This capability
is being approached from many directions. Here we will concentrate on
those under development by NASA Ames and her partners. For a review of
many other approaches see [Goldhaber-Gordon 97].
Carbon nanotubes [Iijima 91]
can be viewed as rolled up
sheets of graphite from 0.7 to many nanometers in diameter. The smaller
tubes are single molecules. [Dai 96] placed carbon nanotubes on an SPM tip thus
extending our ability to manipulate a single molecule with sub-angstrom
accuracy. Not only are the tips atomically precise, but they should
have approximately the same chemistry as C60, and
thus be functionalizable with a wide variety of molecular
fragments [Taylor 93]. Functionalizing carbon nanotube tips will allow
mechanical manipulation of many molecular systems on various surfaces
with sub-angstrom accuracy.
One particularly intriguing possibility along this line is to utilize a
carbon nanotube SPM tip to engrave patterns on a silicon surface. It
should be possible to create features a few nanometers across. These
would be perhaps 100 times finer than the current state of the art in
commercial semiconductor photolithography. Further, in contrast to
approaches such as electron microscope lithography for which the speed
of operation now appears to be an insuperable obstacle for industrial
production, nanotube SPM-based lithography can be accelerated by
utilizing an array with thousands of SPM tips simultaneously engraving
different parts of a silicon surface. Also, nanotube SPM lithography
could provide a practical means to explore various futuristic
electronic device technology ideas, such as quantum cellular automata,
which require exceedingly small feature sizes. Needless to say, if
these ideas pan out, they could literally revolutionize computer device
technology, paving the way for systems that are many times more
powerful and more compact than any available today.
For the near term, it should be noted that
the semiconductor industry is a major market for SPM products.
These are used to examine production equipment. High performance
carbon nanotube tips should be of substantial value. NASA Ames
is collaborating with Dr. Dai, now at Stanford, to develop
these tips.
[Bauschlicher 97a] computationally studied storing data in a
pattern of fluorine and hydrogen atoms on the (111) diamond surface
(see
figure
Among the better probes was C5H5N (pyridine).
Quantum calculations suggest that pyridine is stable when attached to
C60 in the orientation necessary for sensing the difference between
hydrogen and fluorine. Half of C60 can form the end cap of
a (9,0) or (5,5) carbon nanotube, and carbon nanotubes have been
attached to an SPM tip [Dai 96]. Thus, it might be possible using
today's technology to build a system to read the diamond memory
surface.
[Avouris 96] has shown that individual hydrogen atoms can be removed
from a silicon surface. If this could be accomplished in a gas that
donates fluorine to vacancies on a diamond surface, the data storage
system could be built. [Thummel 97] computationally investigated
methods for adding a fluorine at the radical sites where a
hydrogen atom had been
removed from a diamond surface.
As mentioned before, carbon nanotubes can be described as rolled up
sheets of graphite. Different tubes can have different helical
windings depending on how the graphite sheet is connected to itself.
Theory [Dresselhaus 95, pp. 802-814] suggests that single-walled carbon
nanotubes can have metallic or semiconductor properties depending on
the helical winding of the tube. [Chico 96], [Han 97b], [Menon 97a],
[Menon 97b], and others have computationally examined the properties of
some of hypothetical devices that might be made by connecting tubes
with different electrical properties. Such devices are only few
nanometers across -- 100 times smaller than current computer chip
features. For a number of references in fullerene nanotechnology
see [Globus 97].
Several authors, including [Tour 96], have described methods to
produce conjugated macromolecules of precise length and composition.
This technique was used to produce molecular electronic devices in mole
quantities [Wu 96]. The resultant single molecular wires were tested
experimentally and found to be conducting [Bumm 96]. The three and four terminal devices
have been examined computationally and look promising [Tour 97]. The
features of these components are approximately 3 angstroms wide, about
750 times smaller than current silicon technology can produce.
From [Merkle 96]:
One study not conducted by Ames or partners is particularly worth
mentioning since it places a loose lower bound on the computational
capabilities of molecular nanotechnology. [Drexler 92a] designed
a number of computer components using small diamondoid rods with
knobs that allow or prevent movement to accomplish computation.
While this tiny mechanical Babbage Machine is probably not an
optimal computational engine, its calculated performance for
a desktop computer is 1018 MIPS -- about a million
times more powerful than the largest supercomputer that exists today
(Fall 1997).
Note that with very fast computation energy use and heat dissipation
become a severe problem. One approach to addressing this issue is reversible
logic.
[Drexler 92a] proposed a nanotechnology based on diamond and
investigated its potential properties. In particular, he examined
applications for materials with a strength similar to that of diamond
(69 times strength/mass of titanium). This would require a very mature
nanotechnology constructing systems by placing atoms on diamond
surfaces one or a few at a time in parallel. Assuming diamondoid
materials, [McKendree 95] predicted the performance of several existing
single-stage-to-orbit (SSTO) vehicle designs. The predicted payload to
dry mass ratio for these vehicles using titanium as a structural
material varied from < 0 (the vehicle won't work) to 36%, i.e., the
vehicle weighs substantially more than the payload. With hypothetical
diamondoid materials the ratios varied from 243% to 653%, i.e., the
payload weighs far more than the vehicle. Using a very simple cost
model ($1000 per vehicle kilogram) sometimes used in the aerospace
industry, he estimated the cost per kilogram launched to
low-Earth-orbit for diamondoid structured vehicles should be $153-412.
This would meet NASA's 2020 launch to orbit cost goals. Estimated
costs for titanium structured vehicles varied from $16,000-59,000/kg.
Although this cost model is probably adequate for comparison, the
absolute costs are suspect.
[Drexler 92b] used a more speculative methodology to estimate that a
four passenger SSTO weighing three tons including fuel could be built
using a mature nanotechnology. Using McKendree's cost model, such a
vehicle would cost about $60,000 to purchase -- the cost of today's
high-end luxury automobiles.
These studies assumed a fairly advanced nanotechnology capable of
building diamondoid materials. In the nearer term, it may be possible
to develop excellent structural materials using carbon nanotubes.
Carbon nanotubes have a Young's modulus of approximately
one terapascal -- comparable to diamond.
Studies of carbon nanotube strength include
[Treacy 96], [Yacobson 96], and [Srivastava 97a].
[Issacs 66] and [Pearson 75] proposed a space elevator -- a cable
extending from the Earth's surface into space with a center of mass at
geosynchronous altitude. If such a system could be built, it should be
mechanically stable and vehicles could ascend and descend along the
cable at almost any reasonable speed using electric power (actually
generating power on the way down). The first incredibly difficult
problem with building a space elevator is strength of materials.
Maximum stress is at geosynchronous altitude so the cable must be
thickest there and taper exponentially as it approaches Earth. Any
potential material may be characterized by the taper factor -- the
ratio between the cable's radius at geosynchronous altitude and at the
Earth's surface. For steel the taper factor is tens of thousands --
clearly impossible. For diamond, the taper factor is 21.9 [McKendree
95] including a safety factor. Diamond is, however, brittle. Carbon
nanotubes have a strength in tension similar to diamond, but bundles of
these nanometer-scale radius tubes shouldn't propagate cracks nearly as
well as the diamond tetrahedral lattice. Thus, if the considerable
problems of developing a molecular nanotechnology capable of making
nearly perfect carbon nanotube systems approximately 70,000 kilometers
long can be overcome, the first serious problem of a transportation
system capable of truly large scale transfers of mass to orbit can be
solved. The next immense problem with space elevators is safety -- how
to avoid dropping thousands of kilometers of cable on Earth if the
cable breaks. Active materials may help by
monitoring and repairing small flaws in the cable and/or detecting a
major failure and disassembling the cable into small elements.
[Drexler 92b] calculates that lightsails made of 20 nm aluminum in
tension should achieve an outward acceleration of ~14 km/s
per day at Earth orbit with no payload and minimal structural
overhead. For comparison, the delta V from low Earth to
geosynchronous orbit is 3.8 km/s. Lightsails generate thrust by
reflecting sunlight. Tension is achieved by rotating the sail. The
direction of thrust is normal to the sail and away from the Sun. By
directing thrust along or against the velocity vector, orbits can be
lowered or raised. This form of transportation requires no reaction
mass and generates thrust continuously, although the instantaneous
acceleration is small so sails cannot operate in an atmosphere and must
be large for even moderate payloads.
Today, the smallest feature size in production systems is about 250
nanometers -- the smallest feature size in computer chips. Since atoms
are an angstrom or so across and carbon nanotubes have a diameter as
small as 0.7 nanometers, atomically precise molecular machines can be
smaller than current MEMS devices by two to three orders of magnitude
in each dimension, or six to nine orders of magnitude smaller in volume
(and mass). For example, the size of the kinesin motor, which
transports material in cells, is 12 nm. [Han 97a] computationally
demonstrated that molecular gears fashioned from single-walled carbon
nanotubes with benzyne teeth should operate well at 50-100 gigahertz.
These gears are about two nanometers across. [Han 97c] computationally
demonstrated cooling the gears with an inert atmosphere. [Srivastava
97c] simulated powering the gears using alternating electric fields
generated by a single simulated laser. In this case, charges were
added to opposite sides of the tube to form a dipole. For an
examination of the state-of-the-art in small machines see the 1997 Conference on Biomolecular
Motors and Nanomachines.
To make active materials, a material might be filled with nano-scale
sensors, computers, and actuators so the material can probe its
environment, compute a response, and act. Although this document is
concerned with relatively simple artificial systems, living tissue may
be thought of as an active material. Living tissue is filled with
protein machines which gives living tissue properties (adaptability,
growth, self-repair, etc.) unimaginable in conventional materials.
Active materials can theoretically be made entirely of machines.
These are sometimes called swarms since they consist of large numbers
of identical simple machines that grasp and release each other and
exchange power and information to achieve complex goals. Swarms change
shape and exert force on their environment under software control.
Although some physical prototypes have been built, at least one patent
issued, and many simulations run, swarm potential capabilities are not
well analyzed or understood. We briefly discuss some concepts here.
For a summary of swarm concepts see [Toth-Fejel 96].
[Michael 94] proposes brick-shaped machines of various sizes
that slide past each other to assume a variety of shapes.
He has generated a large number of videos showing computer
simulations of simple motions. Although his web site
contains rather extravagant claims, this work has received a
U. K. patent.
[Yim 95] built a small swarm with macroscopic (size in inches)
components
called polypod, built a simulator of polypod, and
programmed it to move in various ways to study locomotion.
There are two brick shaped components in polypod,
one of which has two prismatic joints
linked by a revolute joint.
The second component is a cubic
connector with no mechanical motion.
Polypod is programmed by
tables for each member of the swarm. Each member is
programmed to move at various speeds in each
degree of freedom for certain amounts of time. The
swarm components are implicitly synchronized so there is
no clock signal.
[Hall 96] proposes a swarm with 10 micron dodecahedral components each
with 12 arms that can move in and out, rotate a little, and grab
and release each other. This concept is called the
"utility fog." [Hall 96] estimates that the utility fog
would have a density of 0.2, tensile
strength of 1000 psi in action and 100,000 psi in a passive mode, and
have a maximum shear rate of 100 km/second/meter.
[Bishop 95] proposes a swarm consisting of
100 nanometer brick-shaped components that slide past each
other to change shape.
[Globus 97] proposes a swarm with two kinds of components -- edges and
nodes. The terms "node" and "edge" are chosen to correspond to
those in graph theory.
The roughly spherical nodes are capable of attaching to five
edges (for a tetrahedral geometry with one free edge per node) and
rotating each edge in pitch and yaw. The rod-like edges are capable of
changing length, rotating around their long axis, and
attaching/detaching to/from nodes.
See figure.
Component design, power distribution and control software are
significant challenges for swarm development. Consider that with 10
micron components a cubic meter of swarm would contain about
1015 devices, each with an internal computer communicating
with its neighbors to accomplish a global task.
NASA's mission is divided into five enterprises:
Mission to Planet Earth, Aeronautics,
Human Exploration and Development of Space,
Space Science, and Space Technology. We will
examine some potential nanotechnology applications in each area.
The Earth Observing System (EOS) will use satellites and other
systems to gather data on the Earth's environment.
The EOS data system will need to process and archive >terabyte
per day for the indefinite future. Simply storing this quantity of data
is a significant challenge -- each day's data would fill
about 1,000 DVD disks. With projected write-once nanomemory densities
of 1015 bytes/cm2
[Bauschlicher 97a] a year's worth of EOS data can be stored on a small
piece of diamond. With projected nanocomputer processing speeds of
1018 MIPS [Drexler 92a], a million
calculations on each byte of one day's data would take one second on
the desktop.
Given a mature nanotechnology, it should be possible to build sensors
in balloon-borne systems approximately the size of bacteria. With
replication based manufacturing, these should be quite inexpensive. If
the serious communication and control problems can be solved, one can
imagine spreading billions of tiny lighter-than-air
vehicles into the atmosphere to measure wind currents and atmospheric
composition. A similar approach might be taken in the oceans -- note
that the oceans are full of floating microscopic living organisms that
can sense and react to their environment. Smart dust might sense the
environment, note the location via a GPS-like system, and store that
information until close enough to a data-collection point to transfer
the data to the outside world.
The strength of materials and computational capabilities previously
discussed for space transportation
should also allow much more advanced aircraft. Stronger,
lighter materials can obviously make aircraft with greater
lift and range. More powerful computers are invaluable in
the design stage and of great utility in advanced avionics.
MEMS technology has been used to replace traditional large control
structures on aircraft with large numbers of small MEMS controlled
surfaces. This
control system was used to operate a model airplane in a windtunnel.
Nanotechnology should allow even finer control -- finer control than
exhibited by birds, some of which can hover in a light breeze with very
little wing motion. Nanotechnology should also enable extremely small
aircraft.
A reasonably advanced nanotechnology should be able to make
simple atomically precise materials under software control.
If the control is at the atomic level, then the full range
of shapes possible with a given material should be achievable.
Aircraft construction requires complex shapes to accommodate
aerodynamic requirements. With molecular nanotechnology, strong
complex-shaped components might be manufactured by general
purpose machines under software control.
The aeronautics mission is responsible for launch vehicle development.
Payload handling is an important function. Very efficient payload
handling might be accomplished by a very advanced
swarm. The sequence begins by placing each
payload on a single large swarm located next to the shuttle orbiter. The
swarm forms itself around the payloads and then moves them
into the payload bay, arranging the payloads to optimize the
center of gravity and other considerations. The swarm holds
the payload in place during launch and may even damp out some
launch vibrations. On orbit, satellites can be launched
from the payload bay by having the swarm give them a gentle push.
The swarm can then be left in orbit, perhaps at a space
station, and used for orbital operations.
This scenario requires a very advanced swarm that can operate
in an atmosphere and on orbit in a vacuum. Besides the many and
obvious difficulties of developing a swarm for a single environment,
this provides additional challenges. Note that a simpler swarm might
be used for aircraft payload handling.
Aerospace vehicles often require complex checkout procedures to
insure safety and reliability. This is particularly true of
reusable launch vehicles. A very advanced swarm with some
special purpose appendages might be placed on a vehicle.
It might then spread out over the vehicle and into all
crevices to examine the state of the vehicle in great detail.
Nanotechnology-enabled Earth-to-orbit
transportation
has the greatest potential to revolutionize human access
to space by dropping the current $10,000 per pound cost
of launch, but this was discussed above. Other less dramatic
technologies include:
Space structures with a long design life (such as space station
modules) need high-reliability materials that do not degrade. Active materials might help. The machines monitor structural
integrity at the sub-micrometer scale. When a portion of the material
becomes defective, it could be disassembled and then correctly
reassembled. It should be noted that bone works somewhat along these
lines. It is constantly being removed and added by specialized cells.
To effect timely repairs, space stations require a large store of spare
parts and tools that are rarely used. A mature nanotechnology might
create a "matter compiler," a machine that converts raw materials into
a wide variety of products under software control. Contemporary
examples of very limited matter compilers are numerically controlled
machines and polypeptide sequencers. With a substantially more capable
nanotechnology-based matter
compiler, a space station crew could simply make spare parts and tools
as needed. The programs could be stored on-board or on the ground. New
tools invented on Earth could be transferred as software to the station
for manufacture. Once used, unneeded tools and broken parts could be
ionized in a solar furnace, transferred using controlled magnetic
fields, and the constituent atoms stored for later manufacture into new
products.
An advanced nanotechnology might be able to build filters that
dynamically modify themselves to attract the contaminant molecules
detected by the air and water quality sensors. Once attached to the
filter, the filter could in principle move the offending molecules to a
molecular laboratory for modifications to useful or at least inert
products. A swarm might implement such an active
filter if it was able to dynamically manufacture proteins that could
bind contaminant molecules. The protein and bound contaminant
might then be manipulated by the swarm for
transportation.
With a sufficiently advanced nanotechnology it might even
be possible to directly generate food by non-biological means.
Then agriculture waste in a self-sufficient space colony could
be converted directly to useful nutrition. Making this food
attractive will be a major challenge.
Sleeping crew members in the shuttle experience considerable pain and
sleep disruption when the reaction control system fires and they
collide with the cabin walls. If crew members were connected to the
walls by a swarm, the swarm could absorb most or
all of the force before the crew member struck the wall. The
swarm could then gradually return the crew member to center (without
the oscillations associated with bungee cords) in preparation for the
next firing.
For resupply, spacecraft docking is a frequent necessity
in space station operations.
When two spacecraft are within a few meters of each other, a
swarm could extend from each, meet in the middle, and
form a stable connection before gradually drawing the spacecraft
together.
A swarm could support space-suited astronauts in
simulated partial-g environments by holding them up appropriately. The
swarm moves in response to the astronaut's motion providing the
appropriate simulation of partial or 0 gravities. Tools and other
objects are also manipulated by the swarm to simulate
non-standard gravity.
Active nanotechnology materials (see
active materials) might enable
construction of a skin-tight space
suit covering the entire body except the head (which is in
a more conventional helmet). The material senses the astronaut's
motions and changes shape to accommodate it. This should
eliminate or substantially reduce the limitations current systems
place on astronaut range of motion.
In situ resource utilization is undoubtedly necessary for large scale
colonization of the solar system. Asteroids are particularly promising
for orbital use since many are in near Earth orbits. Moving asteroids
into low Earth orbit for utilization poses a safety problem should the
asteroid get out of control and enter the atmosphere. Very small
asteroids can cause significant destruction. The 1908 Tunguska
explosion, which [Chyba 93) calculated to be a 60 meter diameter stony
asteroid, leveled 2,200 km2 of forest. [Hills 93] calculated
that 4 meter diameter iron asteroids are near the threshold for ground
damage. Both these calculations assumed high collision speeds.
At a density of 7.7 g/cm3 [Babadzhanov 93], a 3
meter diameter asteroid should have a mass of about 110 tons.
[Rabinowitz 97] estimates that there are about one billion ten meter
diameter near Earth asteroids and there should be far more smaller
objects.
For colonization applications one would ideally provide the same
radiation protection available on Earth. Each square meter on Earth is
protected by about 10 tons of atmosphere. Therefore, structures
orbiting below the van Allen belts would like 10 tons/meter2
surface area shielding mass. This would dominate the mass requirements
of any system and require one small asteroid for each 11
meter2 of colony exterior surface area. A 10,000 person
cylindrical space colony such as Lewis One [Globus 91] with a diameter
of almost 500 meters and a length of nearly 2000 meters would require a
minimum of about 90,000 retrieval missions to provide the shielding
mass. The large number of missions required suggests that a fully
automated, replicating nanotechnology may be essential to build large
low Earth orbit colonies from small asteroids.
A nanotechnology swarm along with an atomically
precise lightsail is a promising small asteroid retrieval system.
Lightsail propulsion insures that no mass will be lost as reaction
mass. The swarm can control the lightsail by shifting mass. When a
target asteroid is found, the swarm spreads out over the surface to
form a bag. The interface to the sail must be active to account for
the rotation of the asteroid -- which is unlikely to have an
axis-of-rotation in the proper direction to apply thrust for the return
to Earth orbit. The active interface is simply swarm elements that
transfer between each other to allow the sail to stay in the proper
orientation. Of course, there are many other possibilities for
nanotechnology based retrieval vehicles.
Extraterrestrial materials brought into orbit could be fed into
a high-temperature solar furnace and partially ionized. Magnetic fields
might then be used to separate the nuclei. These are fed in appropriate
quantities to a matter-compiler to build the products desired.
Several authors, including [Freitas 98] have speculated
that a sufficiently advanced nanotechnology could examine and repair
cells at the molecular level. Should this capability become available
-- presumably driven by terrestrial applications -- the small size
and advanced capabilities of such systems could be of great utility
on long duration space flights and on self-sufficient colonies.
Self-replicating systems permit efforts of great scope to be pursued
economically. Adjusting the environment on another planet to suit the
tastes of humans is one such undertaking. Heating and cooling can be
achieved by (among many other methods) using space-based mirrors.
Chemical modifications of the planetary surface and atmosphere can be
achieved in relatively short periods by the use of self-replicating
systems that absorb sunlight and raw materials, and convert them into
the desired products. Much as plants changed the environment of the
earth to what we see today, so self-replicating molecular manufacturing
systems might more rapidly convert the environments of other planets.
As interstellar trips might last many years, the ability to conserve
supplies by maintaining some crew members in a suspended state would
be useful. An extremely advanced nanotechnology might use molecular
manipulations of each cell to provide (a) better methods
of slowing or suspending the metabolic activity of crew members and
(b) better methods of restoring metabolic activity to a normal state
when the destination is reached.
Molecular manufacturing should enable the creation of very precise
mirrors. Unlike lightsail applications, telescope mirrors require a
very precise and somewhat complicated shape. A swarm with special purpose appendages capable of
bonding to the mirror might be able to achieve and maintain the desired
shape.
A very advanced nanotechnology would be capable of imaging and then
removing the surface atoms of an extra-terrestrial sample. By removing
successive surface layers the location of each atom in the sample might
be recorded, destroying the sample in the process. This data could then
be sent to Earth. Besides requiring a very advanced
nanotechnology, there is a more fundamental -- but not necessarily
fatal -- problem: as the outside layer of atoms is removed the next
layer may rearrange itself so the sample is not necessarily perfectly
recorded.
As described earlier in the EOS section,
smart dust
could be distributed into the atmosphere of another planet
to characterize it in great detail.
Low Earth orbit spacecraft generally depend on solar cells and
batteries for
power. According to [Drexler 92b]:
Accordingly, solar collectors can consist of arrays of photovoltaic
cells several microns in thickness and diameter, each at the
focus of a mirror of ~100 micron diameter, the back surface of
which serves as a ~100 micron diameter radiator. If the mean
thickness of this system is ~1 micron, the mass is
~10-3 kg/m2 and the power per unit mass,
at Earth's distance from the Sun, where the solar constant is
~1.4 kW/m2, is > 105 W/kg."
By comparison, the U.S. built Photovoltaic Panel
Module solar cells currently used on the Mir Space Station and
planned for use on the International Space Station generate about 118
W/kg.
A critical component in hydrogen/oxygen fuel cells is the PEM (Proton
Exchange Membrane). This membrane must (a) permit the passage of
protons while (b) blocking everything else. Present membranes do a
rather poor job. One group at Ames is designing and computationally
testing PEMs to study possible energy mechanisms in early life. While
these studies are not meant to design optimal membranes for fuel cell
use, the basic knowledge and approach may be of value. Another proposal
is to design a diamond membrane a few nanometers thick with "proton
pores." The pores might be lined with fluorine, oxygen and nitrogen to
create a region with a high proton affinity. In addition, a
positionally controlled platinum might be held at the mouth of the pore
to verify that H2 can be catalytically split into H+ and e-, and that
the barrier for migration of the H+ into the pore is modest in size.
Nanotechnology must provide precise control over the manufacturing process
of the diamondoid PEM since the pores must be made very precisely.
Studies of H2 absorption and packing in carbon nanotubes and
nanoropes are in progress at NASA Ames and elsewhere. Nanotubes
provide large pore sizes and nanoropes have different pore sizes
depending on interstitial and other locations. [Dillon 97] estimated
that the single walled nanotubes in their sample contained 5 to 10% by
weight of H2. The nanotubes were about 0.1 to 0.2% by weight
of the total sample. Computational studies at Ames suggest that to
store 7-10% H2 in single walled nanotubes at room
temperature the H2s must be stored inside the tubes, not
merely adsorbed on the walls [Srivastava 97d]. This work suggests that
carbon nanotubes might be developed into an excellent H2
storage medium within 3-5 years.
Calculations with oxygen [Merkle 94] suggest that a diamondoid
sphere ~0.1 microns in diameter should easily hold oxygen at ~1,000
atmospheres. While higher pressures are feasible, they offer declining
returns. At higher pressures, the pressure-volume relationship becomes
severely non-linear and the density approaches a limiting value.
Other gases might also be stored if diamondoid spheres can be built, but
the analysis has not been done.
High strength light-weight materials will allow greater efficiency of
energy storage as angular momentum.
Many kinds of ultraminiature electromechanical devices have utility on
a miniaturized space craft. It has been shown that manipulating carbon
nanotubes changes their electrical properties [Srivastava 97b]. This
might be exploited to build nanometer scale strain devices. This may
be achievable within 3-5 years, and simulations along these lines are
in progress.
Similar results have been achieved experimentally with
C60 [Joachim 97]. The electrical properties of a
C60 molecule were changed by applying pressure to the
molecule with an SPM tip.
Smaller, lighter spacecraft are cheaper to launch (current costs
are about $10,000/lb) and generally cheaper to build. Diamondoid
structural materials can radically reduce structural mass,
miniaturized electronics can shrink the avionics and reduce
power consumption, and atomically precise materials and components
should shrink most other subsystems.
Thermal protection is crucial for atmospheric reentry and other tasks.
The carbon nanotubes under investigation at NASA Ames and elsewhere may
play a significant role. Most production processes for carbon
nanotubes create a tangled mat of
nanotubes that has a very low mass-to-volume ratio. Like graphite,
the tubes should withstand high temperatures but the tangled mat should
prevent them from ablating. This may lead to high temperature
applications.
Many of the applications discussed here are speculative to say the
least. However, they do not appear to violate the laws of physics.
Something similar to these applications at these performance levels
should be feasible if we can gain complete control of the
three-dimensional structure of materials, processes and devices at the
atomic scale.
How to gain such control is a major, unresolved issue. However, it
is clear that computation will play a major role regardless of which
approach -- positional control with replication, self-assembly, or
some other means -- is ultimately successful. Computation has already
played a major role in many advances in chemistry, SPM manipulation,
and biochemistry. As we design and fabricate more complex atomically
precise structures, modeling and computer aided design will inevitably
play a critical role. Not only is computation critical to all
paths to nanotechnology, but for the most part the same or similar
computational chemistry software and expertise supports all roads
to molecular nanotechnology.
Thus, even if NASA's computational molecular nanotechnology efforts
should pursue an unproductive path, the expertise and capabilities
can be quickly refocused on more promising avenues as they become
apparent.
As nanotechnology progresses we may expect applications to become
feasible at a slowly increasing rate. However, if and when a general
purpose programmable assembler/replicator can be built and operated, we
may expect an explosion of applications. From this point, building new
devices will become a matter of developing the software to instruct the
assembler/replicators. Development of a practical swarm is another
potential turning point. Once an operational swarm that can grow
and divide has been built, a large number of applications become
software projects. It is also important to note that the software
for swarms and assembler/replicators can be developed using
simulators -- even before operational devices are available.
Nanotechnology advocates and detractors are often preoccupied with
the question "When?" There are three interrelated answers to this
question (see also [Merkle 97] and [Drexler 91]):
We would like to thank Steve Zornetzer, NASA Ames Research Center, for
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Computer Technology
Carbon Nanotube SPM Tips
Data Storage on Molecular Tape
It is possible to store data on long chain molecules (for example, DNA)
and it may be possible to read these data with carbon nanotube tipped
SPMs. Existing DNA synthesis techniques can be used to write data. If
the different DNA base pairs can be distinguished with a carbon
nanotube tipped SPM, then the data can be read non-destructively
(current techniques allow a destructive read). However, the difference
between base pairs is not great. If the base pairs cannot be
distinguished, techniques for attaching modified enzymes to specific
base pair sequences [Smith 97] could be used. Certain enzymes (DNA
(cytosine-5) methyltransferases) attach themselves onto a specific
sequence of base pairs with a covalent bond. The enzyme then performs
its operation and breaks the bond. [Smith 97] modified the enzyme such
that the initial covalent bond was formed but the subsequent operation
was disrupted. The result is that DNA synthesized with the target base
pair sequences at the desired location can force precise placement of
the enzymes. The presence of an enzyme could represent 1 and its
absence 0. Enzymes are sufficiently large that distinguishing their
presence should be straightforward. If the DNA/enzyme approach proves
impossible, a wide variety of other polymer systems could be examined.
Data Storage on Diamond
Carbon Nanotube Electronic Components
Molecular Electronic Components
Helical Logic
Helical logic is a theoretical proposal for a future
computing technology using the presence or absence of individual
electrons (or holes) to encode 1s and 0s. The electrons are constrained
to move along helical paths, driven by a rotating electric field in
which the entire circuit is immersed. The electric field remains
roughly orthogonal to the major axis of the helix and confines each
charge carrier to a fraction of a turn of a single helical loop, moving
it like water in an Archimedean screw. Each loop could in principle
hold an independent carrier, permitting high information density. One
computationally universal logic operation involves two helices, one of
which splits into two "descendant" helices. At the point of divergence,
differences in the electrostatic potential resulting from the presence
or absence of a carrier in the adjacent helix controls the direction
taken by a carrier in the splitting helix. The reverse of this sequence
can be used to merge two initially distinct helical paths into a single
outgoing helical path without forcing a dissipative transition. Because
these operations are both logically and thermodynamically reversible,
energy dissipation can be reduced to extremely low levels. ... It is important
to note that this proposal permits a single electron to switch another
single electron, and does not require that many electrons be used to
switch one electron. The energy dissipated per logic operation can
likely be reduced to less than 10-27 joules at a temperature of 1
Kelvin and a speed of 10 gigahertz, though further analysis is required
to confirm this. Irreversible operations, when required, can be easily
implemented and should have a dissipation approaching the fundamental
limit of ln 2 x kT.
Rod Logic
Aerospace Transportation
Launch Vehicles
Space Elevator
Interplanetary transportation
Active Materials
Swarms
NASA Missions
Mission to Planet Earth
EOS Data System
Smart Dust
Aeronautics and Space Transportation Technology
Active surfaces for aeronautic control
Complex Shapes
Payload Handling
Vehicle Checkout
Human Exploration and Development of Space
High Strength and Reliability Materials
On Demand Spares and Tools
Waste Recycling
Sleeping through RCS firings
Spacecraft Docking
Zero and Partial G Astronaut Training
Smart Space Suits
Small Asteroid Retrieval
Extraterrestrial Materials Utilization
Medical Applications
Terraforming
Suspended Animation
Space Science
Space Telescopes
Virtual Sample Return
Meteorological Data
Space Technology
Solar Power
For energy collection, molecular manufacturing can be
used to make solar photovoltaic cells at least as efficient as those
made in the laboratory today. Efficiencies can therefore be > 30%.
In space applications, a reflective optical concentrator need consist
of little more than a curved aluminum shell < 100 nanometers thick
(photovoltaic cells operate with higher efficiency at high optical
power densities). A metal fin with a thickness of 100 nanometers
and a conduction path length of 100 microns can radiate thermal
energy at a power density as high as 1000 W/m2
with a temperature differential from base to tip of < 1 K.
Power Storage
Fuel Cells
Hydrogen Storage
Oxygen Storage
Fly Wheels
Nano Electromechanical Sensors
Miniature Spacecraft
Thermal Protection
Conclusion
Acknowledgments
References
Web work: Al Globus,
Responsible Official:
Creon Levit.