NASA Applications of Molecular
Nanotechnology
Published in The Journal of the British Interplanetary
Society, volume 51, pp. 145-152, 1998.
Abstract
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.
Introduction
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.
Technology
Computer Technology
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 Nanotube SPM Tips
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.
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
[Bauschlicher 97a] computationally studied storing data
in a pattern of fluorine and hydrogen atoms on the (111) diamond
surface (see figure). If write-once data could be stored this way,
1015 bytes/cm2 is theoretically possible. By comparison, the new DVD
write-once disks now coming on the market hold about 108 bytes/cm2.
[Bauschlicher 97a] compared the interaction of different probe
molecules with a one dimensional model of the diamond surface. This
study found some molecules whose interaction energies with H and F are
sufficiently
different that the force differential should be detectable by an SPM.
These studies were extended to include a two dimensional model of the
diamond surface and two other systems besides F/H [Bauschlicher 97b].
Other surfaces, such as Si, and other probes, such as those including
transition metal atoms, have also been investigated [Bauschlicher
97c].
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.
Carbon Nanotube Electronic Components
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].
Molecular Electronic Components
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.
Helical Logic
From [Merkle 96]:
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
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.
Aerospace Transportation
Launch Vehicles
[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
[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].
Space Elevator
[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.
Interplanetary transportation
[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.
Active Materials
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.
Swarms
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 Missions
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.
Mission to Planet Earth
EOS Data System
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.
Smart Dust
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.
Aeronautics and Space Transportation Technology
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.
Active surfaces for aeronautic control
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.
Complex Shapes
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.
Payload Handling
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.
Vehicle Checkout
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.
Human Exploration and Development of Space
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:
High Strength and Reliability Materials
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.
On Demand Spares and Tools
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.
Waste Recycling
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 through RCS firings
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.
Spacecraft Docking
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.
Zero and Partial G Astronaut Training
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.
Smart Space Suits
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.
Small Asteroid Retrieval
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 Utilization
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.
Medical Applications
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.
Terraforming
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.
Suspended Animation
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.
Space Science
Space Telescopes
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.
Virtual Sample Return
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.
Meteorological Data
As described earlier in the EOS section, smart dust
could be distributed into the atmosphere of another planet to
characterize it in great detail.
Space Technology
Solar Power
Low Earth orbit spacecraft generally depend on solar
cells and batteries for power. According to [Drexler 92b]:
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
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.
Power Storage
Fuel Cells
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.
Hydrogen Storage
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.
Oxygen Storage
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.
Fly Wheels
High strength light-weight materials will allow greater
efficiency of energy storage as angular momentum.
Nano Electromechanical Sensors
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.
Miniature Spacecraft
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
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.
Conclusion
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]):
1.Nobody knows. There are far too many variables and
unknowns. Beware of those who have excessive confidence in any date.
2.The time-to-nanotechnology will be measured in
decades, not years. While a few applications will become feasible in
the next few years, programmable assembler/replicators and swarms will
be extremely difficult to develop.
3.The time-to-nanotechnology is very sensitive to the
level of effort expended. Resources allocated to developing
nanotechnology are likely to be richly rewarded, particularly in the
long term.
Acknowledgments
We would like to thank Steve Zornetzer, NASA Ames
Research Center, for asking us to look into molecular nanotechnology
applications to NASA missions. Special thanks to Glenn Deardorff and
Chris Henze for reviewing the manuscript.
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