Molecular Nanotechnology in Aerospace:

1999
Al Globus, Veridian MRJ Technology Solutions, Inc.

Abstract
Recent progress towards molecular nanotechnology and potential aerospace
applications is reviewed. Great strides have been made in understanding, visualizing,
and controlling matter at the atomic scale. In particular, substantial progress has been
made towards the construction of molecular computers. Some progress has been made
towards understanding biological molecular machines and manipulating these
machines for technological purposes. Also, several polymeric molecules, notably
proteins, DNA, and RNA, can be automatically synthesized from precise
specifications. This example of "programmable matter" has been used to produce at
least one molecular mechanical device. However, integration of molecular
components into larger atomically precise systems has made little progress. Scaling up
molecular nanotechnology to produce macroscopic products of aerospace interest, for
example, launch vehicles, will require large research and development investments. In
particular, self-replication, proposed as a route to macroscopic molecular
nanotechnology products, is a long way from fruition. This paper is a high-level
discussion of molecular nanotechnology and some aerospace applications.
Applications of importance to aerospace include computers, materials, and sensors.
Research reviewed in [Globus 1998a] and [Globus 1998b], for the most part, is not
revisited here. Also, this review is not exhaustive and much important and relevant
work is not discussed.

Introduction
Molecular nanotechnology, for the purposes of this paper, is the thorough threedimensional 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." Atomically precise control of matter is progressing rapidly
in the laboratory today. A particularly dramatic example was the use of a scanning
tunneling microscope to write the characters "IBM" by manipulating xenon atoms on

a copper surface [Eigler 1990]. Controlling the fantastic complexity of atomic scale
matter will almost certainly require "programmable matter," atomic scale products
that are created and/or controlled by computer programs. Current examples include
protein, RNA, and DNA synthesis from an exact specification of the sequence.
Beyond today's state-of-the-art lie molecular machines, although a few biological
molecular machines have been studied, synthesized, and used in laboratory settings.
These technologies should suffice for the production of microscopic products. To
produce macroscopic objects of aerospace interest will require some mechanism to
scale products up in size. Biological systems use reproduction to produce large
objects, such as whales and redwood trees, starting with single cells or small seeds.
The construction of self-replicating programmable machines, while extraordinarily
difficult and dangerous, should enable dramatic improvements in aerospace systems
[Globus 1998a].
Any molecular nanotechnology must be based on chemistry, and the field has taken a
number of directions. Organic chemists have produced a wide variety of small
structures, including testable two junction computer devices [Reed 1998][Rawlett
1999]. Biotechnology has been used to create a wide variety of systems, including 2D
crystal patterns of DNA [Winfree 1998], modified copies of biological molecular
motors [Montemagno 1999] , and covalently bonded molecular tubes with precise
radius [Ghadiri 1993]. Fullerene nanotechnology development has produced
transistors [Tans 1998][Martel 1998] and diodes [Collins 1997]. A wide variety of
theoretical studies have examined the properties of many other potential devices,
including fullerene gears [Han 1997][Srivastava 1997], bearings [Tuzun 1995a]
[Tuzun 1995a], and three junction electrical devices [Menon 1997].
Progress in molecular nanotechnology can be reasonably expected to enable radical
improvement in a wide variety of aerospace systems and applications. Computer
technology will probably be the first to feel the molecular nanotechnology revolution,
with substantial advantages to the aerospace industry. Theoretical and numerical
studies suggest that 1018 MIPS computers [Drexler 1992a] and 1015 bytes/cm2 write
once memory [Bauschlicher 1997] are possible. It may also be possible to build safe,
affordable vertical take-off and landing aircraft to replace personal automobiles [Hall
1999] and eliminate the need for most roads. From [Srivastava 1999b]:
The development of nanotechnology is important for the exploration and future
settlement of space. Current manufacturing technologies limit the reliability,
performance, and affordability of aerospace materials, systems, and avionics.
Nanotechnology has enormous potential to improve the reliability and
performance of aerospace hardware while lowering manufacturing cost. For
example, nanostructured materials that are perhaps 100 times lighter than
conventional materials of equivalent strength are possible. Embedding

nanoscale electromechanical system components into earth-orbiting satellites,

planetary probes, and piloted vehicles potentially could reduce the cost of
future space programs. The miniaturized sensing and robotic systems would
enhance exploration capabilities at significantly reduced cost. Thousands to
millions of such miniaturized devices could help map a planet in a single
launch.
Launch costs might be reduced significantly using molecular nanotechnology. In the
extreme case, [Drexler 1992b] estimated that a four passenger single-stage-to-orbit
launch vehicle weighing only three tons (including fuel) could be built using a mature
diamondoid nanotechnology. More conservatively, [McKendree 1995] estimated
$153-412 per kilogram launched to low-Earth-orbit assuming existing single-stage-toorbit vehicle designs but using diamondoid rather than conventional materials .
Current launch costs are many thousands of dollars per kilogram.
The paper is divided into sections reviewing molecular nanotechnology itself (atomic
scale control and imaging, programmable matter, molecular machines, and
replication), some of the chemistry behind molecular nanotechnology (organic
chemistry, biotechnology, and fullerene nanotechnology), and some of the major
challenges and opportunities ahead.

Molecular Nanotechnology
Manipulation and Visualization of Matter at the Atomic Scale
Laboratories throughout the world are rapidly gaining atomically precise control over,
and views of, matter at the atomic scale. In particular, scanning probe microscopes
(SPM) can image surfaces with sub-atomic precision and manipulate individual atoms
[Eigler 1990] and molecules [Gimzewski 1997] on surfaces. Manipulation can be
accomplished electronically, mechanically, and/or with chemically active tips. An
SPM uses the interaction of a microscopic probe with the surface of a sample to
measure characteristics of the sample at localized points. The probe is typically a
sharp silicon tip, but can also be other materials, including single walled carbon
nanotubes [Dai 1996]. By scanning the sample with a probe in a two-dimensional
pattern (like an electron beam scans a television screen), an image can be produced.
The motion of the sample is usually controlled by piezoelectric materials, sometimes
to sub-atomic accuracy. By measuring the deflection of the cantilever, often with a
laser, the interaction is quantified. A feedback loop between controller and the
deflection measurement system provides extreme accuracy.

Atomically precise image of a carbon nanotube. Note the helical winding. For small diameter
tubes, the helical winding determines the electrical characteristics of the tube. Image due to
[Dekker 1999]. Used with permission.

Scanning tunneling microscopy (STM) involves the tunneling of electrons through

vacuum from the tip of the STM to the sample. STM is very accurate but can only
interact with conductors. STMs can also be used to manipulate molecules. For
example:

These carbon nanotubes were cut by applying a voltage pulse to an STM tip. The images
show a nanotube before and after cutting. Images due to [Dekker 1999]. Used with
permission.

Atomic force microscopes (AFM) usually sense Van der Walls forces from a surface.
This allows measurement of nonconductive surfaces as well as operation in air and
liquid. If a chemically active molecule is placed on the tip, then an AFM can be used
to measure chemical forces between the tip and a surface. [Frisbie 1994] introduced
the term "chemical force microscopy" when they coated an AFM tip with a
hydrophilic monolayer and imaged a surface patterned with hydrophobic and

hydrophilic molecules. While the surface appeared smooth to an unmodified AFM tip,
[Frisbie 1994] was able to measure differences in frictional forces between
hydrophobic and hydrophilic portions of the surface, achieving an estimated
resolution of about 200 nm. To achieve higher resolution, and measure the interaction
of individual molecules with a surface, a sharper tip is necessary. [Dai 1996] was able
to attach carbon nanotubes to SPM tips to achieve atomic precision. [Wong 1998]
subsequently used open ended carbon nanotubes, covalently functionalized with
several different molecules, to image a chemically varied surface achieving a lateral
chemical resolution of approximately 3 nm, "... significantly better than ... obtained
with the use of Si and Si3N4 (15 nm) or multi-walled-carbon-nanotube tips (8 nm)"
[Wong 1998]. Scanning the sample with different functional groups on the tip resulted
in distinctly different images and the differences could be explained on the basis of
chemical affinity between the tip and the surface. Since both closed and open carbon
nanotubes may be functionalized in many ways, Modified nanotube probes may
someday perform extremely well-controlled chemistry at precise locations and use
applied forces to overcome reaction barriers.

A model of a carbon nanotube used as a probe with biotin

binding to streptavidin. Image from Dr. Lieber's
websitehttp://magic.harvard.edu/research.html. Used with
permission.

Carbon nanotubes (see the section on Fullerene Nanotechnology below) have been
manipulated in several recent studies. [Falvo 1999] was able to demonstrate rolling
and sliding of carbon nanotubes pushed by an AFM on mica and graphite surfaces.
Stick-slip behavior was observed in the force curves for rolling. [Skidmore 1999] was
able to build and observe a variety of carbon nanotube structures by placing multiple
SPM tips around a sample within view of scanning and transmission electron
microscopes. Electron beam deposition was used to build up structures in localized

positions from a gas feedstock. The SPM tips could manipulate the carbon nanotubes
and cut them. Three-dimensional structures were built and carbon nanotubes were
weaved around posts. [Skidmore 1999] is probably the most sophisticated
manipulation of carbon nanotubes to date. The sample and apparatus used for
manipulation can be moved from microscope to microscope to take advantage of the
properties of particular devices.

Two artificial nanotube arrangements built by Zyvex, Inc. [Skidmore 1999]. The image on
the left shows a nanotube scaffolding. Numbers show the ordering of tube attachment. The
image on the right shows nanotube weaving. Images used with permission.

Mechanical manipulation of carbon nanotubes may also lead to a new form of

chemistry. [Falvo 1997] was able to repeatedly bend individual multi-walled carbon
nanotubes using a interactively-controlled AFM. This manipulation demonstrated the
high strength of carbon nanotubes and the formation of kinks when nanotubes are
bent. [Srivastava 1999a] computationally predicted that mechanically induced strain
leads to regions of enhanced chemical reactivity as carbon atoms change from a stable
sp2 (planar) configuration with three neighbors towards a less stable sp3 (pyramidal)
configuration with three neighbors and one radical site in regions of greatest strain.
[Srivastava 1999a] presents some experimental data that support, but do not confirm,
this prediction. If the prediction holds true, mechanical manipulation might be used to
induce strain at desired locations along a carbon nanotube leading to selective
attachment of functional groups from a reagent bath at those locations.
Programmable Matter
Programmable matter refers to the fact that machines exist which, when given the
proper instructions and feedstock, will produce physical objects to the given
specification. Examples include numerically controlled machines, "fabbers" (available
for your PC) which take object descriptions from CAD programs and produce plastic

objects of the correct size and shape, and, most important for our purposes,
polypeptide, DNA, and RNA sequencers. These sequencers take a specification of the
desired sequence of amino acids or nucleotides and produce a sample containing
nearly 100 percent of the desired molecules. The cost as of September 1999 is about
$1-10 per base (amino acid or nucleotide) in the sequence (source: the back page
of Science, any issue in the last few years).
In a spectacular example of the power of programmable matter, [Schwarze 1999] use
a portion of the HIV viral protein to insert a wide variety of proteins into mammalian
cells and live mice. Typically, only small therapeutic molecules can enter cells.
[Schwarze 1999] attached an 11 amino acid protein transaction domain from the HIV
virus to a variety of proteins. The protein transaction domain apparently enters cells
directly through the lipid bilayer component of the cell membrane, not through special
pathways. The desired proteins were denatured (unfolded), attached to the transaction
domain, and then passed into the cell where the cell's protein folding machinery
folded the protein into a potentially active form. Since essentially any polypeptide can
be made by sending the sequence specification and a credit card number to any of
several companies, it may be possible to engineer proteins to attack specific points in
the molecular life-cycle of disease organisms and deliver these proteins into infected
cells. With a polypeptide sequencer and related biotech systems on-board a space
station, when disease strikes the necessary medicine for that particular pathogen could
be manufactured on-board from instructions sent up from the ground. This would
reduce the need for large stocks of medicine for every possible contingency. The same
protein sequencer could also be used for research purposes.
DNA has been used to build 3d topological shapes [Chen 1991][Zhang 1994], stiff
structures [Li 1996], crystals [Winfree 1998], and even a molecular machine [Mao
1999a]. This was accomplished by taking advantage of DNA's hydrogen-bonded
complementarity and biotechnology's ability to produce almost any DNA sequence
desired. By cleverly choosing the sequence of base pairs, Seaman's laboratory has
produced a remarkable variety of structures, for example:

DNA cube made from six different cyclic strands. The DNA backbones are shown in
different colors. Each nucleotide is represented by a single colored dot for the backbone and
a single white dot for the base. To get a feeling for the molecule, follow the red strand
around its cycle. Each edge of the cube is a piece of double helical DNA containing two
turns of the double helix. Image due to Ned Seeman [Chen 1991]
and http://seemanlab4.chem.nyu.edu/nano-cube.html. Used with permission.

The image notwithstanding, these cubic molecules are not stiff. In other words, the
complexes are topological cubes but not geometric cubes. To create stiff molecules, a
more complex scheme is necessary. One approach is to use double crossover
DNA strands [Li 1996]. Double crossover DNA complexes are multiple strands of
DNA that cross over each other in a variety of patterns and some patterns form stiff
structures. These strands can be formed into crystals by taking advantage of the single
DNA strands on the edge of the double crossover molecule (the "sticky ends")
[Winfree 1998]. For example:

Two DNA double crossover molecules A and B* use complementarity between their sticky
ends (represented as geometric complementarity) to form a two dimensional crystal. The B*
molecules contain DNA hairpins that project out of plane to allow AFM discrimination. The
molecules are approximately 4 nm wide, 16 nm long and 2 nm thick. When these tiles are
mixed in solution, they form hydrgen bonded 2-D arrays several microns long and hundreds
of nanometers wide. The rows of hairpins appear as stripes separated by ~32 nm when
imaged by AFM (below) [Mao 1999b]. Images due to Ned Seeman
from http://seemanlab4.chem.nyu.edu/two.d.html. Used with permission.

Molecular Machines
Much of the promise of nanotechnology for aerospace applications comes from the
theoretical abilities of atomically-precise molecular machines [Drexler 1992a]. While
SPMs provide positional control at the atomic scale, they are too bulky to build
macroscopic products atom by atom because the parallelism is limited by the size of
the machines. However, very large numbers of molecular machines can fit in a small
space and, properly organized, could provide the parallelism necessary to build
macroscopic products by positioning individual atoms. To date, only one such
machine has been built, although several computational studies have been undertaken
[Tuzun 1995a][Tuzun 1995b][Han 1997] [Srivastava 1997][Cagin 1998]. [Gimzewski
1998] observed a molecular rotor operating on an atomically precise copper surface in

ultrahigh vacuum. The molecule rotated due to thermal energy when separated from a
bearing formed by a hexagonal lattice of the same molecules. Rotation stopped when
the molecule moved into contact with the hexagonal lattice. However, living cells
abound in much more sophisticated molecular machinery built primarily from
proteins. Some of these machines have been isolated, modified and studied.
[Montemagno 1999], building on the work of [Noji 1997], is attempting to integrate
the biological motor F1-ATPase with nano-electro-mechanical systems to create a new
class of hybrid nanomechanical devices. ATPase is used by mitochondria to synthesize
ATP from ADP, phosphate, and proton gradients. ATP is the primary energy source of
our bodies. The F1 portion of ATPase has a sub-unit that turns during synthesis. This
rotation can be reversed by separating the F1 sub-unit from the rest of the protein and
feeding the sub-unit ATP. F1-ATPase can generate >100 pN, has a measured rotational
velocity of 3 r.p.s. under load, and a diameter of less than 12 nm. These characteristics
suggest that F1-ATPase could manipulate currently manufacturable nanomechanical
structures. Since the human body produces ample quantities of ATP, an implantable
sensor in an astronaut's body operated by F1-ATPase would require no other power
source. Such sensors could provide medically important data on an astronaut's health
indefinitely.

F1-ATPase [Abrahams 1994]. The orange sub-unit rotates. The six sub-units in shades of
yellow, green and blue rock back and forth sequentially as ATP is hydrolyzed [Elston 1998].

Another biological molecular motor, kinesin, was deposited on polymer films in order
to guide the motion of microtubules on surfaces [Dennis 1999]. Kinesin has also been
used to manipulate 10 x 10 x 5 m silicon microchips. These microchips were
translated, rotated, and in a few instances turned over by large numbers of kinesin
motors [Limberis 1999]. Normally, kinesin motors operate on microtubule "tracks"
inside cells. From [Limberis 1999]:
The microtubule tracks are hollow tubes, 24 nm in diameter, formed by the
self-assembly of tubulin protein subunits. The engines of this transport system,
kinesins, are remarkable molecular machines. Force production is coupled to
hydrolysis of ATP, a high-energy biomolecule. For each ATP it hydrolyzes,
kinesin steps 8 nm [Svoboda 1993] on the microtubule surface and can generate
forces up to 6 pN ... [Hunt 1994]. With a cross-sectional area on the order of 10
nm2 [Kull 1996], kinesin can be surface immobilized with a packing density
approaching 105 motors per m2. With each motor generating forces as high as

6 pN, cumulative forces on the order of 10s of nN per m 2 are theoretically

possible.
In an attempt to understand the conditions under which kinesin fixed to a surface
could move microtubules, [Unger 1999] demonstrated that kinesin-driven
microtubules can work continuously up to several hours. They can operate on uneven
surfaces with height differences up to 280 nm and in chambers as small as about 100
nm. The height difference was determined by using polished silicon wafers into which
steps were etched. The wafers were covered with kinesin. When microtubules were
added to a solution on the slide, they were observed (with an optical microscope)
gliding across the wafer surface. To determine chamber size, microtubules gliding
between two glass slides were investigated. Microtubules were transferred to a
kinesin-coated glass slide. To produce variable heights, a slightly curved coverslip
was used. An interferometer was used to measure the distance between slides.
[Mao 1999a] created an artificial two-state molecular machine from DNA. [Mao
1999b] connected two stiff double-crossover DNA molecules [Li 1996] with a strand
of DNA that could assume either a right-handed helix (B-DNA) or a left-handed helix
(Z-DNA) conformation depending on certain characteristics of the solution the
molecules were in. By changing the solution and forcing the connecting DNA strand
to wind in the opposite direction, the stiff portions were forced to move. This
movement was recorded by placing dye molecules on each stiff segment. "The
switching event induces atomic displacements of 20-60 angstroms" [Li 1996]. This
experiment was controlled by running the same test on similar DNA molecules where
the connecting segment was not capable (due to the sequence used) of assuming the
Z-DNA conformation. These molecules did not change conformation when the
solution changed.
A very interesting non-biological partly-molecular machine has been fabricated by
Philip Kim and Charles M. Lieber [Kim 1999]. They attached a nanotube bundle to
each of two independent electrodes deposited on a pulled glass micropipette. This
enables independent control of charge on each nanotube. When the nanotubes are
oppositely charged they can be induced to close like tweezers - nanotweezers thereby enabling nanoscale objects to be held and manipulated in three dimensions.
The nanotube nanotweezers have been used to manipulate and measure the electrical
properties of nanoclusters and nanowires.
Replication
While molecular nanotechnology is making great strides with respect to very small
products, many aerospace applications, particularly placing and maintaining humans
in orbit, require macroscopic systems. This requires integration of vast numbers of
microscopic elements. Biological systems scale up from very small beginnings by cell

replication. Thus, some nanotechnology pioneers have proposed building

programmable molecular machines capable of self-replication [Drexler 1992a]. These
hypothetical machines are often called assemblers. Theory suggests that selfreplicating assemblers could lead to fantastic productivity because of their exponential
growth potential. Although molecular-scale assemblers have garnered substantial
attention, the most detailed study of artificial self-replication was a 1980 NASA
summer study that assumed macroscopic machines [Freitas 1980].
[Freitas 1980] studied a hypothetical 100-ton self-replicating lunar factory with access
only to local resources and established materials processing techniques. This study
assumed macroscopic machines, not nanotechnology. While a number of selfreplication strategies are possible, perhaps the most practical is for a computer to
interpret a set of instructions and control a robot to make a copy of itself, a technique
originally proposed by von Neuman. This has the added virtue that the instructions
can be changed so the replicator can make something else. If the computers
themselves are provided using mass production technologies instead of replication,
then the replication can be limited and controlled to avoid the most serious runaway
scenarios.
From [Freitas 1980]:
The Replicating Systems Concepts Team reached the following conclusions
concerning the theory of machine reproduction:
o John von Neumann and a large number of other researchers in theoretical
computer science following him have shown that there are numerous
alternative strategies by which a machine system can duplicate itself.
o There is a large repertoire of theoretical computer science results
showing how machine systems may simulate machine systems
(including themselves), construct machine systems (including machine
systems similar to or identical with themselves), inspect machine
systems (including themselves), and repair machine systems (including,
to some extent, themselves). This repertoire of possible capabilities may
be useful in the design and construction of replicating machines or
factories in space.
It is interesting to note that computer programs capable of self-replication have
been written in many different programming languages [Burger 1980][Hay
1980], and that simple physical machines able to replicate themselves in highly

specialized environments have already been designed and constructed

[Jacobson 1958][Morowitz 1959][Penrose 1959].
[Hall 1998] developed a simple analytical model to describe the performance of selfreplicating machines. This model describes a system "... composed of a population of
replicating machines. Each machine consists of control and one or more operating
units capable of doing primitive assembly operations (e.g. mechanochemical
deposition reactions). Let us define the following:
p(t) -- population at time t
g -- generation time (seconds to replicate)
a -- "alacrity" primitive assembly operations per second
s -- size in primitive operations to construct
n -- number of primitive operating units

Then
and
." [Hall 1998] then investigated the implications of such a
model, particularly with regards to bootstrapping ever more capable replicators. From
this analysis, he derived a potential system architecture detailed in the following
figure:

"System diagram for the bootstrap path of a self-replicating manufacturing system. Each
subsystem in a blue line (as well as the entire system) meets the self-replicating system
criterion." The turquoise ellipses represent components. The black arrows represent material
or information flows. The red arrows represent fabrication. By Hall's criteria, a system is
self-replicating if every turquoise ellipse has an incoming red arrow and all red arrows
originate from a turquoise ellipse. Image from [Hall 1998]. Used with permission.

[Hall 1998] also proposed using the term replication to describe machines producing
exact copies of themselves and distinguishing this from biological reproduction which
implies evolution of species. These conventions are used in this paper, although not in
all of the quotes.
[Freitas 1980] suggested several applications for space based self-replicating
manufacturing systems (SRS). Such systems might use solar energy and lunar or

asteroidal materials. Asteroidal materials might be delivered by large numbers of fully

automated solar sail powered spacecraft that capture and return small (~1 m diameter)
meteoroids [Globus 1999]. From [Freitas 1980] :
Manufacturing. Huge solar power satellites with dimensions 1-10 km on a side
could be constructed in Earth orbit by a fleet of free-flying assembly robots or
teleoperators manufactured by a replicating factory complex using material
from the Moon. Components for very large structures, including
communications, storage, recreational, penal, or even military platforms could
be fabricated, and later assembled, by an SRS. Another exciting massproduction possibility is the notion of orbital habitats, or "space colonies"
[O'Neill 1977][O'Neill 1979], by which increasingly large populations of
human beings could be safely and comfortably maintained .... Additionally, a
replicating factory could build more copies of itself, or new variants of itself
capable of manifesting different behaviors and producing different outputs, in
almost any desired location...
Observation. Exceedingly large sensor arrays for Earth or astronomical
observations could be rapidly constructed from nonterrestrial materials by a
self-replicating manufacturing facility. This technology could be used to make
feasible such advanced missions as optical extrasolar planet imaging (using
millions of stationkeeping mirror assemblies arranged in an array with an
aperture diameter on the order of kilometers); complex multisensor arrays; very
large, high-resolution x-ray telescopy; and other self-organizing optical or radio
telescopic arrays of grand proportions to permit such ambitious undertakings as
galactic core mapping, continuous observation of large numbers of passive
fiducial markers for Earth crustal plate motion monitoring, and various SETI
(Search for Extraterrestrial Intelligence) observations including beacon
acquisition, radio "eavesdropping," or, ultimately, active communication.
Automated mass production will make possible arrays with heretofore
unattainable sensitivity and spatial resolution.
Experimentation. Replicative automation technology will permit a tremendous
expansion of the concept of a "laboratory" to include the Earth-Moon system
and ultimately all of the bodies and fields in the Solar System. A number of
grand experiments could be undertaken which would prove too costly if
attempted by any other means. For example, an Earth orbital cyclotron could be
constructed as a series of thousands of robot-controlled focusing coils and

stationkeeping target assemblies within the terrestrial magnetosphere, with

operating energies possibly as high as TeV for electrons and GeV for protons.
Additional experiments on magnetospheric propulsion and energy generation
could be conducted by free-flying robot drones manufactured on and launched
en masse from the lunar surface. Gravity field probes, including mascon
mappers and drag-free satellites, could be coordinated to perform complex
experiments in kinematics, special and general relativity, and celestial
mechanics.
The construction of artificial molecular self-replicating systems is in its infancy. In a
state-of-the-art study, [Lee 1996] demonstrated the operation of a self-replicating 32residue peptide based on a yeast protein. This peptide acts autocatalytically in a
solution of appropriate 15- and 17-residue fragments that combine to form the 32residue peptide.
Any technology employing replicators will require mechanisms to insure safe use.
Existing replicators such as bacteria and viruses cause severe problems; for example,
human death rates exceeding 50 percent in unprotected populations. The possibility of
accidental or deliberate misuse of replicators must be addressed, preferably before
problems arise. One threat is the production and release of artificial infectious agents,
an extension of the germ warfare in development today. Current efforts to address
germ warfare defense could be extended to address artificial threats. Control
approaches include only designing replicators that require crucial components to be
built by other means, not allowing replicators to program themselves, and only
developing replicators that function solely in artificial environments, such as a helium
atmosphere. A second threat is the rapid production of large quantities of armaments
using the exponential growth capability of replicators. Continuous, high resolution,
and ubiquitous monitoring may be required to meet this threat. Unfortunately, it may
be possible to develop dangerous replicators in great secrecy. Note that the Iraqi
government hid their germ warfare program from onsite inspection for years until a
defector blew the whistle. Thus, it is probably extremely important to develop
molecular nanotechnology out in the open with universal access to results. Requiring
free publication of at least government funded results on the World Wide Web is one
strategy. Many molecular nanotechnology scientists follow this practice today.

Chemistry
Organic Chemistry
For over a century, organic chemists have been developing ever more sophisticated
techniques to construct specific molecules in huge quantities. For organic chemists,

these molecules involve carbon, by definition and because of carbon's uniquely

flexible chemistry which allows linear, planar, and tetrahedral constructs. Although
the individual molecules are, by definition, atomically precise, the collections of
molecules produced by chemists are generally poorly ordered. Still, as chemists scale
up to building larger and larger molecules, and still larger aggregates using selfassembly, the potential for organic chemists to contribute to the development of
molecular nanotechnology is difficult to overestimate. Self-assembly refers to the
process of forming larger, atomically precise aggregates by careful control of intermolecular forces. In this section we examine only an infinitesimal fraction of the
organic chemistry work relevant to nanotechnology. In particular, we examine
attempts to construct computer components using organic chemistry.
[Tour 1998] synthesized a number of organic molecules based on benzene and
investigated their computational possibilities. Since these molecules can be
controllably synthesized in vast numbers, they have great potential as building blocks
for nanoelectronic circuits. Noting that transporting electrons through networks of
such molecules would generate unacceptable amounts of heat, [Tour 1998] proposed
using small changes in electron density to pass information and perform logic
functions. While [Tour 1998] discussed quantum calculations to support the notion of
using electron density changes for logic, it's unclear how the small signals proposed
could be distinguished from thermal noise [Bauschlicher 1999], an issue not addressed
in [Tour 1998]. Nonetheless, if these molecules can be connected appropriately, noise
problems overcome, and a variety of other problems conquered, these molecules
could lead to molecular computers operating at femtosecond time scales.

Three junction device from [Tour 1998].

[Reed 1998] devised a novel mechanically controllable break junction to staticly test
benzene-1,4-dithiol molecules, a component of the devices in [Tour 1998]. Using this
device, [Reed 1998] was able to reproducibly measure the conductance of single
molecules. The observed resistance, approximately 22 Mohm and 13 Mohm
depending on the bias, was within the error bounds derived from measurements on an
ensemble of similar molecules.

Images used with permission [Reed 1998].

[Reed 1998] also built a device to directly measure the conduction through a small
group of organic molecules using self-assembly and semiconductor fabrication
techniques. This device was used to measure a diode-like molecule:

Image used with permission [Reed 1998]. SAM stands for self-assembled monolayer, i.e., a layer exactly one
molecule thick.

[Ellenbogen 1999] designed a one bit adder out of molecular wires [Tour 1998] with
chemical groups added to implement molecular resonant tunneling diodes and
molecular rectifying diodes. These two diodes are sufficient to implement AND, OR
and XOR logic elements. In turn, these logic elements are sufficient to implement a
wide variety of devices, including adders. The full adder would occupy approximately
25 nm by 25 nm of a surface, approximately one million times smaller than current
electronics. However, such a circuit has no gain and probably would not work well in
an extended system. There is also reason to believe that the clock rate of these
molecular devices would be quite low, possibly slower than current electronics.
Quantum calculations suggest that each component of the adder would work properly,
but the entire adder may or may not work due to coupling between the devices.
Nonetheless, [Ellenbogen 1999] is a substantial step towards molecular electronics.
One problem with these molecular electronics devices is the low levels of current
measured experimentally [Reed 1998]. However, [Emberly 1998] used computation to
suggest that the low levels of current are due to the contact with the gold leads, not the
molecule itself. In fact, molecular wires modeled with strong coupling to the leads

were found to have currents orders of magnitude better than observed experimentally.
Molecular wire current computed assuming weak coupling matched experiment
[Emberly 1998]. If these computations turn out to reflect reality, then a different
choice of contacts may lead to higher currents and more practical computer
components based on organic chemistry.
Another problem with the molecules discussed so far is that they contain no fused
rings and are thus fairly floppy. [Hush 1998] proposed using porphyrin chemistry with
fused ring connectors for molecular electronics.

Porphyrin-based molecule with metal contacts at ends.

[Hush 1998] noted that any molecular family used for molecular electronics should
have several properties, including:
1. Synthesizability. Porphyrin synthesis is well-established and it is reasonable to
expect that synthetic problems can be solved. Oligoporphyrin molecules
approximately 12 nm long have been synthesized and longer molecules should
be possible.
2. Stability. Porphyrin is the basis of many important biological molecules and
can survive in the harsh environment of a body for at least a few days.
3. Synthetic flexibility. Over 100 different oligoporphyrins have been synthesized.
4. Solubility. This is required for chemical manipulation. The solubility of
oligoporphyrins can be readily modified.
5. Rigidity. The fused ring structure of oligoporphyrins is much stiffer than the
molecules proposed in [Tour 1998]. Carbon nanotubes are stiffer still. These are
discussed in the section on fullerene nanotechnology below.

6. Self-assembly. Self-assembly may be essential for large-scale rapid

construction of complete systems. Preliminary studies have suggested that
oligoporphyrins systems can self-assemble.
7. Attachment. The metal ends that can be attached to oligoporphyrins should
provide ready coupling to larger devices for input and output.
8. Junctions. Oligoporphyrins are two-dimensional molecules that have four
attachment points 90 degrees apart allowing large molecules to form grid-like
structures. The metals in the porphyrin may provide a mechanism for out-ofplane attachment allowing layer-by-layer assembly of three-dimensional
devices.
9. Functionality. There is reason to believe that oligoporphyrins can transmit
information over long distances via superexchange effects. However, this has
not yet been demonstrated experimentally [Hush 1999].
10.Rectification. Rectification has not yet been demonstrated in oligoporphyrins,
but the strategy used by [Ellenbogen 1999] should be effective.
11. Switching. [Hush 1998] investigated slow switching using chemical means and
provided supporting computational results.
[Heath 1998] built a computer (the Teramac) from a large 2D grid of wires connected
by switches. By design, the Teramac had many faulty components and was not
initially hard wired as a computer. A conventional computer used sophisticated
software to identify faults and configure the Teramac into a correctly functioning
computer. This represents a significantly different architecture than most computers in
use today. The fault tolerance and regular structure is significant for molecular
computers because regular structures with numerous faults are routinely built from a
wide variety of molecules. [Collier 1999] took a step towards building a molecular
scale Teramac-like computer using rotaxane molecules as switches under ambient
conditions when they:
"... developed an electronically (singly) configurable junction that consists of a
molecular monolayer and a tunneling barrier sandwiched between
lithographically fabricated metal wires ... We demonstrate that this junction can
be used as a switch, and that several devices, fabricated in a linear array
structure, could be used as electronically configurable wired-logic gates (an
AND and an OR gate). Because these switches are only singly configurable,
they cannot be used for random access memory (RAM) applications, although
programmable read only memory (PROM) applications are possible. However,
they do have several advantages. First, and most importantly, they should scale

down to molecular dimensions without appreciable loss of performance ...

Second, when the molecular switches are closed, current flows by resonant
tunneling into the molecular electronic states. The net result of this resonant
tunneling is that the high and low current levels of the logic gate truth tables are
widely separated. This should lead to good noise immunity in future logic
circuits built with this technology. Third, molecular switches are voltage
addressable rather than field addressable. This makes the device robust with
respect to dimensional tolerances in manufacturing. In a CMOS-based
configurable device ..., two wires (address lines) are used to configure the
switch, whereas two different wires (data lines) are used to read it. For our
junctions, only two wires are necessary to achieve both functions. One voltage
is applied to read the device, and a voltage of opposite polarity is applied to
configure it. The configuration voltage of our devices can be a factor of two
greater than the logic levels used when they are operating. This means that it
should be easy to design circuits that are safe from accidental reconfiguration
under operation conditions. Finally, this architecture is highly modular. We
report here on the properties of three different molecular switches incorporated
into sandwich devices composed of various combinations of metallic wires."
[Collier 1999]
We see that at least three approaches using organic chemistry may lead to molecular
computers. While organic chemists have been making functional molecules for over a
century, biological systems have made much more capable molecules for over three
billion years.
Biotechnology
The argument for biotechnology applications to molecular nanotechnology was
beautifully made by [Hartgerink 1996]:
Traditionally microscopic devices have been made by being cut or formed from
larger objects, but as these products shrink below the micron level this process
becomes increasingly difficult. Recently chemists have begun to try the
opposite approach, that is, building these nanoscale objects from molecular
building blocks. Although these devices are too small to be manufactured by
traditional materials science approaches, they are also far too large to be
synthesized by classical chemical synthesis. In order to reach these nanoscale
devices from a molecular level up, a massively convergent synthesis is
required. Production of these nanoscale objects is not, however, unknown and
has been occurring for over three billion years -- in living, biological systems.
From microtubules to viruses, nature has used a broad variety of self-assembly
techniques to build its sub-cellular machines that ultimately lead to life.
In a series of papers starting with [Ghadiri 1993], a wide variety of nanotubes were
constructed from artificial cyclic peptides. A peptide is a short sequence of amino

acids. The nanotubes have adjustable pore sizes, easily modified surface chemistries,
open ends for packing metals or passing ions and small molecules, and are relatively
easily synthesized by combining peptide synthesis with self-assembly. From
[Hartgerink 1996]:
Our approach uses cyclic peptides with an even number of alternating D and L
amino acids for the building blocks of the nanotubes. The alternating stereo
chemistry of the cyclic peptides allows them to sample an open, flat
conformation in solution which allows all the side chains of the amino acids to
be pointing outwards which would not be possible in an ordinary all L cyclic
peptide. In this conformation, the amide backbone is able to hydrogen bond in a
direction perpendicular to the plane of the cyclic peptide. When two cyclic
peptides stack upon one another the hydrogen bonding network that is formed
is like an anti-parallel b sheet , which is commonly found in natural proteins.
As this hydrogen bonding lattice propagates perpendicular to the plane of the
cyclic peptide a tubular structure is formed.

"Crystal structure of cyclo-[(L-Phe-D-N-Me-Ala)4] including a partially ordered water

centered in the cyclic peptide" [Hartgerink 1996]. Image used with permission.

"Atomic Force Microscopy image of the self-assembling peptide nanotube formed by the
cyclic peptide cyclo-[(L-Glu-D-Ala)4] . The nanotube shown here has an unusual right
handed super helical form" [Hartgerink 1996]. Image used with permission.

[Pum 1999] used crystalline bacterial cell surface layer (S-layer) proteins to assemble
into two-dimensional arrays on silicon wafers and other surfaces. S-layer proteins, of
which there are many, form surfaces on the outside of cells. [Pum 1999] used these
proteins to position metals on a surface and then removed the protein by heating.
Functional groups were repeated with the periodicity of the S-layer lattice
(approximately 10nm) and this can be used to "... induce the formation of inorganic
nanocrystal superlattices (e.g. CdS, Au, Ni, Pt, or Pd) with a broad range of particle
sizes (5 to 15nm in diameter), interparticle spacings (up to 30nm) and lattice
symmetries (oblique, square or hexagonal) as required for molecular electronics and
non-linear optics" [Pum 1999]. S-layers can have oblique, square or hexagonal lattice
symmetry with a unit cell of 3 to 30nm. S-layers are usually 5 to 10 nm thick with 2 to
8 nm pores.
While living things have shown us something of what nanotechnology might produce,
most biomolecules are far too fragile for many aerospace environments. For example,
it is unlikely that proteins or DNA can survive in rocket engines. One newlydiscovered class of molecules, fullerenes, particularly carbon nanotubes [Iijima 1991],
built from graphene sheets curved into a wide variety of close shapes, may lead to
tougher, higher-temperature materials that can survive in a vacuum and other harsh
environments. Fullerenes also have certain advantages for electronic applications.
Fullerene Nanotechnology

Carbon nanotubes are a novel form of carbon with remarkable electrical and
mechanical properties [Dresselhaus 1995][Globus 1998b]. Carbon nanotubes can be
visualized as rolled up graphite layers formed into cylinders. They may be single-or
multi-walled. The tubes the may be rolled up with different windings (called
chiralities) of the hexagonal sheet. Depending on the winding, small-diameter tubes
have been shown to exhibit metallic or semiconducting electronic properties. From
[Globus 1998b]:
We see that there is some evidence that fullerene based machines and,
conceivably, machine phase materials based on them may be possible.
Combined with the apparently remarkable mechanical and electrical properties
of carbon nanotubes, there is some reason to believe that a focused effort to
develop fullerene nanotechnology could yield materials with remarkable
properties. Materials with electrical properties that could revolutionize circuit
design and increased strength-of-materials leading to, among other things,
opening the space frontier by radically lowering the cost of launch to orbit.
Note: machine phase materials are materials consisting of large numbers of machines
plus supporting structures. Living tissue is a prime example.
Since [Globus 1998b] was written, substantial progress has been made in
manufacturing, controlling, and understanding carbon nanotubes and related
structures. In particular, some of the predicted electronic properties of small-diameter
single-walled carbon nanotubes have been confirmed, and a few devices have been
built and tested. In addition, new numerical predictions have been made of ever more
detailed devices and realistic systems. There has also been progress controlling the
manufacturing process and in connecting carbon nanotubes to electronic components
built by more conventional manufacturing techniques.
When a metallic and a semiconducting tube are joined, a device may be formed. For
example, [Collins 1997] reports using an STM to explore the local electrical
characteristics of single-wall carbon nanotubes. As the tip moved along the length of
the nanotubes, well-defined positions were found where the current changed abruptly,
in some cases exhibiting near-perfect rectification. These observations were consistent
with localized, on-tube nanodevices predicted theoretically [Chico 1996].
[Service 1999] reported that Zettl, McEuen, and Fuhrer discovered an excellent diode
formed from a pair of carbon nanotubes that crossed and didn't touch any neighbors.
The properties of the diode were determined by attaching gold electrodes and passing
current through the device. Unfortunately, the same article reports that Zettl and
Collins discovered that both individual metallic carbon nanotubes and bundles are
extremely noisy electrical conductors. The cause of the noise is currently unknown,
but may be due to the impurities in the sample examined. If this is true, then the

chemistry influences carbon nanotube electronic properties, which may be beneficial

if it can be controlled.
[Tans 1998] reported the construction and testing of a field-effect transistor (a threeterminal switching device) consisting of a single semiconducting single-walled carbon
nanotube in contact with metal electrodes. [Tans 1998] applied a large number of
carbon nanotubes to a surface with pre-fabricated platinum electrodes placed on a
silicon surface with an intervening 300 nm silicon oxide layer. [Tans 1998] found and
measured over 20 individual tubes that were found draped over platinum electrodes.
Some of these tubes exhibited metallic behavior. Others acted as the semiconductor
component of a field-effect transistor. In other words, when a bias was applied to the
gate electrode, the carbon nanotube effectively changed from an insulator to a
conductor. This device, unlike diodes, exhibits gain. Gain is necessary for fan-out,
making up for losses, and is considered essential for practical devices. The estimated
maximum frequency of the transistor is about 10 THz, achievable in part by reducing
the width of the silicon dioxide surface to about 5 nm. [Tans 1998] was able to use
standard bulk material models to qualitatively describe the carbon nanotube based
transistor. Unlike the molecules used in [Ellenbogen 1999], there is currently no
general methodology for controlling the production of carbon nanotubes with the
precision necessary to devise electronic circuits. Also, carbon nanotubes are somewhat
larger than Tour wires. However, if the synthetic challenges can be overcome, the
higher current densities allowed and the fact that gain has been demonstrated makes
carbon nanotubes a prime candidate for molecular computers.

Single-walled carbon nanotube draped across platinum electrodes [Tans 1998]. Image used
with permission. http://vortex.tn.tudelft.nl/~dekker/nanotubes.html.

[Martel 1998] produced field-effect transistors from single- and multi-walled carbon
nanotubes at about the same time. The abstract to this paper is so perfectly written is
difficult to improve upon:
"We fabricated field-effect transistors based on individual single-and multi-wall
carbon nanotubes and analyzed their performance. Transport through the
nanotubes is dominated by holes and, at room temperature, it appears to be
diffuse and rather than ballistic. By varying the gate voltage, we successfully
modulated conductance of a single-wall device by more than 5 orders of
magnitude. Multiwall nanotubes showed typically no gate effect, but structural
deformation -- in our case a collapsed tube -- can make them operate as fieldeffect transistors." [Martel 1998]
It is particularly remarkable that these transistors were fabricated by manipulating
carbon nanotubes on a pattern surface until they were in the correct location for
measurement. [Avouris 1999] reports on a number of interesting advances in fullerene
technology produced by the same research group including the effect vander Waals
forces of a substrate on carbon nanotubes, which is substantial.
Constructing carbon nanotube computers is of no value if they cannot be connected to
the outside world. [Zhang 1999] reported a relatively easy mechanism for connecting
single walled carbon nanotubes to metals and silicon. [Zhang 1999] brought

nanotubes into contact with silicon- and metal-based surfaces in a hard vacuum and
heated the surface. The two materials became joined by carbide (a combination of
carbon and the silicon or metal). Not only did [Zhang 1999] accomplish this with
masses of carbon nanotubes on extended surfaces, they also connected titanium pads
with carbon nanotubes. Current between titanium pads connected by carbon
nanotubes varied linearly with voltage and resistance between them dropped
dramatically after the heat treatment, indicating that a good electrical connection was
created. [Zhang 1999] also used the technique to attach a bundle of single wall
nanotubes to a titanium STM tip. Note that [Anantram 1999] predicted
computationally that electron transport between carbon nanotubes and a substrate
should be substantial, particularly if nanotube defects exist close to the cap.
For the most part, interesting carbon nanotube structures are found by producing large
numbers of tubes in a relatively uncontrolled environment and examining the results
molecule-by-molecule until an interesting structure is found. [Cassell 1999] reported a
notable exception to this pattern. [Cassell 1999] built single-walled nanotube bridges
suspended "... from catalyst material placed on top of regularly patterned silicon tower
structures." Single-walled carbon nanotubes are grown from metal catalysts. The
silicon towers, topped by catalyst metals, were constructed using conventional
techniques, then carbon nanotubes were grown from the catalyst. Most of the tubes
fell over the edge of the catalyst when they become long enough. Those that fell onto
an adjacent tower remained suspended over the substrate between the two towers. By
controlling the location of the towers, specific patterns of carbon nanotubes were
synthesized. For example:

Carbon nanotube "power line" and a square from [Cassell 1999]. The large white objects are
catalyst-tipped towers. The thin lines are carbon nanotubes. Image used with permission.

Major Challenges and Opportunities

Molecular Computers
Molecular computers are an obvious extension of decades-long miniaturization trends
in computing technology. Major progress has been made in the last few years, both
computationally and experimentally, in understanding and manipulating organic
molecules and carbon nanotubes with computational potential. The huge profits
generated by the computer industry effectively guarantees large investments in
molecular computer research and development. Aerospace will undoubtedly benefit
greatly from these investments, but aside from long-term research conducted
primarily by government laboratories and devices designed for high-radiation
environments, the aerospace industry will probably not drive this technology, as most
of the profits are derived from other sectors of the economy. Molecular computers will
very likely be the first commercial fruits of the molecular nanotechnology revolution
expected sometime in the 21st century.
Molecular Machines
In the short term, substantial progress in understanding biological molecular machines
may be expected as the biotechnology revolution proceeds. The more difficult

problem of using these machines in artificial devices can expect a large market for
implantable medical devices. Unfortunately, most biomolecules cannot survive or
function in many environments of aerospace interest because of high temperatures,
extreme pressures, hard vacuum, high radiation, etc. Therefore, molecular machines
based on other chemistry, perhaps fullerenes, must be developed. While there have
been many successful experimental and computational studies of carbon nanotubes,
deployment of operational fullerene-based molecular machines will require a great
deal of research and development.
Macroscopic Products
The realization of enormous launch vehicle performance improvements suggested by
theoretical nanotechnology studies [Drexler 1992b] [McKendree 1995] require
atomically precise macroscopic products. To date, nearly all progress in molecular
nanotechnology relates only to very small things, mostly molecules or partially
ordered molecular aggregates. Integration into larger systems has not been
accomplished. Two mechanisms have been proposed to build larger objects: selfassembly and replication. Self-assembly usually requires an aqueous environment
incompatible with many aerospace applications. Furthermore, the resulting aggregates
are usually held together with relatively weak hydrogen bonds, although sometimes
these weak bonds are a precursor to stronger covalent bonding induced by light or
some other factor. In any case, little progress in producing atomically precise
macroscopic products has been made and producing such products remains a major
challenge. There is one substantial current effort to produce macroscopic products
using molecular nanotechnology, DARPA's moltronics program. This program is
attempting to develop molecular electronics, but required all proposals to directly
address the system architecture (as [Collier 1999] does); not simply develop
individual molecular electronic components.
Replication
Biological systems have used reproduction to build macroscopic objects for over three
billion years. However, artificial replication remains a largely theoretical field,
although simple self-catalytic chemical systems have been developed [Lee 1996].
Current efforts in artificial replication are largely unfunded work by individual
scientists. There's no obvious source of major research funding for this arena,
although the long-term promise is enormous. In addition, there is substantial, wellfounded concern that artificial microscopic replicators might get out of control and do
serious harm. Thus, development of artificial replication faces not only major
technical and developmental hurdles, but substantial safety concerns that must be
thoroughly addressed in practice as well as in theory.

Conclusions
Molecular nanotechnology has enormous potential to improve aerospace systems.
Substantial progress has been made in the last few years, particularly in the
manipulation and visualization of matter at the atomic scale. Increased attention and
funding brought by success will almost certainly accelerate progress in the future.
Molecular nanotechnology, once scorned as "science-fiction" or "a mere dream," is
now comfortably mainstream, as evidenced by frequent references to nanotechnology
in Science, Nature, and other scientific journals and by the fact that the last
three Foresight Conferences on Molecular Nanotechnology have had recent Nobel
laureates as their keynote speakers. Progress, in fact, has been much quicker in some
ways than many practitioners expected. Nonetheless, the closing comments of
[Globus 1998a] are still true today:
Nanotechnology advocates and detractors are often preoccupied with the
question "When?" There are three interrelated answers to this question ...:
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 ... 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.

Acknowledgements
Thanks to Chris Henze for making the image of a molecular motor. Special thanks to
Bryan Biegel, T. R. Govindan, Ralph Merkle, Deepak Srivastava, and Bonnie Klein
for reviewing this paper. This work was performed under NASA contract NAS 214303.

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