This writeup describes the use of mechanosynthetic assemblers to build an orbital tower.
It crosses the domains of the Usenet groups "sci.nanotech" and "sci.space.tech", and tries to stay at a high level in each specialty.

SUMMARY

Space advocates have long desired to decrease the cost of space travel. A method known since the 1960s is the construction of an orbital tower, a long structure or cable in synchronous orbit with one end touching the surface of the Earth. Such a tower could support elevators moving freight and passengers up to synchronous orbit and beyond, and down to the surface, at a cost per kilogram orders of magnitude less than modern rocketry, with passenger safety comparable to a train or subway. However, no ordinary material has the tensile strength needed to build such a structure.

Nanotechnology is the anticipated industrial capability of specifying and building products atom by atom, resulting in atomically perfect structures of any desired chemical composition. A favorite product material of nanotechnologists is diamondoid, a generic word describing any mechanosynthetic object that relies on tetrahedrally (sp3) linked carbon atoms forming a rigid, space-filling lattice as a major part of its design. Diamondoid should be strong enough to serve as a construction material for an orbital tower, and cheap enough to make the tower's construction feasible, given an already orbiting source of carbon and other elements. The construction of an orbital tower would be an excellent bootstrap project for nanotechnology specifically, and a huge benefit to humanity in general.

THE ORBITAL TOWER

The idea of an orbital tower is originally credited to Yuri Artsutanov, a Russian engineer who in 1960 published his idea of a "heavenly funicular" [1]. Other authors [2,3] have expanded on this concept in technical literature, while Clarke [4] and Forward [5] have furthered the idea in popular fiction and non-fiction respectively.

An essential attribute of the orbital tower is that, despite appearances, it is in orbit. In order to keep that perspective, the following visualization exercise is helpful.

Start with a synchronous satellite (technically, an object in a 36,000km circular prograde orbit of zero inclination). Its orbital period is 24 hours, in lockstep with the Earth below. To an observer on the Earth, the satellite appears motionless in the sky, because it is orbiting the Earth at the same speed with which the Earth is turning. This is very useful for communications satellites, because ground antennas can be pointed once and then left alone. (I am ignoring orbital perturbations and station keeping for now.)

Now give the satellite a rotational period of 24 hours, so that it always presents the same face to the Earth. (Commsats do this as well.) The satellite can be any shape, as long as its center of attraction is at the proper distance from the Earth. Now elongate the satellite like a spear, with the point towards the Earth and the tail away from Earth. Again, as long as the center stays where it was (meaning for every bit of stretching of one side towards the Earth, there is a complementary stretching of the other side away from Earth), the situation remains the same, that is, the satellite still orbits the Earth, apparently motionless as seen from the ground. As it gets longer, the near end gets closer to the Earth. Eventually you can stretch the satellite so that one end touches the surface.

What you have now is a solid object, in orbit, that looks like a _very_ tall tower, stretching 36,000km over your head and beyond. If it had an elevator, or an electric car, or steps, you could climb it, right up to orbit. In fact, in order to maintain its center of attraction at 36,000km, the tower must extend significantly further than this, because of decreasing Earth gravity and increasing centrifugal force. Pearson [3] and Forward [5] assume cables extending 110,000km beyond synchronous orbit, rather than Clarke's [4] more solid structure, extending a shorter distance. Despite appearances, the tower is actually in orbit, and its attachment to the ground is for tension, not stability. If the ground attachment were severed, the tower would probably drift upwards in response to its counterweight; it certainly would not crash to Earth.

At the center of the tower (synchronous orbit), a passenger experiences free-fall, because she's in orbit next to the tower's center. At the Earth end of the tower, a passenger experiences 1G, just as if she were standing on the Earth next to the tower. At the far end of the tower, centrifugal force far exceeds the Earth's gravity, and our passenger has to hang onto the tower to avoid being thrown into space. Thus, apparent gravity varies smoothly from 1G at the Earth's surface, to free-fall at the center, to some significant value outwards at the outer end of the tower. The far end is useful for launching objects away from Earth; just wait for the right time and let go of the tower. Alternatively, cargo destined for off-Earth can simply be flung off the end without stopping, or accelerated electrically for even greater range. Pearson [3] and Forward [5]'s design places the end of the tower 150,000km from the center of the Earth, moving at a horizontal speed of 11 km/sec. Thus, simply letting go of the end of the tower at the right time is adequate for a minumum-energy orbit to Saturn, or a faster orbit to planets closer than Saturn. (Nothing is free; the energy to launch an escape payload comes from the Earth's rotational energy.)

Using the orbital tower, the energy cost of placing a kilogram of cargo in orbit is simply the cost of the electricity needed to lift that cargo against Earth's dimishing gravity, counteract any atmospheric friction for the first 100km or so, and stop it at the end of its trip. Forward [5] quotes a price of $2 per kilogram, compared to $5000 per kilogram using rocket-based methods. Note that this price does _not_ take into account the fact that electricity can be generated by the momentum of incoming cargo, so the entire system can be rigged to be pretty energy-efficient.

Like any object in orbit, the tower would be subject to a variety of perturbations that would tend to degrade its orbit. The Moon and the Sun are the chief contributors, along with irregularities in the Earth's mass distribution and shape. Proper scheduling of incoming and outgoing loads can help maintain the tower's orbit. Note that if the tower were to break, the first 25,000km of it could fall to Earth, but anything higher would remain in orbit.

A counterweight is required at the far end of the orbital tower, to maintain tension along the structure. Forward [5] notes that diamond fiber would be a suitable material for construction of the tower, but laments the unavailability of an industrial source of diamond fiber. This situation, however, may change within the next 10 to 50 years, as described below.

DIAMONDOID CONSTRUCTION

The literature on nanotechnology [6,7] describes a new industrial infrastructure based on the precise, mechanical manipulation of atoms and molecules (mechanosynthesis) to build eutactic (atomically perfect) products. Biochemistry provides many examples of the mechanical manipulation of atoms and molecular fragments (enzyme catalysis, protein synthesis, etc.), so the overall concept has precedent. One can visualize a nanometer-scale machine called an assembler, capable of programmable construction of any desired eutactic product, given feedstock, energy, and instructions. Assembly speeds of one million molecular manipulations per second are considered feasible in even the earliest (first-generation) assemblers.

Assemblers themselves are expected to be very small, atomically precise machines. Thus, it is expected that assemblers will be able, once properly programmed, to build additional assemblers. This allows assemblers to be created in geometrically increasing numbers, once the first one is created through some non-nanotechnological means. Such replication is required to build products at a reasonable rate; although a million operations per second sounds fast, a kilogram of carbon contains over 5 x 10^25 atoms.

For a variety of reasons (see [7]), many mechanosynthetic products will be built around diamondoid, a lattice composed chiefly of tetrahedrally-linked carbon atoms. Diamondoid is expected to have many of the mechanical properties of naturally occurring diamond, especially hardness and tensile strength. However, constructing diamondoid products through mechanosynthesis is expected to be no more or less expensive than constructing any product through mechanosynthesis. In fact, because of assemblers' ability to self-replicate, all mechanosynthetic products are expected to be very inexpensive (comparable to agricultural products) and extremely high quality (atomically perfect, with a defect rate less than 1 in 10^15) by today's standards.

THE ORBITAL TOWER AS THE NEXT APOLLO PROGRAM

The combination of geometric assembler growth, eutactic products, and low cost make the construction of an orbital tower using assemblers attractive. Huge numbers of assemblers can construct tower components at whatever speed is desired, limited only by raw materials, energy, and the coordination of product flow. The tower components will be atomically perfect, an almost ludicrous property of such a gigantic object, but a natural property of any mechanosynthetic product, and a necessary property to handle the stresses involved. Estimates vary on the mass of an orbital tower, from a million tonnes (Forward) to a billion tonnes or more (Clarke), depending on the exact design. Enough assemblers can be created through self-replication to convert raw materials into diamondoid at whatever rate is required to complete the construction.

Estimates also vary on the timeframe within which assemblers and other nanotechnological capabilities will be available. Most estimates range from between 10 and 50 years. Progress on a variety of fronts (microscale electronics, biochemistry, human genome decoding, electron microscopy) is encouraging, and seems faster than recent progress in space development. It is possible that mechanosynthetic capabilities will exist to build an orbital tower well in advance of the availability of any off-Earth carbon resources (asteroidal or Lunar) from which to build it.

Many proponents of nanotechnology are concerned about the use of assemblers in any context in which they might be introduced to the biosphere. They believe that a nanomachine is a potential threat to the biosphere because it may somehow compete (with machine-like and potentially brutal efficiency) with lifeforms for some essential resource. Because of their self-reproductive capabilities, if nanomachines "get loose", they could cause irreparable damage to the Earth and its life.

Orbital construction of an orbital tower is an excellent opportunity for nanotechnology to prove its worth and extend its capabilities, with only minimal risk to the biosphere. Working in orbit, with appropriate self-destruct devices as needed, nanomachines can perform useful work in complete safety and isolation, improving along the way as new efficiencies and capabilities are invented. The orbital tower could be to nanotechnology what the Apollo program was to miniaturized electronics and ultimately the computer industry -- simultaneously a market, proving ground, and stimulus.

ADVANCED TOWER PROPERTIES

The earliest nanomachines are expected to provide only the most basic mechanosynthetic techniques, such as the construction of relatively simple eutactic materials (e.g., diamondoid) and a small variety of nanoscale parts (e.g., those in an assembler). However, the capabilities of nanotechnology in both its techniques and its products are expected to grow rapidly, once initially developed. Not only will assemblers become better (faster, cheaper, more general-purpose, etc.), but the products of assemblers will employ active nanotechnology even after their initial assembly. Thus, the first generation of orbital tower components might be relatively static diamondoid blocks, fibers, cables, and so forth. But, later generation assemblers will be able to build active tower components, able to change shape, self-repair, or self-modify in response to orbital perturbations, meteor damage, or other events.

CONCLUSION

The use of nanotechnology to build an orbital tower is a potentially synergistic enterprise for both nanotechnologists and space technologists. It would provide an excellent proving ground for nanotechnology, and the final product would open the solar system to humanity.

REFERENCES

• 1 - Yuri Artsutanov, Komsomolskaya Pravda, 1960.
• 2 - John D. Isaacs, Allyn C. Vine, Hugh Bradner, and George E. Bachus, "Satellite Elongation into a True 'Sky-Hook'", Science 151 (1966), p. 682.
• 3 - Jerome Pearson, "The Orbital Tower: A Spacecraft Launcher Using the Earth's Rotational Energy", Acta Astronautica 2 (1975), p. 785.
• 4 - Arthur C. Clarke, "The Fountains of Paradise", Ballantine Books, 1978 (ISBN 0-345-25356-6).
• 5 - Robert L. Forward, "Future Magic", Avon Books, 1988 (ISBN 0-380-89814-4).
• 6 - K. Eric Drexler, "Engines of Creation", Anchor Books, 1986 (ISBN 0-385-19973-2).
• 7 - K. Eric Drexler, "Nanosystems", Wiley Inter Science, 1992 (ISBN 0-471-57518-6).

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Jerome D. Rosen Martin Marietta Astronautics, a Martin Lockheed company Colorado Springs, CO

The opinions expressed herein are entirely my own. I am entirely self-taught in astronomy, astrodynamics, and nanotechnology, so any errors or misinformation in these areas are due to my own ignorance. Corrections are whole-heartedly welcome.

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