A hot topic in advanced technology in these early days of the 21st century is nanotechnology. While the future trajectory of this technology is very uncertain, it is worth writing a short chapter on the prospect for the use of these as interstellar probes. Nanotechnology is the engineering of systems on the molecular or near molecular scale. These systems are computers with an instruction set encoded on molecules, in a way similar to what nature provides with DNA. These instructions are then parsed or translated by other molecules which enter into a chain of reactions. These reactions are envisioned to perform a wide variety of possible tasks, from production of materials to potentially attacking cancer cells. It is not unreasonable to presume that these nano-bots might also turn out to be employed as tiny spacecraft. The advantage with this is that nano-spaceprobes have little mass and so the energy requirements for sending them to low gamma velocities is far less than with a more massive spaceprobe.
Generically these nano-bots would be a form of von Neumann probe [10.1]. These probes would be an extension of nanotechnology, technology on the scale of molecules [10.2]. This field started with the discovery of buckminsterfullerenes [10.3]. The buckminsterfullerene, C60, the third allotrope of carbon, was discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley. These were found from the laser evaporation of graphite. For this discovery they were awarded the 1996 Nobel Prize in Chemistry. Nano-space probes would require a large enough of a molecular instruction set to be able to take advantage of a range of environments they might encounter at their destination. Upon reaching some asteroid or comet in an extrasolar system they are capable of using the materials available to either reproduce themselves or to fabricate systems and structures on a larger scale. In effect these nano-probes would establish a type of robotic base on
an extrasolar asteroid or planetary surface. It is likely that a nano-probe would first reproduce itself to form a type of colony and to be programmed so each nano-bot in the colony is specialized for distinct algorithmic or processing capabilities. This colony then has an emergent complexity far greater than the original nano-probe. From there it might be able to engage in the exploration of the extrasolar system and to send information back to Earth.
Obviously this requires advances in artificial intelligence (AI) far beyond current technological capabilities. It also will require an AI understanding of emergent complexity, self-regulated, self adapting and self-directed growth. Currently this is not well understood, yet it is clear that biological evolution has through selection of genes permitted this to happen with complex living systems. Since this is the case it is not out of bounds to presume that these problems are tractable in some ways and can be reproduced in synthetic molecular computing systems, such as nano-bots.
It is clear that to probe interstellar space with nano-bots it will require that millions of them might have to be sent to an extrasolar system. This is because most of them are likely to miss any sort of target and end up adrift in interstellar space for billions of billions of years. However, maybe a few will find their way to some asteroid or planetary surface where by they can then begin to exploit resources and develop into a complex self-adaptive system capable of exploration.
Clearly these nano-bots are not likely to be rockets. Just as model rockets can't reach space, a nano-rocket is not likely capable of reaching relativistic velocities. So these have to be sent by alternative means.
During the Reagan administration the SDI program was initiated, which was an ill-conceived idea that Intercontinental Ballistic Missiles (ICBM) could be destroyed before they drop their nuclear payloads on American cities [10.4]. Various ideas were considered for how to shoot down a missile, powerful lasers, particle beams, anti-missile missiles and something called the railgun. The problem with lasers and particle beams is that a target has to absorb the energy from this beam and be thermally deformed to the point of failure. This requires that a laser be pointed continuously on a target spot for some period of time. Conversely a bullet destroys its target not so much be delivering a large amount of energy to it, but by focusing kinetic energy on a spot to cause a material fracture and penetration. A gun does not kill by vaporizing a person, but with a penetrating bullet that passes through a region of tissue, puncturing through vital systems, such as large arteries, the heart or the brain. Further, the rapid shock waves associated with the absorption of a bullet's energy by a target very efficiently delivers that energy in an explosive and lethal way. The railgun was then one of the options for killing a ICBM. This was a way of magnetically accelerating a bullet. Of course for the purpose of space combat this had the deficit that its speed was far slower than a beam of light. SDI turned into a negative sum game, which was then revamped into another organization (BMO) and it continues today. Yet the railgun is a way that nano-bots might be sent to the stars.
A magnetic field induces a force on a moving particle, where this force is perpendicular to the velocity of the particle and perpendicular to the magnetic field. The magnitude of this force is given by the cross product of the velocity of the particle and the magnetic field. This means that if the particle is travelling in a direction parallel to the magnetic field the magnitude of the force is zero, and the magnitude of this force is maximal if the velocity of the particle and the magnetic field are perpendicular to each other. This Lorentz force is then
Of course with Newton's laws the force is F = ma from which an acceleration may be computed. This equation involves the motion of a single charge q moving at a velocity v. A current is the time rate of change of a charge I = dq/dt and so this velocity times charge can be replaced with a current. Hence for a current on two wires bridged by a conducting moving object there exists a force on the object given by the Lorentz equation. The diagram 10.2 illustrates the basic set up. This diagram indicates a transverse rod is pushed by the magnetic field behind it induced by the current. Of course the magnetic fields associated with the two conducting wires are in a repulsive situation. For strong enough fields this can cause the railgun to tear itself apart. This limitation did impede these developments for a railgun which could send a bullet to ~ 20 km/sec in order to hit a missile. However, this technique might be used to send a nano-bot to far higher velocities.
A fairly recent development has been with Buckminsterfullerenes, which are C-60, C-120 and higher order carbon bonded convex polyhedra. A topo-logically different form is that of a carbon nanotube, which has a hexagonal carbon to carbon bond, with p orbitals sticking out from the structure. This is a highly conductive material, which could be configured into wires. These wires might be used to accelerate a nano-bot to very high velocities as a sort of nano-railgun. Due to the dimensions of the problem the force per unit distance on the nano-probe does not have to be that large. Material strengths for carbon nanotubes is considerable, so the gun is not as likely to tear itself apart. The contacts between the nano-probe and the carbon nano-wires requires some addressing, where if the probe is to best to rel-ativistic velocities it can't have actual contact with the nano-wires. The probe must ride above the wires, suspended electromagnetically, and where the current flow between the probe and the wires must be involve electron hopping between the wires and the probe.
An alternative approach would be to put a charge on nano-probes and to accelerate them in much the same way that particle accelerators push protons to relativistic velocities. In this case an electromagnetic pulse in cavities push the charge nano-probe through cavity. There are then quadupole magnets which focus the charge particles into the next cavity. Current accelerators push a proton to 7 ~ 1000. For a hypothetic nano-bot of 1000 daltons a similar system might push it to a 7 ~ 2. For further technical improvements it is not impossible to imagine a nano-bot of some 106 daltons being pushed to a 7 ~ 2.
There is of course the obvious problem of what does the nano-probe do when it approaches an extrasolar system. Its large kinetic energy per unit mass must be dissipated somewhere. It might first deploy some sort of molecular thin parachute which breaks it against the photon radiation from the star. However, a quick calculation indicates this is limited. It is possible that the nano-bot could contain a positive charge, which then is repelled by the outgoing charged stellar wind. Of course entering an extrasolar system with its plasma and charge separation of light electrons and protons in the stellar magnetic field is a complex problem. The nano-bot will require the processing capacity to solve this problem and negotiate its entry into the stellar system and reduce its velocity. This is a problem left to the reader and possible researcher who wishes to pursue this problem. Sending a nano-probe to another star is not the principal difficulty, but breaking it so it reaches a destination is rather problematic.
This short chapter was written due to the precedence of Moore's law on the declining cost and expanding ability of processors at a rapid rate. At this time it is very difficult to predict how this type of technology will progress, but there is a prospect that progress forthcoming decades might make this approach the interstellar architecture of choice
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