Beamed Energy Propulsion for Micronautics: An Introduction

(what Beamed Energy Propulsion can do beyond aerospace applications)

by Andrew V. Pakhomov

Micronautics? Maybe you have heard this word before, but as I typed it in my word processor the spell-checker highlighted it right away. So it is not in the dictionary yet. It will be there one day. How soon? It depends on our technological progress, but we surely need it as soon as possible.

Micronautics was a subject of science fiction for a long time. In one short story, which I vaguely remember from my childhood, a guy has befriended a microscopic robot, which lived inside his body. The robot cleaned guy's veins and entertained him with some shrewd talks, until one day the host almost choked to death on a clam. The robot saved him by pushing the clam out of his throat, but was sadly lost in a spit.

"Being lost in a spit" is a pretty graphic descriptor of dimensions for a nano-vehicle. Comparing to our cars (trailer trucks for safer assumption) measured in meters, nano-vehicle would have similar dimensions in microns. Million times smaller in length than a car: what else could be worth an effort of such a fine contrivance if not a micro-space of a human body? When micronautics, i.e. capacity to navigate and operate in microspace will be developed as much, as modern aeronautics, our benefits from it will be immense. It will be a revolution of medicine: our bodies can be guarded within from the threats of diseases. Imagine such vehicle moving along an artery and cleaning it from atherosclerotic deposits or infectious bacteria or defected blood cells. Hundreds, maybe thousands of deceases can be subdued and maybe even eradicated by such cleaning service.

You just have to "swallow your doctor", as Richard Feynman, great American physicist, once put it. Feynman was using the original idea of his graduate student, Albert Hibbs (see R. Feynman, There’s Plenty of Room on the Bottom, 1959). Of course, there is a plenty of room: one day such micro-robots will establish connection with our brains. Plugging into axons, sending an info, giving orders … Swallowing your doctor, tax accountant, Spanish teacher, parole officer, Big Brother … the possibilities go from marvelous to scary. Perhaps, our freedom of will itself will need a constitutional protection one day. However, before we will get into a need for a new amendment, a lot of technical problems must be solved.

It took a century to develop such sophisticated aeronautics, as one that we have today. How much time do we have before the frontier of micronautics could be taken over as well as our airspace? Perhaps, considering acceleration of technological progress, we will get to that point within a shorter period of time. One of the "simplest" tasks will be design of an engine for nano-robots. The problem is that one cannot just scale down any existing mechanical engine, because comparing to our macroscopic world, material properties and dominating physical processes at micron scales will be different. An engine of nano-vehicle must have very few moving parts, comparing to our vehicles. How one can make such a thing?

Beamed-energy propulsion (BEP) is an answer. Most of currently developing applications of BEP are designed for space. BEP principle is this: energy is beamed to the vehicle from a separate (often remote) source. The vehicle collects the beam and converts its energy into mechanical motion. In space applications the most typical scenario would be a powerful laser, which remotely drives a spacecraft equipped with collecting optics (mirrors) and solid propellant. The laser light will be collected with spacecraft mirrors and focused on some solid fuel. Practically any solid matter exposed to high-intensity focused light will explosively vaporize and turn into a rocket exhaust, which will propel the spacecraft. This process is called ablative laser propulsion. The engine of a spacecraft has no moving parts, but it produces energetic exhaust, and spacecraft is flying using rocket principle. It will work well in space, but what about a micro-space of a human body? Who needs a rocket inside a blood vessel?

Fortunately, at nano-scales there are alternative, soft ways to convert the beamed energy into mechanical motion. There will be no rockets, less dramatic ways of beamed energy propulsion can be used in microspace. Aside from blood cells, which occupy 55% of blood volume, the rest of blood is a liquid, so-called plasma, 90% of which is water. The fastest tool of motion in our blood employed by many bacteria is so-called flagellum, a helical appendage, which acts as a propeller. The same mechanics can be used for nano-robots. Our bodies are transparent to magnetic fields. The energy can be beamed in a form of electromagnetic (EM) waves of relatively low frequency (it can be also done with x-rays, but we don’t need to go there). In the simplest case, the engine, essentially a conducting nano-loop or solenoid, can be made rotating or wiggling under one or several EM beams. With flagellum rigidly attached on a side of the loop or just as continuation of a solenoid, the nano-vehicle will move in the same fashion, as a spirochete or spermatozoon, with direction of motion set by driving EM beam.

In 1959 Richard Feynman predicted that nano-doctors will be moving by means of electric motors, driven by external EM fields. Today, 50 years later, when micronautics moves from the realm of sci-fi to the real science and becoming a subject of study by physicists and engineers, we have a choice of several approaches to the problem. The electrical nano-propulsion was sketched here, there are others, though. For example, in 2002 at First International Symposium on Beamed Energy Propulsion (ISBEP) professors from Tokyo Tech University, Shiho and Yabe, with co-authors*, have presented a possibility to drive nano-robots with x-ray lasers. The next, Sixth ISBEP will be held in November 2009 in Scottsdale, Arizona. As expected, the discussion on micronautics will be continued there.

*Makoto Shiho, Kazuhiko Horioka, Yuji Kiriyama, Sadao Aoki, and Takashi Yabe, Generation and Focusing of High Brightness Pulsed X-rays - Toward the X-ray Driven Micro-Ship, Proceedings of the First International Symposium on Beamed Energy Propulsion, ed. by Andrew V. Pakhomov, American Institute of Physics Conference Proceedings, v. 664, Melville, NY, 2003, pp.475-484.

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