2. Introduction to Tailored Force Fields

2.1 Breakthrough Opportunity

In Space, minor forces exerted over long periods can achieve major results. This fact offers a way to solve some of the basic problems which hinder human ambitions to develop a Space-based economy with permanent, large-scale habitats. In the 1970s, O’Neill [1] and Johnson et al [2] considered the problem of building large habitats in some depth.  The well-known artist’s conception from the 1970s of the inside of the “Bernal Sphere” habitat is shown in Figure 2.1.  Three basic points emerged:

1.Large habitats for a distributed economy were ideally situated in orbit, not on or below planetary surfaces.

2.Long-term human residence in Space required artificial gravity, spin rates below 1 rpm, and most of all, radiation shielding which would stop all ionizing radiation.

3.Human labor for construction would be prohibitive in both hazards and cost.

Given these constraints, there was no practical solution. The mass needed for full radiation shielding was immense, and techniques for assembly of the outer shell and shield of any such habitat demanded millions of human work-hours in unshielded Space.

 

If a method could be found to build large-scale infrastructure protected from radiation in orbit, commercial activity could accelerate, and the human presence in Space would grow rapidly in a synergistic Space-based economy. The past decades have led to the growing realization that such an Economy is the top priority for Space endeavor. [3,4]

Figure 2.1: “Island One” concept for a spherical colony in Space, described in [1,5]



 
 


 
2.2 Steady and Unsteady Potential Fields 

Various kinds of force fields are used today.  Forces exerted by radiated energy on objects in their path, have been proposed for space propulsion [6-8]. NASA uses Electrostatic Levitation (ESL) for non-contact positioning involving small particles of some materials.  In the vacuum of Space, weak forces acting over long periods, can achieve large results. Familiar examples are microthrusters and solar sails for deep space craft. The relevance of these observations is that automatic construction of large/complex objects from random-shaped material is feasible.

Forces can also be generated by the interaction of unsteady potential fields with matter. In such interactions, the nature of the interaction depends primarily on the intensity and the intensity gradient of the radiation, the transmissivity of the particles for the particular wavelength of radiation, and the ratio of the wavelength to the size of the particle.  A beam with a “waist” (focal region) can both “push” and “pull” particles. Very roughly, it may be stated that particles with high transmissitivity get pulled towards the beam waist from either direction – this is used in Optical Tweezers [9-11]. This phenomenon has been explained partly using geometric optics and the refractive index of the particle. The interaction is complex when the particle size is comparable to the radiation wavelength – the Mie scattering regime. In this regime, the interaction of the incident and scattered radiation has a strong directional dependence, and is difficult to compute, especially for non-spherical particles.


Acoustic

Beams are used to position particles, in both optics and acoustics applications. Ultrasonic beams are used to hold small objects (mm scale) away from solid surfaces for non-contact processing. The Optical Tweezer concept is used in microscopy with particles in the micron to nanometer size range.  Ultrasonic “Fingers of Sound” are used to hold particles in the millimeter size range in space applications as well as ground experiments. In this NIAC Phase 1 term,McCormack [12] is studying laser beam / particle positioning in orbit to form mirrors for space telescopes.

When the particle size is much smaller than the wavelength (less than 0.05 l ) the interaction is described by Rayleigh scattering. Here the scattering is independent of direction, and largely independent of particle shape. Thus it is much easier to compute.

Figure 2.2: Stable traps in acoustic and optic fields.

 

2.3 Resonators


A resonator can be used to increase the intensity of the field by a large factor above that of the incident beam. In a standing wave field, the trapping force can be 1000 times the force obtained with a single beam. The “trapping stiffness” at the stable positions can be seven orders of magnitude above that in the focal region of a single beam.  Such phenomena have been considered in detail in the context of optical (electromagnetic) fields interacting with solid particles inside a waveguide in [13]. Figure 2.2 schematically illustrates the creation of a resonant standing wave pattern with multiple beams.

Higher-order modes correspond to complex shapes of the trap regions. This is the aspect which enables the formation of stable walls of desired shape. Figure 2.3 illustrates contours of pressure-fluctuation intensity on a wall of an acoustic resonator. 

As mentioned above, with standing waves in a low-loss resonator,  small input intensity suffices to produce substantial forces on particles. Various mode shapes can be generated by varying frequency and resonator geometry.

Figure 2.3: Pressure distribution for a higher-order mode in a rectangular acoustic resonator
 


 

2.4 Time Line / Application Map

The implications of the above reasoning are explored in a time-line / application map in Figure 2.4. Steady potential fields are commonly used, and have continuing applications for the future. Steady beams of sound and light are already used in positioning small particles, and these will presumably see greater applications in the next few years. The regime of “acoustic shaping” using standing-wave fields offers potential for automated construction of parts ranging in size from millimeters to perhaps 3 meters. This capability can be taken to a technology readiness level for Space Station applications within 5 years, but application to the larger sizes (on the order of 1 meter) must wait until there are facilities large enough to accommodate such manufacture. There is no fundamental obstacle there except the absence of suitable pressurized, enclosed volume in Space – a problem which can be remedied by such solutions as the usage of empty STS main propellant tanks.

At the far horizon is the large size application to building radiation shields for habitats using extraterrestrial material, to form sheltered bases for commercial exploitation of Near-Earth Objects (NEOs). The Near-Earth asteroids will be the most probable source of local mass for building these habitats. Because of the difficulty of obtaining fine-grain material from asteroids, it is probable that the raw construction material will be in the form of rubble of arbitrary shape, with sizes in the range of ten centimeters. Radio-frequency waves will be most suited to move such particles into walls several centimeters to about 2 meters thick for the outer shells of habitats. A set of powerful radio-frequency antennae will be required. While conceptual calculations of the system are possible at this time, credibility demands that we describe the process for initiating large-scale activities beyond Earth, creating a demand for the commercial activity which would justify the building of such habitats.

For these reasons we also undertook the exploration of a system for initiating a space-based economy closer to Earth. Studies in the 1970s (and basic reasoning valid today) showed that the best location for a self-supporting human habitat away from Earth would be in orbit, not at the bottom of the gravity well of a massive body such as a planet or a moon.  The need to create a local “artificial gravity” close to 1-G, and to maintain a rotation level less than 1 RPM to accommodate the physiological constraints of most humans, dictate the rim diameter of such a habitat – roughly 2km. The assembly of the massive radiation shield for such a habitat without using large amounts of human labor in Space is the primary challenge. Thus we looked at the process for creating a 2km diameter, 2km-long cylinder shielded with lunar regolith to a wall depth of 2m, located in the Earth-Moon system.
 
Time  /

Size

1 – 5yrs

Ultrasonics

5-20 yrs

Standing Wave Acoustcs

20-30yrs

Steady Magnetic

/ Telepresence

30 – 50yrs

Long-Wave

Electromagnetic

10-3m

Steady Beam 

Acoustics

ISS Parts

Formation

Flight

Asteroid Reconstruction

100m  

Heat Shields

Habitat Parts/

Fuel Tanks

 
103m    
Habitat Construction

Figure 2.4 Timeline / Size scale and application map for construction using Tailored Force Fields

 
 
 
Abstract 
Intro
Theory
Near-term: Acoustic
Mid-Term: L2 Habitat
Space Economy 
Far-Term: Radio-Wave Construction
Comments
Issues
Conclusions
Acknowledgements
References