37 – Where is power beaming by laser a viable space technology?

Power transfer via laser is frequently proposed for space launch, in-space propulsion, in-space remote power supply, and space-to-earth power supply. The intent is the same of any modern power system—to organize power collection/generation, distribution, and utilization into an optimum configuration tailored to the mission needs. This posting focuses on how power transfer can and probably should not be used in our emerging spacefaring capabilities?

Nature defines our natural EMR environment

sun source nasaThat power can be transferred by electromagnetic radiation (EMR) is obvious when standing in bright sunshine on a summer day. Our sun—a natural fusion reactor—conveys power to the Earth’s surface, some 93 million miles distant, with a continuous EMR power level of around 1000 watts per sq. meter at the surface (1367 watts per sq. meter outside the atmosphere). We work, play, and relax bathed in fusion-originated EMR. Of course, there are limits to what we and most other animals can tolerate. Instinctual behavior—moving out of the bright sun, not looking directly at the sun—and the autonomic responses to sweat or pant to prevent over heating and the constriction of the eyes’ pupils, to restrict the EMR intensity within the eyes, further control our exposure to the natural environment to prevent harm.

What makes life possible on the Earth is the natural inverse-square law of power transfer by a non-coherent radiation source where power decreases by the square of the distance from the EMR source. This law enables the Earth’s surface to exist within the solar EMR zone where water is liquid and our form of life is possible. Mercury, for comparison, at 39% of the Earth’s distance from the Sun, experiences about 9126 watts per sq. meter and is a barren, lifeless rock.

Lasers change nature’s rules for EMR power transfer

Diagram of first ruby laser
Diagram of first ruby laser

From the earliest coherent radiation work in first, microwaves, and then the shorter frequencies generally referred to as “light”, the goal was to overcome the inverse-square law. The result would be that the intensity (power per unit area) emitted at the source would then be available wherever the EMR was directed as long as the EMR remained coherent. If such an ideal or perfect coherent source could be produced, the beam would travel across the universe at the same power per unit area.  The fact that lasers only closely approximate this ideal still means that over meaningful distances, the EMR intensity remains nearly the same. Taking advantage of this near perfection, since the first ruby laser was invented in 1960, an impressive breadth of capabilities came to be realized through technologies enabling the  control of power level, frequency, duration, and beam structure.

Lasers create a new EMR environment in which humans and our civilization exist

USAF laser experiment
USAF laser experiment

Fundamental to civilization is the ability to harness and control energy sources and its delivery in useful forms of power. In some forms of power delivery, significant additional EMR is added to the natural environment. When the total EMR exposure exceeds the tolerable natural surface environment in terms of power and frequency, special protections are required. Workers near blast furnaces, for example, wear EMR protective clothing to limit exposure to the intense infrared EMR being released by the furnace. In the accompanying photograph of laser research, the researcher is wearing protective goggles for the same reason that welders were such protection—to keep the eyes’ exposure levels within normal natural conditions. When significant sources of EMR are added to the human environment, added protections must be provided in a way that controls the human/natural environmental exposure.

Laser power transfer will exceed natural levels

While lasers are now quite common, so is our understanding of the need to warn of their dangers and protect against these dangers. This site provides an explanation of the hazards of laser radiation to the eye. This illustration helps to make the hazard clear. For example, a 100-watt incandescent frosted glass bulb, at 5 meters distance, will create a power level of 150 watts per sq. meter over a very small portion of the retina. This power level is within human tolerance. A 1 milliwatt laser—one hundred thousandth of a power of the bulb—at the same distance will create a power level of 300 million watts per sq. meter on the retina over a very small portion of the retina. This is near at or near the threshold of causing damage. The ability of coherent radiation to transfer eye-damaging power even with low power output is well understood. Consequently, even such low power lasers carry warning signs of the potential hazard.

Sign danger laser radiationThe danger from lasers is not just with direct exposure, such as a laser pointer being directed into an eye. Laser warning signs also warn of the exposure to scattered laser radiation. Reflected laser light remains coherent. Thus, the reflected laser light also poses a danger because its power intensity does not diminish according to the inverse-square law.

It is also very important to recognize that most industrial lasers operate at frequencies that are not visible to the human eye. Thus, unlike a bright non-coherent light source that, like the sun, can be visually detected and avoided, many lasers cannot. The potential for extreme power concentration within the eye means that damage may occur prior to any physical sensation. Because of this, the burden of protection falls on the designers and operators of the laser system to build in suitable protections either through shielding or access control.

Dangers from lasers used for space launch

Lightcraft in flight powered by laser
Lightcraft in flight powered by laser

Ground-breaking experiments to demonstrate the potential for laser-powered space launch have been undertaken by Lightcraft Technologies, Inc. (Laser Pumped Flying Saucer Spacecraft). In the linked video, note the discussion of safety and the use of safety measures in the intense laser environment. Now scale this demonstration up by multiple orders of magnitude and understand that the application of laser power will, unlike the demonstration program, extend downrange tens of miles as the laser-powered vehicle climbs in altitude and accelerates toward orbital velocity. This mode of producing propulsion adds significant coherent EMR to the natural environment. Should the flight system tumble or wobble or the flight system’s reflective collector not be aligned with the beam, then scattered laser radiation would appear likely. The practicality of providing traditional protection from direct and scattered laser radiation would, obviously, be quite challenging for humans and animals. Yet, such protections are needed to make this approach practical.

While the Lightcraft approach uses a ground-based laser directed upwards, other concepts use space-based lasers, or ground-based lasers reflected from orbiting mirrors, to provide the propulsive power over at least a portion of the ascent trajectory. As with the Lightcraft approach, the dangers of scattered laser radiation would exist. Also, it must be expected that not all of the laser energy will impinge and be absorbed on the flight system. With the downward path of the laser beam, some of this energy will intersect the surface creating a direct or scattered exposure hazard. How will these protections be provided?

A final area of concern is the intense non-natural EMR environment created in Earth orbital space from laser-powered space launch. Whether originating on the ground or in space, the high-powered laser beams must also pass through Earth orbital space. As mentioned in the video, this creates a hazard to satellites and, eventually, to human space operations. How will suitable protections be provided?

Dangers from lasers used for power beaming to the surface

The power intensity (watts per sq. meter) for lasers has been noted as benefiting the transmission of space-generated power to the Earth’s surface because it would (ideally) require relatively small ground receiver footprint compared to that of a microwave-based system.

Orbiting satellites and debris
Orbiting satellites and debris

The first issue that must be addressed is how to deal with satellite and human space operations. With a geosynchronous-based laser, essentially a fixed beam, relative to the Earth, would be established transiting lower orbits to illuminate the ground receiver. Every satellite and human space operation at lower orbital altitudes would then be at risk of passing through this beam or being impinged by scattered radiation from debris passing through the beam. For any reasonable space-based power infrastructure, hundreds, if not, thousands of transmitting beams would be needed. Sunlight is about 1.4 kW per sq. meter in space spread across a broad spectrum. These power beams would be in the tens of MW per sq. meter concentrated into a specific frequency. Even momentary exposure could prove very damaging. The generally benign space environment would become very dangerous.

At ground level, a similar significant safety hazard would exist. Airplanes and birds in flight would be exposed to the beam directly and airplanes and birds in flight and people and animals on the ground could be exposed scattered radiation. The notion of hundreds of these power beam hazards in the U.S. raises concerns about the practicality of this approach.

Another risk is the accidental or intentional loss of control of the beam. If the control of the laser aiming system is lost or hijacked, if control thrusters malfunction, or should the system is impacted by space debris, then the beam could move outside the ground receiving site presenting added danger. (While there are proposed control system methods that are intended to address this, there is also something called Murphy’s Law.) While the same could happen for a microwave-based system, the substantially lower mass of the laser beam generator, the substantially greater beam intensity, and the eye danger make this a more serious problem. For any reasonable space-based power infrastructure, hundreds, if not, thousands of GW-class transmitting beams would be needed implying that the threat of such a malfunction is not trivial.

Potential space laser power applications that may work with acceptable hazards

The advantage of coherent EMR power beaming in overcoming the inverse-square law comes with some significant disadvantages when proposing its use within a natural EMR environment in which humans and nature must coexist. Does this mean that power beaming has no future in our emerging spacefaring civilization? Certainly not. On the lunar surface, power beaming may be a very useful means of providing distributed power from a remote nuclear power plant, for example, without the need to lay extensive physical cables. Solid-state lasers, relay mirrors, clear lines of sight, the ability to completely control the human environment and hazard exposure, the lack of any “nature” to impact, and the ability to physically shield human habitats from accidental exposure may make this a very useful technology.

MoonSpace launch from the moon may also benefit from laser propulsion without bringing many of the inherent hazards discussed above. The reason is that the fact that the Moon always has the same orientation with respect to the Earth. This means that the direction of the laser beam could be kept well away from the Earth and its orbital space. In fact, the launch site may be located on the “back” side of the moon or in a deep crater near the visible limb where the Moon’s surface physically prevents any threat to the Earth.

Finally, lasers may be used over short distances to transfer power. For example, space solar power satellites—which may be on the order of miles in size—could use laser power beams to transfer power within the satellite. This could be done within EMR-shielded conduits that would minimize the risk of scattered radiation.