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Beamed Energy for Space Exploration: Giant Leap or Incremental Steps?

Les Johnson

The countdown was going according to plan, which shouldn’t be unusual, but Elaine had been involved in too many space projects to believe that any countdown, no matter how well the engineers planned, could proceed to launch without a hitch on the first try. But she had to admit, this one was going very smoothly.

Elaine scanned the status displays and saw that the first twenty chipsats were deployed from the seeder ship with their 10-square feet reflective graphene sails fully unfurled. Each of the tiny chipsats weighed less than a gram, about the same as a dollar bill. Each sail was attached by three tiny buckytubes that, if all went well, would soon be pushed to the limits of what materials science could do to protect them from the hellish environment that was about to be unleashed upon them.

When the countdown clock reached zero, a series of events beginning in the Andes mountains would send the chipsats on their way to a flyby of the Alpha Centauri star system in about twenty-five years—on the scale of true interstellar travel, this was practically the next day.

She began to sweat. She knew it was nerves, because the temperature in her control room always seemed to be kept just barely warm enough to prevent ice from thawing. That, of course, was an exaggeration, but the rooms in the building always seemed cold to her no matter what the weather outside was doing. She kept a portable heater under her desk that she sometimes had to use in the middle of the summer—Atlanta air conditioning at its energy efficient finest. She was just nervous. Propelling the spacecraft from Earth orbit to Alpha Centauri was her responsibility and the last ten years of her life culminated here. Now.

Soon, the phased-array, solid-state lasers sitting atop a mountain in the Andes would tap into a huge supercapacitor battery for power, unleashing a one hundred-billion-watt continuous laser beam into space. The beam would be directed to hit the first chipsat’s sail, designed to reflect nearly all the light energy falling upon it. It had to be as close to one hundred percent reflective for two main reasons: To maximize the thrust produced by the reflected light and to prevent the sail from absorbing too much energy and vaporizing. Any light not reflected or transmitted by the sail would be absorbed by it, and that would be a problem.

The laser light would continue to impinge the sail for several minutes, accelerating it at over sixty-thousand gees until it reached the phenomenal speed of twenty percent the speed of light. After that, it would illuminate the next one and the one after, repeating the process until all were on their way. The chipsats would coast the rest of the way to Alpha Centauri where they would use their tiny onboard cameras to take high-resolution pictures of the planets there and then send the images back to Earth for analysis.

Building a fully functional spacecraft on a chip was as large a challenge as building the propulsion system. Elaine was thankful she was “only” responsible for the laser sail propulsion system. There was another team that worked on the chipsats, extracting one engineering miracle after another as the design proceeded.

Thankfully, the clock reached zero without a hitch, and the artificial intelligence started the launch process, controlling the whole system—coordinating the dance of the chipsats with the laser and sending them on their way.

Humanity was sending its first probes to the stars and Elaine was part of it. She smiled and, for a moment, felt immortal.

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Breakthrough Starshot

The Breakthrough Initiatives, founded by billionaire Yuri Milner, is all about giant leaps in space exploration and trying to foster the next one. Among their many initiatives is also perhaps the most audacious in recent memory, Starshot. Starshot “is a $100 million research and engineering program aiming to demonstrate proof of concept for a new technology, enabling ultra-light unmanned space flight at 20 percent of the speed of light; and to lay the foundations for a flyby mission to Alpha Centauri within a generation.”i

Given the distances involved, sending a spacecraft outward at twenty percent of the speed of light is a great idea since it would reduce the trip time to our nearest star from over seventy thousand years (for a spacecraft propelled by chemical rockets) to less than twenty five-years. How do they propose to achieve this feat?

Starshot is all about power beaming. An originator of the Starshot idea is Dr. Philip Lubin, a professor at The University of California in Santa Barbara. His analysis shows that a small spacecraft can be accelerated to these extremely high speeds using high energy lasers and highly reflective light sails.ii (Figure 1)

While no known laws of physics are violated in this approach, the engineering challenges are daunting:

- The spacecraft, with all its controls systems, power, communications, and scientific instruments to study the new solar system when it arrives, must weigh about a gram.
- The lasers needed to generate enough light pressure required to send the spacecraft on its way have to be many orders of magnitude larger than any available today—with output powers of hundreds of megawatts to gigawatts or more.
- The laser light needs to reflect from a sail for all this energy to be transferred to the spacecraft. (Technically it is the momentum transfer from the reflected light that propels the sail—light does not have rest mass, but it does have momentum.) The sail, while not physically large, would have to reflect nearly all the incident billions of watts of power and absorb nearly none. As of this writing, no known material can sustain this sort of energy loading and survive.
- Assuming the above three elements are developed successfully, the laser-accelerated spacecraft would then fly through the target solar system at 20 percent the speed of light since it will be unable to slow down. All scientific data will have to be taken during the fast flyby and then have the data somehow transmitted back home from across the gulf between the stars—another seemingly unsolvable (at least for now) problem.

Figure 1. In Starshot, giant lasers on Earth would illuminate a highly reflective sail in near-Earth space and use the momentum of the reflected light to send very small spaceships toward another star system at twenty percent the speed of light. (Image courtesy of Breakthrough Initiatives.)

To meet these engineering challenges, Breakthrough Initiatives is spending about $100 million dollars for research and development in all these categories and others, seeking the technologies that will be needed to achieve the giant leap that is Starshot. Will they be successful? No one knows, but it is certainly worth trying.

An early success of Starshot has nothing directly to do with the actual project at all—it is the idea of Starshot. With the prospect of getting money (“research funding” for those who are hesitant to be blunt and say they are motivated by money), many are putting forward creative ideas of how beamed energy for power and propulsion might benefit space exploration and development—even if the capabilities are far short of those required for Starshot. The prospects are . . . promising. And all, from the point of view of investment, pacing and utility are more incremental steps than “giant leaps.”

Laser Communications

Guglielmo Marconi would be proud. The Italian inventor credited with developing the world’s first radio and for broadcasting in 1901 the first transatlantic radio signal, would undoubtedly be impressed with how far we’ve come with radio and wireless technology. From television and FM radio to two-thirds of the human population having a cellular phone, his invention has been a resounding success. Equally impressive has been the application of radio to space exploration. As our robotic emissaries journey far from home in the outermost reaches of the solar system and beyond, they send their data back home to Earth using variations of the same technology Marconi invented.

But there is a problem. Radio is essentially omnidirectional, with nearly all the power used to create the signal being lost by radiating in all directions. This can be useful for a local television or radio station that wants to blanket its service area with signals to be accessed by anyone, anywhere, anytime. But when trying to communicate with a single spacecraft many millions or billions of miles from home in the near emptiness of deep space, wasting most of the energy by broadcasting in all directions is, well, wasteful. Granted, we can do better than send the radio energy off in all directions—antennas can be somewhat directional, sending a signal outward to something in one general direction and not another. At our most creative, we can narrow the broadcast beam to something under a few degrees of sky, which sounds impressive, until you consider the distance the signal has to travel and how much the beam of radio energy will spread out while crossing that distance according to the inverse square law as illustrated in Figure 2.

Figure 2. The inverse square law describes how the intensity of light decreases with distance. Applied to laser sailing, it means that the intensity of the laser light falling on the sail drops to ¼ its initial value if the distance is doubled; 1/9 if the distance is tripled; etc. (Image courtesy of Borb)

Thanks to the inverse square law, our narrow radio beam will drop off in strength rapidly as the distance increases—double the distance and have only one fourth the initial power; quadruple the distance and the power there is 1/16 of its initial strength. For example, the radio on the most distant spacecraft yet launched, Voyager 1, has an output power of 23 Watts. The strength of the signal received at Earth from the spacecraft, now at about fourteen billion miles, is measured in attowatts, or billionths of a billionth of a watt. This is a ridiculously weak signal and very hard to detect.

With the advent of optical communications, the situation can get much better. We haven’t solved the inverse square law problem completely, but it can be managed. Instead of a wide area broadcast of radio waves, in-space optical communications systems use lasers and telescopes to efficiently communicate across astronomical distances. Lasers, as popularized, appear to be the perfect medium for transmitting information across long distances—and they are, to a point.

The concept is simple. The data is encoded in a laser beam and shot toward the receiver, either on a repeater spacecraft in deep space or on the ground. The receiver detects the tightly focused beam of light and converts the optical signal into useful data. In the ideal case, the complete energy of the transmitted beam is received and processed. This assumes, of course, that the transmitter knows exactly where the receiver will be when the light arrives. Remember that in space, everything is moving relative to everything else and the distances are so great, it can be minutes to several hours after a transmission is sent before it reaches its destination since the signal is limited to traveling at or below the speed of light (186,000 miles per second).

In an ideal world, the biggest challenge would be to know exactly where you are pointing the laser and when to point it.

But we don’t live in an ideal world and there are other problems. Lasers, like all other sources of electromagnetic energy, are subject to the inverse square law. (Just because lasers operate in optical frequencies and not radio doesn’t make them exempt!) That means the light coming from the laser source immediately begins to diverge and weaken as the inverse square of the distance. But since the initial diffraction of the laser beam can be made relatively small, little or none of the energy used to transmit the beam is wasted sending electromagnetic energy in directions other than those of direct interest and within the pointing capabilities of the transmitter. So, the laser beam may spread out and provide a weaker signal, but it starts with more directed energy than its radio counterpart and is therefore able to efficiently transmit over much larger distances. Some of the beam divergence, causing a weaker signal, may be overcome by adding a telescope at the receiver—collecting photons of laser light and concentrating them onto a focal plane for counting and decoding.

Laser optical communication has been tested in deep space by NASA and is expected to be used on more and more missions in the years to come.

Space Based Solar Power from Power Beaming

Wireless power transmission is becoming commonplace. Every night, I charge my cell phone by placing it on top of a wireless charger and when I awaken the next morning, the phone is charged—no wires were ever connected to it. This type of near-field wireless power transfer uses magnetic fields and is very limited in range.

Beaming power over long distances, like from the Earth to the sail, requires very high frequency radio or light, such as masers or lasers. Laser power beaming takes advantage of recent breakthroughs in the development of high-power lasers to turn electrical power into laser light. Collimated laser beams can traverse large distances, keeping more power on the desired location or target at greater distances. (Though they are still subject to that darned inverse square law.) Using a photovoltaic cell tuned to be most efficient at the wavelength of the laser being used, one can then convert the energy in the laser beam back into electrical power for use where it is needed.

In principle, power beaming should be able to be used anywhere electrical power is abundant to send that power where it is needed.

There have been many studies over the years of how plentiful electrical power can be sent to the surface of the Earth from space-based solar arrays, providing green energy anywhere on the planet, all the time. Therefore, rather than going into too much detail, I will refer you to these sources for more information:

- The Case for Space Solar Power by John Mankins (Virginia Edition Publishing, 2014)
- Sky’s No Limit: Space-Based Solar Power, the Next Major Step in the Indo-US Strategic Partnership by Peter A. Garretson. Institute for Defence Studies and Analyses, New Delhi.

When thinking about space based solar power, a salient figure of merit to remember is 600 Megawatts (600 million watts). This is the amount of power generated by a typical coal fired power plant that we would want to replace with a space-based solar power station.

Beamed Power for Lunar Bases

If we are ever going to have a permanently occupied base on the Moon, then we need to solve the power problem. Keeping people alive and functioning in the harsh lunar environment, especially the harsh temperature variations between the very hot lunar day (212 degrees F) and the unbelievably cold lunar night (-279 degrees F), will require electrical power and lots of it.

During the lunar day (14 Earth days), it is conceptually simple to deploy solar arrays to generate the estimated 35 kilowatts of continuous power required by an early, permanently occupied, lunar base. Unfortunately, generating this level of power during the lunar night (also 14 Earth days) is not going to be easy. Most lunar base studies solve this problem by advocating the development of space-qualified nuclear reactors and power plants to generate the needed energy or they locate the base at the only place on the Moon that has access to continuous sunlight for power generation—the lunar south pole. If you want to have the flexibility of placing your base, hotel, or other habitat anywhere on the Moon, then you need another option. Fortunately, there is one—place a space-based solar power station in lunar orbit to beam the needed energy to the base.

Using an orbiting space based solar power station to generate electrical power and beam it to a base sited anywhere on the Moon is a near-term option that should be considered. The technology to collect sunlight, generate greater than the estimated 35 kilowatts of required power, and beam it to the surface of the Moon using microwaves or lasers is available today. Furthermore, since the amount of needed power is relatively low compared to that required for a terrestrial space-based solar power station (thirty-five thousand watts versus six hundred million), such a station can be built and launched to a suitable lunar orbit by one flight of NASA’s Space Launch System or a multiple commercial rockets.iii

Having a lunar space-based solar power station providing continuous, “green” power to astronauts on the Moon could be a steppingstone, or proof of principle, toward developing much larger systems for use at Earth.

Beamed Power for Robotic Exploration

We humans are pretty good at powering our robotic spacecraft near the sun. The size, efficiency, and overall capability of solar photovoltaic cells to generate electrical power from sunlight continue to increase, allowing missions to carry more powerful instruments or more capable propulsion systems, many of which use highly efficient solar electric propulsion.

Solar electric propulsion (SEP) is a type of space propulsion that takes electrical power to accelerate a propellant using electrical and/or magnetic fields. Unlike chemical systems, SEP systems require very little mass to accelerate their spacecraft. The propellant is accelerated to speeds ten to twenty times faster than from a chemical rocket and therefore the overall system is many times more mass efficient. NASA, the European Space Agency, the Japanese Aerospace Exploration Agency, and others routinely use SEP systems for missions in the inner solar system and out to Jupiter (Figure 3).

Figure 3. Electric propulsion is real and has been used on multiple space missions. Shown here is an electric thruster being tested at the NASA Glenn Research Center. Yes, when operating, some electric thrusters have that science-fictional blue glow . . . (Image courtesy of NASA.)

Beyond Jupiter, SEP systems aren’t nearly so useful because of the rapid loss in sunlight falling on the solar panels to provide power for the spacecraft. This is due to that pesky inverse square law which says that a spacecraft solar panel operating at Jupiter will produce only 4 percent the power it would at Earth. (Jupiter is approximately five times further from the sun than the Earth; 1/52 = 1/25 = 4 percent the power generated at Earth from a spacecraft’s solar panels operating there.)

Engineers and scientists are today building very large, lightweight solar sailsiv that can be scaled in size to be several hundred square feet in area—allowing them to be covered in solar cells might allow these large solar collectors to generate a lot of power in the inner solar system and barely enough power in the outer solar system where sunlight is dim. If, however, a near-Earth-based laser illuminates the array in deep space, it is possible that far more power could be beamed to and used by the target spacecraft, providing both power and solar electric propulsion.

In a future envisioned by visionary Dr. John Brophy at the NASA Jet Propulsion Laboratory, a multi-hundred-megawatt laser would be used to transmit power to a spacecraft that converts the beamed energy back into electrical power for use by an electric propulsion system, accelerating the craft to speeds sufficient to reach nearby interstellar space within a human lifetime—up to five times faster than Voyager.v (Unfortunately, this is still woefully short of what we need to reach another star—but it would be a step!)

Once we have this capability, it would be a relatively simple step to extend its use to rapidly and efficiently propel spacecraft, mostly uncrewed, throughout the solar system. A steady stream of supplies aboard laser-beamed energy cargo ships using highly efficient electric propulsion systems might traverse the routes between Earth and Mars. Robotic mining ships could be sent throughout the asteroid belt prospecting for whatever materials our future solar system-wide civilization might need. As on Earth, abundant, low-cost power is a key ingredient to successful economic growth—why should this be any different in deep space?

Giant Leap or Incremental Steps?

Progress in space exploration and development used to come in “giant leaps.” Neil Armstrong’s “one giant leap for mankind” is perhaps the most well-known leap. As Armstrong stepped on the Moon for the first time, he was the living fulfillment of a decade-long endeavor that took us from barely being able send someone into space all the way to sending multiple people to the Moon and back again. In 1977 the Voyager spacecraft were launched, taking us on a leap that gave us our first closeup looks at Jupiter and the planets beyond. Since then, we’ve achieved significant milestones of exploration and science, but, sadly, they’ve come at a much slower pace and few, if any, have achieved the status of being a giant leap. And that might not be a bad thing.

Giant leaps tend to be expensive and take many years to develop, fly, and achieve their goals—leaving them open to the vagaries of politics, changes in the economy, and the whims of those who fund them. So, what about the alternative? Is the incremental approach better or at least more sustainable?

Consider Telstar. In 1962, NASA launched Telstar 1, the world’s first real telecommunications satellite. In its meager seven-month life, it broadcast the first live television images between the United States and Europe, showing the benefit of space for providing global communications. While the high-visibility Space Race raged on, private companies began developing more and more sophisticated communications satellites and began launching them into space. Today, space-based telecommunications networks from many countries and companies span the globe and form the basis of a greater than $240B annual economy.vi This is hugely significant, yet almost no one can tell you much about it, how it got there, or when the tipping point was reached at which space-based communications became an integral part of the global economy—demonstrating space development incrementalism at its finest.

The same may be said of the robotic exploration of Mars. We launched the twin Viking landers to Mars in the mid-1970s and, by all measures, they were hugely successful in achieving their scientific objectives. Yet we didn’t go back to the surface of Mars for over 20 years. In 1996 the much-smaller Pathfinder rover launched for the red planet and ushered in a new era of smaller, and ever-more-capable rovers and orbiters being dispatched to Mars, resulting in our current “fleet” of more than six active Mars spacecraft. We are launching a new mission to Mars at a rate of approximately one every two years, a trend that is likely to continue—and is yet another example of successful space incrementalism.

Thus, it may be better to take the incremental approach to space development, with each achievement building upon the successes of those that came before, so that a truly usable new space capability can be not only developed and used but sustained.

Beamed energy for spacecraft propulsion and power might just be the technological path that provides a growing infrastructure for the space exploration and development that many have envisioned since the dawn of the space age. As space science, exploration, development, and settlement advocates, we should applaud grand efforts like Starshot and wish them success. But the incrementalists among us are likely to be the primary benefactors of their efforts. If Starshot is only ten percent successful, then the technologies developed and proven will dramatically expand our capabilities in space and begin to give us the solar system wide civilization that may be a necessary next step before we are truly ready to travel to the stars.

i https://breakthroughinitiatives.org/about

ii A Roadmap to Interstellar Flight, Philip Lubin, JBIS, Vol. 69, pp.40-72, 2016

iii Orbital Space Solar Power Option for a Lunar Village, Les Johnson, IAA Symposium on the Future of Space Exploration, 2017

iv https://www.nasa.gov/content/nea-scout

v A Breakthrough Propulsion Architecture for Interstellar Precursor Missions: NASA NIAC Phase I Final Report, Brophy, John, James Polk, Leon Alkalai, Bill Nesmith, Jonathan Grandidier, and Philip Lubin, 2018

vi According to the Survey of Current Business, the Journal of the U.S. Bureau of Economic Analysis, the global space economy in 2018 was $414.8B, of which 58 percent was consumer services—primarily telecommunications related. https://apps.bea.gov/scb/2019/12-december/1219-commercial-space.htm

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Les Johnson is a husband, father, NASA physicist, and author. Publishers Weekly noted that “The spirit of Arthur C. Clarke and his contemporaries is alive and well . . .” when describing his 2018 novel, Mission to Methone. His 2018 non-fiction book (with co-author, Joe Meany, Graphene: The Superstrong, Superthin, and Superversatile Material That Will Revolutionize the World was reviewed in the journal Nature, excerpted in American Scientist and on Salon.com. His latest anthology, Stellaris: People of the Stars, coedited with Robert Hampson, was released by Baen Books in 2019. You may learn more about Les on his website: www.lesjohnsonauthor.com

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