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Space Solar Power: An Extraterrestrial Energy Resource for the U.S.
August 18, 2021

Space Solar Power: An Extraterrestrial Energy Resource for the U.S.

It is time to reconsider SSP as a valuable tool in the nation's
decarbonization strategy

BY Daniel Oberhaus

Emerging Technology

August 18, 2021
Download PDF

Outline

  * EXECUTIVE SUMMARY
  * INTRO TO SPACE SOLAR POWER SYSTEMS
  * AMERICAN SSP: PAST AND PRESENT
  * SSP APPLICATIONS
  * ECONOMICS OF SSP
  * GEOPOLITICS OF SSP
  * FUTURE OF SSP
  * References

Space Solar Power: An Extraterrestrial Energy Resource for the U.S.

Space Solar Power: An Extraterrestrial Energy Resource for the U.S.

It is time to reconsider SSP as a valuable tool in the nation's
decarbonization strategy

Emerging Technology

EXECUTIVE SUMMARY

It's been 20 years since the first astronauts took up residence on
the International Space Station (ISS) and marked the beginning of a
continuous human presence in space. Yet rather than kickstarting a
new era of space megaprojects, the ISS remains the largest space
infrastructure program ever undertaken. Insofar as it catalyzed a
robust American commercial space industry, it is considered a
resounding success. Launch costs have plummeted and commercial
operators have gained the experience they need to take over key
operational activities from NASA such as resupplying cargo and crew
to the space station. The increased efficiency and lower costs have
allowed NASA to once again set its sights on more ambitious deep
space efforts, particularly sustained crew operations on the lunar
surface and a crewed mission to Mars.

At the same time, the reduced cost of space access has created unique
commercial opportunities in low Earth orbit. Although we are still in
the early days of in-space manufacturing, several American companies
have made great strides in laying the technical foundation for an
industrial cislunar economy. Moving heavy industry off-world would
reduce greenhouse gas emissions and would accelerate our transition
to a low carbon economy. This is an old dream -- most famously
articulated by the late Princeton physicist Gerard K. O'Neill^1 -- but
the technologies for realizing it are no longer science fiction.

What is often left unsaid in discussions about extraterrestrial
industrialization and deep space settlement is how to supply the
energy needed for large scale infrastructure projects. Nuclear energy
has long been the power source of choice for deep space missions.^2
This is largely because nuclear power systems can operate for decades
without intervention and in locations where there is limited or
non-existent sunlight. But nuclear energy is limited in its ability
to scale and also creates serious health hazards for near-Earth
operation.^3 In this paper, we make the case for space-based solar
power (SSP) megaprojects as relatively low-cost, scalable, renewable,
and always-on power source for on-and-off world applications.
Although SSP is a space-based energy asset, it has the potential to
rapidly accelerate decarbonization on Earth while also fulfilling
space exploration priorities.

SSP is a decades-old idea that has only recently become economically
viable due to the rapidly falling costs of space access and
technological advancements such as higher efficiency electronics,
low-cost mass-production of modular space systems like satellites,
robotic in-space construction, and wireless power transmission. NASA,
the Department of Energy, and several other research agencies have
conducted in-depth studies and limited experiments on SSP, but the
development of this energy resource was hindered by unfavorable
economics. Things have changed and it is time to reconsider SSP as a
valuable tool in the nation's decarbonization strategy.

This paper shows how the development of SSP can serve several
national imperatives at once. In space, it can provide a renewable
and cost-effective source of energy for moon bases and deep space
missions. SSP can also provide a valuable source of energy -- both
electric and thermal -- for industrial processes in cislunar space.
This will facilitate the transition of heavy industry from Earth to
space, which will mitigate carbon emissions in the medium-to-long
term on Earth. Critically, SSP will have a massive impact on
terrestrial greenhouse gas (GHG) emissions in the near term through
wireless energy transfer from space to Earth. This is SSP's original
"killer app," and multiple studies have shown that SSP can meet a
substantial portion of Earth's energy needs. Unlike terrestrial solar
power, SSP is always on. It can provide solar power rain or shine,
day or night. It is also flexible and can be quickly redirected to
ground stations in geographically distant locations to meet rapidly
changing energy needs.

The dream for SSP is to have a source of clean baseload energy that's
available regardless of weather, location, or time of day. The
baseload is the minimum electrical energy demand on a grid, which has
historically been provided by power stations that are able to
generate large and relatively constant amounts of energy. But as more
renewables penetrate the grid and create fluctuations in electric
supply, the base load power stations of the future must be flexible
enough to rapidly ramp up and down to meet the evolving supply and
demand dynamics of the grid.

Much like the advent of GPS, a robust SSP capacity would have
profound geopolitical implications. China is investing heavily in SSP
and plans to have the first operating SSP plant in orbit by the end
of the decade.^4 The Department of Defense (DOD) is also pursuing SSP
research for military applications. Notably, the Air Force Research
Laboratory recently created a $100 million program to advance key SSP
technologies.^5 This paper concludes that the U.S. must allocate
substantially more human and financial capital to SSP as part of its
national security, domestic energy, and space exploration strategies.

INTRO TO SPACE SOLAR POWER SYSTEMS

Space solar power (SSP) is the catch-all term for orbital systems
that collect sunlight and convert that solar energy into microwaves
or lasers and transmit that energy to receivers on Earth. On the
ground, the electromagnetic energy is converted into electricity and
delivered to the grid. A practical solar power satellite was first
conceptualized in 1968 by Dr. Peter Glaser, an aerospace engineer at
the American engineering firm Arthur D. Little.^6 He proposed a
microwave transmission system to beam power from a solar satellite in
geostationary orbit (GEO). This is a unique equatorial orbit 22,236
miles above the surface that allows a satellite to maintain a steady
position relative to a point on the Earth.

Glaser's idea was ahead of its time. The first solar-powered
satellite had only been launched a few years earlier and solar energy
comprised a negligible part of power energy generation. A lot has
changed in the past few decades. Today, solar power is widely used
for terrestrial applications and is rapidly increasing its share of
the power generation mix as a result of the plummeting cost and
rising efficiency of solar cells.^7 Yet the expansion of terrestrial
solar power is clouded by fundamental limitations of the energy
resource. These include physical limits on solar cell efficiency as
well as natural fluctuations in the availability of solar energy due
to weather, seasonal variations, and the Earth's rotation.^8

AVAILABILITY OF SOLAR ENERGY: EARTH AND SPACE

The Earth is constantly bathed in enough solar radiation to meet
humanity's energy needs several times over.^9 But only a small
fraction of this sunlight can be economically harvested for power
generation on the ground. The best conditions on Earth for collecting
solar energy are found on a clear day at noon along the equator,
where the power intensity of sunlight is approximately 1 kW/m^2.^10
But most systems have access to significantly less solar power. For
example, the day-night cycle reduces the available sunlight by about
half each day depending on the latitude. Likewise, seasonal
variations can result in up to 60% loss of sunlight intensity
depending on the latitude above or below the equator, with a greater
latitude corresponding to wider seasonal variations. In fact, there
can be up to a three-fold difference in available sunlight between
the summer and winter months depending on the location of the solar
asset. Weather variation is also a major factor: cloud cover can
result in anywhere from a 20% to 90% reduction in solar energy
depending on its thickness and persistence.^11

[Solar-Irradiation-U]

Note that the annual total kWh peaks at around 3000 kWh/m^2 in the
American southwest. This is equal to about 125 full 24-hour periods
of peak irradiance (1 kW/m^2). A SSP platform would be exposed to a
higher peak irradiance of 1.4 kW/m^2 24 hours a day, 365 days a year
for a total annual irradiance of 12,264 kWh/m^2.

Terrestrial solar power is highly variable and limited to a power
intensity of about 1 kW/m^2 under ideal conditions. In typical
locations in the U.S., peak intensity is closer to 50-100W/m2 in the
winter and 250 W/m2 in the summer, which makes solar poorly suited to
supplying baseload power on Earth. Since space-based solar assets are
not subject to atmospheric interference or seasonal variations and
can always face the sun, they can always harvest solar energy at its
peak intensity. In geostationary orbit, solar intensity is roughly
1.4 kW/m2.^12 This is roughly 40% more solar energy than reaches the
surface of the Earth under ideal -- yet transitory -- conditions.

SSP ENERGY EFFICIENCY

No physical system is perfectly efficient, which means that not all
of this solar energy can be captured by a SSP satellite. A space
solar power system has three main components -- the solar panels/
collector, a transmission system, and a ground receiver -- and each
bleeds energy due to intractable hardware inefficiencies.

The cells used in solar panels on Earth are typically single-junction
silicon cells, which have a maximum theoretical efficiency of around
33%,^13 although most commercial solar panels operate at closer to
20% efficiency.^14 Although significantly higher efficiencies have
been achieved by using different materials, multi-junction cells, or
solar concentrators,^15 these systems are substantially more
expensive and many remain in very early stages of development.
Currently, the world record for solar cell efficiency is just over
47%, which was achieved on a single experimental multi-junction cell
using a solar concentrator.^16 This suggests we can expect that any
cells used in a SSP satellite in the next two decades are very likely
to have an efficiency below 50%.

When the solar energy is converted to electricity, it is delivered to
a transmission system onboard the spacecraft to beam the energy to a
receiver on Earth. These transmission systems come in two main types:
microwave and optical.

Optical SSP systems use lasers operating in the near infrared part of
the electromagnetic spectrum, which means they are not visible to the
human eye. The main advantage of optical SSP transmitters are the
short wavelengths of the laser, which allows for substantially
smaller receivers on the ground.^17 (Beams of radiation spread out as
they travel through space; longer wavelengths and larger distances
create larger beam spots on the ground, which require larger
receivers.) The primary disadvantages of lasers are they don't pass
through clouds and are less efficient than microwave technologies.
Typical solid-state lasers today have maximum efficiencies around 30%
and photovoltaic receivers have efficiencies around 50%. When coupled
with typical atmospheric absorption for infrared wavelengths, this
means that the entire transmission system would have a total,
end-to-end efficiency of around 10%.^18 Even assuming significant
progress on the laser and photovoltaic receiver, future optical
transmission systems are likely to max out at around 15% end-to-end
efficiency.^19

The limitations of optical systems have led many SSP researchers to
focus on microwave transmission systems, which are not subject to
atmospheric interference and can achieve significantly higher
efficiencies. These transmitters would operate at wavelengths between
2 and 20 centimeters, which is thousands of times longer than the
optical systems.^20 The longer wavelengths result in a corresponding
increase in the size of the ground receiver on Earth. In most
microwave SSP concepts, the ground receiver consists of an array of
rectennas -- a type of receiver that converts electromagnetic energy
into direct current electricity -- that work together to form the
equivalent of a single large receiver. These arrays are massive: In
most designs they cover areas on the order of several square
kilometers. To put this in perspective, the largest radio telescope
array in existence is the Square Kilometer Array (SKA) facility in
South Africa, which will cover a single square kilometer when it is
finished.^21 Although the size of a ground receiver for a microwave
system is large, it is relatively modest compared to its power
output. A 2,000 MW SSP system would require a ground receiver
covering about 30 square kilometers. By comparison, the surface area
of Lake Mead is about 600 square kilometers and feeds the Hoover Dam,
which has a peak generating capacity of 2,000 MW.

The primary challenges associated with the ground portion of a SSP
platform is the size of the receiver and the energy intensity at the
point of reception. The actual receiver technology itself -- rectennas
in the case of microwave transmission and photovoltaics in the case
of optical transmission -- are relatively simple. In the case of a
rectenna array, it would consist of millions of antennas each about 6
inches in size that could be fabricated in large sections in an
automated factory and easily interconnected in the field.^22 A recent
study found that the rectenna receivers could be deployed at a cost
of about $10 per square meter assuming they are produced at
sufficient scale (e.g., millions of receivers).^23

The size of the receiver can vary greatly with the architecture of
the SSP platform and the components used for wireless transmission.
Two of the main determinants of the size of the receiver are the
location of the SSP platform in space and the wavelengths used for
wireless power transmission.^24 Generally speaking, the higher the
orbiting altitude for the platform, the larger the beam footprint
will be on the ground. Likewise, shorter wavelength systems, such as
high-frequency microwaves or near infrared optical systems can enable
substantially smaller receivers on the ground. The design
requirements of a SSP platform requires choosing either an optical or
microwave transmission system rather than a combination of both.
Thus, any SSP system must be developed in the context of a particular
application to justify the tradeoffs between the efficiency and cost
of both the transmitter and receiver.

A key policy issue associated with SSP platforms will be where to
site the systems and how to ensure that they do not exceed safe
energy intensities on the ground. Both factors suggest that any
receiver will likely have to be placed in a sparsely populated area,
such as the desert, open fields, or on a floating platform in the
ocean.^25 In all of these cases, it would be necessary to build
high-capacity transmission infrastructure to connect the receivers to
the local grid.

The power intensity on the ground is related to the design of the
space-based SSP components, but research shows that it is possible to
build systems that will not pose a health hazard to humans or other
life on the ground near the receivers. This would be accomplished by
limiting the energy intensity of SSP to levels below 1,000 watts per
square meter, which is comparable to full summer sunlight at the
equator.^26

AMERICAN SSP: PAST AND PRESENT

Despite SSP's promise, most research on the subject over the past
half century has been conceptual with limited hardware
demonstrations. In the decade after Glaser's groundbreaking paper,
NASA and the Energy Research and Development Administration -- the
precursor to the Department of Energy -- collaborated with industry on
a handful of SSP studies. NASA's Jet Propulsion Laboratory and
Raytheon, which invented microwave wireless power transmission,
partnered for the first major experiment involving SSP in 1975. That
test involved beaming more than a kilowatt of microwave energy from a
Deep Space Network dish -- typically used to communicate with NASA's
exploration spacecraft -- to a receiver built by Ratyehon located a
mile away in the California desert. It remains the longest distance
and most powerful demonstration of wireless power transmission to
date.

In 1979, NASA and the Department of Energy published the first major
study on space solar power that examined the costs and technical
challenges associated with building a SSP platform. Known as the 1979
SPS Reference System, it consisted of a rectangular array of solar
panels 20,000 meters long by 5,000 meters wide. The system would be
capable of delivering up to 10 GW of energy -- about 1% of today's
installed generation capacity in the U.S.^27 -- to a receiver array
consisting of millions of small antennas -- known as a rectenna --
arranged in a circle 10 kilometers in diameter. The limits of 1970s
technology meant the concept would have required assembling the
platform in orbit using multiple space stations and the labor of
hundreds of astronauts working side by side with thousands of robots.
It would also require the development of fully reusable launch
vehicles that would have been about five times larger than the space
shuttle that was under development at the time.^28

The 1979 SPS Reference System was as expensive as it was ambitious.
The report estimated that the completed system would cost between
$300 billion and $1 trillion (in current dollars) and take 20 years
to build.^29 It would have been possible to begin using the system
prior to its completion, but the report estimated that it would still
require a $400-500 billion outlay (current dollars) to get to first
power.^30 The federal response to the 1979 SPS Reference System study
was overwhelmingly negative, and the congressional Office of
Technology Assessment and National Research Council both issued
scathing indictments of the concept.^31^, 32 Not only were most of
the technologies in a very low stage of development, but the
exorbitant cost of the system was a non-starter. In 1980, the U.S.
federal government eliminated funding for any further work on space
solar power.^33

Throughout the 1980s and early 1990s, there was little research on
SSP conducted in the United States. Some American companies,
including Boeing, sponsored small private research efforts, but most
of the modest R&D work that happened during this period occurred
abroad, particularly in Japan.^34 However from 1995 to 1997, NASA
revisited the topic with its "Fresh Look Study." The goal was to
determine whether new technologies had emerged since the 1970s that
would make space solar power economically viable and several SSP
concepts were evaluated.^35 The study concluded that the technologies
required for SSP had matured enough to make the system economically
viable, but NASA still rejected the proposal at the completion of the
study. Possible reasons for NASA's continued rejection of SSP
included the agency's focus on nuclear power for space systems and a
general view among NASA's upper management that building space
systems to generate commercial power is outside the agency's mandate.
^36

Although NASA leadership wasn't enthusiastic about the prospects of
SSP, it did catch the attention of policymakers elsewhere in the
federal government. Following the Fresh Look Study, both the House
Subcommittee on Space and Aeronautics and the White House Office of
Management and Budget recommended follow-on efforts.^37 As a result,
Congress allocated additional funds to NASA's budget to conduct more
research on SSP over the next three years with a focus on systems
studies and limited technology demonstrations.

In 2000, about midway through NASA's follow-on SSP research program,
NASA contracted with the National Research Council to produce an
independent review of its SSP roadmap. The NRC concluded that the SSP
roadmap was a "credible plan for making progress toward the goal of
providing space solar power for commercially competitive electric
power," but also acknowledged the significant economic and technical
headwinds that would be faced by any SSP project. Still, the message
from the report was clear: 20 years after NASA's 1979 SPS Reference
System, space solar power had become an achievable and desirable
goal.^38

Internal resistance to SSP at NASA once again effectively halted
research on the subject in 2003 and it wasn't until 2007 that the
topic was revisited with federal dollars. This time, it was the
Department of Defense carrying the SSP banner with its first-ever
study on the concept for the National Security Space Office. The U.S.
military was interested in SSP as a way to supply affordable,
on-demand energy to remote DOD installations around the world. While
the DOD study did not delve into technical details of a SSP system,
it did underscore the valuable role for SSP in national security.^39

Around this time, a handful of SSP startups were also formed in the
United States. Arguably one of the most notable American SSP startups
is Solaren, which was founded in 2001 and is still active today.
Solaren's patented SSP system would use a solar concentrator to focus
sunlight onto solar panels and beam the electrical energy back to
Earth using a microwave transmission system. Although Solaren has yet
to launch any hardware to space, it plans to launch a demonstrator
unit within the next few years.^40 It also has the notable
distinction of signing the first space solar power purchase agreement
with PG&E in 2009.^41 The terms of the power purchase agreement
required Solaren to begin providing service by 2016 and it is unclear
if the purchasing agreement is still in effect.

Federal research on SSP has increased dramatically in the past five
years driven by the rapidly falling costs of space access and
geopolitical concerns. (See "Geopolitics of SSP" below.) The
resurgence of SSP R&D has both civilian and defense dimensions. In
2012, NASA's Innovative Advanced Concepts (NIAC) program, which
focuses on high risk, high reward space technologies, sponsored a
research project on a new solar power satellite called ALPHA, which
was the agency's first major SSP study in a decade.^42

The ALPHA study was led by John Mankins, who was also a lead author
on the agency's Fresh Look Study in the mid-1990s. ALPHA is arguably
the most advanced space solar power reference concept produced to
date and demonstrates how thinking around SSP systems has evolved
over the past few decades. Whereas the 1979 space solar power
Reference System was a monolithic structure that would require a lot
of astronaut labor to assemble, ALPHA is a modular system designed to
be assembled by autonomous robots.

The ALPHA platform would consist of thousands of relatively small
reflectors that would be capable of moving independently of one
another to maximize sunlight collection. The combined solar energy
from these modules would be routed to a single large microwave
transmitter at the aft end of the structure. Mankins proposed several
different configurations for ALPHA, although most reference designs
look similar to a bellflower whose petals are composed of
interconnected solar panels. But the critical feature of the ALPHA
system is its modularity, which dramatically drives down costs by
enabling the mass production of standardized components.^43

[SPS-Alpha]

The SPS-ALPHA design developed by John C. Mankins as part of a 2012
NASA Innovative Advanced Concepts study.

Another notable development in civilian SSP was the creation of
Caltech's Space Solar Power Project in 2015.^44 The 3-year program
was funded with $17.5 million from Northrop Grumman and aims to
develop ultralight SSP hardware.^45 In 2017, Caltech unveiled a
series of prototypes of its SSP hardware and demonstrated that they
could be used to collect sunlight and wireless transmit energy.^46
Although the scale of the demonstration was small, it represented an
important advance from the realm of theory to functioning SSP
hardware. Furthermore, the team demonstrated remarkable progress on
reducing the weight of SSP modules, a critical design element that
accounts for a substantial portion of the cost of an SSP system.
Their prototypes were more than 10 times lighter than previous
examples.^47 In 2021, Caltech announced a $100 million gift that it
had received in 2013 from a private donor to work on SSP. The private
funding will culminate in the launch of a 6-foot-by-6-foot space
solar power platform that will demonstrate solar power generators and
wireless transfer technology in 2023.^48

There has also been notable progress on SSP in the defense sector
driven by the Naval Research Laboratory. In 2019, NRL demonstrated a
wireless power transmission system that beamed 400 watts of power
between two receivers situated on each end of the David Taylor Model
Basin, a large research wave pool in Maryland. Although this
technology was developed to beam power from the surface to aerial
drones, the researchers also highlighted its potential application to
space solar power, which is another research area under active
investigation at NRL.^49

For the past few years, NRL engineers have been focused on developing
hardware for a so-called "sandwich module" for SSP applications.^50
These modules consist of photovoltaics, DC to electromagnetic
converters, and the transmission antennas stacked as layers in a
"sandwich" configuration. This arrangement is desirable because it
substantially reduces the complexity of distributing power from solar
panels to a transmission system in a non-integrated system.^51

After extensive testing on the ground, NRL launched a 12-inch
sandwich module to orbit for further testing on the military's
secretive X-37B spaceplane in May 2020.^52 The goal of the
experiment, which is still ongoing, is to test how efficiently the
sandwich module converts sunlight to microwave power in the vacuum of
space. Preliminary results from the experiment published in early
2021, reveal an 8% efficiency for the module.^53 This is roughly half
the efficiency of a low-end commercial solar panel on Earth and
suggests the technology still requires significant development before
it is ready for operational use.  Although no energy will be
transmitted from the space plane to Earth during the NRL experiments,
they mark the first time that SSP hardware has been tested in orbit
and marks a major milestone on the road to a full scale SSP system.

SSP APPLICATIONS

Space solar power is primarily designed to meet terrestrial energy
needs by providing a form of safe, scalable, renewable, and flexible
baseload power. The United States must rapidly decarbonize its
electricity generation if it is to meet its commitments to the Paris
Climate Accord. SSP is an attractive option both for its ability to
directly provide meaningful amounts of baseload power to
geographically distant locations and for its role in facilitating
extraterrestrial manufacturing, which can reduce domestic CO2
emissions by transitioning heavy industry from the surface into
space.

FLEXIBLE BASELOAD POWER

The primary application for a SSP platform is to provide a source of
flexible baseload power. Global electricity demand is predicted to
grow by almost 50% by 2050.^54 To meet these growing energy
requirements while simultaneously decarbonizing our energy systems is
an immense challenge. Fortunately, the cost of conventional renewable
energy resources like wind and solar have plummeted over the past two
decades.^55 But these two resources are ill-equipped to provide
baseload power due to the variability of the energy source.^56
Although this variability can be moderated by storing excess energy
in large-scale battery packs, current battery chemistries are
expensive and only able to store a few hours worth of energy.

There are a number of solutions available to alleviate the challenges
posed by variable renewable energy resources. For example, advanced
nuclear and geothermal power stations are both capable of providing
carbon-free baseload power. But given the uncertainty around the
international and domestic political support that is needed to
support key technical breakthroughs to scale these technologies, it
is prudent to pursue an "all-of-the-above" approach to energy
decarbonization that includes SSP. Compared to SSP, the deployment of
nuclear power stations is further complicated by negative public
sentiment about the energy resource. Much like nuclear energy, SSP
will require a sustained public engagement and education to overcome
the perceived dangers of the technology. In the past, proposed SSP
projects have created public concern about the possibility of these
systems being weaponized or unintentionally causing harm by exposing
humans and animals near the receiver to high doses of electromagnetic
radiation. While numerous studies have shown that the SSP systems can
minimize these risks and don't pose health threats, the success of an
SSP program will critically depend on the clear communication of its
safety features to the general public.^57

Many of the enabling technologies for SSP, including in-space
assembly robotics and wireless power transmission systems, still
require substantial R&D before they can be deployed in an operational
system.^58 Most of the core technological competencies for a SSP
platform -- in space solar arrays, wireless power transmission, and
microwave receivers -- have only been demonstrated on a small scale.
Still, SSP is an attractive solution for flexible baseload power that
avoids many of the challenges faced by these alternatives and also is
uniquely equipped to increase the resiliency of the American grid.

AUGMENTING REGIONAL ELECTRICAL GRIDS

An SSP platform in a geostationary orbit has access to approximately
one-third of the Earth's surface at any given time, which allows it
to beam power to geographically distant locations. Current designs of
SSP systems would allow the platform to switch its power beam from
one receiver to another thousands of miles away in a matter of
seconds. This is an incredible benefit from a grid management
perspective, particularly in a country like the United States that
lacks a fully integrated national grid. If there is a widespread
power outage like the one that occurred in Texas in February 2021^59,
a SSP system can rapidly reroute its power transmission to make up
for the energy deficit. This is also a benefit during non-crisis
situations since energy use reliably peaks at certain times of the
day (morning/evening) and the SSP platform can be used to smooth the
load spikes across time zones.

In this context, the cost and complexity of constructing an SSP
system must be weighed against the cost and complexity of integrating
the United States' grid. Connecting the balkanized U.S. power grid
into a national grid is widely seen as the key to transitioning the
country to 100% renewable energy.^60 The reason for this is due to
the variable nature of key renewable energy resources such as wind
and solar, which requires a careful balancing of supply and demand on
a national scale. For example, there may not be enough local solar
energy available to meet the spike in demand in the evening on the
east coast even though solar facilities on the west coast are
overproducing solar energy. Today, there is no high-capacity
transmission infrastructure to export carbon-free electrons from
regional grids to other areas of the country to meet local demand
spikes.^61

Addressing this problem will require substantial investment in new
transmission infrastructure that will consolidate America's regional
electric grids into a unified national grid. The Biden
administration's recent infrastructure proposal allocated $73 billion
to expanding and modernizing the U.S. transmission system.^62 Some
studies have found that the cost of modernizing and expanding the
U.S. transmission and transformer infrastructure could cost upward of
$2 trillion to reach 100% renewable energy penetration.^63 But even
setting the costs of modernizing the U.S. electric grid aside,
building thousands of miles of new transmission lines faces
intractable policy challenges. The methods for allocating the costs
of these projects can vary markedly from region to region or state to
state, transmission planning zones are extremely fragmented, and new
transmission lines are a perennial NIMBY hot button issue that is
likely to draw strong opposition at the local level.^64

A SSP platform is not necessarily an alternative to a unified grid,
but it is a parallel pathway to further enhance the energy security
of the United States. A SSP system could provide many of the same
benefits of a unified grid at a reduced cost in addition to many
unique capabilities that are not found in any terrestrial generation
or transmission systems. Thus, an SSP platform can be a stopgap that
bolsters grid resiliency and accelerates renewable penetration while
new transmission infrastructure is built on the ground. Once the
domestic grid is unified, it will benefit from the enhanced security
and resiliency provided by the United States' SSP assets.

SPACE MANUFACTURING

The dream of moving heavy industries such as steel, cement, and
chemicals manufacturing into space is nearly as old as the space age
itself. It's a vision most famously articulated by the late physicist
Gerard K. O'Neill, who envisioned a future with millions of humans
living and working in orbital habitats. For now, this remains a
distant goal. But the burgeoning commercial space sector is rapidly
taking steps to turn it into a reality. Jeff Bezos, the founder of
Amazon and the rocket company Blue Origin, has expressed his desire
to turn O'Neill's vision into a reality on multiple occasions.^65 For
Bezos, the ability to manufacture in space is a critical vector for
decarbonization. Heavy industry contributes roughly one quarter of
global greenhouse gas emissions each year and resist easy solutions
for decarbonization due to high energy requirements of the processes
and the long lifetime of the assets.^66 Moving these industries off
world would significantly accelerate decarbonization efforts.

There are still a number of fundamental challenges that must be
addressed before large scale in-space manufacturing becomes a
reality. For example, basic manufacturing techniques such as cutting
or welding metal have still not been demonstrated in microgravity.
This is about to change in the near future during an uncrewed mission
launched by Made In Space, which will demonstrate metal cutting in
orbit for the first time.^67 While this will be a purely experimental
mission, it is a critical first step toward scaling manufacturing
practices beyond Earth.

But even if the critical in-space manufacturing tools were ready
tomorrow, they would still lack a reliable and large source of energy
to drive the processes. Heavy industry depends on substantial thermal
and electrical energy inputs, but all spacecraft launched prioritized
energy conservation. We simply haven't yet launched a substantial
integrated energy resource (e.g., >1MW) to space.^68 (To give a sense
of the scale of an SSP platform, SpaceX's Starlink constellation has
the capacity to generate about 5 MW of electricity, but it is spread
out over nearly 1,500 satellites.)^69 While an orbital nuclear
reactor is one solution, this is still a developing technology and
the first flight demonstrations of a space nuclear reactor aren't
expected to occur until the middle of this decade.

In-space manufacturing presents something of a chicken-or-the-egg
problem for space solar power. All SSP concepts proposed to date have
assumed the ability to assemble the structure in space. But the costs
of launching all the components from Earth are still high. Many of
the raw materials needed for a SSP system can be sourced from
asteroids or the lunar surface and if these could be used to
manufacture the SSP components in orbit it would cause the cost of
the system to plummet. In fact, in-space manufacturing may be key to
making SSP a cost-competitive energy resource. But in-space
manufacturing can't happen without access to a substantial energy
resource. This suggests it may be necessary to absorb a significant
capital outlay for an initial SSP system used in an in-space
manufacturing context before the costs can be brought down to the
point that SSP is competitive with other clean energy resources on
Earth.

DEEP SPACE EXPLORATION

In 2017, the Trump administration directed NASA to land humans on the
Moon by 2024.^70 Known as the Artemis Program, the space agency aims
to establish a permanent human presence on the lunar surface (i.e., a
moon base) and use this outpost as a proving ground for the
technologies that will eventually carry humans to Mars.^71 One of the
main goals of Artemis is to advance the technologies required for
in-situ resource utilization, which will allow astronauts to make
productive use of lunar resources.^72 For example, a heat treatment
process known as sintering can fuse lunar dust to create a
concrete-like material for rocket landing pads, and frozen water can
be broken down into its constituent elements -- hydrogen and oxygen --
which are the main ingredients of rocket fuel.^73

The problem is that all these activities, including simply sustaining
a human habitat, require far more energy than has ever been
artificially generated in space. Moreover, the agency has shortlisted
the Moon's south pole as a likely site for its first landing, which
is a particularly inhospitable piece of lunar real estate.^74 To meet
the energy needs of its planned lunar missions, NASA is developing
small nuclear reactors capable of providing up to 10 kilowatts of
power.^75 While these should be sufficient to meet the needs of a
small lunar crew, they also come with important limitations. They
must be sited sufficiently far from the lunar base to limit
astronaut's exposure to nuclear radiation. Furthermore, these
reactors are effectively immobile once they are operational. Because
of the infrastructure required to transport the energy from the
reactor to the load source, they cannot easily be moved to meet
evolving exploration needs.

Conventional solar power on the lunar surface may also be an
insufficient solution for future moonbases, especially around the
lunar south pole. There are craters on the Moon's south pole that are
permanently shadowed and others that receive near constant sunlight.^
76 The challenge is that it may not be possible to simply put solar
collectors in the permanently sunny craters and route this power to
where resource extraction is happening in the shadows. These craters
may be separated by kilometers of rugged terrain, which would require
substantial investments in lunar surface infrastructure to route
energy from the source to the end user. While there may be areas
closer to the extractive operations that receive sunlight, this light
will be intermittent. And like nuclear power on the surface, any
large-scale solar installations are likely to be immobile.^77

Space solar power is a strong alternative candidate for providing
power for lunar surface operations. It is fundamentally safe for
humans, it can be rerouted to different locations to meet changing
mission requirements, and it can be scaled as necessary. While the
size of both the solar collector and ground-based receiver is a
limiting factor for lunar operations, the tenuous lunar atmosphere
means that smaller systems that use laser light can be a viable
option on the Moon.

ECONOMICS OF SSP

The economics of SSP has been one of the most pernicious issues for
the technology since it was first conceptualized more than a
half-century ago.  There are two primary cost drivers of SSP: launch
services and hardware.^78 There has been substantial progress in both
areas over the past decade.

DECLINING LAUNCH COSTS

For most of the history of space exploration, the rockets used to
place assets in orbit have been expendable, which substantially
increased the cost of satellites and spacecraft. NASA's space shuttle
was the first launch system to be partially reusable, but the costs
of refurbishing the shuttle and its boosters after flight only
resulted in marginal cost savings.^79

In 2015, SpaceX launched and landed an orbital-class rocket booster
for the first time and marked the advent of truly reusable rockets.^
80 Since then, SpaceX has successfully landed boosters on more than
80 occasions, including the boosters used on a reusable heavy-class
launch vehicle capable of delivering up to 50,000 kg to LEO and more
than 6,000 kg to GEO. SpaceX has also demonstrated its ability to
reuse its boosters several times and recently completed its ninth
successful mission using the same first stage of a Falcon 9 rocket. 
Today, the company is rapidly advancing its reusable super-heavy
class rocket, Starship, which will be capable of delivering more than
100,000 kg to LEO and about 20,000 kg to GEO. But if Starship can be
refueled in orbit, it should be able to deliver up to 100,000 kg of
payload to GEO and return to Earth.^81

SpaceX's reusable rockets are a success story of NASA's commercial
cargo program that supported their development. Between 1970 and
2000, the average cost to launch a kilogram to LEO was around $18,500
/kg, but this could vary dramatically depending on the vehicle. The
average cost of launch on the space shuttle, for example, was around
$54,000/kg.^82 By contrast, the advertised cost delivering a payload
to LEO with SpaceX's Falcon 9 rocket is just $2700/kg -- a 20-fold
improvement over the cost of the space shuttle. The economics improve
even more in the case of SpaceX's heavy-class rockets, which cost 40
times less to deliver a kilogram of payload to LEO compared to the
space shuttle.

The cost of space access will continue to decline with the arrival of
still more powerful rockets. SpaceX's Starship is expected to deliver
payloads to LEO -- and perhaps even the lunar surface -- at a cost
below $100/kg. (SpaceX CEO Elon Musk has cited a possible price point
as low as $10/kg to LEO on multiple occasions, but this aspirational
goal depends on several variables such as the payload destination and
annual launch cadence.)^83 And while SpaceX has been a clear leader
in the development of reusable rockets, it is not the only company
working on the technology. Blue Origin, the rocket company founded by
Jeff Bezos, is also developing its heavy-class New Glenn rocket that
will be capable of delivering 45,000 kg to LEO and around 14,000 kg
to GEO.^84

A competitive market that features multiple launch services providers
with reusable rockets will further reduce the costs of space access.
Additionally, the development of new technologies such as on-orbit
refueling and "space tugs," a type of spacecraft used to boost cargo
into higher orbits once it's in space, create the opportunity for
lowering costs further by dramatically increasing the amount of
payload that can be delivered to geostationary orbit. This is a
critical development for SSP, which will require launching thousands
of tons of hardware to GEO for a commercially viable platform.

DECLINING SPACE HARDWARE COSTS

Low launch costs are imperative for a large-scale SSP system, but
arguably the primary cost driver is the cost of the SSP hardware
itself. In the past decade, there has been a substantial decrease in
the cost of space hardware as a result of advances in materials
science and new approaches to manufacturing spacecraft.

In the context of SSP, one of the biggest advances in recent years
has been the declining cost of solar panels. Between 1975 and 2020,
the cost of photovoltaic modules has decreased from more than $100/w
to less than $1/w.^85 Of course, the cost of PV modules varies
substantially depending on their design, the materials used, and the
scale of production, but the overall trend toward dramatically
cheaper solar panels is clear. At the same time, the efficiency of
photovoltaics has markedly improved. Between 1995 and 2020, the
efficiency of crystalline silicon PV systems has risen from around
10% to over 15%, while the efficiency of thin-film PV systems has
increased from just below 5% to just under 15%.

Whereas most solar systems on Earth are likely to use relatively
low-efficiency, single-junction silicon cells, the photovoltaic
systems used for spacecraft tend toward high efficiency photovoltaics
such multi-junction or gallium-arsenide cells.^86 Historically, the
high-performance solar cells used in space applications were far too
expensive to produce for large-scale terrestrial power applications
or SSP. But the cost of these cells is also falling rapidly driven by
new manufacturing techniques developed in the U.S. National Labs.^87
While a full discussion of the merits of various PV technologies in
the context of SSP is beyond the scope of this paper, the bottom line
is that a large-scale SSP platform has become substantially more cost
effective due to the development of low-cost, lightweight, and
high-efficiency solar cells over the past few years.

A SSP platform capable of providing baseload commercial power will
necessarily be incredibly large. This means it will require
inexpensive, lightweight materials that can be mass manufactured for
its core structure. While nothing on the scale of an SSP platform has
ever been built in space, the International Space Station
demonstrated fundamental concepts such as the robotic assembly of
modular components that will be required to build an SSP platform.^88
But despite the ISS being a sort of proving ground for large-scale
space infrastructure, it came at an incredible cost. By the time the
final core modules were added to the ISS in 2008, the cost of
building the space station clocked in at around $100 billion.^89
While this is not a perfect comparison (e.g., the ISS must support
crew, which adds substantial costs), it's clear that if SSP is going
to be commercially viable on Earth, the costs of building large-scale
infrastructure in space must be dramatically reduced.

Since the beginning of the space age, the approach to building
spacecraft has been to treat them as unique objects. This required
building specialized hardware and creating spacecraft-specific
building processes, which substantially increased the cost of a
spacecraft. But the past decade has seen the rise of large-scale
manufacturing for space hardware, which opens the door for low-cost,
large-scale space infrastructure.^90 This was first driven by the
development of cubesats in the 1990s, which use standardized form
factors and hardware to enable the rapid and inexpensive development
of on-orbit testbeds. Today, cubesats are used for everything from
Earth observation to Mars missions at a fraction of the cost of
entirely bespoke satellites in the past.

More recently, very large-scale manufacturing of space hardware has
been given another boost by the emergence of satellite
mega-constellations. These are networks of hundreds of satellites in
low-Earth orbit that are typically designed to provide broadband
access across the entire planet. The largest planned constellation is
SpaceX's Starlink network, which has already been approved for 12,000
satellites and is currently petitioning the FCC to permit up to
30,000 additional satellites for the network.^91 Since the company
first started launching operational Starlink satellites in 2019, it
has single handedly doubled the number of operational satellites in
orbit. By the time it completes its 12,000 satellite constellation,
SpaceX will have increased the total number of operational satellites
in orbit by a factor of six.^92

Achieving the satellite production cadence that is required to create
a mega constellation like Starlink would have been impossible with
conventional satellite manufacturing techniques. By standardizing the
spacecraft hardware, however, SpaceX was able to mass manufacture its
internet satellites and now produces more than 30 tons of operational
satellites with a total of 500-600 kW of power generation capacity 
at its facilities every month.^93 This is roughly the cadence that
would be required to build a commercial SSP system and is a critical
proof of concept that it is possible to build substantial amounts of
flight-ready space hardware through the use of modular, standardized
components.

These are key foundational breakthroughs required to create a
large-scale SSP platform, especially one based on a modular design.
We believe that the plummeting costs of space access and hardware
make it possible to build a 2 GW SSP platform that can provide
flexible baseload commercial power to the grid for under 10 cents/
kwh, which would make it competitive with terrestrial renewable
energy resources.

GEOPOLITICS OF SSP

The geopolitics of space solar power are best understood along two
dimensions: diplomacy and national security.

SSP is valuable as a diplomatic tool both during its construction and
operation. The assembly of such a massive piece of space
infrastructure will almost certainly involve international
participation due to the substantial costs of construction. But even
if SSP projects are undertaken on a national or regional basis, they
will still involve cooperation from the international community to
establish frameworks for their safe operation.

In each instance, there are important historical precedents to
consider. The International Space Station began its life as a
national project of the United States called Space Station Freedom.^
94 But as costs ballooned and policymakers began to sour on the
project, NASA opened the station to international partners. The
lessons learned from the internationalization of the space station,
such as bringing partners into the conversation early, the clear
delegation of responsibilities, and equitable cost-sharing agreements
are directly relevant to the construction of a space solar power
platform. Similarly, there is also a precedent for establishing
operational norms for space. The 1967 UN Outer Space Treaty remains
the guiding document for all spacefaring nations and also establishes
a basic framework for the operation of SSP assets (e.g., their beamed
energy cannot be used as a weapon). Fortunately, the community of
researchers working on SSP are already well acclimated to
international cooperation and consensus building from decades of
workshops and conferences dedicated to the subject.

SSP can also play a vital role in national security.^95 Just in the
past year there have been multiple high-profile failures of the U.S.
electricity grid from extreme weather events, such as the
unprecedented wildfire season in California and the cold snap in
Texas. The vulnerability of our energy supplies to cyberattacks was
also exposed during the recent breach of the Colonial Pipeline.
Cybersecurity experts expect that state-sponsored attacks of critical
infrastructure, particularly energy infrastructure, will become more
frequent in the future and have raised concerns that the U.S. grid is
unprepared to adequately manage this threat. SSP can bolster the
resiliency of the U.S. electric grid by rapidly delivering clean
energy to affected regions of the U.S. in the immediate aftermath of
a natural or man-made disaster.

SSP can further play a role in national security by supplying cheap
and readily available clean energy to remote U.S. army facilities and
U.S. forward operating bases.^96 It would prove to be especially
valuable in the latter case given the high cost of supplying these
bases with electricity. Typically, they rely on fossil fuel-powered
generators, but delivering these fuels to the base can be dangerous
and incredibly expensive. During the early days of the Iraq war in
the mid-2000s, for example, the cost of generator fuel for these
bases ranged from $15 to well over $100 per gallon.^97 A 2009 study
from Deloitte underscored the risk that fuel transportation poses to
American soldiers, concluding that "During the modern age of warfare,
the use of fossil fuels

to power these vehicles has increased exponentially and this
dependence has itself created casualty risk."^98 If SSP is used to
reduce the U.S. military's reliance on fossil fuels to power their
forward operating bases, it could save both money and lives.

A key limiting factor in the use of SSP for remote military
installations is the size of the ground receiver required for SSP.
According to researchers at the U.S. Naval Research Laboratory,
however, it should be possible to build modest receivers that are
able to generate power on the order of 10 kilowatts to 10 megawatts,
which is sufficient to meet the needs of most forward operating
bases. In the baseline model proposed by the researchers, such a
receiver would occupy 0.8 square kilometers or less. This would also
require a substantially smaller space-based collector and
transmission system compared to the massive platforms required to
deliver baseload energy to the grid.^99

Finally, SSP can also be a powerful diplomatic tool that can be used
to help developing nations decarbonize their electric grids through
power purchasing agreements. Climate change is ultimately a global
problem and extraterrestrial energy assets are particularly
well-suited to address the challenges associated with decarbonizing
the world's electricity.

The economic, political, and security benefits of SSP are
well-understood by both allies and adversaries of the United States.
In particular, Japan and China have made the development of SSP a
central priority of their space exploration programs. Japan has been
actively researching and developing experimental SSP hardware since
the early 2000s.^100 In early 2019, China announced its intention to
build a megawatt-scale SSP platform by 2030 and create a
gigawatt-scale SSP station by 2050.^101 India and the European Union
are also pursuing their own SSP projects.^102^, 103 In late 2020, the
UK conducted its first major assessment of space solar power and
identified it as an important option to achieve its national carbon
net-zero goals by mid-century.^104 This makes the United States the
only major space faring nation whose national space agency does not
have a serious plan to develop a SSP platform.

FUTURE OF SSP

The United States' reluctance to pursue SSP can be attributed to a
number of causes. In the 1970s and 80s, the exorbitant projected
costs of an SSP station guaranteed that the project would not be
pursued by NASA, the DOE, or the DOD. At the same time, the agency's
emphasis on developing nuclear space technologies -- a trend that
continues to this day -- undermined enthusiasm for other ambitious
energy projects like SSP. Finally, the fact that SSP is a space
project meant to provide commercial levels of electrical power on
Earth meant that it wasn't obvious whether it fell within the purview
of NASA or the DOE, and so both agencies were reluctant to allocate a
substantial portion of their budget for its development.

Today, the low cost of natural gas and renewables like wind and solar
makes it seem challenging to justify a space energy project of this
scale. But SSP offers several unique benefits as an energy resource,
including its resiliency, its ability to provide flexible baseload
power to geographically distant locations, its capacity to accelerate
decarbonization directly by providing clean energy and indirectly by
expediting the transition to off-world heavy industry, and its
strategic benefits as a tool for diplomacy and national security.

Given SSP's benefits and the interest in the technology from most
other space agencies, it's puzzling that policymakers in the United
States have not prioritized SSP R&D. The development of key
technologies such as reusable rockets and thin film solar panels has
finally made SSP economically and technically viable. But there is
still a lot of fundamental research on SSP that needs to be done and
it is in the United States' national interest to begin this research
program as soon as possible.

So far, the only glimmer of hope for an American SSP program has come
from the DOD's efforts. In 2019, the Air Force Research Lab awarded a
$100 million contract to Northrop Grumman as part of the new Space
Solar Power Incremental Demonstrations and Research (SSPIDR) Project,
which aims to develop hardware for in-orbit SSP experiments based on
the design developed at Caltech.^105 This is by far the United
States' largest federal expenditure on SSP R&D, but it is only a
fraction of what will be required to build a large-scale SSP station
and the specific technologies included in the SSPIDR program will not
result in a system that could ever provide commercial power to
civilians.

SSP is a key tool for ensuring the prosperity and security of the
United States in the latter half of the 21st century. It is
imperative that NASA and the DOE prioritize the development of SSP.
We believe the federal government should earmark approximately $1
billion for SSP research over the next five years with a special
emphasis on advancing emerging technologies and in-space hardware
demonstrations.

Congress must take the first step in establishing a civilian SSP
platform by directing NASA and the DOE to collaborate on a
public-private initiative similar to NASA's commercial crew program
or its more recent commercial lunar payload services program. The
directive must clearly delineate responsibilities between the
agencies in order to avoid leadership paralysis that has stymied
domestic SSP research in the past. Furthermore, a public-private
program must be structured so that there is competition among
multiple private companies, which must hit key milestones in order to
continue receiving contracts. These contracts should be awarded with
a fixed-price structure to avoid the massive cost overruns and delays
that are typical of cost-plus contracts in the aerospace and defense
sector. This is also an approach likely to find support among new
launch providers and spacecraft manufacturers that have demonstrated
the innovation that occurs when operating within the relative
constraints of fixed price contracts. In fact, the main trade group
for the aerospace sector has advocated for the increased use of
fixed-price contracts in the past.^106

Alternatively, it may be more efficient to establish a focused
research organization (FRO) dedicated to SSP technologies to avoid
delays associated with collaboration between two federal agencies on
multi-year--and perhaps multi-decade--projects. FROs are independent
entities that exist outside of national laboratories and
universities. They are effectively a startup for basic research and
deep technological development that requires large-scale engineering
collaboration on technologies that may not yet have a market or are
not readily monetizable.^107 Recently, the U.S. Congress created five
FRO-like centers in the DOE's national labs as part of the National
Quantum Initiative Act, which can serve as a framework for the
creation of similar FROs dedicated to space solar power.^108

While there are several approaches to a large-scale SSP system, we
believe the most fruitful pathway is to focus on cost reduction over
energy efficiency. This would prioritize highly modular systems
similar to ALPHA, which benefit from the substantially reduced costs
of mass manufacturing standardized components. We believe that it is
possible to conduct a civilian SSP demonstration in low-Earth orbit
within three years of the program's start with less than $250 million
in funding. The first phase of this program would involve conducting
a series of ground tests with prototype systems over the course of
about 18 months. Based on the results of this program, a system could
be selected for an in-space demonstration capable of generating up to
300kw of power in low-Earth orbit.

After a successful LEO demonstration mission, the next step would be
to build a larger SSP system in mid-Earth orbit capable of producing
commercial amounts of power (e.g., 1-10 MW). While this orbital
altitude is not sufficient for maintaining the SSP system over a
fixed spot on the Earth, it would stay on a fixed path so that it
always passed over the same spots on the Earth. While the power from
this MEO demonstrator would not be competitive with terrestrial
electricity prices -- we expect a cost of about $1/kwh -- it would be a
critical step toward proving the system's ability to provide
commercial power. We expect that the MEO demonstrator could be built
and launched for approximately $1 billion.

The success of the MEO demonstrator would lay the foundation for an
SSP system in geostationary orbit that would be large enough to
provide meaningful amounts of baseload power. We expect the initial
version of this SSP system to be capable of delivering around 2 GW of
solar energy to the surface. We expect that a 2 GW SSP system in
geostationary orbit could be built for about $10 billion. Here we
start to see the cost savings of mass manufacturing modular SSP
components. This system would be capable of delivering more than 200
times more power than the MEO demonstrator for only 10 times the
cost.

We believe that a public-private SSP program jointly led by NASA and
the DOE could result in a commercially viable SSP platform in
geostationary orbit by the end of the decade. In addition to
providing a critical pathway for SSP, it also has the potential to
lead to substantial advancements in solar power and wireless power
transmission technologies that would be useful on Earth. If
policymakers do not take action on advancing domestic SSP
capabilities soon, the United States will find itself losing its
leadership position in space and increasingly vulnerable to natural
and human-made disasters on the ground.

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all research

Based in Washington, DC and part of the Progressive Policy Institute,
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