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Disclaimer—This paper partially fulfills a writing requirement for first year (freshman) engineering students at the University
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THE PROMISE OF LASER ABLATION IN PROPULSION
Jacob Rosenberger, jtr54@pitt.edu, Sanchez 10:00, Nils Aho, nca7@pitt.edu, Lora 6:00
Abstract- Our nation’s space program has been on the
decline for decades and little effort has been put into potential
long distance space flights due to the length of these travels
and their costs. However, advancements and research in laser
ablation propulsion systems have massive potential to solve
these issues by increasing the thrust and efficiency of
propulsion systems, reducing flight time. Researchers in
propulsion are experimenting with integrating lasers into
more traditional propulsion systems to increase thrust. Lasers
can be used to heat propellants such as liquids, gases or solids
in order to produce an ion cloud that is directed using the
cone of a thruster, creating thrust. Research has been
especially focused in using solids as fuels. In laser ablation,
a laser is shot at a solid piece of material which becomes hot,
burning off material in a plasma cloud that can propel the
craft. Researchers are working on integrating the ablation
process effectively into propulsion systems for future use in
space flight. They are also making the ablation process more
efficient and effective by experimenting with the qualities of
the laser and the shape of the thruster nozzle. This topic is of
significance as our goals of space travel have become more
ambitious. One of the biggest problems facing the mission to
Mars is the distance of the trip. This technology can help
reduce the time of the trip by increasing the speed of the ship
through higher powered propulsion. It is also possible that
this technology can be applied to in-atmosphere aircraft one
day, resulting in faster travel.
reducing the amount of fuel used and lowering the weight of
the spacecraft [1]. These types of propulsion systems can also
be applied to smaller spacecraft such as microsatellites,
allowing for more precise movement. The laser ablation
process and how it will be integrated into a propulsion system
will be discussed in the following sections as well as the pros
and cons and impacts of such a system.
In a broad definition, ablation is the process of removing
a material by heating it and turning it to vapor. This can be
achieved in different ways but our technology uses lasers to
heat the material. The process of laser ablation is heavily
rooted in physics which dictates the results of the laser being
focused on a piece of material. In most materials, the focusing
of the laser on it causes it to become heated to such a degree
that it vaporizes [2]. When the material is vaporized by the
laser, it produces a plume made up of several possible
substances including free electrons, molecular fragments,
ions, neutral particles and products of chemical reactions that
may occur when the laser interacts with the material [2]. This
plume of vaporized materials is often referred to as the plasma
plume. Ablation can be performed on solids, liquids, and
gases but we will focus on the use of solids in this paper.
The extent to which the material is vaporized is dependent
on several factors of both the material and the laser. One of
the more important factors is the degree to which the material
can effectively absorb the laser energy [2]. Essentially, how
deep the laser can penetrate the material affects how
effectively the material is vaporized. A material with little
ability to absorb the laser helps the ablation process as the heat
is concentrated on a smaller volume of the material. A smaller
volume of material is heated and vaporized more effectively
than if the heat created by the laser was occupying a large
volume of material.
In most cases, metals are used as fuels in the ablation of
solids. This is due to their low penetration depth which results
in more ablation. The drawback to using some metals in
ablation is their high thermal conductivity [3]. The high
thermal conductivity allows the heat to spread out over a
greater area, diminishing the ablation process. The threshold
intensity needed to create ablation can be tens of times higher
than materials with a low thermal conductivity [3]. This
requires a greater intensity laser to be used in the ablation
process to produce vaporization of the material. Other
materials that are being used include polymer compounds
such as polyvinyl chloride (PVC), polyoxymethylene (POM),
Key Words- laser ablation, plasma cloud, propulsion, space
flight, thruster nozzle
THE PHYSICS OF LASER ABLATION
Propulsion systems form the basis of our space program
as well as modern air transportation. As our goals of
spaceflight grow more ambitious with goals of a manned
mission to Mars, our current propulsion systems pose
limitations. The main limitation of the mission is the long
distance, which given our current propulsion systems would
make for a lengthy trip. A key step into making our goal of a
manned mission to Mars become a reality is finding a way to
reduce the time of the trip. Developments in integrating a
process known as laser ablation into a propulsion system
could hold the key to making this goal a reality [1]. This
process can also help in making the flight more efficient by
University of Pittsburgh Swanson School of Engineering
2016-03-03
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Jacob Rosenberger
Nils Aho
and polyvinyl nitrate (PVN) [3]. These polymer compounds
have a low thermal conductivity unlike many metals which
allows the heat to stay more centralized at the location where
the laser is striking it [3]. A specific class of polymers that
are of high potential in ablation are energetic polymers such
as hydroxyl terminated polybutadiene (HTPB). These types
of materials are desirable because they burn more cleanly and
are more environmentally friendly [4]. This would be
important if integrated into propulsion systems on earth in the
future as they would be more ecologically friendly than
normal propulsion systems. However, polymer compounds
have their own set of limitations as they have high penetration
depth, which causes large amounts of mass dissipation when
compared to metals [3]. A large mass dissipation means that
larger amounts of the fuel are burned up during the ablation
process. This causes the material to be used up faster which
would make it a less efficient fuel when used in propulsion
and would require a greater amount of fuel to be taken in a
craft when used for spaceflight. The greater amount of fuel
needed is a detriment as it adds weight to the craft and thus
adds cost to the flight.
As shown, the properties of the material being used as fuel
have large impact on the ablation process. This is not the only
factor in ablation as the degree to which the laser can be
absorbed into the material is also affected by properties of the
laser such as the wavelength of the laser and the energy
density of the beam [2]. In most cases, the laser energy in
ablation is delivered in pulses, so the duration of these beam
pulses will also be a factor. Overall, these factors of the laser
affect the amount of energy delivered by the laser to a specific
area of material. The measurement of energy per area
delivered by a laser is known as fluency, which is measured
in Joules per centimeter squared (J/cm^2) [5]. The more
energy delivered by a laser the more ablation of the material
will occur. Although more laser energy produces more
ablation, it is ideal to create systems that do not require large
amounts of laser energy to produce effective ablation. In later
sections, methods used to maximize ablation compared to
laser energy will be discussed. There are also other factors
regarding the material used as fuel and the qualities of the
laser that affect the ablation process. These factors will also
be discussed in more detail in later sections.
opposite direction, propelling the craft forward like in
traditional gas propulsion systems. The plasma plume created
by ablation can be the only source of thrust used to propel the
craft forward. Another option that is getting attention from
researchers is using the laser ablation process alongside a
supersonic gas flow created by another system [1]. In these
cases, the ablation process is used as a supplement to
significantly increase the thrust and efficiency of a normal
propulsion system.
There are different options for the exact positioning of the
laser in regards to a propulsion system. Some designs place
the device creating the laser in the actual system while others
are designed to have the laser shot at it from a remote location.
It is possible that in the future these lasers would be located
on earth and would be fired at vehicles in space to help propel
them [1]. There are different methods of ablation propulsion,
each with their own advantages and drawbacks which will be
discussed in this paper. These different methods of ablation
propulsion suit different systems and goals.
To focus the laser effectively on the fuel and to create
ideal ablation, different methods of concentration can be used.
One way that has been used in micro-propulsion is the use of
a transparent substrate layer that is positioned on top of the
layer of fuel that is to be ablated [3]. Figure 1 shows the
alignment of the substrate and the fuel, in these cases, a layer
of either glass or water was used as substrate. Image A shows
the use of glass as a substrate layer and Image B shows water
used as a substrate In this method, the substrate confines the
movement of the plasma plume and focuses the laser more
precisely on the material that is being used as fuel. This type
of ablation is known as confined ablation [6]. The layer of
substrate directs the vaporized material around it,
perpendicular to the material being used as fuel. The
vaporized material is then pushed into the walls of the device
that is enclosing this system, in this case a nozzle. The
vaporized material creates a force against the walls of the
nozzle and this force creates a driving shock wave that pushes
the material out of the nozzle, creating propulsion [6]. This
substrate layer on top of the fuel has the added benefit of
preventing the vaporized material from being directed back
into the optics, obscuring the laser and diminishing the power
of the laser and therefore decreasing the ablation. The
confinement method also had an effect on properties of the
plasma plume. This included an increase in the temperature
of the plume and an increase in the electron density of the
plume [6]. This method of ablation created a more efficient
result with a higher coupling coefficient value.
The coupling coefficient is an important term when
discussing laser ablation and will be used frequently in this
paper. The coupling coefficient is defined as the ratio of the
thrust produced to the laser power used in ablation, Cm= T/P
[3]. The coupling coefficient is measured in Dynes per Watt
(N/W). Essentially this measures how effective the ablation
process is at producing thrust with a certain amount of energy
delivered from a laser. Tests between methods are often run
at the same laser power to compare the coupling coefficient
INTEGRATING LASER ABLATION INTO
PROPULSION
The laser ablation process has been used for processes
other than propulsion such as micromachining and
nanoparticle production [2]. The use of ablation in propulsion
would be expanding it to a wider scale than used in most other
industries. The key to integrating the laser ablation process
into propulsion is harnessing the plume created by the
ablation and directing it out of the cone of a thruster [1]. As
the vaporized material is ejected from the fuel in a plume, the
acceleration of that mass produces an equivalent force in the
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Jacob Rosenberger
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values. The coupling coefficient is a good indicator of how
efficient and effective the ablation is at producing thrust and
therefore propulsion. A higher coupling coefficient value is
desired and research is often focused in increasing this value.
In experiments performed to investigate the use of substrates
to focus laser energy, trials were run using layers of glass or
water as substrates. It was found that the water layer was
highly effective in increasing the coupling coefficient of the
ablation. At 53 mJ of energy delivered by the laser, the
coupling coefficient of water was 308 (D/W) to glass’
coupling coefficient of 116 (D/W) [3]. This makes using
water as a substrate an effective method of increasing
propulsion, more effective than using glass.
The use of substrates mentioned before is just one of the
methods being developed to focus the laser energy more
effectively and create better ablation and thus better thrust.
Another promising method that is in development is based on
the interaction of the plasma plume generated by ablation and
a supersonic gas flow that is normally used in propulsion [1].
In this method, two types of laser concentrators were
experimented with recently. One type of concentrator used is
a parabolic beam concentrator, these structures contain a
mirrored inner surface which focuses the beam on a small area
of the fuel’s surface creating ablation. The shape is similar to
that of a satellite dish and its symmetry allows for effective
focusing. Another type of beam concentrator is the off -axis
parabolic concentrator [7]. These types of concentrators have
a less symmetrical shape than the normal parabolic
concentrators but work in the same way.
In systems that combine laser ablation and a supersonic
gas flow, the interaction of the vaporized plasma plume with
the gas flow causes an increase in thrust. Propulsion system
designs of this type are known generally as LIGHTCRAFT
[7]. They are made up of an engine cowl, a parabolic
afterbody concentrator and a conical forebody as pictured in
Figure 2. This diagram shows the design of a thruster nozzle
using an off axis parabolic concentrator. 1 represents the
engine cowl, 2 is the off-axis parabolic afterbody
concentrator, and 3 is the conical forebody
The forebody shapes the incoming flow going into the
system. The forebody shape and the engine cowl increase the
gas pressure inside the nozzle [7]. The concentrator obviously
works to focus the laser energy being shot at the system. The
shape of the concentrator allows for some margin of error
when shooting the laser at it.
FIGURE 1 [3]
Focusing Laser using substrates
Another important term to use when talking about laser
ablation is the specific impulse. The specific impulse is
defined as the ratio of the coupling coefficient to the ablated
consumption mass [3]. This essentially means the amount of
thrust produced when a certain amount of the ablated solid
(fuel) is used up. The higher the specific impulse, the better
the method. With a high specific impulse, we are using up
little of our fuel while generating the thrust. Materials with
high specific impulses make for longer lasting fuels in
ablation propulsion and would help reduce the fuel load of a
spacecraft with an ablation propulsion system. As mentioned
previously, polymer compounds have a lower specific
impulse compared to metals due to their high penetration
depth, this makes them less efficient fuels. In the experiments
described previously, when water was used as a substrate, the
specific impulse value was better than when a glass substrate
layer was used [3]. This is another way in which the use of
water as a substrate helps propulsion.
The use of substrates to increase ablation has been shown
to be an effective method however at the time it is only being
used to perform micropropulsion. This makes the process
useful for the precise movement of satellites and other smaller
craft in space. This form of propulsion also has potential for
on earth uses such as micro-robots in nuclear reactors or ships
[6]. However, this form of propulsion is not useful for larger
craft such as space shuttles but there are other methods of
laser ablation propulsion that will work better in these larger
vehicles.
FIGURE 2 [7]
Thruster nozzle with off-axis parabolic concentrator
The increase in thrust is due to the way in which the laser
ablated plume alters the flow of the gas out of the nozzle.
When the ablated materials interact with incoming gas flow,
a stationary shock wave is created. When using an off-axis
parabolic concentrator, this shock wave was created in a
critical space of the nozzle [7]. It causes an increase in
pressure in the area of the nozzle close to the afterbody which
generates more thrust. Other laser propulsion techniques
using regular parabolic concentrators were limited by
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Jacob Rosenberger
Nils Aho
shockwaves as they occurred in positions detrimental to the
thrust, but the shock waves created using an off-axis parabolic
afterbody create beneficial shockwaves.
The interaction of the plasma plume and the normal
supersonic gas flow also causes a reshaping of the gas flow
out of the nozzle. In tests using the off axis parabolic
afterbody and ablation, the gas flow was directed closer to the
walls of the nozzle. This difference in the gas flow out of the
thruster nozzle is shown in Figure 3. Section A shows the gas
flow out of the nozzle without laser ablation and Section B
shows the gas flow with ablation. As visible, with ablation the
gas flow is more powerful along the walls of the thruster
nozzle. The gas flow being closer to the walls of the nozzle
created an axial force perpendicular to the nozzle walls equal
to 1340 Newtons, this extra force produces more thrust [7].
This type of flow is also characterized by a large amount of
acceleration near the engine cowl. In multiple tests, the offaxis concentrators proved to be more effective than the
normal on-axis parabolic concentrators in increasing thrust.
The downside to both regular parabolic concentrators and offaxis parabolic concentrator systems is that they require a high
powered laser to be used [7]. These systems would most likely
be large remote systems as discussed before.
addition, the lack of heavy, bulky nozzles, combustion
chambers and insulated fuel tanks makes laser ablation
technology ideal for smaller craft in particular [1]. If a
powerful external laser system were used, the laser could also
have other uses including destroying space debris that can
potentially damage satellites.
Another advantage of a laser ablation propulsion system
is the relative simplicity of such a system. Rather than the
combustion chambers, high temperature nozzles, and fuel
pumps necessary for a traditional rocket, a laser ablation
system requires very few moving parts. This has two
advantages: reliability and cost. Obviously, when it comes to
space travel, reliability is king. Even the smallest of errors
can spell utter, sometimes fatal, disaster. Every moving part
compounds this risk, thus making laser ablation systems more
reliable. The issue of cost is also central since simpler
systems are almost always more affordable than more
complex ones. This relative cheapness could make space
travel available to a wider range of nations and institutions in
the future.
Finally, laser ablation propulsion systems are extremely
scalable. It is very easy to make very small microthrusters,
perhaps even putting out micronewton levels of force, with a
very weak laser. Similarly, it is possible to make arbitrarily
large laser propulsion systems, with the only limitation being
the power of the lasers that are available and practical. These
could potentially be used for heavy lift and even
interplanetary applications in the future.
It would be foolish to pretend, however, that there are no
disadvantages to laser ablation propulsion systems. A laser
ablation system using remote lasers is only effective when
there is a direct line of sight to the laser source, and other
factors such as atmospheric interference can further inhibit
laser transmission. This can make operation at extreme
distances or on the other side of obstacles from the laser
source difficult or downright impossible. The need for
massive laser systems on earth or in space also presents
problems involving international law as these lasers could be
viewed as weapons systems by other nations. There is also the
risk of unintentionally damaging satellites or other objects in
space with the lasers. In addition, laser ablation systems not
coupled with traditional gas flow often do not produce as
much thrust as traditional propulsion systems, which often
makes them unsuitable for larger vehicles. In other words,
laser ablation propulsion may be limited when it comes to
very large applications, particularly when a base on or near
earth is required.
FIGURE 3 [7]
Gas flow with and without ablation
PROS AND CONS OF LASER ABLATION
PROPULSION
Propulsion of any kind, whether it be internal combustion
or rocket power, is a constant tradeoff between power, weight,
and efficiency. Standard rockets are exceedingly powerful
but are extremely inefficient, often burning hundreds of
gallons of rocket fuel per second in their larger
variations. This makes traditional rockets particularly
unsuitable for small spacecraft where light weight and fuel
efficiency are the priorities. It is here that laser ablation
propulsion systems step to the fore. The advantage of such a
system lies partially in the fact that the laser in question need
not necessarily be on the craft itself, but rather can be placed
in a remote location, such as another space vehicle or even on
the surface of the earth, in order to help save weight. In
POSSIBLE EFFECT ON U.S. SPACE
PROGRAM AND WHY IT IS IMPORTANT
The space program of the United States has not been in
the forefront of citizens minds for some years. The last
manned mission to the moon was Apollo 17, which took place
in 1972 [8]. For the most part, large ambitious projects
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Jacob Rosenberger
Nils Aho
involving human spaceflight have been abandoned. The space
program has stuck mostly to what it has accomplished before,
such as putting people in the International Space
Station. NASA has experienced some revitalization due to
the landing of the Curiosity rover on Mars in 2012. This
created excitement although it still is a far cry from the awe
generated by the first manned mission to the moon. The
Curiosity rover did generate buzz around the potential for a
manned mission to Mars, which has long been thought of as
only possible in the far distant future. With the advancements
in laser ablation propulsion and other developments in
spaceflight technology, it appears the mission is not as far off
as previously thought. One of the biggest factors holding back
the mission is the length of the flight which presents a
multitude of problems including the supplies needed for such
a flight as well as the unknown effects of such a long flight on
astronauts. The length of the trip also adds to its budget and
thus makes it less likely of occurring due to the government's
unwillingness to fund it. Laser ablation propulsion, with its
added thrust and efficiency, can cut the time of the flight to
Mars making it a more achievable goal in the nearer future.
Eventually, all spaceflight vehicles from satellites to space
shuttles can be equipped with laser ablation propulsion. Work
is also being done to integrate laser ablation into satellites as
a way to destroy space debris.
To many people, the mission to Mars may seem
unimportant to our country and society. People often say that
we need to focus more on our problems on Earth and that the
space program just costs taxpayers money. This may appear
to be true on the surface but when you look into the benefits
the space program has caused in the past, both economically
and socially, it is hard to argue against investing in the space
program. This is especially true when you look at how
proportionally low the budget of NASA is. NASA’s 2016
budget saw an increase of almost 1.3 billion dollars to a total
of roughly 19.3 billion dollars [9]. These numbers may seem
like a staggering amount of money but they only make up
about .5 percent of the United States’ annual budget. NASA’s
budget is dwarfed by other aspects of the national budget such
as military spending, which is about 600 billion dollars,
making up roughly 16 percent of America’s total budget [10].
The benefits of the American space program are not as
obvious as other investments. There are many technical
benefits including driving innovation and creating new
technologies. Laser ablation propulsion technology can be
included in this category as it's mainly researched now to be
integrated into space flight systems, but in the future it can
potentially be applied to civilian aircraft, creating faster air
travel for the average citizen. The technology will also most
likely be applied to military aircraft in the future. There are
many technologies that have come from the American space
program in the past including developments in satellite
technology, medicine, and consumer goods [11]. These
spinoff technologies include advancements in firefighting
equipment, solar energy, water purification systems and even
household items such as the cordless vacuum [11]. These
advancements in technologies help stimulate the economy by
creating industries and jobs. The space program also has
many social benefits that are intangible, such as giving a
greater understanding of the universe to humans and inspiring
future generations to be involved in the sciences [12]. The
American space program has also had a large cultural impact
as well. Laser ablation propulsion has the potential to have
huge impact on the American space program, which if
revitalized can in turn provide massive benefits to the
American people, the economy, and the international
community.
THE FUTURE OF LASER ABLATION
PROPULSION
Laser ablation propulsion is a relatively young
field. Although the principles of laser ablation have been
known for quite some time, research into its application in
propulsion is relatively recent. Luckily there is some amount
of research going on to try and make it a more practical option
for the future. Much of the most research has come out of
Russia, and has focused on the combination of laser ablation
and high pressure gas ejection. This field has shown great
promise in breaking laser ablation propulsion free of the
technical challenges that have prevented it from coming into
its own as a propulsion technology. As was discussed above,
this type of hybrid system has a much increased thrust
potential while still being more efficient than a conventional
rocket system. This avenue of study is quite new and very
exciting for the future of laser propulsion systems.
Another field of study that has recently come into the fore
is the use of laser propulsion systems in
“microthrusters’. Microthrusters are used for tiny satellites
and very small course corrections, putting out millinewton
and even micronewton levels of force. These are very small
applications too small for conventional rockets to be practical
and not requiring very much power, a combination that is
ideal for small laser ablation propulsion systems. Several
space agencies and even universities have explored
microsatellites as a potentially cheap way of performing
experiments in space, but the level of precision required to
construct rockets on such a small scale has, up until this point,
made the construction of extremely small craft still
prohibitively expensive to many [13]. If laser ablation
microthrusters are adequately developed, we may see a
proliferation of tiny satellites, making space more accessible
to a wider range of programs.
The final exciting area of study occurring in the laser
propulsion field is that of materials science: which fuels,
substrates, and containment materials make for the best
ablation systems? As has been discussed throughout this
paper and our sources, there are several substrate materials,
namely glass and water, being explored, and there is still
research into these and other materials ongoing. Hopefully
even more ideal materials and fuels will be discovered,
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Nils Aho
allowing laser propulsion systems to achieve even greater
levels of power and efficiency.
2011/12/7/076101/pdf;jsessionid=71C9FE7F14111200A793
2C2451E7CA12.c4.iopscience.cld.iop.org
[7] Y. Rezunkov, A. Schmidt (2014) “Supersonic Laser
Propulsion”
Applied
Optics.
(online
article)
https://www.osapublishing.org/ao/fulltext.cfm?uri=ao-5331-I55&id=303516
[8] “Apollo 17 (AS-512)”. National Air and Space Museum
(online
article).
https://airandspace.si.edu/explore-andlearn/topics/apollo/apollo-program/landingmissions/apollo17.cfm
[9] D. Dickinson. (2015). “NASA’s Budget Gets a Boost”.
Sky
and
Telescope
(Online
Article)
http://www.skyandtelescope.com/astronomy-news/nasasbudget-gets-a-boost-1230201545/
[10] S. Gould. J. Bender. (2015) .“These charts show the
immensity of the US' defense budget” Business Insider
(Online Article). http://www.businessinsider.com/the-usdefense-budget-is-massive-2015-8
[11] E. Howell (2015) “What are the Benefits of Space
Exploration?”
Universe
Today
(Online
Article).
http://www.universetoday.com/37079/benefits-of-spaceexploration/r/pdfviewer?sid=0a0f4eb1-b05e-4258-b67d833f9668dcc5%40sessionmgr113&vid=6&hid=128
[12]M. Griffin. “The Real Reasons We Explore Space”. Air
and
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Space Exploration”. USA Today.
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http://www.usatoday.com/story/news/2015/10/27/rocketsspaceships-space-exploration/74704540/
OUR FINAL THOUGHTS
Space exploration is cool. This simple, childish
enthusiasm alone would be more than enough to make laser
ablation propulsion an exciting technology to us. More to the
point, however, space is important. From the development of
new technologies to the expansion of our knowledge of our
place in the universe, there is perhaps nothing more exciting
than the ability to send our payloads to other
worlds. Although mankind has achieved great success with
our probes, rovers, and other craft, these ventures have been
extremely expensive and achievable only by extensive
government efforts by major world powers.
Laser ablation propulsion promises nothing less than a
total change in the way we view space travel. It has the
potential to bring space exploration from the domain of large
governments to within the reach of other nations and, perhaps,
even private companies and universities.
Who knows,
someday the University of Pittsburgh may have its very own
satellite in orbit.
Through our research we have concluded that laser
propulsion is one of the most exciting and promising
technologies being researched today. We believe that the
development of laser ablation propulsion will lead in turn to
development in space exploration and technology as a whole.
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ACKNOWLEDGEMENTS
We would like to thank our writing center instructor
Julianne Mcadoo for providing useful feedback on our paper
as well as our conference co-chair Aaron Wannemacher for
his input.
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