Future Sources of Energy for Space Travel
Math 89S: Mathematics of the UniverseRafay Malik
Professor Hubert Bray
What if you could send your iPhone into space?
While this isn’t possible yet, Yuri Milner, a Russian entrepreneur, plans to do
something similar. Milner is the head of Breakthrough Slingshot, a venture that aims to
send a large fleet of tiny spacecraft no bigger than iPhones to the nearest star system
4.37 light years away, Alpha Centauri. The project reportedly will cost 10 billion dollars
and will take decades to complete. It would work by sending a rocket carrying a “mother
ship” containing miniature spacecraft into Earth’s orbit. Once there, powerful lasers from
Earth would target sails on the spacecraft and accelerate them to 1/5th the speed of
light. Within two minutes, the spacecraft will have traveled 600,000 miles and be on
their way to Alpha Centauri. Once there, the ships will beam back measurements and
photos to Earth (1).
Joining Milner on the mission as part of the board of directors are Mark
Zuckerberg and Stephen Hawking. The project will be led by Pete Worden, a former
director of NASA’s research centers. Milner approximates that it will take 20 years to
launch the mission, another 20 years to travel to Alpha Centauri, and another final four
years to send the observations from Alpha Centauri back to Earth. One of the largest
hurdles for this mission will be the mile-long array of lasers that fire in sync needed to
accelerate the spacecraft. To power them, a 100 GW of energy will need to be
generated for 2 minutes, approximately 100x the output of a typical nuclear power plant.
The laser beams produced need to be extremely accurate and also account for
atmospheric turbulence, or else the the spacecraft and their sails will be vaporized. The
sails of the spacecraft also need to be specially designed to be extremely thin yet also
be able to reflect the laser beams without absorbing any of their energy (1).
While this project is very interesting, it’s also highly hypothetical. There are much
more realistic alternatives for space travel that actually exist at this time.
Solar Sails
A similar idea that already exists and is in use are solar sails. Solar sails are put
on spacecraft to allow them to “sail” on solar wind through space. While there is no
actual wind in space since it is a vacuum, solar sails instead are propelled by light and
gases expelled at high speed by the sun. Solar sails have a large surface area and are
made up of thin mirrors that harness these light and gases from the sun (2).
While the actual impulse generated from the light and gases is small, spacecraft
with solar sails can still reach high speeds since there is virtually no friction in space.
Eventually, the craft will reach a fraction of the speed of light due to the constant
acceleration from the impact of the sunlight and high-speed gases. To accelerate this
process even more, lasers could be aimed at the sails from the moon or another
satellite (2).
This method is not ideal though. Once the spacecraft is far enough from the sun,
it will no longer accelerate and must rely on its own inertia to carry it towards its
destination. To decelerate, the spacecraft must also be able to redirect its sails towards
another source of light. These barriers didn’t stop Japan in 2010 though when it
Japanese Ikaros Solar SailPhoto Credits: NASA
launched the Ikaros probe which was equipped with solar sails. The probe is currently in
orbit around the sun and is expected to arrive at Jupiter in the next few years, proving
that solar sails are a viable option for space travel (2).
Ion Thrusters
Another viable option for space travel are ion thrusters. In fact, NASA is already
using this technology and is developing even more ion propulsion technology. NASA
uses these thrusters to keep more than a hundred satellites in position as well as
sources of propulsion for spacecraft like Dawn which is currently orbiting the asteroid
belt between Mars and Jupiter. One of the largest benefits of ion thrusters are that they
have high specific impulses, or a high amount of thrust produced compared to the
amount of propellant consumed. As a result, much less propellant would be needed
than if rocket fuel is used. The only drawback is that this propulsion only works in the
vacuum of space. Ion technology would best be used in conjunction with rocket fuel,
where rocket fuel could launch the spacecraft into outer space and then ion thrusters
could take over from there (3).
Ion propulsion works by taking a propellant, usually xenon gas, and ionizing it by
bombarding it with electrons to to knock electrons of the neutral gas. As a result, a new
neutral gas of positively charged ions and negatively charged electrons, or a plasma, is
produced. The plasma is then passed through two grids, one positively charged and the
other negatively charged. As a result, the ions are accelerated to high speeds of almost
90,000 mph. Once the ions are accelerated out of the thruster, thrust is produced. To
make the exhaust beam neutral, a neutralizer expels an equal equal amount of
electrons. Without this neutralizer, a significant negative charge would build up in the
thruster and eventually pull ions back into the thruster. This would eliminate the thrust
produced as well as cause the thruster to erode (3).
Some of the past uses of ion thrusters were NASA’s Deep Space 1 probe which
was launched in 1998 to collect data on asteroid 9969 Braille and later Comet Borrelly.
Japan’s Hayabusa probe, with four xenon ion engines, also successfully landed on an
asteroid and collected samples, and returned to Earth in 2010. Thus, ion thrusters have
proven to be viable. While they are suitable for missions in the solar system like those
motioned before, they lack the power for interstellar travel. If we wish to travel outside
our solar system, more powerful sources of thrust will be required (2).
Nuclear Propulsion
One of the quickest and most affordable ways to travel through our solar system
would be through nuclear propulsion. Theoretically, nuclear propulsion could launch a
spacecraft to about 12% the speed of light. This is fast enough to travel around the
Earth in two seconds and to the moon in thirteen seconds. Compare that to the Apollo
missions, which took four days to reach the moon (2).
At this speed, we could also reach Proxima Centauri, 4.25 light years away, in
about 35 years. With an ion thruster, this same journey would take 19,000 years. The
NASA Evolutionary Xenon Thruster Photo Credits: NASA
technology for this form of travel is quite simple. Small nuclear bombs are released from
behind the spacecraft and detonate, launching the spacecraft forward. This process is
repeated until a desired speed is reached. To shield the spacecraft from the radiation
and the blast from detonation, a large, reinforced plate would be needed. Dampeners
would also need to be used to ensure smooth acceleration (2).
Research on this topic has been very limited though. In 1958, the US military
began researching the idea of nuclear propulsion but was forced to end the project due
to the Partial Test Ban Treaty which prevented the detonation of nuclear bombs in
space. Penn State University’s Nuclear Engineering department also explored a similar
concept with two projects known as Project Ican and Project Aimstar. The lack of
exploration of nuclear propulsion is likely due to the numerous technological hurdles
involved. In order to travel with nuclear propulsion, thousands of nuclear bombs must be
carried which poses huge safety risks. Another issue is designing a protection plate to
shield the spacecraft from bomb blasts. A special material would be needed that could
withstand the effects of thousands of nuclear blasts. Finally, how would the nuclear
fallout produced by the bomb blasts be dealt with? A possible solution would be to
launch the craft from the polar regions, or to use rocket fuel to first get the spacecraft
into space before using the nuclear bombs for thrust. Ultimately, this technology
appears that it will be unfeasible for some time until these large barriers are overcome.
Antimatter
An even more hypothetical source of fuel, but the most potent, is antimatter. A
few milligrams of antimatter would be enough to launch a spacecraft to Mars. Antimatter
is identical to regular matter except that its properties have been reversed. For example,
electrons which have negative charge in regular matter become positively charged (and
are called positrons) in antimatter. When antimatter comes into contact with regular
matter the two combine to annihilate each other and produce a tremendous amount of
energy (4).
The process of harnessing antimatter is simple. A stream of positrons is released
towards a metal plate and the two annihilate, providing an explosion which can propel a
spacecraft. In an alternative method, a stream of antimatter could be pointed at a sail
and react with the sail to propel the spacecraft forward. The theoretical max speed that
could be reached by this method is 70% of the speed of light. Instead of the 35 years to
reach Proxima Centauri with nuclear propulsion, the star could be reached in about 6
years with antimatter propulsion (2).
The challenges for actually using antimatter for energy are complex. Antimatter
reactions produce gamma rays, which according to NASA, are like “X-rays on steroids”.
Gamma rays penetrate cells easily and break apart molecules, making them highly
carcinogenic. Gamma rays can also make a spacecraft radioactive through the
fracturing of atoms of the spacecraft’s material. Obtaining antimatter is also a financially
difficult task. Antimatter is produced at particle accelerators through collisions of atoms
at nearly light speed. To produce 1 gram of antimatter, nearly $1 trillion would be
required. Storing it also poses a unique challenge since antimatter would annihilate any
container composed of regular matter. Antimatter would have to be suspended in a
vacuum at low temperatures. Even more difficult is accounting for the fact that positrons
repel one another, often in an explosive manner (4).
VASIMIR
Another possibility for space travel would be NASA’s VASIMIR, or Variable
Specific Impulse Magnetoplasma Rocket, which is an electromagnetic thruster. The
idea was proposed by former NASA astronaut, Franklin Diaz, who now operates his
own company, Ad Astra Rocket Company. According to Diaz, the thruster will enable
travel to Mars in about 40 days while only using 1/10 of the fuel of traditional rockets. To
fund this project, NASA provided $10 million to Diaz’s company (5).
VASIMIR works in a method similar to ion thrusters. A neutral gas like argon or
xenon is injected into a cylinder surrounded by electromagnets and heated to become a
“cold plasma”. Electromagnetic waves, or radio waves, then bombard the neutral gas to
remove electrons and produce a plasma. In the final stage, the plasma is heated and
compressed by electromagnets to be forced out of the engine and produce thrust (6).
One of the largest benefits of VASIMIR engines are their durability. These
engines do not require electrodes, actuators, and pumps like typical rocket engines.
Instead, they only require magnets to protect the rest of the engine from the plasma and
Diagram for a VASIMIR EnginePhoto Credits: Wikipedia
other minimal parts like gas valves. By reducing the amount of hardware, less
maintenance is required, reducing costs and enhancing longevity (6).
Criticisms of VASIMIR do exist though. While Ad Astra plans to power its engines
through solar-electric power, it cannot meet its goal of reaching Mars in 40 days without
a more powerful source of energy like a nuclear reactor. Robert Zubrin, head of Mars
Direct, a competitor company who plans to make colonizing Mars affordable, believes
VASIMIR will not be able to meet its goal since the nuclear reactor technology required
does not currently exist and that VASIMIR’s electric propulsion system will not be as
efficient as Ad Astra proposes (6). Whether these claims will be unfounded or not will be
demonstrated when Ad Astra performs their first mission to the International Space
Station later in 2016.
Conclusion
While we are currently limited to traditional rocket based space travel, there are
number of exciting alternatives being developed. Some already are in use like the solar
sail and ion thrusters. Projects like VASIMIR and Breakthrough Slingshot also provide
interesting methods of space travel but they are still in the early stages of development
and far from being put into use. Finally, completely theoretical ideas like antimatter and
nuclear propulsion engines are fascinating but will definitely not be seen for many years.
It is likely that we will use these methods in conjunction with rocket fuel until they are
fully developed. Whatever happens, it will be interesting to see where these energy
sources will take us in the future.
Works Cited
1. Overbye, Dennis. "Reaching for the Stars, Across 4.37 Light-Years." The New York
Times. The New York Times, 12 Apr. 2016. Web. 20 Apr. 2016.
2. "What Is The Future Of Space Travel?" Zidbits. 23 Dec. 2012. Web. 20 Apr. 2016.
3. Dunbar, Brian. "Ion Propulsion." NASA. NASA, 11 Jan. 2016. Web. 20 Apr. 2016.
4. Dunbar, Brian. "New and Improved Antimatter Spaceship for Mars Missions." NASA.
NASA, 14 Apr. 2006. Web. 20 Apr. 2016.
5. Schilling, David R. "NASA's New VASIMR Plasma Engine Could Reach Mars in 39
Days." Industry Tap. 03 Jan. 2016. Web. 20 Apr. 2016.
6. "Variable Specific Impulse Magnetoplasma Rocket." Wikipedia. Wikimedia
Foundation, 20 Mar. 2016. Web. 20 Apr. 2016.