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.