Ion Propulsion: A story of the Conservation of Momentum

     Behind every theory for future forms of spacecraft propulsion lies a terrible truth. They are all either too costly, to difficult, too dangerous, or too inefficient for practical use today for reaching towards the planets. But what if there were a form of propulsion for which the fuel would cost 33 times less than traditional fuel? What if there were a fuel that was non-toxic, non-volatile, and unlikely to cause an explosion or other catastrophic failure? What if there were an engine that had an average efficiency rating of 70%? And what if it were available and ready to launch today? Ladies and gentlemen, this is the truth behind the xenon ion propulsion drive.

     The ion thruster is a feasible solution for space travel in the not-so-distant future. The truth is, it has already been done. In 1998, NASA launched Deep Space 1, a probe designed to do a close flyby of an asteroid. What was supposed to be a fairly short, straightforward mission turned much more complicated when they realized that Deep Space 1 also had the opportunity to study the attributes of passing comet Borrelly first hand. Though the encounter with the asteroid was only considered a partial success, the information gathered from Deep Space One's flyby of comet Borrelly turned out to be a huge success. And it was all thanks to the highly efficient and decently cheap ion propulsion system mounted on board.

     The theory behind an ion drive is actually quite simple. Xenon atoms are initially stored in a containment unit. Since xenon has a full valence shell of electrons, it is non-volatile and non-toxic. This also means that xenon is very easy to ionize. Ionization is a process where a stream of electrons are shot at regular atoms in such a way that electrons get knocked loose from the atom, creating a net positive charge on the atom. Since positive charges are attracted to negative charges, these ions are then subjected to two powerful charged plates, the one behind the ions being positive, and the one in front of them being negative. As the positive plate forces the ions away from the craft, the negative plate pulls the ions out into space. The potential difference accelerates these positive ions out of the exhaust tube at speeds upwards of 40 kilometers per second! Since leaving a trail of positive ions behind the spacecraft can induce a charge upon the spacecraft, which can be dangerous, a stream of electrons is shot into outgoing ions to neutralize them so they have no net charge.

Theory Behind an Ion Drive

     However, at this point you may have noticed the problem. Though the exhaust velocity is 40 kilometers per second, we can only accelerate a few atoms of xenon at a time. This makes acceleration a very difficult and tedious process. It would be like instead of pressing the gas pedal to accelerate your car to work, you threw grains of sand at extreme velocities to slowly accelerate your car to its top speed. Granted, you would save gas money, and your car would actually achieve a higher speed (neglecting friction), but it would take you hours or days to even get your car to go a modest speed. An ion powered spacecraft would either need to be constructed in space, or blasted into orbit via chemical rockets. This is the only major downfall of ion propulsion.

     The ion propelled thruster works on a very simple level. The law of conservation of momentum states that the mass multiplied by the velocity of one object which exerts its energy upon a second object is equal to the other mass multiplied by the other velocity. In simpler terms:

m1v1 = m2v2

     Where m1 and v1 are the mass and velocity of the xenon atoms, and m2 and v2 are the mass and velocity of the spacecraft. We know that the mass of one xenon atom is 2.18*10^-25 kg, and that they are being expelled from the spacecraft at 40 km/s, or 40,000 m/s. Let's say that we wanted to launch a 500 kg probe to Mars. How much xenon would we need. Well:

m1 = 2.18E-25

v1 = 40,000

m2 = 500

v2 = ?

(2.18E-25)*(40,000) = (500)*(v2)

v2 = 1.74E-23 m/s

     Let's say we want our top velocity to reach 20,000 m/s, about 5 times the speed of a conventional rocket:

(20,000)/(1.74E-23) = 1.15E27 xenon atoms = 250.7 grams.

     This mass of xenon will get the craft to 20,000 m/s. The craft will then coast at that speed until it is time for it to begin to slow down to arrive at its destination. This does mean, however, that it will need another 250.7 grams of xenon to slow back down to starting velocity, bringing the total amount of fuel to about 502 grams. But how fast can we get up to this speed? Let's say that our engine has a diameter of half a meter. This will give it an output of about .044 Newtons of thrust. For our 500 kg craft, this is equal to an acceleration of  8.8*10^-5 m/s^2. This means that, starting from 0 m/s, it would take upwards of 6 hours to reach our desired speed of 20,000 m/s. So time is the only sacrifice of the ion engine.

     But what about the benefits? The cost of 501 grams of xenon fuel for a 500 kilogram spacecraft would cost just proud of $600. Compare that to the $20,000 of liquid fuel that it would take to get a space vessel from low Earth orbit to Mars's orbit. And even with all that expensive fuel, it would still only be able to achieve a peak net velocity of around 4000 m/s, though it may only take a few minutes to get there. This is only a fraction of the top speeds achievable by the ion drive. Plus, with no explosive or volatile chemicals on board, the journey would be much less prone to accidents. The far future may hold the keys of space exploration in nuclear and antimatter drives, but getting us to our celestial neighbors today may be much more feasible by means of the xenon ion drive.

A Trip to Mars: A New Form of Propulsion

     On November 6th, 2011, NASA launched the Atlas V space vehicle en route to the red planet, nearly a nine month trip ahead of it. On board the Atlas V, a small rover, about the size of a car, was curled up within the nose. That rover would later land on the surface of Mars to discover that the seemingly cold, dry dust of Mars once long ago had the ability to support life as we know it. The Curiosity project is regarded as one of the most informative and successful missions to Mars to date; but it didn't happen overnight. The unsatisfying truth of the Curiosity rover's trip to Mars is that it took 253 days from liftoff at Cape Canaveral, to touchdown at Gale Crater on Mars's dusty surface. Whats more is that this rocket requires 284,000 kg of fuel just to escape the strong pull of Earth's gravity and maintain a stable orbit. Since time is one of humanity's most valued possessions, and the cost of rocket fuel is not expected to drop in the near future, its time to start searching for new sources of fuel and propulsion to get us to the surfaces of our nearest celestial neighbors.

Atlas V Rocket

     At its closest approach, Mars is 55 million kilometers away. Since the Atlas V accelerates the Curiosity probe to about 20,000 kilometers per hour, one would think that you could just the distance and divide by the speed to get the length of time required for a trip to Mars. This, however, is not the case. Since everything that travels around the Sun goes in elliptical orbits, the rocket will not be an exception. It's path too will be curved by the gravitational pulls of the Earth, the sun, and Mars. At 20,000 kilometers per hour, a spacecraft launched from Earth when Earth and Mars are in sequence (in line with the Sun), the craft will have arrived at Mars after Mars has done one half orbit in its path around the Sun. So, the actual distance traveled by the craft itself is the average of the lengths of Earth's and Mars's orbits divided by 2. This gives us a total distance of about 593 million kilometers. 

     However, this is (clearly) not the most efficient way to launch a craft to Mars using conventional methods, seeing that it would take the craft more than three years to reach Mars. If the craft is launched at a higher velocity, the curved orbit from Earth to Mars will have flattened out. The only down side to launching spacecraft at these trajectories is that once at the red planet, the vessel will have to begin to slow down to enter her orbital velocity.

     In this image, the distance traveled between Earth and Mars is closer to 300 million kilometers, slicing the trip time down to only about 18 months. The truth is, there's no limit to how fast you can fly a ship from Earth to Mars (except the speed of light), thus the path between Earth and Mars will grow continually shorter with velocity. If you could accelerate a craft to the near the speed of light, the shortest possible distance would only be about 55 million kilometers. Take that divided by the speed of light and you get a trip time of about 3 minutes. So, if there is no limit, the only question is how fast can we go?

     There has been a lot of talk in recent years about the possibility of using nuclear fusion power to propel a rocket to Mars. The theory behind nuclear fusion is actually quite simple. Take, for example, the fusion of deuterium, an isotopes of hydrogen. Deuterium is simply a hydrogen atom with two neutrons. In the image below, blue spheres represent neutrons while red spheres represent protons. When heated to extreme temperatures, these hydrogen atoms will begin to gain velocity. Once a certain velocity is achieved, these atoms will slam into each other with enough force to combine into a single helium atom and release a tremendous amount of energy.

     Let's compare this to the energy released by liquid hydrogen, currently the most efficient and powerful rocket fuel in use today. The specific energy of a substance tells us exactly how many megajoules of energy are released per kilogram of whatever it is you are using as fuel. The specific energy of liquid hydrogen is about 141.86 megajoules per kilogram. That is a lot of energy. And that is in just 1 kilogram of liquid hydrogen. putting that into perspective, 1 kilogram of liquid hydrogen could power an average American household for 31 hours.

     So how about nuclear fusion? The net reaction of one deuterium-tritium fusion (tritium being another isotope of hydrogen, actually more efficient than a deuterium-deuterium reaction) creates about 12.5 megaelectron volts, or about 2*10^-12 joules. This may seem like a tiny amount of energy, but this is only in one reaction of just one deuterium and one tritium atom! Since one mole of hydrogen is equal to one gram, we just multiply that by Avagadro's number (the number of atoms in one mole of hydrogen) to obtain how much energy is released in one gram. This turns out to be 1.2*10^12 joules in one gram, or in a kilogram, 1.2*10^15 joules. Using nuclear chemistry, we find that one gram of reacted dueterium-tritium fuel generates 1.2 billion megajoules of raw energy! Modern experiments show that at best, a fusion engine could only utilize about 30% of the total energy created. This means that the energy density of the nuclear fusion of deuterium and tritium fuel at 30% efficiency is 361 million megajoules per kilogram. This is 2.5 million times more efficient than liquid hydrogen fuel. Recall from before that the Atlas V takes 284,000 kg of traditional fuel to launch off, leave Earth's orbit, slow to Mars's orbit, and land on the surface of the planet. Using a vessel of the same mass and a fusion powered rocket, we find that a voyage to Mars would only require about 113.6 grams of fuel! If it were a manned mission, we must assume that it would be nice for the crew of the craft to return home, bringing the grand total to about 227.2 grams of both deuterium and tritium fuel, or 454.4 grams of total fuel (about 1 pound for all you Imperial system goers).

Proposed NASA Nuclear Fusion Rocket

     Using the ideal gas law, we can determine the volume of two spherical fuel tanks to hold the deuterium and tritium as well. For a system at reasonable temperatures and pressures, PV = nRT, where:

P = Pressure
V = Volume
n = moles
R = .08206 (constant)
T = Temperature

     We want a pressure of 1 earth atmosphere, and since one gram equals one mole for hydrogen, n will equal 227.2. Room temperature in Kelvins is 294. Thus:

(1)*V = (227.2)*(.08206)*(294)

V = 5481.3 liters, or 5.48 cubic meters

For a sphere:

V = (4/3)*pi*r^3

Thus:

r = 1.09 meters

     So we would need two spherical fuel tanks just over 1 meter in radius to bring a manned crew to Mars and back in a vessel about the mass of the Atlas V. This is an incredibly small amount of fuel compared to the thousands of kilograms needed to launch the Atlas V towards Mars. Nuclear fusion is definitely a cheaper source of spaceflight, but what about faster? According to NASA, the exhaust velocity of a nuclear fusion powered rocket is likely to be upwards of 30 kilometers per second. Compared to that of the Atlas V rocket, whose exhaust velocity is approximately 4 kilometers per second, this would cut the trip time from here to Mars by more than 7 times, leaving the ETA at about 36 days. Not to mention that the only byproduct of nuclear fusion is helium and neutrons. So there you have it; cleaner, faster, and cheaper. So, why are we not using it?

     The sad fact of the matter is that fusion isn't easily achieved. For deuterium and tritium atoms to smash together with enough force to fuse and release energy, we must first heat them to 700 million degrees Kelvin, a temperature hotter than the core of the sun. Not only do we not have the means with current technology to heat anything to such a temperature, but no known material could contain such heat without deformation. Furthermore, though hydrogen is very abundant, deuterium and tritium only compose a fraction of a percent of the total amount of hydrogen in the universe.

Temperatures for Achieving Fusion (Blue line is Deuterium-Tritium)

     So fusion may not be practical as a source of rocket propulsion today. It is only a matter of time, however, until we are able to find a way to heat hydrogen to such temperatures, possibly by way of microwaves, and keep it contained maybe in an extremely rigid futuristic material or with a powerful electromagnetic field. In the near future, we may even be able to synthesize deuterium and tritium fuel by somehow combining excess neutrons with hydrogen atoms, using the strong nuclear force as a glue for the new isotopes. Though fusion may seem difficult today, we will, as a human race, find way to overcome these setbacks in order to further mankind. Mars is our nearest celestial neighbor besides our own moon, and we know so little about it under its mysterious, dusty red crust. With the nuclear fusion rocket, a trip to the red planet may be only a few pounds of fuel and 36 days away. So what are we waiting for?

Life in our Galaxy: How Many Other Intelligent Species Are There?

     It seems pretty unlikely that there is other life in the galaxy from our perspective right now. As a human race, we have been communicating radio waves for 78 years giving extra-terrestrials a chance to respond to our inadvertent hails. We have projects like SETI that have been actively searching for and attempting to communicate with potential visitors of the Milky Way. We even have powerful telescopes that have accurately mapped thousands of nearby stars and extrasolar planets with no traces of life, intelligent or otherwise. But what if I told you right now that there is almost certainly at least 126 advanced, technological civilizations that have the willingness and ability to communicate with Earth right now in our own stellar neighborhood, the Milky Way? It would probably sound a lot like science fiction. Maybe so, but it is a real possibility. Let me present you to the famed Drake Equation:

     This equation is literally the key to telling us exactly how many alien civilizations are out there right now, alive and well, and able to communicate. So, what's the problem? why haven't we found extra-terrestrial life in our galaxy if we know where to look? The truth is, we don't. This is because four of these variables, f_l, f_i, f_c, and L, we simply don't know the values to. If the Drake equation is broken down into its variables, we find that:

N = The Number of communicating civilizations in our galaxy right now.
R = The Rate at which stars are formed each year.
f_p = The Fraction of those stars which have Planets.
n_e = The Number of Earth like planets that can potentially support life.
f_l = The Fraction of these planets which actually forms Life.
f_i = The Fraction of these planets with life that develops Intelligent life.
f_c = The Fraction of these civilizations that develop a means to Communicate effectively.
L = The Length of time that these intelligent species communicate for.

     Many of these variables are very clear, and have been accurately measured and tested. R, for example, has been calculated and known for many years. NASA has clocked the average star birth rate to be approximately 7 stars per year based on patches of radioactive aluminum throughout our galaxy. so, for our equation, R = 7. Now, with the launch of the Kepler space telescope, f_p has also been determined relatively accurately over the past decade or so. Of the 153,000 stars that were accurately measured for planets using multiple means, they discovered that approximately 34% of stars have some form of planets around them. However, this does not even account for the number of planets that were missed or not observed long enough to discover a planet. Recent studies have now estimated f_p = .80, meaning about 80% of stars form planets. But of these planets, how many are in the "Goldilocks" zone for the potential formation of life or habitability? Studies show that approximately 22% of stars have Earth-sized planets within the habitable zone. Out of the 34% that had planets, this means that about 65% of the planets are habitable. This percentage seems to be quite accurate closer to home. Even in our own solar system, there are 2 or 3 roughly Earth-sized planets within our habitable zone (depending of whether or not you count Venus or Mars), and only one of them supports life. So, averaging the 65% with the low estimate of 33% in our solar system, then multiplying by the 22% of stars with planets in the habitable zone, we can equate n_e = .11 or about 11%.

Goldilocks Zones of Sun-Like and Other Stars

     Alright, about half way done. Now here's the real problem. The last four variables in the Drake equation are impossible to determine precisely with our current knowledge of the galaxy. Since we have never discovered another example of life in our own Milky Way, the estimation range of these last four numbers is quite vast. So now we must start using ranges. Of the Earth-sized planets in habitable zones of their sun, how many of these will develop life? According to some, the answer to this is 100%, meaning if a planet has the potential to support life, it will develop life. This is not an unreasonable suggestion. Geological records of the Earth show that once our home planet developed conditions stable enough for life, it developed rather quickly. However, we do not know if our creation was a chance accident or a common occurrence. This number could literally be anywhere between 0% and 100%. Averaging these two will assign a probability of approximately f_l = .5, or 50% of planets will develop life at some point after they become habitable. Once life is created, what are the chances that this life becomes intelligent? So, lets step back a moment and take a look at what humanity had to overcome to become an intelligent community. Our journey from a puddle of primordial goo to walking, talking, bipedal beings had a few speed bumps along the way. We endured billions of years of evolution, a total of 5 mass extinctions, and factors that we can't even begin to imagine! However, if humanity is indeed "average", then over our 3 billion years of evolution on planet Earth, there were 5 chances that could have extinct life on this planet. This puts the value of f_i = .2, at around 20%. This is assuming, of course, that if life exists on a favorable planet, that it will "strive" to become intelligent on its own. This is, however, still reasonable due to the effects of evolution that can be seen across the globe.

Some Extrasolar Planets Discovered by Kepler Project that may Host Life

     So now we have an intelligent species that has overcome all odds on an Earth-sized planet around a star in its Goldilocks zone. Will they develop a means to communicate. If they are truly intelligent, then yes, of course they will. Think about it. There were civilizations on opposite ends of the globe, like the Mayans and the Egyptians, that both developed independent, effective languages without having any prior knowledge of each other. An intelligent species strives to share their intelligence with others. Assuming that in the 10,000 years from intelligence to when they develop some means of interplanetary communicative means, like radio waves, they don't kill themselves or are wiped out by a natural disaster (the latter of which is unlikely due to the fact that an average mass extinction only occurs once every 700 million years), the civilization will communicate. This puts the value of f_c = .9, allowing for a 10% chance that they will either never develop communications or they will be wiped out somehow. Good. So the final variable remains; how long will the civilization communicate for before they either are wiped out by natural disaster, destroy themselves, or move on to a more advanced form of communication that we cannot detect? Once the civilization has become intelligent, they will almost immediately become space-faring and colonize and terraform many other worlds. This makes destruction by means of natural disaster unlikely. What is more likely is that they will destroy themselves.

     Since we have developed radio communication in 1936 (and yes, it was a radio broadcast by Hitler), we have sustained 2 world wars, a cold war, and multiple terrorist groups that threaten to rid of all others who don't agree with their beliefs. The first world war was mostly waged in Europe and had little chance of wiping out the entire human population, but with the invention of the atom bomb in the 1940's, this threat is all too real. There have been two circumstances in the last 70 years where the world almost destroyed itself with nuclear warfare, the first of these being World War II in 1945, and the second being the Cold War nuclear scare of 1983. Both of these events could have ended humanity, making our radio transmissions cut short at a mere 9 or 37 years. however, it has been 31 years since the nuclear scare in 1983 and no such event has occurred since then this means that tensions of humanity (though far from gone) are generally decreasing as time moves onward. I will give a civilization a 60% chance of surviving their own destruction, leaving us with just one final element; how long will this civilization communicate for with simple radio waves? History has shown that technology advances exponentially. However, we are still using radio waves after 78 years. I would estimate that we will sacrifice this open source of communication to a more specific, private form of communication like lasers or some form of telepathic enhancement within the next 500 years. averaging that with the civilizations that kill themselves within 20 years, we can calculate 20*(.4)+500*(.6) 300 to give us an approximate value of 300 for L. So, lets put it together:

R = 7 stars per year.
f_p = .8 ratio of stars with planets.
n_e = .11 ratio of these planets which are habitable.
f_l = .5 ratio of these habitable planets which develops life.
f_i = .2 ratio of these life forms that become intelligent.
f_c = .9 ratio of these intelligent life forms that will develop effective communication.
L = 300 years of active, effective communication.

N = (7)*(.8)*(.11)*(.5)*(.2)*(.9)*(300) = 16.63

     So what exactly does this mean? There are currently about 17 civilizations in our Milky Way galaxy right now that are intelligent and willing and able to communicate. This, however, does not mean that there are only 17 intelligent civilizations within the milky way. This is only the number of them that we are likely to find and communicate with in the next 200 years or so. If we assume that species are more friendly at communicating, the value of L may shoot up to however long that civilization has been alive for, which, in a 13 billion year old galaxy, may be close to 10 billion years. The chances that an alien civilization that advanced would ever have the motive to talk to us would be astronomically small, however, as they have probably already discovered millions of other civilizations just like us in galaxies all across the universe. So the true question for "L" is how long will a civilization be interested in contacting other life. Some studies report that it is humanity's drive to discover other life in the galaxy. But if there really is 30,000 civilizations in the galaxy right now, that would get pretty boring after a while. life would no longer be unique. so how many civilizations would it take for an intelligent species to consider life "common"? Lets say a civilization discovers the nearest 15 civilizations before either becoming bored with communicating with lesser beings or conjures laws against tampering with the development of these civilizations.Well, the galaxy has approximately 5,500,000,000,000 (5.5 trillion) cubic light years of area and about 200,000,000,000 (200 billion) stars. This means that there is an average of 27.5 cubic light years per star, or, if you prefer, a stars nearest average neighbor is about 1.87 light years away. This means our civilization will have communicated with every other civilization within 65450 cubic light years, or about 25 light years in all directions. Taking into account that it takes them about 50 years to go from a communication era to a space era, then another 100 years from space era to interplanetary era, then another 150 years from an interplanetary to an interstellar era, then another 200 years to explore all of their nearby star systems before becoming bored (with the speed of light as a barrier), the value of L can now be expanded to L = 500, then:

N = (7)*(.8)*(.11)*(.5)*(.2)*(.9)*(500) = 27.72

     This is almost 28 potential intelligent life forms, willing to communicate just within our galaxy alone right now! But this still doesn't answer the question as to how many intelligent species have lived in our galaxy ever. To find this, we simply eliminate the first and last factors of the Drake equation to find out what percentage of stars in the milky way have had an intelligent species living on a habitable planet at one point in time.

N% =  (.8)*(.11)*(.5)*(.2)*(.9) = .0079 or 0.79%

Multiplied by the number of stars in our galaxy:

N = (.0079)*(200,000,000,000) = 1,580,000,000 life forms

     1.58 billion intelligent life forms have existed in the galaxy at one point or another. Since that number is approximately the length of time in years that life could have been erupting in our galaxy, we can assume that these civilizations would have spawned proportionally with time. This means that every six years, a new intelligent species is born in our galaxy on average! This leaves us with about 80 alien civilizations within just 500 years of our own technology that may be willing to communicate with us! Now, since the galaxy is so vast, the closest of these civilizations would be on average 1800 light years away (as can be seen from the graph above), but still, there is a chance! So, in conclusion there is approximately:

• 17 intelligent civilizations in our galaxy that are not too much more advanced and are within the technological realm of communicating with us.
• 28 intelligent civilizations in our galaxy that may or may not be more advanced than us, but still are willing to communicate with us.
• 1.58 billion intelligent species that have ever existed in the Milky Way.
• A new intelligent species is born every 10 years on average.
• About 80 intelligent civilizations in our galaxy right now that are within 500 years of our technology.
• The nearest intelligent civilization that is willing to communicate with us is most likely over 1800 light years away from us.

     And what about humanity? Where do we fit into the Drake equation? Well, if we consider ourselves an intelligent life form, we just entered into the vast network of communicating aliens just 78 years ago, and we are still alive. This means we know for a fact that there is no willing or able species within 39 light years that will communicate with us (given that they have to respond to our radio waves). Given time and technology advances in the coming years, there is a high chance that we will discover extra-terrestrial life within the next 1000 years. It is only a matter of time.

The Big 10: The Largest Ships in Sci-Fi

     Pick your favorite science fiction television show or movie. Good, now go ahead and pick your favorite star ship or vessel from that TV show or movie. If you are a Star Trek fan, chances are you picked the USS Enterprise (hopefully either the original or the 1701-D). If you love Star Wars, maybe you picked the triangular-shaped Imperial Star Destroyer. Or maybe you're a fan of the Cylon vs. human conflict in Battlestar Galactica, in which case you may have picked the Galactica herself. Okay, now take all of their lengths and add them together, end to end. All three of these massive science fiction icons only collectively could span 35% of the shortest space vessel on my Big Ten countdown.

     After many hours of research and studies (that maybe could have been spent doing homework) I have conglomerated the single most accurate countdown of massive science fiction spacecraft to date. But see now, we have two problems. How do you classify "big", and what really qualifies as a "spacecraft"? As far as size goes, this countdown will be based solely upon the length of the vessel. Now it comes down to what is the definition of a spacecraft. For the purpose of this countdown, I have come up with three necessary qualifications that must be met for a ship to be considered for the list. They are as follows:

• The spacecraft must be from a "credible" TV show or movie. This means that I will not be including novels, video games, or animated comics or shows (otherwise Warhammer 40k would take home all of the ribbons).
• The vessel must be mobile and able to transport itself from one destination to another. This basically means that there are no space stations, ring-worlds, or Dyson spheres allowed in the competition.
• The ship must not simply be a modified asteroid, planet, or moon. Sorry Phobos.

     So, now that a spaceship has been defined, the question remains... which science-fiction spacecraft is the mother-ship of them all? The answer may (or may not) surprise you!

10) Varro Generational Ship

Length: 9.3 kilometers (5.8 miles)

TV Show/Movie: Star Trek Voyager

     This very long stick of a ship was encountered by the crew of the Voyager in the 2370's. They helped her repair her warp drives and get on their way through the Delta Quadrant of the galaxy. The vessel is actually composed of multiple individual pods constructed to carry a large number of people for a long time through the bleakness of space.

9) Voth City Ship

Length: 9.8 kilometers (6.1 miles)

TV Show/Movie: Star Trek Voyager

     Yes, another contender from Star Trek Voyager, but this ship is not only long; its massive! Encountered by the crew of the Voyager when the city ship used a powerful tractor beam to take the entire ship aboard, this immense shell of a craft could fit the volume of 3,500 Voyagers within her gaping hull. Not much was learned about the Voth in the series other than that their technology was advanced and that their ships were huge.

8) Lexx

Length: 10.0 kilometers (6.2 miles)

TV Show/Movie: Lexx

     Lexx is a 10,000 meter long bio-ship that, despite its amazing technology and size, somehow still manages to look like a gigantic... amenity... Joking aside, Lexx is a massive vessel. Haven't heard of Lexx? Neither had I until i started doing research on the largest ship in sci-fi. This may be because Lexx only appeared in Great Britain and Canada and never gained as much popularity as many of the sci-fi shows during the time. None the less, Lexx is a very lengthy ship with a very... interesting... shape.

7) Super Star Destroyer

Length: 19.0 kilometers (11.8 miles)

TV Show/Movie: Star Wars

     "It's a trap!" Admiral Ackbar could not have worded it better when he encountered the emperor's Executor-class Super Star Destroyer. clocking in at 19,000 meters, this ship dwarfs a typical Star Destroyer or rebel star ship and with turbo-lasers and laser cannons mounted all across the hull, it is unmatched in strength as well. The term "Star Destroyer" is actually used loosely throughout the Star Wars saga, and refers to a large selection of vessels, all of different classes, which are all around 19 km in length. The Executor class, however, is the longest and most powerful of them all.

6) Alien City Destroyer

Length: 24.0 kilometers (14.9 miles)

TV Show/Movie: Independence Day

     Whats 24 kilometers across and looks like a pancake? When Independence Day came out in 1996, it shattered the sixe scale for science fiction with its massive alien vessels. 36 of these massive saucers came from space and hovered over Earth's largest cities to destroy them (hence their names). The aliens were attempting to rid of all life on Earth to harvest her resources so sustain their ships. However, it would seem that through all of their advances in technolegy and science, these aliens couldn't crack the code to computer programming. All it took was one good 20th century virus and the whole fleet of City Destroyers were brought down. Earth wins. The end.

5) Vorlon Planet Killer

Length: 45.0 kilometers (28.0 miles)

TV Show/Movie: Babylon 5

     This is the next best thing that destroys planets and isn't a small moon! Tipping the scales at 45 kilometers in length, the Vorlon Planet Killer is truly an enormous vessel. Through all of Babylon 5's effort in ridding of the Vorlon threat, only one of these gigantic vessels was ever destroyed. the others escaped into hyperspace. driving your car at 100 kilometers per hour (62 miles per hour) it would still take you 27 minutes to get from one end of the ship to the other! now imagine that you work at the tip and communications go out but you needed to tell the captain something...

4) Whale Probe

Length: 74.0 kilometers (46.0 miles)

TV Show/Movie: Star Trek the Voyage Home

     What happens when by the 23rd century, all whales have gone extinct and a massive, metallic cylinder is emitting a dangerous energy field threatening the inhabitants of Earth demanding to speak to the deceased species? Time travel, transparent aluminum, and nuclear wessels, that's what! he whale probe can be seen in Star Trek the Voyage Home dwarfing a Federation star base in the foreground as it threatens to eliminate Earth's population unless it can speak to a Humpback whale. So, Kirk and his crew travel through time to retrieve two whales from the past (because that's easier than genetically cloning one from DNA) in order to drive the probe away. Why whales?! That's Star Trek for you...

3) V'Ger

Length: 98.0 kilometers (60.9 miles)

TV Show/Movie: Star Trek the Motion Picture

     This simply huge spacecraft is renowned as the single largest ship in Star Trek. at nearly 100 kilometers in length, the Enterprise crew would not have stood a chance had they not found what it was the massive ship was looking for. So, what was it looking for? Its creator, of course. so, then, what is V'Ger? Its in the name, but I'll leave that to you if you want to find out. Mystery aside, this ship marked a very important milestone in digital graphics for movies in the late 70's and early 80's.

2) Death Star

 Length: 140 kilometers (87.0 miles)
TV Show/Movie: Star Wars

     That’s no moon indeed. And though Obi Wan later refers to the immense vessel as a “space station”, it is indeed mobile and has the ability to travel from planet to planet… even if it is only to destroy them… The Death Star was initially constructed by the empire when the Jedi were first wiped out to solidify their tyranny throughout the galaxy. However, it was destroyed by pesky Luke Skywalker and his ability to use the Force. Why there is a tube that leads strait to the main power generator and when shot down destroys the entire station, one will never know. But one thing that is for certain is that the Death star truly is a monster of a spacecraft, station or otherwise. The station was then reconstructed (almost) only to be again destroyed by Luke Skywalker in Star Wars Return of the Jedi. This immense structure is number 2 on the largest sci-fi starships and there were TWO of them?! Go figure. At any rate, at 140 kilometers across, this mobile space station is (was) a giant.

1) Alien Mother Ship

Length: 800 kilometers (497.1 miles)

TV Show/Movie: Independence Day

     What could possibly be larger than a small moon?! How about a medium sized moon? That is basically the size of the Mother Ship of the alien species that attacks Earth in the film Independence Day. This colossal vessel is nearly the diameter of Ceres, the largest asteroid in the solar system which, by the way, has enough gravity to pull itself into a sphere. If the Mother Ship were real and were composed of, say, steel, it would have similar gravity to the asteroid. This immense space faring vessel could fit 30 Death Stars within its gaping hull if it were hollowed out. Laid out across the United States, this craft would be the size of the state of Minnesota. This may not seem that huge until you consider that a craft the size of Minnesota made of steel would weigh approximately 4,000,000,000,000,000 (four QUADRILLION) pounds! Holy crap! They don’t call it the Mother Ship for nothing!

Conclusion:

     So there you have it, the 10 largest star ships in science fiction. Hope you enjoyed!