Worst Ideas for Space Travel. Ever.

This idea is good compared to some on this list...

     As a race, humanity has used brilliant and unconventional ideas to take our spacecraft to the Moon, Mars, and most recently Pluto. Since the 1920's, scientists and aerospace engineers have been designing and tweaking rocket propulsion to more efficiently reach for our nearest celestial neighbors.

     So far, we haven’t done a bad job, conjuring a wide range of propulsion methods varying from conventional chemical rockets, to electrical and nuclear rockets of the near future.

     However, the pressure induced upon the space travel industry during the Cold War lead to a lot of improbable, impractical, and downright awful ideas for propelling payloads into orbit and beyond that make Han Solo’s ‘bucket of bolts’ look like a space travel masterpiece (no disrespect to Star Wars or the Millennium Falcon). So with no further ado, I give you the top 5 worst ideas for space propulsion!

5) The Space Tether

Space tether (cord shortened for detailed craft)

     Take a small spacecraft in orbit. Now take a 4 kilometer long rope and tie it to another spacecraft. Now have them both fire up their engines and whip around each other like a ladder ball, until the centrifugal force on both objects is barely enough for the rope to withstand. Finally, cut the cord. This is the space tether.

     In theory, the idea of a space tether is sound. If you have the two crafts attached to each other in orbit, then they are both already traveling at orbital velocity. If you get them both to swing around each other at significant speeds, then you could send one of the crafts whizzing into space at pretty astonishing speeds.

     But what about the other craft? Well, the added velocity to the first craft is identical to the subtracted velocity of the second craft (if they are the same mass). Either we use more fuel to speed the second craft back into orbit, or we just let it fall back down to Earth. So even though the first spacecraft gets off using very little fuel, the second will end up using all of the necessary fuel that the first craft didn’t use just to return to its initial orbit. Either that, or fall back to Earth, and with NASA charging $10,000 to put up a pound of material in space, I don’t think anyone would want to launch up another space tether to try this experiment again.

     All things aside, the space tether would actually work, which is more than some of the other items on this list will be able to prove.

4) Star Tram Magnetic Launch System

Star Tram proposed magnetic launch system

     This whole idea was a disaster waiting to be constructed and then happen. Fortunately, it never got to phase 1.

     The Star Tram launch system was proposed as a way to completely eliminate the use of rocket fuel at all. It was to be a 5 kilometer long magnetic track propped up against a mountain to force a payload to astonishing velocities in order to blast them into orbit. If you have ever taken a physics introductory course, you can already see every problem with this idea.

     First of all, the amount of power necessary to launch even a small payload to high enough velocities to blow through the atmosphere and into orbit is unbelievable; upwards of 3000 Gigajoules. That’s enough energy to power 100 households for a year.

     Secondly, the shear material and construction prowess required to build such a track is immense. And to build it up against a mountain no less.

     Another major issue with such a launch mechanism is the acceleration undergone for the payload. Astronauts reach low Earth orbit in about 8 minutes, accelerating the entire way up into space. The Star Tram system intends to launch the payload to orbital velocity in 30 seconds within a few kilometers of the ground where it still has the entire atmosphere to traverse. Atmospheric conditions are chaotic, and often difficult to predict.

     Finally, the payload would still need some form of propulsion on board to guide it into a stable orbit, at which point a scientist on Earth should probably ask himself if it was worth the $40 billion to build what is essentially a giant space gun.

3) Project Orion

Artist's rendition of Project Orion

     It takes a lot of expelled scientific knowledge and prowess to remember that at one point, Project Orion was a real idea. In fact, the idea was so prominent that in the early 1960's, the math for such a craft had already been worked out and was actually small scale tested! Project Orion was proposed as a means for interplanetary travel in the near future.

     Devised by Ted Taylor in 1958, Project Orion was the study of using nuclear bombs to propel a spacecraft at extreme velocities (upwards of 3% the speed of light) so it could cover large distances relatively quickly. Yup. This was a thing.

     The craft was to be designed as such: The payload (or crew) would be positioned at the front of the craft. Stowed away just below them would be hundreds upon hundreds of nuclear warheads. During flight, the craft was to launch a nuke out the back of the craft, and detonate shortly after exit. The resulting explosion would push on a shock absorber and consequently force the craft forward. What could possibly go wrong?

     Temperatures within a nuclear explosion exceed 67,000 K. Even if you could find a material that remains solid in such heat, repeated subjection to a nuclear blast isn’t ideal for structural integrity. This temperature would also correspond to a ton of high energy radiation, dousing any crew or equipment in a deadly shower ultraviolet photons.

     On top of this (or on the bottom of this), the shock absorber would be a feat to engineer. Subjected to such rapid fluctuations in heat and pressure, the absorber would most definitely be the first piece of equipment to fail on the ship.

     Even if you could design a safe absorber and crew compartment to shield from radiation, this craft would be dangerous to launch on, or anywhere relatively near to Earth. Even in space, radioactive particles could still get caught in Earth’s magnetosphere and directed back to the Earth’s surface.

     And even if one could engineer a safe, practical, and reasonable design for a Project Orion spacecraft, it would still be illegal due to the nuclear testing ban treaty of 1963! That’s right, this idea was so bad, they banned it. Under world law.

2) A Space Cannon

A 1940's space cannon proposition

     The other ideas on this countdown wouldn’t be half bad if they could overcome a few key issues. This idea for spacecraft propulsion, however, is the issue.

     The theory is as simple as it sounds; to use an extreme explosion to thrust a projectile into space. As one can already predict, this one is going to have a pile of problems.

     Like the Star Tram, we face the issues traveling through the atmosphere for the majority of the trip and the fact that we would need some form of propulsion to guide the craft into a stable, circular orbit once at altitude. The cannon, however, has even larger issues.

     To launch a projectile from the base of a cannon to orbital speeds (about 7 kilometers per second) would require an impossibly immense explosion. So much so, that you probably wouldn’t have a cannon anymore after launch. But now that you’ve got your payload up to speed, it’s all smooth sailing, right? Nah.

     Unlike Star Tram, where you have at least cleared the bulk of the atmosphere by launching from a mountain, a cannon would face the difficulty of actually burning up its projectile within a couple kilometers of the ground. Meteorites, which are composed of mainly iron, burn up in just a few kilometers traveling just 5 or so kilometers per second. Name a material that can sustain that heat for 60 kilometers of atmosphere traveling 40% faster and you’ve got a lot in the aerospace industry!

     What could be worse that a space cannon?

1) Current Conventional Rockets

The Saturn V, a conventional rocket

     Shots fired? Maybe, but let’s take a step back and analyse our current methods of spacecraft propulsion.

     Since the dawn of the space age, we have utilized the same basic formula for rocket propulsion:

Liquid hydrogen (LH)+ liquid oxygen (LOX) + fire = thrust.

     Granted, the reaction between LH and LOX has the highest specific impulse of any potential chemical rocket (with some minor exceptions, but these are mostly dangerous and volatile reactions). Hydrogen and oxygen are also two of the most abundant elements in the universe, making them cheap and ease to get a hold of. Both gasses are easy to store and transport as well, and not difficult to synthesize into their liquid counterparts.

     Okay, here comes the kicker. We have been using this method for 60 years. If we were still using the same computers from 60 years ago, we would all be using Touring Machines. Most of our parents don't even know what those are!

     You may say to yourself, “so what? why fix something that isn’t broken?” Well brace yourself. Here comes more.

     Conventional rockets are extremely dangerous. When you launch a LOX-LH rocket into space, you are basically putting a pile of astronauts on the top of a massive chemical bomb. About 500 people have been to space. 18 have died, 14 of whom were killed in fuel related explosions. This equates to roughly 3%, which is a huge fatality fraction. If NFL football had this magnitude of fatality ratings, 300 players would have been killed on the field since the dawn of the sport, compared to the actual value of 1.

     Chemical rockets are also insanely inefficient. The space shuttle has a mass of 75,000 kg. The culmination of LOX and LH fuel weighes in at 754,000 kg, meaning that only 10% of the space shuttle’s mass at launch is the mass of the shuttle itself.

     Traditional propulsion is also highly expensive. As I have already discussed, it costs $10,000 to send every extra pound of material into space due to the shear cost of fuel for a space flight.

     Finally, though exhaust velocities for conventional rockets are suitable for orbital flight, they are incredibly slow for reaching out to farther destinations. Using our best chemical engines today, it takes days to reach the moon, years to reach Mars, and hundreds of millennia to get close to any neighboring stars. If we ever want to leave our doorstep, we are going to need more powerful propulsion systems.

Conclusion

     Are chemical rockets really a worse method than a cannon for achieving space travel? No, obviously not. But we have been squeezing every last ounce of efficiency out of these engines since the 1960's, and it’s about time that someone made a change. Whether it be some form of fusion powered rockets, or a higher impulse ion drive, we need to replace traditional propulsion in the near future if we ever expect to get off the ground and begin to study and colonize other planets and star systems. Earth is a great place, but there is so much more offered out there than we can ever achieve by staying here.

     The universe is a big place. And some forms of propulsion just won’t cut it.

Where Should we start Looking for Habitable Planets?

Artist's Depiction of extrasolar Earth-like planet.

     It's impossible to look up at the stars and not wonder about the ever growing possibility of life elsewhere in the universe. As I discussed in my previous blog regarding the Drake Equation, life may be as abundant as dirt in our galaxy alone. Many scientists disagree on whether or not meeting intelligent extraterrestrials will result in the wetting of our feet in the vast ocean of interstellar travelers, or the end of humanity as we know it. One thing, however, is certain; humanity is upward bound. Regardless of whether or not we have any desire to make first contact with the Vulcans from Star Trek, it is necessary that we, as a human race, find another planet hospitable for life. We are an ever growing species. Our energy, food, and resource needs will soon outpace the population (if it already hasn't) and we will be left in the devistating wake of our own needs. Furthermore, consider this. An asteroid just nine kilometers across wiped out the dinosaurs. There are at least 100,000 asteroids this big or larger in our asteroid belt alone. We all live on this one planet. Care to gamble humanity?

     Besides extraterrestrial influence and safety of the human species, finding an extrasolar planet hospitable to life would be an immense impact on the scientific realm. Just think of the astrophysical and geologic knowledge we could gain from the data obtained from an entirely different star system hosting a planet similar to our own. We would gain insight on what kinds of stars are suitable for life, how planet mass, density, and temperature influence that life, and finally, how that life affects us. We may have only explored 1% of earth's oceans, but we have observed just a minuscule fraction of a percent of all the stars, clusters, and galaxies in the universe. So, where do we start?

Hertzprung-Russel diagram of stars.

     The first step in finding a planet that is hospitable to life surely begins with finding the right star. Many wrongly assume that size and temperature of a star do not matter in finding such a planet, citing that, 'if the star is hotter, the planet just has to be farther away'. This is not entirely true. We must first look at stellar mechanics. The first thing to know about main sequence stars (normal stars, see diagram above) is that everything about them can be determined by their mass and composition. A star that is more massive has more gravity, which crunches its contents inward towards the core. This makes the pressure very high. As the internal pressure of a star increases, so does its temperature, and in turn, its luminosity. Luminosity, or brightness of a star, can also be thought of as energy consumption. This means the brighter the star, the faster it burns up its fuel. The lifetime of a star, then, is dependent upon two things: its mass (how much fuel it has), and its luminosity (how much fuel it is burning away). Knowing that our own sun has a lifetime of 10 billion years (10^10 years. There is a formula for computing this too, but it's much longer and not necessary for this blog), the formula for the lifetime of a star is:

T = (10^10)(M/Ms)(Ls/L)

Where:

T = stellar lifetime
M = mass of star
Ms = mass of sun
L = luminosity of star
Ls = luminosity of sun

     Now, I know what you're asking, "what does stellar lifetime have to do with finding a planet suitable for life?" It turns out that planets need time to form. With that formation, a planet needs time to settle and mature before it would ever be suitable for life. On Earth, it took about 2.5 billion years for our planet to develop conditions that could satisfy life. If a star burns up faster than that time frame, a suitable planet can never form around it and can thus be discluded in our search for life-harboring planets. So, stellar lifetime (T) must be greater than or equal to 2.5 billion. This leaves us with two unknowns: M and L for the star in question. Fortunately, main sequence stars have predictable luminosity for known masses. Plugging in the numbers with Ms = 1 and Ls = 1:

2.5*10^9 = (10^10)(M/L)

M/L = 0.25

     For main sequence stars with M/L = 0.25, M = 1.59 Ms (solar masses) and L = 6.39 Ls (solar luminosities). This mass corresponds to a mid-sized F class star, a little larger than our G class sun. This may seem like we're really narrowing our search, however, stars much more massive than our sun are incredibly rare. It turns out that only 1% of stars exceed 1.59 solar masses, so we really haven't cut much off of our search. It would seem like the planetary hunt can begin, but (as always), there's another hurdle in the way of finding a star acceptable for planetary life. Obviously a star can't be too large, but what if a star is too small?

An 'A' class star, about 2-3x the mass of our sun. Burns up too fast to form habitable worlds.

     In many ways, small stars (or red dwarfs) seem even more hospitable to life than our own sun. They burn for trillions of years, allowing plenty of time for life to evolve, and they are incredibly abundant, making up about 75% of all stars in the galaxy. However, there are some unforeseen dilemmas with life sustaining planets forming around small stars. The first problem is their magnetic activity. Smaller stars tend to experience increased solar activity, such as solar storms, much more frequently and violently than our own sun. This makes the survival of life around such a star quite difficult. In more massive stars, the interior stabilizes and consequently mellows any violent solar activity that might occur.

Artist's depiction of a magnetically active red dwarf star.

     Another major difficulty with life in a red dwarf system is a process called tidal locking. Tidal locking is the phenomenon where one side of a planet or moon always faces its host. This occurs when that object (like our moon) orbits too closely to the host and the tidal "bulges" of the host actually tug on the object gravitationally and slow its revolution. Over millions or billions of years, the object becomes tidally locked. In our solar system, this isn't a problem because our planet orbits so far away from our sun. However, the habitable zone distance for a red dwarf is much closer to the star, leading to accelerated tidal locking. One can see where this would be a problem with a life-stable planet around a red dwarf star. If one side of the planet always faced the star, that side would always be a baked crisp while the other would be a frigid tundra, both sides devoid of life. Fortunately for us, there is a formula for how long it would take for a star to tidally lock a host planet as well! The equation is:

T = (w*(a^6)*I*Q)/(3*G*(Ms^2)*k*(R^5))

Where:

T = Time to tidal locking
w = initial radial velocity of planet
a = distance from star
I = moment of inertia of planet
Q = dissipation factor
G = gravitational constant
M = star mass
k = Love's number (no, not 'love' the phenomenon)
R = radius of planet

A tidally locked planet, half desert, half tundra.

     Some of these variables are difficult to explain, but basically, tidal locking depends on the distance, radius, revolution and orientation of the planet at its formation, and the mass of the star at hand. Remember, we want the planet to be stable for at least 2.5 billion years, so we will set 'T' equal to that. 'a' is the distance from the star, which is really just the habitable zone distance of a red dwarf star. You may have noticed that we can't calculate 'a' if we don't know the mass of the star, which we are trying to solve for. To overcome this, we must write out 'a' as a function of luminosity which, in itself, is a function of mass for main sequence stars. Also, the variables 'Q','G', and 'k' are all just constants for any system. These numbers can easily be found or equated from information online. Finally, for the variables 'w', 'I', and 'R', I used the equivalent Earth values (because we are solving for an Earth-like planet). The formula is rough, but it comes out with a reasonable answer:

M = 0.568*Ms

     This number indicates that any star with a mass of less than 0.568 solar masses will consequently lock any earth-like planet in a death stare for the remainder of its existence, quenching all and any chances of finding life there. This is the mass of a small K type star. This number is also a devastating blow to the percentage of habitable stars systems out there. 80% of stars in our galaxy have masses of 0.568*Ms or less, meaning that there is little chance of finding a life supporting planet around this vast majority of stars. Adding this to the 1% of stars that are too big for habitable planets, this leaves us with just 19% of stars in our galaxy that can construct life harboring planets.

A 'G' class, sun-like host star.

     And the onslaught doesn't end there. Even if a star is the right size for a planet doesn't necessarily mean it will be in the right system to form planets. I am talking, of course, about multi-stellar systems. Though it is possible for binary, trinary, and even quadrinary systems to form planets (as has been proven with the kepler space telescope), it is unlikely that any of them would ever remain in a stable habitable zone because their distance from the host star is constantly changing. This would cause dramatic seasonal changes. On top of this, multiple host star systems are notorious for causing wild planetary orbits, shooting planets way out into the cold only to drag them back through the inferno of two suns. It is estimated that 1/3 of all star systems in the Milky Way, are in multiple star systems. This means that 1/3 of our 19% of "Goldilocks" stars are in multiple star systems as well. Oh, I also forgot to mention that only 90% of stars in the galaxy are main sequence stars... Sorry about that. Simple multiplication will lead us to the inevitable conclusion that about 5.7% of stars are really "Goldilocks" stars.

A quadrinary system, a very unstable environment for habitable planets.

     In the large scale of the galaxy, 5.7% isn't a small number at all. This is equivalent to roughly one stable, single star for every 18 stars around it. In our galaxy alone, this is still 17 billion stars. The math seems to hold true for our plot in the galaxy as well. Within 20 light years of our own sun burns 150 stellar objects. of these 150 stars, 9 of them are single, main sequence stars between 0.568 and 1.59 solar masses (including the sun), or 6% of them. These stars may be the hosts of planets with strange and complex extrasolar life, or these planets may serve as a second home for humanity in the distant future. Whatever the future holds the most important thing is that we, as a race, remain curious as to what might be out there. Who knows, life might surprise us after all.

THE LIST: These are the 9 single stars between 0.568 Ms and 1.59 Ms within 20 light years of Earth. Enjoy!

1) Sun
   -distance: 0 ly
   -mass: 1.00 Ms
2) Epsilon Eridani
   -distance: 10.52 ly
   -mass: 0.82 Ms
3) Tau Ceti
   -distance: 11.89 ly
   -mass: 0.78 Ms
4) Lacaille 8760
   -distance: 12.87 ly
   -mass: 0.60 Ms
5) Groombridge 1618
   -distance: 15.85 ly
   -mass: 0.67 Ms
6) Wolf 1453
   -distance: 18.53 ly
   -mass: 0.57 Ms
7) Sigma Draconis
   -distance: 18.77 ly
   -mass: 0.88 Ms
8) 82 Eridani
   -distance: 19.71 ly
   -mass: 0.97 Ms
9) Delta Pavonis
   -distance: 19.92 ly
   -mass: 1.05 Ms

Straight out of Star Trek: Antimatter Propulsion

An actual proposed antimatter rocket

     As a child, I grew up watching Star Trek. I grew up watching a lot of science fiction TV series, but Star Trek was different. Whereas Star Wars was a spectacular story aliens and spaceships of a different galaxy far, far away, Star Trek was closer to home. It depicted a future version of humanity, where we had all given up wars and savagery for a greater good; the exploration and discovery of the cosmos. With the recent death of renowned Star Trek actor Leonard Nimoy, many of us are wondering if the Star Trek legacy will continue to remain true to its standards. I cannot speak for Paramount on that issue, but I can say that the ideas and technology that Star Trek set the standards for are becoming closer to reality every single day. Communicators, computer pads, and even the universal translators first conceived by Star Trek are becoming analogous to the technology that we see around us every day. It is only a matter of time before our top scientists sit down and discuss the reality of a mission to another star. And the technology behind that mission was also predicted by, you guessed it, Star Trek.

Star Trek inspired many of today's technologies

     Antimatter propulsion has been on the scope as a potential form of space propulsion for decades. The idea is actually really simple. E = mc^2. Whereas other forms of space propulsion require muddy formulas that may give us the energy output of the engine estimating on a feasible efficiency, the formula for an antimatter rocket depends solely upon how much antimatter you are supplied. Okay, let's do a little experiment. Let's say that CERN has decided to use their atom smasher to collect antimatter for the use of energy yield. Slaving away for thousands of years (our current technology is not very suitable for collecting antimatter), they have amassed a grand total of one gram of antimatter. Plug that into our formula for energy yield:

E = mc^2
E = (.001)(3*10^8)^2 = 9,000,000,000,000 J = 90,000,000 MJ

     One measly gram of antimatter provides us enough energy to power a 100 watt light bulb for an astonishing 2854 years, providing that the light bulb doesn't burn out. Okay, but what about traversing the cosmos? If you can recall my post about nuclear fusion propulsion (you might want read that one first if you haven't yet), one gram of tritium-deuterium fuel gets you 3.6*10^11 joules of useful energy in one gram, after you multiply by efficiency. Simple math will tell you that antimatter is a whopping 250 times more efficient than nuclear fusion. Recall also that a mission to Mars required roughly 460 grams of total fuel. That's right, a trip to Mars using an antimatter propelled rocket would be able to make a round trip to the red planet and back with just 1.8 grams of total fuel, or just 0.92 grams of antimatter!

A matter-antimatter collision results in energy and gamma rays.

     Slap me twice and call me stupid, but that number is shockingly low. If the human race were to ever harness that kind of technology, traversing within our solar system would become a snap! Any spacecraft would only need a few grams of fuel to reach their destination, and return home with fuel to spare. But the real question remains; is it enough to get us to the stars? Mars is (on average) 225 million kilometers away. Our nearest neighbor, Proxima Centauri, is 4.22 light years. This equates to 4,000,000,000,000, or four-trillion kilometers. I could simply divide Mars's distance by Proxima Centauri's distance and multiply by 0.92 grams to find how much fuel it would take to perform a round trip, but let's be real. There are a lot of limitations and specifications that need to be addressed before we begin calculating our fuel consumption.

     First of all, traveling across the cosmos does not happen instantaneously. We are restricted by the speed of light. This means that any manned mission to another star would need enough volume for not only the crew, but also the food, the water, the engine, and all other electronics and systems on board. Secondly, a trip to another star is very different from a trip to a planet. We would most likely be dealing with relativistic speeds which require more complex math (which I will get to) to accommodate for the acceleration of the craft. Finally, due to the duration of such a mission, it is unlikely that our first manned mission to another star will involve a return trip. The crew of whatever craft leaves the Earth, is most likely destined to remain upon whatever planet they decide to go to.

What a habitable exoplanet might look like around Epsilon Eridani

     Now the year is 2150 and we are finally ready to send a manned mission to another star. Top scientists have identified a habitable planet in the Epsilon Eridani system, using gravitational microlensing, approximately 10 light years away. They have constructed a massive craft in high Earth orbit weighing in at over 840 million kilograms (roughly 420 times the mass of the space shuttle) that is suitable to support 1000 human passengers for a multi-year mission to this strange new world. The ship is designed so that it will accelerate at one Earth gravity until it reaches its maximum speed at 70% of the speed of light, where it will coast until it nears Epsilon Eridani. From there, it will spin around and reverse accelerate at 1*g, taking in hydrogen using a Bussard collector from interstellar space to use as matter for her matter-antimatter engines, until it comes into orbit around the young, orange star. The ship will maintain artificial gravity while coasting by having the crew live on a giant rotating hub, which spins just fast enough to simulate the gravitational forces of Earth. Now here's the kicker; how much fuel do they need? Imagine that one on your next physics exam!

     Let's start off small. How long will it take the crew to accelerate to 70% the speed of light (0.7*c) at 1 Earth gravity. The formula for that is decently simple:

v = a*t

     Where "v" is desired velocity, "a" is the acceleration, and "t" is time. 0.7*c is equivalent to 2.1*10^8 m/s accelerating at 9.81 m/s^2. Plugging in the numbers, we get that:

t = 21.4 million seconds = 0.68 years

     End of story? Not exactly. Because the ship will ultimately be traveling at relativistic speeds (which really just means speeds close to the speed of light), we will need to use a formula that compensates for the relativistic effects. This "relativistic acceleration" formula, is as follows:

v = (a*t)/(1+(a*t)/(c))^0.5

     Wait a minute, so why don't we see this formula when calculating the acceleration of a ball as it falls towards the Earth? Because the ball is not traveling near the speed of light. This formula is only useful for objects that accelerate to velocities close to "c". Once again solving for "t", we get:

t = 3.0*10^7 seconds = 0.95 years

     Assuming a constant acceleration of 9.81 m/s^2 (Earth gravity), this gives us that the total distance traveled is about 0.45 light years. Since it took the craft 0.45 light years to accelerate to 0.7*c, it will take that same distance to slow down. this means that the craft only needs to burn fuel for 0.9 light years of its 10 light year trip! 9.1 light years will be spent coasting at 0.7*c. Due to the fact that we are accelerating to relativistic speeds, our fuel percentages will shift as well. For any engine that is 100% efficient at converting mass to energy, the projected weight of the fuel for a 0.9 light year voyage is approximately 2.1 kg for every kg of spaceship. Recall that our spacecraft is 8.4*10^8 kg, meaning that our fuel mass is 1.8*10^9 kg! Yikes, this number seems pretty high. but let's break that down, quite literally. What I mean is, let's take that amount and split it into six spherical fuel tanks, each of which is equipped with powerful magnetic fields to ensure that the antimatter does not touch the walls of the tanks.

An antimatter rocket in the interstellar mod of Kerbal Space Program

     For the purpose of this calculation, I will assume that the vessel uses anti-hydrogen, which should have all of the same properties as regular hydrogen except its charge, which is reversed. The density, then, of compressed, liquid anti-hydrogen is the same as that of liquid hydrogen: 70.85 kg/m^3. Take our mass and divide by the density to find the volume of fuel we need:

(1.8*10^9)/(70.85) = 2.54*10^7 cubic meters

     Now we take that volume, and divide it into six identical spherical fuel tanks:

(2.54*10^7)/(6) = 4.23*10^6 cubic meters per fuel tank

     Finally, we set that volume equal to the formula for the volume of a sphere to find the radius of each tank:

4.23*10^6 = (4/3)*pi*r^3

r = 100 meters

     So that's the answer. Our proposed interstellar craft would need six spherical fuel tanks containing compressed anti-hydrogen each having a radius of about 100 meters. Is this realistic? Definitely not with today's technology. But who's to say that in a hundred years or so scientists won't come up with a way to produce antimatter in bulk? Criticisms aside, how long would this voyage really take? Assuming acceleration is constant at the beginning and end of the trip, and that speed is a constant 0.7*c for the 9.1 light year trip, calculation of duration becomes quite simple:

t = 2*((d_1/(0.5*a)))^.5 + (d_2)/(v)

d_1: distance traveled in ly during acceleration  phase(0.45)
d_2: distance traveled in ly during coasting phase (9.1)
a: acceleration (9.81)
v: velocity of coasting in "c" (0.7)

t = (((0.45)/((0.5)(9.81)))^.5)/(9.46*10^15) + (9.1)/(0.7) =  14.87 years

     That is, 14.87 Earth years. Due to the fact that the craft is traveling at relativistic speeds, strange things start to happen to the crew of this ship. Time actually passes slower for the crew of the ship than it does for a bystander on Earth. The factor of this time dilation, as it is called, can be derived as such:

y = 1/((1-(v^2)/(c^2))^.5)

y: time dilation factor
v: velocity
c: speed of light

     Plugging in 0.7*c for "v", we find that our time dilation factor, "y", is 1.40. This equates to time being 1.4 times slower for the crew of the ship at 70% of the speed of light. While central command at Earth would record the vessel as taking nearly 15 years to reach Epsilon Eridani, the crew of the spacecraft would have only aged less than 11 years. 15 (or 11) years is a very realistic time span for an interstellar crew. It is not so long that it would require a generational ship, but that is still 11 years of supplies that need to be carried along with the ship on its epic voyage to Epsilon Eridani. Alas, there are other problems with antimatter propulsion that need to be addressed.

Artist's rendition of the known Epsilon Eridani system

     Besides the fact that you need to amass a crew of 1000 who are willing to depart on a decade long mission to a planet light years away from which they will never return, there are many hurdles that need to be overcame before we make a trip to another star. As I have already mentioned, antimatter has proven difficult to produce. So far, we have only been able to produce a few atoms of anti-hydrogen, far short of the two billion kilograms needed for an interstellar mission. There is also the problem that matter-antimatter annihilation results in the formation of a lot of heat and dangerous gamma radiation that would be harmful to a crew that was near such an engine. Internal problems, however, are the least of the crew's worries when they realize that we need to come up with a material or energy field capable of protecting them from the dangerous blue-shifted mass of cosmic radiation that is capable of penetrating even the densest materials known to man.

     However, we have 100 or more years to devise ways to overcome these problems. If there is anything that the human race has taught me so far, it is that we don't stop when we believe that something is impossible, we try even harder. Though antimatter engines may not be able to take us to Vulcan, or fly us at Warp speeds, they are the first step in reaching humanity's ultimate goal of achieving a higher existence. It is only a shame that Leonard Nimoy will not be around to see the wonders that our brilliant race will create in the centuries to come. But I hope now that he can rest in peace knowing that his character has sparked the intuition multiple generations to reach for the stars, quite literally. Star Trek has already correctly predicted the invention of the cell phone, the iPad, and Google translate. Next stop: Epsilon Eridani.

In memory of Leonard Nimoy
-"Live long, and prosper..."