With the discovery of about 230 exoplanets and the possibility of habitable planets around the nearest neighboring star, it is time to start thinking seriously about how to get there. Weather it will be to visit our galactic neighbors or to colonize the planets ourselves, the journey would probably be worth it.
So here on ReducedMass.com, we decided to look at 5 methods making it possible for humankind to go on light year journeys. Note that we’ll base this short list on what seems scientifically feasible today (but might not be), so don’t expect any warp-drives, worm-holes or other faster-than-light methods. That said, some kind of speculation, subjectivity and guess-work can’t be avoided, so please feel free to let us know what you think in the comments section below, and do tell us about all the brilliant methods that should’ve made it to the list.
Enough with the babble… let’s get started!
5. Ion thrusters
If you’ve ever fired a gun and felt the recoil numb your hand or shoulder, you understand the basic principles of Ion thrusters, namely Newton’s 3. law. But in stead of bullets being ejected, Ion thrusters fire ions into space, thus recoiling the spaceship to gain a velocity in the opposite direction.
This method scores on feasibility because it has actually been used in space crafts already, e.g. Deep Space 1. However, it would probably take too long for such a space craft to go to other stars unless we begin to talk about human hibernation or other preservation methods, decreasing the feasibility significantly.
Power: 1
Feasibility: 8
Result: 4.5
4. Antimatter rockets
Antimatter is strange stuff, my friend. The name describes particles that are in many ways opposite to the corresponding normal matter particle, but the magic doesn’t stop here. When normal matter particles and their antimatter particle meet, they DISAPPEAR, leaving in their wake super energetic electromagnetic radiation, called gamma-rays, that carry the energy of the particle pair.
Antimatter rockets work by using the energy output from these matter-antimatter annihilations to propel the spacecraft towards unimaginable speeds. Although they are probably the most powerful engines in this list (meaning they have the highest specific impulse), there are just sooo many practical things that we have to master before we can actually use it. These problems include how to obtain large amount of antimatter and how to store it for later use.
Power: 9
Feasibility: 2
Result: 5.5
3. Fusion rockets
Fusion rockets work by utilizing the energy of, surprisingly, nuclear fusion, the process also powering the Sun. Currently we cannot create fusion under controlled circumstances and harvest the energy, but many scientist believe we will be able to do so in the not so distant future.
The fusion rocket could use the energy in several ways to gain forward thrust. It could be by firing the result from the fusion process from the back of the spacecraft, generating thrust just like the ion propulsion system explained above, only much more powerful. Another, more feasible way would be to create fusion explosions (yes, bombs) in front of some sail that caries the ship forward from the energy of the explosion.
In either way, the big down-side is the relatively high amount of fuel needed to go on the monster journeys that is interstellar travel.
Power: 6
Feasibility: 6
Result: 6
2. The Bussard Ramjet
Named after the late physicist Robert W. Bussard, this type of spacecraft is related to the fusion rocket above, as it uses the same method for propulsion, but it solves the fuel problem in an ingenious way. In front of this whoping spaceship would be some kind of collector of matter from the interstellar medium. In between the stars lie huge amounts of matter, called the interstellar medium, consisting mostly of hydrogen, which can be collected and used in a fusion reactor for an unlimited amount of fuel without the weight of carrying it. The huge sail and the collection of particles would however create a drag in the opposite direction, and there are still discussions on the feasibility of the method, depending mostly on the density of the interstellar medium.
Power: 8
Feasibility: 5
Result: 6.5
1. Beamed propulsion
Now to the winner. And no, this has nothing to do with beaming people like fans of a certain sci-fi franchise would love, but with an idea almost as cool, and it involves enormous lasers! Just never mind what xkcd has to say about it :-)
This spacecraft would have a giant sail, and I’m not talking about normal giant, I’m talking really giant here… I’ve read sail size proposals ranging from a few kilometers to Earth-sized. A monster laser beam is fired from Earth or another stationary point, slowly pushing the reflective sail away from it and allowing for velocities around 10% the speed of light. This would allow for a trip to Alpha Centauri in around 40 years.
Variations on this includes using microwaves (masers) in stead of visible light lasers, so the sail wouldn’t have to consist of one giant, perfect mirror, but more of a grid.
As we have already begun experiments with very small objects being propelled by a laser from the ground, the idea is not so far-fetched. There are of course still some technical difficulties for using the method for interstellar travel. E.g. the precision of the beam and/or the size of the sail would have to be extra-ordinary for it not to miss the sail to much, vasting precious amounts of energy. The laser would of course also have to be much more powerful than any lasers today, but judging from the last hundred years of human advancements, who knows what can happen in another hundred?
Power: 7
Feasibility: 7
Result: 7
So, if you’re not too tired from all this, go find your own method of interstellar travel, and we’d be glad to hear from you. As you can see in this list, you have to have a pretty good imagination and know a lot of physics to get on the journey towards the stars.
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March 21st, 2008 at 4:41 pm
I think you should mention that the major problem with antimatter is momentum. Combining a particle with an anti-particle gives you a great deal of energy but you end up with the same momentum you started with… namely zero. The two gamma rays go in opposite directions. That isn’t going to propel you anywhere.
To use antimatter you only get to “use” the energy. So all you are getting is a very dense source of energy. You have to carry some reaction mass and some machine to convey the energy to your reaction mass.
If you have all that to get antimatter to work, then you may as well do it with lasers. Forget the momentum of the earth-based lasers, capture the energy on the ship (solar panels?), transfer that to your propulsion machine and away you go.
Or maybe even better, make your “machine” itself a laser, bounce it back-and-forth a couple million times off a mirror here at home. When you’re far enough away, switch to antimatter for energy for the same laser. Maybe drop extremely light (but wide) mirrors behind you as you go, to bounce your laser off. You could do the same thing to slow down in the other direction.
Add all this up and you have a pretty good interstellar ship, actually. I’d go with fusion, personally.
1) Ground-based lasers for initial energy, powered by fusion power plants.
2) Ship-based collector panels tuned to the frequency of the lasers, to capture the energy (and a little of the momentum).
3) Ship-based laser, same frequency, aimed initialy at earth-based mirrors for momentum and reabsorbed for energy on the ship via the same collectors.
4) Ship-based fusion (or antimatter if you insist) for energy after the earth-based lasers can’t reach you anymore.
5) Payload of collapsed mirrors on board the ship. Eject these (for thrust) with a railgun at high velocity (relative). They expand to provide a closer reflector for the ship-based laser.
6) Reverse direction to decellerate at the destination.
Where do I sign up? I’m ready to leave anytime.
*grin*
March 21st, 2008 at 7:44 pm
Well Corey, awesome comment. I’ll sign up right there with you !
And your totally right. A practical interstellar spacecraft should probably combine various methods, but as you’ve surely found out, the point of the article was more to give a very brief introduction, giving people something to investigate futher if they disire.
Regarding the antimatter method, there are some thoughts on how to use the antimatter reaction itself for propulsion. For example we could hope that somone invented a gamma-ray mirror so one of the photons would be reflected. Surely, the least energetic anihilations produce only gamma-rays, but higher energy anihilations can actually produce small mesons that can be channeled in other ways (eg. magnetic nozzle) to produce thrust.
But you’re right, its not easy, and antimatter methods are generally futher in the future than any of the other methods, hence the low placement on the top 5. Other uses of antimatter in rockets could be as catalyst for fusion processes or similar. You make a small amount of antimatter anihilations, that creates the temperatures and densities required for fusion, and get the rocket going in that way.
Its hard, but I’ll stop here. Keep the comments comin’
March 24th, 2008 at 11:49 pm
Something seems funny about the idea of shooting a mirror out ahead of yourself to use as a method of slowing down. Presumably the mirror will be a lot less massive than the ship minus the mirror. So when you shine your laser on it won’t you accelerate the mirror away from the ship a lot faster than you will decelerate the ship?
And here’s a down to earth problem you guys may be able to answer for me. Everybody talks about battery technology for cars, but it’s hard for a non-techy to get a grip on how good batteries can theoreticly get. A 15 gallon tank of gasoline weighs about 90 pounds and contains enough usable energy to move my Ford Taurus about 450 miles at highway speed. Assuming one could build a perfect battery - how much total energy does 90 pounds of Lithium and whatever ions it reacts with contain?
March 25th, 2008 at 1:21 am
Hi Sully
I’m not totally sure what you mean about the mirror for slowing down. In the beamed propulsion method I was talking about, the mirror is on the ship, and a laser is fired from Earth to push the ship away from it towards whatever star system we want to visit. The slowing down when we get there part is a toatally different matter that also has to be considered, and can be difficult if the ship is not carrying an engine.
And your question on electric cars is dificult. As far as I can tell from Wikipedia, there’s actually more energy in 90 pounds of gasoline than in a 90 pound lithium-ion battery of today. The difference lies in effeiciency, where batteries uses almost all of its potential, while combustion engines are very inifficient (around 20-40%).
In the line of the article, if we’re talking fusion or antimatter reaction, though, we’re talking whole new energy potentials. Imagening we are able to convert the entire weight of some material to energy. We can use Einsteins famous equation E = mc^2 to calculate the energy of 90 pounds, and following your example of how far you can get on 90 pounds of gasoline, you would now be able to go a thousand billion miles… that is, to the Sun and back again around 5000 times ! I guess your Ford Taurus, although trusty, will give in before that :-)
March 25th, 2008 at 4:08 am
Hi Brink,
It was actually Corey I was reacting to - He wrote “Maybe drop extremely light (but wide) mirrors behind you as you go, to bounce your laser off. You could do the same thing to slow down in the other direction. ” As I see it you shouldn’t be able to gain much acceleration by bouncing a laser off a mirror which is not very massive because any acceleration to be gained will push the mirror away from you at a multiple of the acceleration that your much more massive ship will gain.
I understand the use of the mirror or space sail for acceleration away from a fixed laser (as put forth by Niven and Pournelle in The Mote in God’s Eye). N+P had their aliens (the Moties) use their light sail for acceleration away from lasers on their home planet which where made more and more powerful as they moved farther away. They then used the light sail for deceleration by diving directly at the destination star (our sun in the case of the novel). As I recall the idea was that both the light from the star and the solar wind of particles would work to decelerate the sail and thus the ship.
If you’re at all into science fiction and haven’t read The Mote in God’s Eye you should definitely get a copy and then act sick and spend the next 10 or 15 or so hours reading it.
My question about the gasoline and the lithium was poorly phrased and was so totally off the subject of the original space-faring article that I confused you. I figured I was among materials scientists, so I was asking whether it is theoretically possible for a lithium-ion battery to be made which can propel a car as far as an equal weight of gasoline.
March 27th, 2008 at 11:19 pm
Duh, its amazing I could miss that you were talking about Corey’s comment. And I must admit that I can’t figure out what he ment by that either. Maybe Corey himself can explain?
And you know what, I love science fiction novels, and have read a lot of Larry Niven already. This one I havn’t read yet, but I’ll definitely keep an eye out for it.
Regarding the battery question, my quick research says no, you will not be able to gain the same amount of miles from an equal weight lithium battery than gasoline. Looks like the power to mass ratio of current Li-Ion batteries are about 1% of that of gasoline, and with the efficiency already at 99% or similar, it doesn’t seem to get much better I’m afraid. What they can improve though, is time of recharge etc.