Which form of Interstellar propulsion seems most viable and why do you feel this way?

Leaving aside my suspicion that the original post is misplaced, probably the most viable interstellar propulsion technology within something vaguely close to current technology is either Freeman Dyson's application of nuclear pulse propulsion or something like AIMStar.

Ion engines powered by photovoltaic panels receiving power from beamed energy.

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Ion engines wouldn't work, beamed or not, there isn't enough fuel efficiency (you still need a reaction mass.)

Only engine with a high enough specific impulse would be a "dusty plasma" fission fragment engine. It would be possible to do a 50 year mission to Alpha Centauri with one, but constructing an engine powerful enough to do so is a long way off.

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funny enough i think solar sails are the answer

at first they seem clumsy and daft...a giant umbrella in space and a giant laser on the moon

but the more i looked into it the more it made sense

the advantage is not having to carry the fuel on board the space ship

im not totally convinced about the techniques of a return journey (bouncing of a smaller detached sail onto the reverse side of the returning ship)

i thing it is valid for one way trips...and to be honest no point coming back until you have established that colony on alpha centuri anyway....by which time u will have a moon base with a laser to help you on your way home

the solar sail can be used to slowdown at the other end by approaching a star (or multiple close flybys in stages )

My two favorite methods are Geoffrey Landis's relativistic particle beam propulsion and my own relativistic kinetic impact powered rocket.

Geoffrey Landis's system is simple, elegant, and efficient, and it uses mostly technology that we already have. You simply have a big honking particle accelerator in space, which emits a stream of atomic particles at relativistic speeds. This is aimed at a starship featuring a large superconducting magnetic loop. The particles "bounce" off of the starship's magnetic field, producing thrust until the starship reaches relativistic speeds.

This method has only two flaws:

1) As originally proposed, it only works for "outward" acceleration. It does not provide for braking at the destination other than to rely upon Zubrin's somewhat speculative interstellar medium mag-brake. This flaw can be addressed by assuming the use of a sacrificial particle beam drone.

and

2) We simply don't know how easy or hard it will be to focus a particle beam over multi-AU distances. It might be easy. Or it might be impossibly hard. Depending on how hard it is, the entire idea may be impractical, or it may be rescued by the use of many "short range" beams instead of one "long range" beam.

My relativistic kinetic impact powered rocket is more complex than Landis's RPB method.

It involves a long range free electron X-ray laser, focused by a 1+km wide zone plate, to accelerate a swarm of kinetic impactors.

The starship is similar to Landis's in that it's mainly a superconducting magnetic loop. Unlike Landis's starship, mine also has an onboard supply of sacrificial propellant.

This propellant is sprayed into a puff either "behind" or "ahead" of the starship. When a kinetic impactor hits this puff, the result is an explosion more energetic per gram than a fusion bomb or even an antimatter reaction. Ions from this explosion "bounce" off the starship's magnetic field, producing thrust.

This scheme is about an order of magnitude less efficient than Landis's RPB propulsion system (losses due to generation of the X-ray beam, losses due to focusing the beam through a zone plate, and losses due to transfer of photon momentum to pellet momentum), but it addresses the two flaws:

1) It works "both ways". It can be used to produce thrust either away from or back toward the source system. Thus, this system may be used to efficiently brake at the destination, as well as return back to the source system.

2) We know that an X-ray beam can be focused over many light years through space, because of the sharp X-ray images of far away X-ray sources.

My favorite method of interstellar propulsion would combine both concepts. Landis's RPB propulsion for the outbound acceleration leg, due to its much higher efficiency. My kinetic impact powered rocket for the deceleration and return legs, due to its inherent capability to do so.

This isn't exactly the deal-killer. In principle, an ion engine can have any exhaust velocity up to the speed of light. You can use a particle beam accelerator as an ion engine, in principle.

The deal-killer is thrust/weight ratio. An ion engine with a relativistic exhaust velocity would have the thrust/weight ratio of a flashlight. It would take centuries for the engine to even budge.
Fission is lacking in specific impulse; it's too slow. Using RPB and/or kinetic impact powered rockets, it would be possible to go to Alpha Centauri AND BACK within 50 years.

The key is to use energy/power provided externally rather than internally. If you rely upon internally stored energy, then the only really viable energy source would be antimatter. However, antimatter is mind-bogglingly expensive to produce using known science.

In fact, though it can be done, focusing X-rays is not that straightforward, given that they get easily absorbed. You want to make them reflect at shallow angles in order to being oriented in specific directions.

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Interstellar travel is done using gravity waves.

It is straightforward if you use the proper method. In this case, a large zone plate is the appropriate method. We don't use them for X-ray telescopes because they are extremely frequency dependent. For a free electron X-ray laser, this is not a problem.

A zone plate works using absorbtion and diffraction, and is inherently limited in efficiency to 50% or less. Using a sinusoidal zone plate, half of the beam gets absorbed/reflected by the zone plate; the other half of the beam gets focused onto the desired Airy disk. For this and other reasons, a long range X-ray laser beam will not be as efficient as a particle beam (IF a particle beam can be focused over those long distances).

Ion engines with on-board power supply don't work because of the mass of the required power supply. A high exhaust velocity ion engine craft using beamed power and a propellant mass fraction similar to that of a more typical chemical rocket would work: if exhaust velocity is 0.1 c, and 9/10 the initial mass is propellant, final velocity is (0.1*c*ln(1/0.1)) = 0.23 c.

I have researched this and maybe the answer is a onboard laser for the return trip. From what I read this can produce speeds of .10c which is a 40+ year trip to the closest star in the Alpha Centauri system. That is feasible. Just would love to see us do this in my life time.

Actually, I posted this for my follow-up which is now that we have choosen a possible form of travel, which star system seems most promising to find planets in its star's habitable zone that might support life? Or do you think we should just go for the closest star system? (I.e. Alpha Centauri)

Where would this on-board laser get its power? The laser that accelerates any laser-powered sailcraft would be very large, very heay and require a lot of power. That would be okay if you build it in the Solar System, effectively stationary and powered by solar energy or some other convenient local source of power.

However an on-board laser would need to be carried along with the payload, and would need stored energy in some form (fuel, in other words) to produce any thrust required. An on-board laser is not a very efficient option.

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I was actually combining Isaac Kuo's Idea above with the Solar Sail. (Xray Laser)

Personally, in a way that I would consider seriously proposing as a goal for the next century or so? I'd pick Proxima Centauri, simply because it's closest. It may not have any planets in its habitable zone, but it is a radically different star than the Sun, and getting data from a fairly close approach would be useful for astrophysics. If there were to be a much closer brown dwarf found, I'd suggest that.

For a hard sf story constrained by currently known physics with a story line about some interstellar colonization, I'd pick α Cen (either A or B) as either may have planets in the HZ, and even if they don't they're likely to have useful things like planetoids or gas giants.

Unfortunately, the amount of time it would take to reach 0.23c would be far too long to be practical.

This depends on the thrust/weight ratio of the starship, which depends on the power/weight ratio of the power system and ion thrusters. Let's assume a specific power level of 1 kW/kg (better than the estimated specific power of the VASIMR thruster alone--not even including the power system or propellant tanks).

With a specific power level of 1000 W/kg, and an exhaust velocity of 30,000km/s, we get an acceleration of 0.000667 m/s/s. After 10 years worth of acceleration, velocity is still only 21km/s.

In order to reach a speed of 0.23c, you'd need to accelerate for 33,000 years!

There is a certain amount of inefficiency here, due to Doppler shift, as I've mentioned before. But this method does sound promising.

All methods of decelerating interstellar spacecraft have their problems; in some ways this is the most difficult part of the process. I think we might have to use some brute force method in practice, at least until deceleration stations are established in the target system.

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The most practical option in this case would likely be on board nuclear reactors.It may not be a very efficient option, but it is a straightforward option. The basic idea is:

1) HUGE solar orbital laser accelerates BIG laser drone along with small starship.

2) BIG laser along with small starship cruise to destination star system.

3) BIG laser shoots a laser back toward the home system, decelerating the small starship.

4) Small starship arrives at destination; BIG laser sails through the destination system and goes off to infinity.

It's not the most efficient option, and it does require a honking HUGE laser in the home system. But it's straightforward. It's easy to understand and easy to do the math on.

I would agree with you on the target. I am also hoping in the next few years with the advancement of telescopes that we may find closer systems and or more promising evidence of earth like planets in Cen binary system.

The upper limit for possible planets around Alpha Cen A is 0.3 Jupiter masses, and around Alpha Cen B is 0.5 Jupiter masses, according to this paper
http://arxiv.org/abs/0802.3482

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Proxima Centauri and Alpha Centauri are close enough to each other that an interstellar mission should investigate both of them.

Within the next century, we're looking at unmanned probes. I have doubts that we could perfect Landis's RPB propulsion system within a century, so I'd go with the less efficient but less technologically challenging method of an X-ray ribbon laser sail.

Either way, these are beam-based acceleration systems which can be used many times with little marginal cost. Therefore, it doesn't cost much more to send three probes (one to Proxima Centauri, two to Alpha Centauri) than it does to send just one probe.

Deceleration is the annoying point. Flyby probes are hardly worth the effort. They would zoom through the system so fast, that they don't get a chance to get a decent look at anything (too few photons to capture in too little time). Frankly, advanced long range telescopes in our own system could get better scientific results than flyby probes--at a fraction of the expense.

To minimize overall costs, I'd go with Zubrin's ISM mag-brake to slow down the probes. This means adding on agonizing decades of deceleration time, as the mag-brake is expected to be very ineffective at slower speeds. But if we're looking at doing a mission within the next century...I think this may be the best we can do.

Considering that Earth is considerably smaller than either, I'd not consider that a show-stopper. I also feel that by the time interstellar colonization is possible, we'd be able to quite cheerfully colonize asteroid belts.

Far from being a show-stopper, consider this line:

"All simulations lead to the formation of multiple-planet systems with at least one planet in the 1-2 MEarth mass range at 0.5-1.5 AU"



All simulations resulted in at least one Earth-like planet in the range between Venus and Mars orbit? If those simulations hold water, then that's a pretty compelling reason to visit Alpha Centauri B, isn't it?

Considering how different Venus, Earth-Moon, and Mars are, surely an Earthlike planet around Alpha Centauri B is going to be very interesting.

Too bad Zubrin's salt-water fission is so contentious! How about a Kuiper belt fuelling stop, a near-miss dive into the sun, with good Vasimr drive and a solar-sail deployment at perihelion?

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A fast solar sail using a near-miss sun dive may get up to 0.0005c--too slow. Using something like VASIMR to get up to a fast speed before solar sail deployment doesn't really help much, because the increased initial speed means that much less time near the Sun for solar photons to push.

VASIMR by itself could get you up to 0.001c, so the whole solar sail thing is not necessary. But this is still too slow. It would take four thousand years to reach Alpha Centauri.

I'd like to see your numbers, I'm sure you'd get some boost from the sail, but you're right the transit speed is so fast that the utility is minimal, better to devote sail and rigging mass to fuel. Too bad we're stipulated to (near)current technologies, could probably add a few boost lasers throughout system to increase sail's utility, I don't think an ICAN system has as good a terminal velocity as the Vasimr. not sure a pure Orion interstellar varient would be within the (near) current technology spectrum, even if so, might be best used as an initial boost stage, and then use a higher ISP, longer term system (ion/plasma) for mission propulsion. 0.001c is okay for exploration of nearer interstellar space, and to help serve as relays for later faster missions. Really need 10-50x that terminal velocity before you really can begin to reasonably expect to eventually explore other star systems, but that didn't seem to be a component of the original post?

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http://www.rbsp.info/rbs/RbS/PDF/aiaa05.pdf

The specific impulse of the design I'm referring to is over 1,000,000 Isp.

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I'm going to suggest that the minimum speed for a practical exploratory voyage to α Cen would be about 0.1c -- which would result in a trip time of about 40 years -- although the minimum speed for a colonization voyage would be less, perhaps as low as 0.045c, for a trip time of about a century.

In the former case, especially, faster is better.

Regarding this point... We're flying to a place with a star and (presumably) several planets, some of them Jupiter size or larger. That's a LOT of gravity. Isn't there a way to utilize that to at least assist with deceleration, even if it couldn't do the entire job?