Very little deceleration could be gained by flying past a gas giant, although such a flyby could be useful for adjusting the angle of the approach into the local plane of the ecliptic.

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Which is still only good for around maybe 5%c, for flyby only. If you include braking, you're looking at speeds of less than 3%c. It's just too slow.

The sorts of interstellar propulsion I favor would be good for 60%c or better including the capability to brake and even return. For an Alpha Centauri mission, a "sweet spot" of around 25%-30%c would be better than faster speeds, to keep costs at least somewhat reasonable.

I really don't see any use or need for any speeds in excess of a few percent of c, with a possible exception being military applications. If nothing else shielding and the requisite energy begin to get beyond any reasonable consideration.

(however)

If you use a mag-sail drag chute system, you can considerably reduce the amount of propulsion fuel/energy you need for deceleration. There's a NIAC study by Zubrin, indicating that a Mag-Sail can potentially slow a vehicle from 0.95c to just 0.0054c in just 820 days – with no reaction mass, of course, we're all talking about much lower speeds, so I'd think that would be a much better option,...at least worth considering.

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Iyam what Iyam

Impressive. Of course 0.0054c is still much too fast, but it is much better than 0.95c (or whatever your initial speed might have been).

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If you are starting out at a much lower Vi then you will achieve a much lower Vf given a similar time of application, though as noted in the accompanying calcs, the Mag-Sail is actually more efficient at higher velocities, so it isn't a direct 1:1 ratio for say a Vi of 0.6c, or 0.2c. Plus all of this is done before you enter the energy dense regions of a solar system proper where the magsail can actually begin using stellar winds to do real power drags and manuevering. Zubrin is more the maverick propulsionist than his Mars colonizing image most often fronts him up as.

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Iyam what Iyam

I don't see anything in these interstellar schemes that does anything but slide by a destination at the ultimate velocity attained by it's intial scheme.
If and when you get there you will either slam into what ever is there at terrific speed or travel on for millions of years. Kind of like Wile E. Coyote.

In http://home.comcast.net/~mbmcneill7/...2361329d7f1ce7 the mass of the interstellar vehicle, the amount of energy, and how the energy is obtained are each specified. Roundtrips to the moon and each solar planet, the Kuiper belt, and the postulated Oort cloud are also specified. If the system is ever implemented, the realism of actual problems encountered and their solutions will probably alter performances that double or triple the time intervals.

Energy levels support the delivery of 10^17 to 10^18 watts by collimated beam to the power receiver of the interstellar vehicle which permit ion engine powered trips to, and stopping at, the Alpha Centauri system in 40 or less years. A similar system will have to be built on some AC planet (or transported from the solar system) to make the return trip to the solar system. The system should easily accommodate 10 adult humans and may be capable of accommodating several hundred with minor specification changes that remain within the energy envelope. The human payload should be 80% female and a few hundred human frozen fertilized eggs should be carried along to rapidly establish a functioning population with moderate genetic diversity.

The interstellar vehicle will be asembled in geosynchronous orbit and will not be designed to land on sny planet, but it will have shuttle craft capable of landing and also powered by beamed photon energy and using whatever material is around for propellant mass. We should assume there will be no human favorable ecosystem in the AC system and allow for not even a planet that lends itself to terraforming; however, telescope technology may improve sufficiently to inform us of what sorts of planets to expect by the time we are ready to go.

The motivation for developing interstellar transportation technology is human survival, nothing more. Trading between habitats in stellar systems won't be economical for several thousand years, if ever. If there are unusual amounts of some rare elements found in some systems, some minimum trading may be initiated. Most stuff can be more economically produced in each system by its populace.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

This depends on how patient you are. Here's an example manned mission plan:

1) 10y - Aggressive prep work for interstellar mission.

2) 15y - Starship travels to Alpha Centauri at .3c

3) 10y - Starship conducts research, operating various drones

4) 20y - Starship returns to Sol at .25c (the return propulsion system is less efficient than the outbound propulsion system)

With this plan, the overall mission time is 55 years, with the crew members having aged 45 years. They may leave as young post-graduates and return just in time to retire.

Depending on human lifespan improvements, this sort of mission may be practical with a lower speed, but a few percent c? Let's look at it again:

1) 10y - Aggressive prep work for interstellar mission

2) 150y - Starship travels to Alpha Centauri at 3%c

3) ???y - Aged crew conducts research until they die

This plan may be practical if human lifespans are improved to, say, 200y. But it's a hard sell compared to a much quicker return mission.

This is assuming a high performance mag-sail with optimistic performance characteristics. It's possible for all we know, but it also might not be possible for all we know.

Also, we don't quite know enough about the interstellar medium to know how well an ISM mag-brake could work.

Indeed, how patient we are, how important the mission is, and how much we as a society are willing to invest in the project. I'm of the mind that slower, cheaper, multiple purpose missions will always win out over faster, narrow focus, horrendously expensive missions, with the possible exception of military missions.

Then we certainly don't know it well enough to propose relativistic manned missions through it. Ultimately my confidence is much higher in the published proposals of Robert Zubrin than they are in the doubts of IsaacKuo, but if you have a compelling argument to make with regards to the Mag-Sail concept or this specific application, I would be interested in reading it.

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Iyam what Iyam

Until the amount of energy required and the method of acquiring it are specified, it will be difficult to evaluate whether 10 years is sufficient for the prep work. I doubt that the prep work can be done in less than 300 years to achieve trip times of 20 years.

If on the way there an object of 2 or more kilograms is dicovered to be on a collision path with the interstellar vehicle and the vehicle's velocity is at 0.000001c or greater, the system will require the capability to avoid the collision with carefully controlled lateral acceleration. Note that the object will not be illuminated and that sudden lateral motion to avoid the object will subject the vehicle's contents to traumatic accellerations. Hence an early warning system must be included as an essential part of the infrastructure. As the velocity approaches 0.01c and greater, dodging becomes ever more tricky especially as the objects become larger.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

I'm of the opinion that a mission which will not produce results within the lifetime of the people who initiate it are much less likely to ever get done.

The Moon missions produced results within a decade. The Voyagers produced results within a decade. I think that space missions that produce results within a few decades may also be feasible. I do not think a space mission that produces results after a few centuries are feasible. In between? I suppose 150 years might be acceptable if we assume a doubling of human lifespans.

And "cheaper" and "multiple purpose" are at odds with each other. The reality is that cheaper missions are the ones with more narrow focus. Sometimes those missions are the ones that get done. Sometimes it's the more expensive multiple purpose missions that get done.
Which is why I consider alternative methods of braking other than Zubrin's ISM mag-brake.
Robert Zubrin did not claim to have the scientific knowledge to be certain of the ISM mag-brake concept, just has he never claimed to have the scientific knowledge to be certain of the nuclear salt water rocket concept. He wrote papers on these concepts by making various assumptions--which he's perfectly explicit about within those papers. He was studying the feasibility of an interesting concept based on speculative guesses about the interstellar medium and future high temperature superconducting materials. The paper says if we use these assumptions, then we get these numbers.

In more recent years, Zubrin has shifted his attention to concepts which require less speculation and which are doable in the near-term.

Or even smaller! shielding against even the normal background interstellar gas concentrations becomes a serious issue, as your own velocity relatively changes them into penetrating radiation. Yet alone dust and cold gas bubbles and pockets. Not that these aren't an issue with some slower transit proposals, but rather that because of the need to minimize mass when you are trying to achieve high relativistic fractions, it is much more difficult to easily cope with and account for.

Certainly can be done, but it increases the risk and the necessary investment. For something like a relativistic bomber the risks and investment may well be considered acceptable. For exploration and/or colonization issues, I don't see the need for speed.

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Iyam what Iyam

I'm not talking about prep work from today, I'm talking about the prep work from the start of the mission go ahead.
I favor protection systems which are passive and do not involve dodging. My prefered method is a lightweight cloud of smoke many kilometers ahead of the starship, along with a magnetic field around the toroidal starship (this is the magnetic field of the starship's main superconducting magnetic loop sail).

A lightweight cloud of smoke will do two things to any potential impactor. First, it will vaporize it into an exploding puff of plasma. This alone will mean most of the impactor will outright miss the starship. Second, it will ionize the impactor's atoms into charged particles. The small fraction of the explosion which is headed toward the starship will be deflected by its magnetic field.

A sufficiently large obstacle could still overwhelm this system, but the probability of running into a large obstacle is very low. We know this because looking through interstellar space is not like looking through pea soup. The overwhelming majority of photons make it through 4.3 light years without running into anything significant. Our starships will also.

You are going to need it to be quite a bit more than a few kilometers ahead of the vessel if you are travelling at .6c (roughly 180,000km/sec). And there is of course the issue of continually replenishing the cloud as it will steadily and actively spread and disperse. Of course, none of these are deal breakers but they aren't negligible considerations either.

If you have them handy, or get the time and inclination over the next few weeks, I'd be interested in seeing what kind of numbers you come up with in regards to requirements for density of shielding cloud, and how much replenishment it will take in order to maintain that density.

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Iyam what Iyam

Interestingly, the speed of the starship does not have much effect on the required distance. The expansion rate of the explosion is proportional to the impact velocity, so things even out that the resulting plasma density at the starship depends only on separation distance.

At the starship, the magnetic field needs to have a certain strength/size in order to deflect particles of the cruise velocity. This field strength/size turns out to be the same strength/size required for the operation of the relativistic particle beam or relativistic kinetic impact powered rocket. Both of these systems are most efficient when the relative particle velocity is about the same as the desired cruise velocity.

So again, things even out.
Yes, these are considerations. I favor a smoke cloud over a gas puff because it would disperse more slowly. But it would disperse more quickly than a foil or foam shield.

My gut intuition is that a smoke cloud would be better than a foil/foam shield because the dispersal rate would be acceptably low and this flaw is made up for with the ease of deploying/replenishing a smoke cloud. Deploying a foil shield seems mechanically complex. A foam shield, I think, would end up pretty heavy.
The required density is largely determined by what density is low enough to consider a plasma as individual charged particles rather than something more complex (a dense plasma can "push around" magnetic field lines complicating calculations).

Unfortunately, I lack the plasma physics knowledge to know what this density should be.

Well, this doesn't seem intuitive, particularly since your vessel is going to intercept a point a few kilometers ahead of it in less than a couple hundredths of a second. The problem here is that any dust particulate more than a a few microns thick is going to need time for either erosion by a volume of the smoke cloud, or time for conduction to transmit the energy throughout the particulate and for it to react. Most of what you are going to encounter will be hot ionized H and a bit of He and other nuclei. A properly designed charged sheilding system should handle them up to .6c without undue effort, but even that begins to break-down as you get truely relativistic.

The foam and foil are difficult to properly replenish. I don't know that I'd go with "smoke" but a low temp jet something like Cesium or Barium periodically blasted forward would probably work pretty well.

such problems are probably more than could be quickly or easily handled by anyone, I was looking for some rough BOTE assessments, but if you'd prefer I can see what I can come up with on my own. I'll shoot them up when I get a chance to play around a bit.

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Iyam what Iyam

The premises that have informed you about why to develop interstellar travel and just how difficult it will be are far reaching. I agree many egos will have to be set aside such that achieving individual glory is made subserviant to preserving the human species. The missions you reference are mere child's play compared to developing a system that can carry humans safely and comfortably to AC. When you consider the energy and propellant mass required and how each will be provided, you'll see why the interstellar trip will take a few hundred years just to build the infrastructure. I hope you can prove me wrong!

I agree that cheaper and multiple purpose have many aspects which are at mutual crosspurposes. Again motivation and the premises on which the motivation is based are the primary drivers. The multipurpose system I have described will cause us to begin our trip much later than otherwise; however, the design lends itself to incremental improvement over thousands, even millions of years, such that we will be able to make the MW our garden and the universe our field of play (Kardashev types I through VII)

Concerning the low probability of objects "lying in wait or slowly moving to ambush the ship" consider the number of "minor planets" discovered in the solar system in recent years. The milliarcseconds of solid angle subtended by Pluto sized objects located more than a half light year away aren't likely to be dectable until our telescopes are improved immensely. The Oort cloud is postulated to have a thickness of from 0.5 to 0.8 light years with an inner radius of 0.5 light years. I choose to define the solar system's outer boundary to be coincident with that of the Oort cloud. Note that we have no detection of any Oort cloud object although we believe there are many objects within the cloud. Detecting and avoiding is more easy and likely than surviving a collision, although it is a challenging design feature.

__________________
For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

Deceleration could use a magsail or similar method, "parachuting" against the stellar wind.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

This doesn't really matter. You're still talking about a total number of objects which are spread out over such a large volume that they don't block photons. The fact that we can't resolve individual objects is not relevant to percentage of photons deflected/absorbed.

The fact is, looking through interplanetary space is not like looking through pea soup. Looking through the Oort cloud is not like looking through pea soup.

Looking through pea soup IS like looking through pea soup, even though the individual oily droplets are microscopic and far too small for human eyes to see. Looking through a smoke cloud is like looking through pea soup, even though the individual smoke particles are far too small to see. Resolution isn't the issue. The percentage of deflected/absorbed photons is the issue.

That's right, it's not intuitive. But it's the way it works.

There is no "erosion" going on. The smoke cloud hits the object at .6c. If the object is, say, 10m in diameter this will affect the entire object on the order of 1/18000 of a second.

Basically, the faster you go, the faster that smoke cloud will hit the object. The harder that smoke cloud will hit the object. The blast velocity will be proportional to the smoke cloud's velocity (it depends on the mass ratio between the smoke cloud and the object).

The velocities involved are all proportional to your cruise speed. The times involved are all inversely proportional to your cruise speed.
Yes, but I can't see the utility in going "truly" relativistic. I don't see the point of going faster than, say, 85%c. The costs get truly ridiculous and you don't end up getting to your destination much sooner (15% faster at best).

I must have gotten off track here. The absence of photon blocking is not proof of no objects in the interstellar medium; rather it is proof that the ratio of photons from any particular star that are deflected/absorbed is not significant to our telescopes. Our not having seen the objects with our telescopes does not mean they are absent, just that we can't see them from here. To say it another way, there could be many planet sized objects between us and AC and we have no way of detecting them until we are within a few AU's of them. There are too many unknowns out there to ignore something this potentially hazardous. There is enough chaotic behavior in each stellar system for some objects to have been hurled into independent galactic orbits by the gravitational forces at play. There is no guarantee that the path between Earth and AC will be free of them.

__________________
For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

There could be billions of wandering objects in interstellar space between the Sun and Alpha Centauri- but they are going to be so far apart that any interstellar mission would not come within several AUs of them. Even the Oort cloud is incredibly sparse- the planets of our solar system are more closely spaced than Oort cloud objects, but the Oort Cloud is crowded compared to interstellar space.

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show me the numbers

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Iyam what Iyam

I agree the objects could be widely separated from each other except for those mutually orbiting (clustering is not prohibited), but I want to be as sure as I can that they stay far from the space ship. The space ship occupants are the only ones interested in dodging. It would be foolhardy not to provide a maneuvering capability.

The universe is without passion for our survival or our demise.

__________________
For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

Well to be honest, even a "slow" (0.06c) colony is going to have to deal with these issues, and dodging isn't terribly viable at any of these velocities for the types of structure masses we are talking about. The difference is in the options and time for dealing with these issues.

Now these colonies may well develop and choose to use some high relativistic exploration/scouting craft of their own, but these make much more sense once the colony is within a light year of the approaching system than it does all the way from Earth, especially as such systems would already be starting out with the colony's transit speed as an initial velocity.

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Iyam what Iyam

I accidentally used 300,000m/s instead of 300,000km/s. The actual amount of time it takes is on the order of 1/18,000,000 of a second.

.6c is 180,000,000m/s. So it would take only 1/18,000,000 of a second for it to travel 10m. Now, the actual rate of the explosion front will depend upon the relative masses of the cloud and the object. Assuming the masses are on the same order, then this will only slow down the explosion front by a factor of sqrt(2). Note that the "explosion" starts across the entire front face of the object. Assuming the object is roughly spherical, then the rearmost tip is only 7m away from the outer front edge. In practice, the object will likely be non-spherical, so even less distance is needed.
First off, consider the JPL solution to protecting Voyager as it passed through Saturn's rings. The "protection system" was to simply calculate the probability of a collision and decide no protection system was warranted. The probability of a collision was too low for it to be worth spending so much as a gram of payload on a protection system. Voyager simply sailed right through Saturn's rings.

Saturn's rings are a lot denser than the rest of interplanetary space, which is a lot denser than the Oort cloud, which is a lot denser than interstellar space. Space is remarkably empty. If it weren't, then looking through it would be like looking through pea soup.

Now, let's consider that the Oort cloud and interstellar space is also remarkably dark. It is HARD to see anything in this space. What would be the easiest way to detect any potential objects out there? They wouldn't glow with reflected sunlight the way Saturn's rings glow. You'd have to provide illumination of some sort. The most efficient method would be a cloud of smoke particles ahead of you by many kilometers (maybe 300,000km to give you one second's worth of warning time). Unlike a radar beam or laser beam, this cloud doesn't cost you oodles of energy to maintain, and unlike a photon beam even the smallest of incoming particles will produce a bright explosion to make it easy to see.

However, the sheer violence of this bright explosion is not merely good enough for you to see the incoming object. The collision is so violent that the incoming object is utterly vaporized. Sure, you see the incoming threat...after it's a moot point.

I'm not sure I understand how your cloud would work. How dense will it be, and how do you keep it from dispersing or collapsing? My guess is that objects more than several cubic meters in volume will punch right through the cloud and clobber the ship. Would even pea soup survive a single collision with an object of several cubic meters?

__________________
For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

I think the odds of hitting a large object the size of few cubic metres is so highly unlikely as to be almost irrelevant. I mean there are trillions of such objects in our own solar system, but not only have no space craft ever crashed into one, they've never even come close to one (except by design). I'd say it's not impossible for a ship to hit such an object, but the odds would mean that you might loose a ship ever million flights or something. A truly freakish accident that would be so rare it's not worth thinking about.

As for the idea of a Smoke screen ahead of the ship, the main problem that I see is; how do you keep it there when the ship is accelerating? I mean sure it will probably just sit there and coast along with the ship when you're on the long interstellar journey and don't use the engine/beam power for decades, but what do you do in the acceleration and deceleration phases?

If you try to puff out a smoke screen when you're accelerating, the ship will immediately leave the cloud behind (because it's not accelerating). Likewise for stopping, how do you maintain it when you ship is changing speed?

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The Sky is no longer the Limit

A smoke cloud would dissipate slowly, if it could be kept cold.

In interstellar space you would think this might be easy, as the spaceship would be far from any star - but the cloud would constantly interact with the interstellar medium, especially the hydrogen, and this would heat the smoke cloud, causing it to disperse more rapidly.

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The relativistic velocity of a rocky pencil eraser is going to slice right through this 'cloud of smoke' and then through the ship like crud through a goose . That is plain to see.