INTERSTELLAR SCENARIOS

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There have been many books written about different ways to travel to the stars; a few are listed in following text. This chapter describes a few theoretical and feasible travel methods and contrasts them with this most practical, TE's g-force acceleration method.

More to explore: 

Physics of the Impossible: by Michio Kaku The Physics of Star Trek by Lawrence M. Krauss

THEORETICAL interstellar concepts have been thought about, discussed, and even written about, but it'll definitely be a while before we use them. Examples include: warp drives and wormholes.
WARP DRIVES. Star Trek's "warp" drive is not just science fictional, it's definitely science theoretical.  Roger Highfield, UK Telegraph Science: "Dr Gerald Cleaver, associate professor of physics at Baylor, and Richard Obousy have come up with a new twist on an existing idea to produce a warp drive that they believe can travel faster than the speed of light, without breaking the laws of physics. In their scheme, in the Journal of the British Interplanetary Society, a starship could 'warp' space so that it shrinks ahead of the vessel and expands behind it. By pushing the departure point many light years backwards while simultaneously bringing distant stars and other destinations closer, the warp drive effectively transports the starship from place to place at faster-than-light speeds.

Suggested Reads: American Journal of Physics: Visualizing Interstellar's Wormhole by Kip S. Thorne

Cosmic Wormholes: by Paul Halpern

A new class of solutions of the Einstein field equations describe wormholes available to humans. Such wormholes must have a throat with no horizon; this would tightly constrain the wormhole's space-time curvature. This constraint likely violates "energy conditions" for general relativity; however, the existence of such material is still theoretically possible.  In fact, quantum field theory provides tantalizing hints that such material might actually exist.

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IN CONCLUSION, "warp drive" and "wormholes", as well as other theoretical concepts, definitely require huge advances in current technology before we can actually use them.  Therefore, we don't plan to include them in our thought experiment.

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FEASIBLE scenarios require much less advancement in current technology . Some examples follow.
SOLAR SAILCRAFT.  
BACKGROUND:  In 1873, James C. Maxwell’s discovered that  reflected light applies pressure to the mirror.  Later, Einstein states that photons have mass.  Thus, a sailcraft might use the very low friction coefficient of space to travel from A to B without bulky propulsion devices and large stores of onboard fuel.

Suggested Read Solar Sails: by Vulpetti, Johnson and Matloff

Discarding the propulsion engine and the massive propellants would greatly decrease the gross weight of any transportation system, say a modern day cargo ship or cargo aircraft.
There's no doubt that sail ships and blimps are much lighter than their heavier counterparts. However, blimps and sail ships are not now popular methods of transportation, because moving air particles (i.e., wind) hasn't proven to be sufficiently reliable. In like manner, dependence on sunlight (photons) to accelerate spacecraft doesn't now seem feasible. Perhaps future advances will enable fuel-less travel on both earth and in space.
GENERATIONAL STARSHIP

Some futurists speculate that a hollowed out asteroid 
would make an excellent, long term habitat;
thus, it would likely be an adequate generational spacecraft.

This is a hypothetical starship that travels across great distances between stars at a speed much slower than that of light (see interstellar travel). Such a ship might take a century to reach the nearest stars; indeed, distant stars might take many generations.  The original occupants will likely die during the journey and leave their descendants to continue traveling.

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Want more? Interstellar Travel and Multi-Generational Space Ships:  by Yoji Kondo

Genetic diversity requires an initial population at least 160. However, normal human interactivity might suggest a much larger population of perhaps 10,000.  Sperm banks and/or egg banks might reduce this requirement. Also, the ship would be almost entirely self-sustaining (i.e., biosphere and life support), for sufficient food, air, and water. Ship's systems must be maintained by the ship's crew during the long voyage.

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PERHAPS, humans might create large, self-sustaining, space habitats before sending generation ships to the stars. Each space habitat could be effectively isolated from the rest of humanity for a century or more, but remain close enough to Earth for help. This would test whether thousands of humans can survive on their own before sending them away.

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CONCLUSION:  Though solar sails and generation ships might be feasible, Thought Eperiment (TE) still considers them to be impractical for interstellar travel. Even if successfully implemented, they would take way too long to travel to even the nearest star. No enterprise would sponsor them; no crew/passengers would want to participate in them. Practical methods of star flight need a much shorter flight duration.

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PRACTICALITY for interstellar travel is what this thought experiment is all about. Recent spacecraft history has an example of the start of practical space travel. BACKGROUND: Deep Space 1 pioneered ion-electric propulsion in interplanetary space from 1998 to 2001.  VASIMR ion drive is another leading edge technology.
ION ENGINE  removes electrons from a designated propellant gas, such as xenon, to transform gas atoms into charged ions which then respond to electro-magnetic fields. These ions accelerate to exit the engine at extremely high speeds; resultant momentum propels the spacecraft faster.
Solar-Electric Propulsion (SEP). Requisite electrical power comes from arrays of photovoltaic cells converting sunlight to electricity; thus, this technology is also called Solar Electric Propulsion. SEP systems can run continuously for many months or even years so that, despite the low thrust,, they may ultimately build up to a higher total impulse (specific impulse times propellant mass) for continued gains in velocity.

Exiting high speed ions provide spacecraft with momentum for acceleration in the opposite direction. Thus, ion engines have much higher performance than chemical engines.
IONIZATION. Electron bombardment ionizes propellant gas by displacing many electrons from their original atoms. Thereafter, a magnetic field guides these charged particles (ions) into a magnetically charged, discharge chamber.

Want more, try: Fundamentals of Electric Propulsion: by Dan M. Goebel, Ira Katz

At the aft end of the chamber, high voltage metal grids electrostatically "pull" the ions to increase their speed (to 31.5 km/sec) as they exit the exhaust nozzle as a focused beam. Finally, a neutralizer injects excess electrons into the ion beam as it exits the spacecraft. This prevents a large negative potential from trailing the spacecraft.
CONCLUSION:  With their high specific impulse (due to high nozzle exit velocities), ion engines can achieve the high velocities for interplanetary or even interstellar flight.

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THOUGHT EXPERIMENT (TE) ASSUMES following factors favor humankind's exodus to interstellar destinations:

1. INTERPLANETARY EXPERIENCE. Many years of successful interplanetary flights will yield considerable technical improvements; thus, spacecraft propulsion systems will routinely use onboard particle accelerators. With improved reliability and efficiency, we can better control numerous ions leaving the spacecraft at near light speeds. The consistent momentum exchange from these exiting high velocity ions will impart a small velocity increase to the much larger spacecraft; precisely control this exit rate for constant g-force.
2. CONSTANT G-FORCE. Reliable, efficient particle accelerators enables small velocity increase of 9.80665 meters per second for every second (m/sec/sec = m/s²) throughout powered flight. This not only shortens the travel time, but it imparts a gravity like force (g-force) upon the spacecraft, cargo, and most importantly, the crew and passengers. Current space medicine data indicates that consistent gravity is essential for normal health of humans. HOWEVER, constant g-force limits the range of powered flight duration.
3. STARFLIGHT DURATION. While g-force acceleration shortens interplanetary flights to days, interstellar travel will take years. Of several relevant problems, perhaps most obvious is the range limitation. Powered flight requires fuel; thought experiment assumes daily fuel consumption to be much less than one percent of entire ship's mass.  This is not a problem for a multi-day, interplanetary flight. However, an interstellar flight will last years; such a flight will consume fuel mass greater than 100% of the ship's gross weight. This is a problem; however, another, more subtle, problem is relativity.
4. RELATIVITY. From Einstein's Special Relativity (circa 1905), an observer measures c, speed of light in a vacuum, as a constant regardless of observer's velocity. Consequently, time and mass must change according to the observer's speed. Relativistic change can be quantified by the Lorentz Transform:

   mr =	mo √(1-v2r/c2)

   EXAMPLE: Let an observer travel at .866 c in relation to Earth, he still observes c as 299,792,458 m/s.  HOWEVER, his mass doubles and his aging process slows by 50%. He ages an hour on the spacecraft while an Earth borne observer ages two hours; a time dilation rate of 50%.

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   Time dilation is no problem for an interplanetary flight; after only 3 days of powered flight, g-force acceleration will take us halfway to Jupiter and achieve 1%c.  HOWEVER, a year of g-force flight takes our notional starship in excess of 50%c. Thus, TE assumes initial interstellar voyages will approach near light speeds in a controlled manner; carefully increasing successive flight's maximum inflight speed to observe relativistic effects.

Within above constraints, there are still some practical flight profiles. This thought experiment will consider several such profiles in the next chapter.

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SLIDESHOW

VOLUME 0: ELEVATIONAL
VOLUME I: ASTEROIDAL
VOLUME II: INTERPLANETARY
VOLUME III: INTERSTELLAR

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ADVANCED PROPULSION

CONCEPTS AND PROJECTS

Nuclear Propulsion  Laser Propulsion Antimatter Propulsion

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