Monday, January 01, 2007


INTERSTELLAR flights will take generations if a spaceship attempts interstellar voyages at currently achievable speeds which barely exceed Earth's escape velocity (recall e = 11 km/sec = 2.3 AU/year). During the voyage, initial crew would grow old and train their children to succeed them in their duties. These children would themselves age and train their children to succeed them, and so on. Unfortunately, it's likely the fifth or sixth generation who would eventually arrive would likely have forgotten the reason and lost the motivation for making the trip in the first place.
While several science fiction stories use the intergenerational space trip scenario, this thought experiment presumes this to be impractical as well as infeasible. For example, our Polynesian ancestors probably would not have made the trip from their home islands to Hawaii if they had to spend generations in their canoes. Another step up the technological ladder, Captains Magellan, Cook, Drake were willing to ride their ships in multiyear voyages around the world; however, it's unlikely they or their royal sponsors would have pursued these ventures if the trips would have taken generations. Furthermore, these trips had lots of landfalls to replenish supplies and take breaks from sea duty; it's doubtful that these intrepid explorers would have stayed for years confined to their vessels.
In like manner, present day as well as future entrepreneurs are unlikely to fund an interstellar venture if payoffs won't possibly accrue until their great-great-grandchildren are elderly. This totally discounts the problems from consumption of food, energy, space and many other resources during this very long flight. Thus, we presume that interstellar flights need a much quicker way to get there.
Recall Einstein's "Equivalence" thought experiment of an elevator with no windows;
1) the elevator could accelerate through space at rate g (9.8 m/sec/sec),
2) or it could be static in a building on Earth's surface.
Either way, the occupant feels a force pressing him against the elevator's floor. In a similar way, constant g-force acceleration could simulate gravity on a spacecraft throughout powered flight. To notionalize quick, effective interplanetary travel; we extend this concept to interstellar travel. Thought Experiment (TE) considers g-force travel as a practical method to routinely travel to nearby stars.
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 travel methods and contrasts them with this most practical, TE's g-force acceleration method.
More to explore? Try these two books.
Physics of the Impossible:
A Scientific Exploration into 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 get any mileage from 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, Science Editor, UK Telegraph, writes: "Now 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:
The Search for Interstellar Shortcuts
by Michael S. Morris and Kip S. Thorne
Cosmic Wormholes:
by Paul Halpern
A new class of solutions of the Einstein field equations is presented, which describe wormholes that, in principle, could be traversed by human beings. It is essential in these solutions that the wormhole possess a throat at which there is no horizon; and this property, together with the Einstein field equations, places an extreme constraint on the material that generates the wormhole's spacetime curvature:

In the wormhole's throat, material must possess a radial tension τ0 with the enormous magnitude 
τ0~ (pressure at center of most massive neutron star)×(20 km)2

Moreover, this tension must exceed the material's density of mass-energy, ρ0c2
No known material has this τ00c2 property, and such material would violate all the "energy conditions'' that underlie some deeply cherished theorems in general relativity. However, it is not possible today to rule out firmly the existence of such material; and quantum field theory gives tantalizing hints that such material might, in fact, be possible."

These and other theoretical concepts definitely require huge advances in current technology before we can actually use them; so, we don't plan to include them in our thought experiment.

scenarios require much less technology advance to be utilized. A couple of feasible examples are described below.SOLAR SAILCRAFT. ITSF posts following: "The idea of leaving the engine of a craft behind and using the endless fuel supply of solar or star light seems like a great way of reducing the mass of a craft. The obvious disadvantage is the dependence on a high flux of photons to give the craft the needed acceleration. For interstellar travel, light-sail craft have to depend on extremely large-scale constructions such as huge solar-power relays around Mercury and enormous Fresnel zones in the outer Solar System.
Since light applies pressure to surfaces, the stream of photons can be used for propulsion in a near-frictionless environment. This concept is the background for light (or solar) sails. It is a method of space travel that negates the need for onboard fuel. Sails using the solar wind or only the light from stars are less efficient at larger distance from the Sun. In science fiction, solar sails are encountered from the 1920s, with early models being giant, multiple-sail craft. In other works, light sails are used to propel ramjets up to ram speeds.
To increase efficiency, ground based lasers can be used to push the craft – using monochromatic light increases the reflectivity of the sail material and gives more acceleration. The efficiency decreases with distance, but much slower than if the craft was riding sunlight alone. The laser beam can be refocused by gigantic Fresnel zones. Light sails will have enormous areas, but the craft will carry no fuel or bulky engines. Alternatives to light sails include microwave sails, particle sails, magnetic sails, and laser or solar thermal or electrical propulsion.
Suggested Reading
Solar Sails:
A Novel Approach to Interplanetary Travel
by Vulpetti, Johnson and Matloff
Basically, the light sail is a use of James C. Maxwell’s discovery in 1873 that light reflected in a mirror applied pressure to the mirror. Since photons according to Einstein have mass then, by using the rather low friction coefficient of space, a craft is able to travel from A to B without having to carry bulky propulsion devices and especially without the need for onboard fuel. This is a large plus in terms of logistics. The fuel is supplied from nearby stars or by high-power lasers."
{NOTE: lack of gravity & deceleration capability}Wikipedia Affiliate Button
GENERATIONAL STARSHIPS. Wikipedia calls a generation ship
Some futurists speculate that a hollowed out asteroid would make a good, long term habitat, and therefore, an adequate generational spacecraft.

a hypothetical starship that travels across great distances between stars at a speed much slower than that of light (see interstellar travel). Since such a ship might take from as little as below a hundred years to tens or even hundreds of thousands of years to reach even nearby stars, the original occupants might either grow old or die during the journey and leave their descendants to continue traveling.....
genetic diversity during a centuries-long trip, a generation starship would require at least 160 inhabitants[1]. Sperm banks or egg banks can drastically reduce this requisite number. Also, the ship would be almost entirely self-sustaining (see biosphere and life support), providing food, air, and water for everyone on board. It must also have extraordinarily reliable systems that could be maintained by the ship's inhabitants over long periods of

Want more info, try reading

Interstellar Travel &
Multi-Generational Space Ships:
Apogee Books Space Series 34

by Yoji Kondo
It has been suggested that humans 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 beyond the reach of help." Though feasible, thought experiment still considers above technologies as impractical for interstellar methods. Even if successfully implemented, they would take way too long to travel to even the nearest star. No entreprise would sponsor them; no crew/passengers would want to participate in them. Practical methods of starflight need a much quicker flight duration.

PRACTICALITY for interstellar travel is what this thought experiment is all about. Recent spacecraft history provides at least one example of an attempt to make space travel more practical. Deep Space 1 pioneered ion-electric propulsion in interplanetary space from 1998 to 2001. For more about Deep Space 1. For more about ion engines, read the following.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. The ions accelerate to extremely high speeds and exit the engine. Electrical power comes from arrays of photovoltaic cells converting sunlight to electricity; thus, this technology is also called Solar-Electric Propulsion (SEP). 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) and hence much greater velocities.Ejecting mass at extremely high speed provokes a reaction by the spacecraft to accelerate in the opposite direction. The much higher exhaust speed of the ions compares very well to chemical rocket exhaust; thus, ion engines have much higher performance.Ionization. Electron bombardment ionizes propellant gas. A heated cathode emits electrons, an electric charge accelerates these electrons towards the anode and into a magnetically charged, discharge chamber. Propellant gas enters this chamber for the purpose of colliding propellant gas atoms with electrons. As cathode emitted electrons collide with the xenon atoms, to strip off xenon electrons for positively charged xenon ions which move about very quickly.Thrust comes from high voltage metal grids at the back of the chamber.
Want more, try this book:
Fundamentals of Electric Propulsion:
Ion and Hall Thrusters
(JPL Space Science and Technology Series)

by Dan M. Goebel & Ira Katz
These grids electrostatically "pull" the ions to increase their speed. Ions reach a velocity of 31.5 km/sec and focus into an ion beam for final exit out of engine nozzle. Finally, a neutralizer injects the excess electrons into the ion beam. This prevents a large negative potential from trailing the spacecraft. With their high specific impulse(due to high nozzle exit velocities), ion engines can achieve the high velocities for interplanetary or even interstellar flight.

TE ASSUMES. For initiating interstellar flights, thought experiment assumes the following:

  1. INTERPLANETARY EXPERIENCE. Many years of successful interplanetary flights will have happened. This thought experiment presumes interplanetary flights to become routine due to many years of technical improvements to onboard particle accelerators as spacecraft propulsion systems. As these accelerators gain reliability and efficiency, we'll be better able to control the momemtum exchange of numerous ions leaving the spacecraft at near light speeds. The consistent exit of high velocity ions from the spacecraft will impart a small velocity increase to the much larger spacecraft. This exit rate will be controlled to produce constant g-force.
  2. CONSTANT G-FORCE. Reliable, efficient particle accelerators enables small velocity increase of 9.80665 meters per second for every second throughout powered flight. This not only shortens the travel time, but it imparts a gravity like 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. Unfortunately, there'll be a limit on powered flight duration.
  3. STARFLIGHT DURATION. While g-force acceleration shortens interplanetary flights to days, it will still take years for interstellar travel. Of the several relevant problems, consider just two: range and relativity. Powered flight requires fuel; thought experiment assumes daily fuel consumption to require a fraction of a percent of entire ship's mass; this is not a problem for a multiday, interplanetary flight. However, an interstellar flight will last years; such a flight will require more then 100% of the ship's mass just for fuel. This is a problem; however, a more subtle problem is relativity.
  4. RELATIVITY. In 1905, Einstein published his famous thesis on Special Relativity. In brief, Einstein stated that c, speed of light in a vacuum, must be a constant for any observer regardless of observer's velocity. For that to be true, time and mass must change according to the observer's speed. Fortunately, this change can be quantified by the Lorentz Transform:
    mRel = mOrig

    For example, if the observer travels at .866 c in relation to his original position, he'll still observe c as 299,792,458 meters per second, but his mass will double and his time will dilate by 50%. Time dilation is no problem for an interplanetary flight; after 3 days of powered flight, g-force acceleration will take us to only 1%c. But after years of g-force flight, our notional starship will have easily exceeded 50%c. Thus, the thought experiment assumes initial group of interstellar voyages will approach near light speeds in a cautious manner; carefully incrementing successive flight's maximum inflight speed and observing relativistic effects.
Within above constraints, there are still some practical flight profiles. This thought experiment will consider several such profiles in the next chapter.

Web Sites of Interest

NASA - Emerging Possibilities.... The ideal interstellar propulsion system would be one that could get you to other stars as quickly and comfortably as envisioned in science fiction ...
NASA's Interstellar Probe will be the first ...
Interstellar Propulsion Research: Realistic Possibilities and ... Interstellar. Propulsion? • Managed Interstellar Propulsion Research Project in the late. 1990's and early 2000's for NASA ...
Interstellar Probe Studies at NASA - This page at NASA's Office of Space Science gives a brief overview of NASA's current studies into interstellar exploration.
NASA - Some Emerging Possibilities 1994 Belbruno: Conference assessing: "Practical Robotic Interstellar Flight: Are We Ready?" 1995 Hujsak & Hujsak: Formation of the "Interstellar Propulsion ...
BEACON eSpace at Jet Propulsion Laboratory: Space Communications ... NASA Logo - Jet Propulsion Laboratory, + View the NASA Portal ... Title: Space Communications Technologies for Interstellar Missions. Authors: Lesh, J. ...
Interstellar Propulsion Research Within NASA Author(s): Johnson, Les Abstract: NASA is actively conducting advanced propulsion research and technology ...
NASA - Warp Drive, When? This web site focuses on the propulsion related issues, explaining the challenges of interstellar travel, existing propulsion ideas, and the possibilities ...
Sidebars: Time Dilation
BEACON eSpace at Jet Propulsion Laboratory: Interstellar ... May 1, 2005 ... Title: Interstellar Exploration: Propulsion Options for Precursors and Beyond. Authors: Leifer, S. Johnson, L.
· Setting Sail for the Stars "This will be humankind's first planned venture outside our solar system," said Les Johnson, manager of Interstellar Propulsion Research at the Marshall ... · Dashing and Coasting to the Interstellar Finish Line Aug 19, 1999 ... to the Interstellar Finish Line. NASA Funds Advanced Propulsion Concept ... the Mini-Magnetospheric Plasma Propulsion - M2P2 - concept that ...



OpenID leonjwilliams said...

Could you elaborate more as to why you think the fifth or sixth generation would lose motivation.

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