Chapter 2 - Limits of Interstellar Flight Technology

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Notes by David A. Roffman on Chapter 2 of


(Chapter by Robert A. Frisbee, Jet Propulsion Laboratory, CAL Tech)



       Voyages to other stars are currently science fiction, and it appears that it will stay that way for a long time.  Proposed (and approved by the Textbook) methods for propulsion though are: Light-Sails, Matter-Antimatter annihilation rockets, and the Fusion Interstellar Ramjet.  Of course, these methods are currently infeasible, and would require much work and funding to develop; both of which are in great shortage unless there are efforts in this field that are backed by so-called black budget funds as was the case with stealth aircraft development. 



     The first of these three proposals (Light-Sails) relies on the use of a laser to fire a high energy beam at a “sail” for propulsion.  A laser must be constructed on or around Earth for this plan to work.  There are no practical applications of this laser except for space travel.  This method does not involve solar energy, which is important because solar radiation available deceases in accordance the inverse square of the distance from the sun. 

     The laser receiving sails would only be 63 atoms of aluminum thick (if that element is selected).  Such a thin layer couldn’t easily be manufactured on Earth, so the best approach may be to construct the sails in zero-G space.  The Textbook proposes that Al be sprayed onto a plastic sheet, and that the sun be allowed burn away the plastic template, leaving only the Al sheet behind. Low density for the sails would be a complete antithesis to the size of the sails (i.e. length and width).  Total length might be as long as the distance between the Earth and Moon. 

      Size “impossibility” is also coupled with the heat produced by the laser on Earth.  Too much intensity could cause agglomeration (droplet formation of Al in this case on the surface).  Evaporating Al could prove unpleasant for any mission itinerary.  Furthermore, the laser’s power would be reduced by relativistic effects (i.e. more mass + big problem).  Also, the red shift to follow would alter the reflectivity and absorbance of the sail. 

      The craft as a whole could be a two stage adventure, with one stage reflecting laser lights to another.  Although this design is a possibility, it seems like way too much work and “impossible” designs (with luck).  Also, the “sails” would be subject to tear by interstellar particles.  However, there could be things that get in the way of the laser.  The question is what effects they would have on propulsion.



      The interstellar particles are some of the same “dust” that can be harvested for the Fusion Ramjet.  The ramjet approach utilizes Hydrogen (Deuterium specifically - it has one proton and one neutron) and Helium-3 in a reactor, and fuses the Deuterium together to create Helium.  All of the fuel is made in-situ, that is, it is collected from the environment.  There is a problem though.  Efficient fusion is currently impossible, except in stars.  There are fusion reactors today, but they require a much greater input of energy than the output.

      Impossibility also dogs the anti-matter approach.  It can barely be harvested today.  However, if production obstacles can be overcome, it would be possible to generate enough energy to travel at least 50% the speed of light.  There are two key reactions.  The first is where a proton and anti-proton are combined to yield two uncharged pions, 1.5 position pions, and 1.5 negative pions.  Those uncharged particles transform into deadly gamma rays.  The charged particles become muons and neutrinos.  The second reaction involves a positron (positively charged electron) and an electron.  This yields two gamma rays that are 355 times less deadly than the two produced in the first reaction.

      A problem associated with all types of non-space bending propulsion is the amount of time required to reach peak velocity.  In many of the theoretical cases, deceleration after reaching that velocity also poses a problem.  Thus, the speed up and slow down periods would require too much time.

      The desired great speeds also have another weakness: Interstellar dust.  It is proposed to build a dust shield, but how effective can it be?  Although the impact speed is great, the thin size of the sail (again, only 63 atoms of aluminum) does not allow for great heat generation in the short period of time involved during the impact (at .1C, just 5 X 10-16 seconds).

      This dust would also impede on the ramjet.  Interstellar dust would slow down the craft with drag.  The hydrogen also may not be useful for the ramjet.  Deuterium (a form of hydrogen) is scarce compared to regular hydrogen.  So, much of the impacting dust is “waste.”

      A more conventional, but no less wasteful idea for transport is the pulsed fission propulsion.  This process releases explosives charges behind the craft for acceleration.  These charges are nuclear devices, providing an opportunity of a life time to surf the “big one.”  They would allow speeds of 3.3% of light to be reached.  However, the amount of nukes required would be in the hundreds of thousands, with a one megaton yield each.  Unfortunately for this craft’s bold designer (Freeman Dyson), the world does not possess that many warheads.  Also, one warhead must be released every few seconds.  Another complication is that the propulsion would be too slow, taking 130 years to reach Alpha Centauri.  On the positive side, Dr. Strangelove (see Part 10 of 10 for the movie) would love such an idea.



      Yet another nuclear idea is to shoot waste out of the pipe of a rocket (fission fragment propulsion).  As the name suggests, fragments of U-235 fission are used as exhaust.  Some of the favorites in the book are Sr-90 (Strontium) and Xe-136 (Xenon).  Like all “reasonable” propulsion options, maximum velocity is little (in this case about .03C).  The “waste” will be “excreted” via magnetic fields.

      A variant of the fission fragment propulsion rocket is fission fragment “Sails.”  A fissionable sail is used here in which the layer that “burns” releases particles to be intercepted by the absorbing layer.  The sail will degrade and become less efficient over time.



    The Anti-matter approach can lead to much energy, but currently only 10 nanograms of antimatter are produced a year.  Storage is also a problem.  An Anti-Proton is subject to the space-charge limits imposed on any ion plasma; with current magnet technology this equals about 1010 to 1012 ions/cm3 or 10-14 to 10-12 g/cm3.  Electromagnets make storage an expense too.  Due to these problems, a rocket may seek to use liquid Hydrogen and solid anti-hydrogen (in floating pellet form).  The Anti-Hydrogen must be at 1K to avoid contact with container (destruction).

      Although Anti-Matter production is not currently efficient, we can use anti-matter as a catalyst.  It can be used to lower the activation energy of fission, which in turn can be used to generate fusion.  Negative muons could be used to generate 16 neutrons in fission rather than the standard 2 to 3.  This hybrid approach could allow a 130-day Mars mission (roundtrip with 30 day stop over).  It would also be possible to reach Jupiter in a roundtrip in 1.5 years (with 30 day stop over).  The approaches (technologies) are currently available, and quite feasible.  This travel would be useful for in solar system travel, but isn’t it a little dangerous (nuclear, fission, and anti-matter risks)?

     Gilster discusses how human habitation of other worlds may require journeys in the hundreds of years.  If so, multiple generations must be born, raised, and die on board ships.  The minimum initial crew size must be 50 in order to have a genetically stable crew over 2,000 years (to prevent genetic drift and excessive inbreeding).  However, more realistic mission plans should probably be for no more than 40 years so that the results can be viewed within the lifetime of scientists working on the project.  The ethical issues of multiple generation ships are problematic.  Kids would be denied a home to remember or to look forward to occupying. 

      Such ships of this size, without a dramatic breakthrough, are probably going to be impossible for centuries.  The amount of power and/or antimatter would take anywhere from hundreds to trillions of years to produce at our current rates.  In the short-term, high specific impulse speeds (ISP)s equal slow velocities.  However for the long-term, there is no tradeoff.  Because of the short-term tradeoff, high ISPs have not been used.  Furthermore, spaceships would also require enhanced communications technologies. Improved navigation systems are required.  A slower rate of damage (longer life spans for equipment) may require less efficient, but more reliable technology.

     With the Anti-Matter rocket, we must first build an antimatter factory.  For laser sail power it is possible to use the sun at close range as a power source (for the laser), but this would require a huge sail.  With the laser sails, our lasers aren’t accurate enough for very long distances out to the stars.  The most simple space design is the solar sail.  Although solar sails are the most simplistic, they are the hardest to build.  A dust shield must also be built for effective use.  In conclusion, we do not know if the technologies mentioned here will ever be used, but they are possible from a physics standpoint.  From an engineering and practicality standpoint, all the propulsion systems mentioned may not be possible.   But, as we shall explore, there are others options that are covered in Frontiers in Propulsion Science.


For a story about the relationship between antimatter and lightning, click here.




A light sail that accelerates at .0488 g (a g is about 9.81m/sec2) and that reaches Vcruise of 0.5C on a 40 light year flyby would take 84.97 years to reach its target star, but the signal would not reach the Earth for another 40 years.  Thus the mission would take 124.97 years to get the signal back to Earth.  There are about 1,000 stars within 40 LY of Earth.