Part 7: Book Review - Prospects for Interstellar Travel

In this continuing book review series we look into Chapter 11 where the author reviews the search for other intelligent beings and explores the related issues this has with interstellar travel. Whether life can begin on other planets is still an open question as so far there is no strong evidence to suggest so. The author points out that despite all the research papers published on this subject, conferences held etc, "The work has been informed speculation due to the lack of experimental and observational results". However given what we do know about life here so far, a number of interesting educated guesses can be made on the possibility of life elsewhere.

The Search for Extra-Terrestrial Intelligence (SETI) is a vast subject incorporating many disciplines including: "astronomy (which stars might have planets), planetary studies (which planets might have conditions for life), biology (what sorts of biospheres can evolve on planets), a mix of biology, sociology, and philosophy (what kinds of intelligent beings can arise and how would they act), technology (what means would "they" use to send and receive messages, or themselves, across many lightyears), mathematics (expected to be a common language), and other fields (as we define them). SETI includes Communication with Extra-terrestrial Intelligence (CETI) and can include sending messages as well as receiving them."

Interest on the possibility of other beings has been around for a long time looking back in our history, authors going back from ancient Greece to 16th Century Giordano Bruno for eg. [CI: my favourite quotes from Christiaan Huygens and part of Edward Young's Night Thoughts followed below]

Several terms over the years have been used to refer to extra-terrestrials, such as "ETs" (from the movie), "humanoid" (somewhat assuming human like two legs, two arms and a head) and "aliens" (blood thirsty bug eyed monsters). The author uses the term intelligent beings or simply "beings", implying: "creatures who have evolved to the level of recognizing their own existence and therefore likely to study their place in the Universe ... We assume intelligence also includes rational thinking (which some humans practice some of the time), problem solving, artistic creation, development of science, technical competence (tool use expanded to building, flying, computing, transmitting messages, etc.), complex societies with evolving cultures, and more." The study of life as it might arise elsewhere in different conditions and its evolution not necessarily leading upto intelligent life, is known as exobiology. So far we are only familiar with carbon-based lifeforms as on Earth. Carbon based molecules are very versatile for life however there is no basic proof showing that life cannot use other chemistry .

Mention is made of the Drake equation (shown above) which makes an attempt to predict the number of civilisations in our galaxy at any given time which have achieved technology for radio communications by multiplying a series of probabilities together which include the number of new stars per year, those with habitable planets, the probability of life on those planets, the probability of that life achieving intelligence, the probability that those beings develop the ability to send and receive interstellar messages and the average lifetime of a technological civilisation. As pointed out, most of these factors are very difficult to estimate and the result of the equation may be no better than a wild guess however the number of stars formed per year in our galaxy is given to be between 1 and 10 per year. Some estimates given on the number of civilisations in our galaxy range from 10 for short lived ones and millions for long lived ones giving a geometric mean of 10,000 civilisations [CI: see also this interesting paper].

[CI: we haven't been able to leave the Solar System to explore possible habitable exoplanets to find closeup evidence to make more informed guesses. We would need to send out starships, interstellar probes or large space telescopes could provide some indirect evidence, however given the number of galaxies in our observable Universe and the number of stars per galaxy (in the hundreds of billions, upper guess 400 billion for the Milky Way alone) and given that astronomers are finding more and more planets around stars (ie planetary formation may be a common phenomenon around certain stars) and given that some of the base molecules required for life as we know it have been confirmed in the interstellar medium, my point of view on this is despite the fermi paradox (explained later), it is highly unlikely that the only lifeforms (intelligent or otherwise) reside only on Earth.]

An average density of civilisations in our galaxy can also be guessed and notes that they are less likely to be near the galactic core because of the shortage of heavier elements and the possible violent black hole at the center: "For galactic volume about 10^13 CLy (Cubic Lightyears), excluding core, each civilisation is surrounded by 10^9 CLy, and the average spacing is thus about 1000 Ly, or about 1/6 the thickness of our galaxy." which would be problematic for communications and any plans to meeting them in person.

[CI: Photo: A panoramic view of our galaxy with closeup towards the center. Anyone out there? (ESO)]

The author then moves on to discuss the Fermi Paradox or "the great silence" and explains the key link between other beings and interstellar travel: "Life has been possible in our galaxy for so long that some other civilisations should have been around a long time and engaging in widespread travel, if only by probe. Yet they are not known to have visited us." and hopes that this paradox is an apparent one and an explanation will eventually remove this paradox. Many reasonable and wild explanations have been put forth as to why no contact has been made so far and the author outlines many of them. Some have suggested that "absence of evidence is evidence of absence", others that habitable planets may be rare or life may not start easily on habitable planets or intelligent life might be a rare step in evolution, technical civilisations even rarer or some civilisations may not want to or are unable to venture out to other star systems or we are in a region of the galaxy where no beings have ventured or sent messages or we already have been detected and unknowing to us a fleet of starships are on their way to Earth or because of a "Codex Galactica", Earth has been quarantined by "them" and no contact allowed, or etc. etc.

Searches for evidence of other beings (SETI) are ongoing and have been mostly in the radio spectrum using large radio dishes and very sensitive receivers searching for any meaningful signals buried in the galactic noise. Some of the technology outlined was already discussed by the author in Chapter 7 and as pointed out SETI searches are difficult: "The million directions in space (or million stars) at which to point a receiving dish, the billions of one-cycle channels to be examined, the short time available to "listen", and the rapid decrease of power with distance are some of the factors making search very difficult. Power is not the main problem. Presently we have a receiver (using the Arecibo 300m dish) which could detect messages sent with a similar dish from 15,000 Ly away, and further if they use more power than Arecibo engineers have." and extensive searches of several hundread stars have been conducted at and between 1420 Mhz, 21 cm wavelength (hydrogen) and 1666 Mhz, 18cm wavelength (hydroxyl molecules) the band being referred  to as "the water hole", the logic being the two combine to form water which is a necessity for life, other beings might decide to search or transmit radio signals in this band which is also relatively quiet.

It is pointed out that several intelligent signals may be reaching us right now however we may not be looking at the right directions or frequencies and furthermore: "SETI has been thought of as a cheap alternative to interstellar travel (still requiring billions of dollars and much donated effort for a semi-thorough search) [a-Finney 1985]. But SETI would be useful in finding destinations for travel, and could be a program parallel to a starship program."

What if we receive an interstellar message? According to the author this would profoundly affect the world. Principles and protocols have already been prepared for initial action on confirmation of the message. Basically scientists have agreed to cooperate in the confirmation then announce the evidence to the public fully and openly "presumably this includes all data on the content of the message even though deciphering it early is unlikely." [CI: for an interesting movie on all this see Contact]. If it comes from a star nearby, it is pointed out that a serious study of traveling there would begin and many subsequent messages planned in preparation.

So far our main SETI work has has been listening however one message has been sent intentionally from Earth in 1974 using Arecibo. The message was transmitted to the Hercules Globular Cluster M13 25,000 Ly away at an effective power of 3 TW [CI: M13 shown below with the depicted message and a relevant quote from James Jeans, see also this 35th anniversary message sent last year from Arecibo.]

Interesting point made by the author on p268:  "Considering the time and cost of a full search in the radio spectrum, not to mention other possible channels and sending our own messages, interstellar travel is not necessarily slower or more costly in the long run." Further considerations are also given to search in other parts of the spectrum such as laser signals (10 KJ pulses would outshine stars). Further studies are also looking into the possibility of detecting other possible signatures left by large and advanced galactic civilisations such as excess heat for a given class of star due to a hypothetical Dyson sphere partly surrounding the star, such gradual change over time may also indicate their space engineers have been busy working on the structure.

What would happen if visitors actually came to Earth? Apart from the expected mass histeria, scientists would get real busy with almost every field of study deeply affected: "visits with the visitors would need to be leisurely to treat them kindly and respectfully, and new complex programs and procedures would be needed to allow many people fair and full access to the visitors and their knowledge." hopefully they won't be hostile beings with sinister purposes in mind and keen to exchange knowledge with us.

The author then moves on to discuss habitable planets which are defined: "(somewhat anthropocentric) as planets in the pleasantly warm biozone around a star with adequate water, breathable (to us) air, stable orbit, low eccentricity, reasonable rotation and axis tilt, absence of very violent weather and high radiation (including UV), and other factors." The interstellar medium must be relatively stable with very little instellar dust clouds passing by (which would block the precious sunlight). Also mentioned are the dangers of closeby supernovae which can increase radiation levels by 1000 to 10,000 if within 30 Ly and would be harmful for developing life. One study suggests that a supernova is expected in any spiral arm galaxy every 100 million years or so. Long term stable stars (with life spans in the billions of years) are also required which are stars of

   [CI: Diagram: Theoretical habitable zones for various stellar masses.]

types F, G, K and M spectral classes which make up 90% of all stars. A, B and O type stars are more massive than our Sun and are said to change luminosity too rapidly with fast rotations and unstable habits not very good for a stable biosphere for life which requires some stability in the system over billions of years. The past ice ages here alone are said to be due to variations in the Earth's orbital distance and orientation from the Sun by only a few percent. Current theories also suggest that planetary formation from collapsing nebulae can only form around a single star however some research has suggested otherwise: "A system with two or more stars would not permit stable orbits for planets unless the stars are far apart, at least four times the distance to the best place for habitable planets."

[CI: Photo: The Orion Nebula with closeups of planetary

systems in formation around their host stars. (APOD)]

Other interesting points mentioned: "Low-mass planets do not have the gravity to hold the lightest gaseous elements hydrogen, helium, nitrogen, and oxygen, especially if they are warm. A small difference in gravity or temperature has a large effect on the time over which gas molecules can escape [recall molecular speed in Appendix B for 1.2]." and many other considerations are given as to the possiblity of habitable planetary formation. Taking account these various factors and considering planetary formation models, the author gets back to the Drake equation and mentions that the number of stars with planets could be 1/2 but the number of habitable planets per such star (not including M stars) could be less than 1/10 and some have suggested a factor of 0.001.

The origin and evolution of life is also discussed, how life begins on a planet is very important to whether intelligent life eventually develops or not. We also don't fully understand life as is on Earth and have only one biosphere to study however: "The self-regulation of the biosphere would be an important explanation of how a planet can remain habitable over long times and extend the chances for life and intelligence. Indeed, the dominant form of life on Earth, by mass and by global effect, is micro-organisms, mainly bacteria. Bacteria are a large source of oxygen. Plankton in the ocean surface remove much CO2 and release dimethyl sulfide that helps nucleate clouds (which helps reflect excess solar input)."

How life began on Earth is still uncertain in detail however it is mentioned that organic molecules may have originated from comet collisions and points out the Miller-Urey experiment which produced amino acids and other biochemicals with UV radiation in a simulated primordial Earth ocean and atmosphere [CI: some more recent studies have also indicated that early genetic code was based on a smaller number of amino acids (only those available in prebiotic nature), see this interesting paper and the Last Universal Ancestor.] Spectral analysis of the atmosphere of exoplanets using sophisticated telescopes may give an indication of life on those planets. Some molecules to look for would include oxygen, methane, ammonia, nitrous oxide, dimethyl sulfide (from Planckton as explained above), terpene vapours, chlorofluorocarbons, and other gases emitted by lifeforms would be an atmosphere not in equilibrium. NASA's exobiology program is studying the above and many other factors including the possiblity of life on Mars, Titan and other places.

The author also points out that several studies have also looked into the possiblity of whether life can develop in environments other than luke warm water: "A few other liquids besides water (e.g. ammonia, oils, silicates) might be good solvents for the complex chemistry needed for life at other temperatures from cold (-50°C) to very hot (1000°C)." however notes that no experiments have been succesful in making interesting chemistry with other base multi-bonding elements such as silicon which could provide the basis for complex "organic" structures at higher or lower temperatures. Several moons in our Solar System do have interesting compounds including liquid methane, ammonia, nitrogen, sulfur etc however no evidence of intelligent beings have been detected on them.

Even if life turns out to be spontaneous on planets with the right conditions, according to some evolutionary biologists, the development of intelligent life (the next Drake equation factor) with humanoid bodies would be very unlikely however other scientisits suggest intelligence would readily appear within a few billion years however: "More generously, at present no one can say how likely a repeat evolution of humanoid form and/or abilities is under similar conditions. Biologists are familiar with the nearly endless number of variations and branchings in evolution and the lack of ultimate purpose or design (teleology) in biological nature. Thus they are inclined to a view of uniqueness for any life form. And almost any form may appear as time goes on and the environment changes." The chance of intelligent life developing communications technology is given for the next Drake equation factor between 1/100 to 1/10 and opinions are speculative.

Several speculative theories on the spread of civilisations throughout the galaxy are also outlined and some interesting numbers for thought are given: "If a high estimate of 10,000 civilizations are actively spreading in our galaxy in an epoch of 10,000 years and typically advance 100 Ly (or 0.01 Ly/year), then the chance of encountering one in a given region is only about 0.001, rather low." A scale known as the Kardashev scale lists three types of civilisations based on energy usage: I Earth scale, II stellar scale, and III galactic scale, we are currently below Type I. The author also mentions the interesting cosmological princliple: "the Universe must be physically (therefore biologically) about the same everywhere, that nothing special occurs in one place that cannot occur elsewhere. This principle is a modern version of the Copernican view in astronomy, that Earth is not the center of everything."

Several possible case scenarios of humans visiting a habitable planet without life, with primitive life and/or with intelligent life are then outlined. If the civilisation is capable of radio or otherwise communication, contact should be made first via this avenue to agree on the timing, method and purpose of the visit instead of just dropping by. If no communication is possible for them to space then a cautious landing may be needed for first contact however as discussed in a previous chapter by the author, landing has severe technical problems attached to it notably the problem of getting back to space. One interesting point raised on p284: "Just by their presence humans would begin altering local life." If a planet is found which lacks habitability, making a suitable biosphere suited for humans may be possible over a very long time. Just as was done on Earth, bacteria could be used to make an oxygen atmosphere, remove methane and CO2 and even aid in warming or cooling it, this is known as "ecopoiesis".

 [CI: Image: Terraforming of Mars one day a possibility?]

Many ethical issues would need to be considered if there are already lifeforms (intelligent or otherwise) on the planet. The chapter concludes with an interesting discussion on how humans might coexist with possible beings on their planet (hopefully peacefully) and discusses various issues including possible competition for resources, developing a human outpost on their planet while not interfering with them and even possibly continue to explore the galaxy with those beings to seek out other new life and new civilisations together.


In the next part of this book review series we'll look into the last chapter of the book where the author outlines other more speculative energy and propulsion ideas related to interstellar travel and some conclusions on the various topics discussed in the book.

Part 6: Book Review - Prospects for Interstellar Travel

Continuing on with this book review series, we look at Chapter 9 where the author considers the various biological factors for the starship habitat design. A deep understanding of ecological and biochemical systems is required if one wishes to consider the case of sending a starship with 100 people operating over 30 generations for 1000 years. Discussion of the various factors that might make it difficult for humans or other biological beings to survive for these long periods are also considered however: "it is expected that if the physical problems of interstellar travel can be solved, determined humans are likely to solve the biological and social ones." The social and ethical issues for humans remaining in this closed system for these long periods are also considered.

Several systems of the closed ecosystem of the starship are outlined by the author. Heating is stated not so much as a problem as a human idles at about 100 Watts and 200 W when moderately active and gives a value of 1000 W total per person when one takes into account the typical equipment that the person will use such as lighting, computers, cooking, shop tools, lab instruments etc. Other equipment outlined for the habitat would include instruments, water pumps, ventilation system, environment recycling system, light for food growing etc. All these produce heat and for 100 persons an estimated total of 1 MW is given. Keeping temperature at say 20°C wouldn't be much of a problem because of the habitat's insulation and any excess heat can be sent to storage areas as a dump heat. Most of the lighting in use would be needed for food plants which need at least 1/10 of Earth's solar intensity or more than 100 W/m^2 with a certain spectral quality.

Atmospheric requirements for human needs requires a pressure of at least 13,000 Pascals (bit over half of standard Earth atmospheric oxygen) together with Nitrogen at 27,000 Pa which is relatively inert for humans but necesaary for some organisms. CO2 levels must be kept below 400 Pa and humidity should be kept below 20% Rel. to let people cool without sweating too much.

An estimate of this habitat volume is given at 30,000 m^3 for a volume of 300 m^3 per person. Water is vital to all organisms and a sophisticated water recycling system will be needed where: "purified water must come from waste treatment discussed later and can be of several grades: drinking and cooking; washing bodies, clothes, habitat, and equipment; watering plants; medical and laboratory; and industrial processes." Use is then mentioned of deionized water to prevent corrosion in the plumbing pipes, small amounts of minerals can be added to drinking water later for taste. Waste water will need non-corrodible pipes. Upto 100 tonnes (upto 1 tonne per person) is given as the total requirement with most of this going to watering the plants.

What would life be like for the starship inhabitants on their way to the stars? The author gives us an idea what the ecosystem needs to cope with: "People must wake, toilet, eat, wash body, clean special parts (nose, ears, hair, nails, teeth), clean living area, snack, work on familiar and new tasks, toilet, converse, make love, wash, cook and eat, rest, exercise, work, eat, meet, converse, repair clothes and other personal equipment, snack, wash hands, have fun individually or with others, toilet, and try to relax and sleep after another domestic day in a small starship hurtling through space. In zero gee all these activities would be different, with some harder, some easier... sleep and sex might be fun since one can be comfortable in any position."
[CI:  For the curious, read this interesting article on sex in zero gee]

Studies have shown that for current humans if they remain for long periods of time in a zero gee environment then this will present some biological consequences which can include: "blood redistribution with blood puffing up the face, water loss, space sickness with motion and orientation problems, muscle loss, bone loss, cardiovascular deconditioning, and irregular heartbeat." Very long periods at zero gee may cause permanent changes and humans may over time adapt to a different physiology suited to the zero gee environment if artifical gee systems turn out unfeasible on starships. Current astronauts on the International Space Station regularly exercise with special threadmills to give the muscles something to push against however for a large starship complement with many people it is mentioned this may not be practical. Space medicine recommends 0.9 gee continually however humans can adjust to 0.5 gee and readjust back with treatment and rehabilitation.

Several options are outlined to creating artifical gee in a starship. There is no known propulsion method that would provide continuous linear acceleration near 1 gee. Large rotating structures are considered, notably a colony ship about 0.5 Km in radius rotating once a 1 minute will generate 0.5 gee however these rotation habitats or wheels have many technical problems apart from shielding and strength issues, the bearings at the center of the structure need to be "frictionless" using superconductors for magnetic support. Other problems can include vibrations and gyroscopic effects on the starship when altering course heading. [CI: some examples are shown below from the Mission to Mars movie.]

Photos: Examples of rotating habitats providing artifical gee for the crew (The Space Review)

Food issues are then discussed: "On a starship producing and preparing food must be very efficient and reliable so that most voyagers are free to spend most of their effort on other activities."  If 8 MJ or 2000 kilocalories is taken daily, this corresponds to a minimal power input of 100 W for a human (just idling and awake). Eating animal products seems unfeasible however people can survive well on grains (which have protein), vegetable and fruit diets even without milk however "milk" and "cheese" equivalents can be made from soybeans as the Chinese have done for the last 3000 years. Issues regarding growing plants and food engineering are also discussed. According to NASA one would require 20m^2 of growing area per person however 10 m^2 could be used if using efficient plants. In order to study all this NASA and others have funded studies which have looked into all the various issues related to a closed habitat ecosystem, known as Controlled Ecological Life Support Systems (CELSS). A diagram is given on p218 showing a simplified flow chart of this ecosystem:

The waste treatment system shown in the diagram above would have to process 0.5 tonnes of sewage and food residues each day for 100 people and at least 10 tonnes of water. [CI: Current research on efficient waste recylcing systems is being undertaken by various space agencies.] Several considerations are then given to population control and reproduction over the generations onboard the starship. An interesting point raised: "If aging as a fundamental biological problem is solved, interest in interstellar travel would change."

Another option given is hibernating people for the long voyage which would eliminate the need for food production and save considerable power, and touches on issues which are related to cryonics: "Frogs are the highest animal yet to survive being frozen solid." [CI: today some people have signed up to be placed into cryonic suspension shortly after their clinical death, the alternative of burial or cremation results in certain information-theoretic death with no chance of revival in the future.] A discussion is also given on what to do with dead bodies in a starship, recycle into the ecosystem, jettison overboard or some may choose to be frozen for later burial are possible options. Some strange proposals have also been outlined sending human embryos by probe to distant planets with sophisticated robots onboard which would see them through to adulthood to start a society on a new habitable planet [CI: reminds me of the nurse robots in The Matrix movie which grow humans for other purposes]. This would reduce the starship size by not sending adult human crew however as the author points out the robotic equipment might not function for that long and supporting equipment and shuttles would still be needed but most importantly: "there is plenty of evidence that children developing without commited human parents come out lacking essential qualities. A human colony that we would recognize as such might not be the result."

[CI: Photos: Left: can understanding how bears hibernate help humans hibernate one day in their own high-tech caves? (Miller-McCune)  Right: Robotic nurseries growing humans, very strange but one day possible? (Dansego)]

The author then discusses radiation (energetic particles and photons) issues and possible effects on living organisms: "At 0.01c the forward radiation has energy about 50 KeV per particle and a flux or intensity much greater than cosmic. At 0.1c forward radiation at 5 MeV per particle is very serious." Energy dumped in tissues or the ionization can disrupt fragile biological microstructures such as DNA and cause permanent damage to the retina and neural cells which don't replace themselves. In our local space the average cosmic radiation is given at 10 rem/year if it reaches living material or equivalent to about 100 simple medical x-rays. However the allowed dose for people is about 0.5 rem/year (0.005 Sievert per year). Shield systems were discussed in the previous chapter (2m solid shielding is needed from all sides the ecosystem which amounts to 5 tonnes/m^2) however several radiation dangers are outlined notably the danger of radiation from solar flares which can give doses of hundreds to thousands of rem over a few hours at the distance of Earth. 500 rems in any short time can be lethal. NASA suggested that astronauts could take about 200 rems if received over several years.

[CI: Photo: X-ray image of one of the most powerful recorded solar flares in history (APOD)]

Other more familair hazards outlined include dangers from noxious chemicals, injuries from equipment, electric shock, electric and magnetic fields, fires and other dangers. The equipment needs to be designed to be people friendly (if human crewed) that is no sharp edges or corners, hot spots, slippery handles, no toxic emissions etc. For example it is mentioned that plastics, glues and some fabrics keep low levels of solvents which are released over time, these and other harmful gases would need to be removed by the atmospheric filtration system.

Some medical considerations are outlined and notes that: "A different approach to medicine seems necessary, perhaps one based on a small set of medicines, full understanding of human genome (our 3 billion unit DNA code) and immune system, and a computer model of human physiology that can specify individual drugs to be synthesized." and mentions the possible use of a universal biochemical synthesizer and genetic engineering. There are upto a million human deseases to keep the medical staff busy and any epidemic on a starship would be a catastrophe. Good advice for anyone is given including keeping a good diet, hygiene, exercise, good sleep and safety. [CI: see also the future possibility of using nanotechnology such as nanobots (atomic-scale robots comprised of individually arranged atoms and molecules) to seek and destroy known and new planet unknown deseases at the cellular level inside the body, see below.]

Video: Nanobot destroying an unhealthy cell (Rutgers University)

The chapter concludes with several considerations given to large colony "worldlet" ships and several references are given including some from Johnson and Matloff among others. One outlined example: "A rotating cylinder colony of 1 Km radius and 10 Km length has an inner surface area of about 60 million m^2 (plus end caps), could support upto 600,000 people, and masses about 500 million tonnes shielded [b-Johnson, fuel not counted]." Propelling these massive ships would be a major problem but it is mentioned interior lighting would be even a larger problem: "The electric power needed to provide 1/10 sunlight on 60 million m^2 with broad spectrum 30% efficient lamps is about 20 GW." or equivalent to 20 nuclear power plants and this is an underestimate because food growing needs more light intensity. Some energy diversion from the main propulsion system may be possible depending on what system is used.

We now move on to Chapter 10 of the book where the author examines some of the psychological, social, philosophical, political and economical aspects of interstellar travel. The mission is again assumed to function for 1000 years and considers both human crewed and some of the benefits of robotic only missions.

Regardless of the mission duration (very slow or ultra-relativistic), consideration is given to if people would volunteer for such a dangerous mission in the first place: "if the call went out for a hundred or a thousand super-multi-skilled brilliant specialists and generalists, plenty of volunteers would appear for this ultimate trip...[however]... It is difficult to say how anyone would personally benefit after the glory of starting the great voyage wears off in years or decades and isolation sets in. " Some have suggested that 10 people for a very fast starship mission would be the bare bones minimum if the starship is "ultra-reliable" and has near automatic food production. Two of the people might need to be expert medical doctors. However the author's estimate and others give a minimum of 100 people for the skills required onboard and considers the problem of population number fluctuations in a multi-generation starship and genetic variability. One interesting point made on p236:

"The people who leave Earth will not be the ones who arrive at a different star, barring some very major improvements in human longevity, body preservation, or starship propulsion."

Many psychological issues are touched on by the author and notes that for a succesful mission people who decide to go should be doing so for positive reasons as those with negative reasons may not find what they are looking for on a starship: "People would choose to take a long journey for positive reasons such as scientific interest, need for new adventure and challenge, or deep curiosity (not all would have a scientific frame of mind)." and notes that: "People would choose to build a starship if it were at all feasible because some people (mostly men thus far) like to build large difficult, dangerous, systems which move fast and amplify and project personal power." Some further considerations are also given to the social structure in a starship and as suggested for any large technical system, a hierarchical command structure is most efficient especially when dealing with emergencies which would be too slow to be dealt with via a democratic vote by committee. Other social aspects are also discussed noting that for long term success of the mission social stability and commitment is required over several generations.

Several pages are also devoted to discussing political and economical aspects with regards to sending a starship on a long interstellar mission and notes that: "the starship should not be taking significant sums away from the ability to pay for education, health, local research, pensions, and the like." otherwise the whole enterprise would not be popular by the community at large and people have to survive here and now and also notes: "The money spent on the starship is somewhat like money spent on the military: it is spent with no immediate return." and more importantly:

"Already over 10 T$ have been spent on military armaments, enough money to build 10 space colonies or send a small interstellar probe instead. Surely a modest mission to a star would provide more inspirational, philosophical, scientific, and economic benefits to many nations than preparing for war has." 

Cost estimates for a starship would depend among many other factors including the propulsion system used however the author gives some comparaisons. The Daedalus project was estimated to cost 1 T$ to 10 T$. The Apollo moon program cost 30 G$ (billion dollars) and suggests because of the million times greater Kinetic Energy that would be required for an interstellar mission, this would be an adequate cost factor estimate however something of the order of 1 T$ would be the minimum for a prototype 1 tonne probe (affordable over 10 years to a determined nation). The author also reminds us that starship construction will have to be built from materials already in space or transported from locations with shallower gravity wells such as the Moon.

In the next part of this book review series we'll look at the following chapter which deals with the possibility of extra-terrestrial life, SETI, galactic civilizations and first contact issues.

Part 5: Book Review - Prospects for Interstellar Travel

Continuing on with this book review series, we look into Chapter 8 where the author discusses further some of the technical issues which apply to a starship (probe or human crewed) leaving the propulsion side of things mainly out which were already covered in earlier chapters. It's pointed out that "In accord with earlier findings on propulsion, reliable operations for a thousand years is assumed to be needed for a starship unless discussed otherwise." [CI: 1000 years! Back to Engineering Rule 6 ?]

The chapter starts off by considering starships as a closed system (it isn't strictly a closed system because of mass-energy equivalence and propulsion exhaust energy flow) and considers leakage of material and energy to the outside space. Remember that the assumption is for a very long journey so if material and energy leak past the boundary or envelope of the starship then there won't be much left over that time. Small leaks could be by small gas leaks or water vapour, erosion by dust collisions, loose atoms from the outer hull of the starship "avoid painting the starship", radioactive decay from many materials etc. In the diagram on p184, the author sums this all up and also mentions the use of a scoop to make up the loss of material and try to acquire material and fuel along the way from the interstellar medium:

The author then discusses various issues concerning deterioration and reliability of the starship: "Everything except atoms themselves deteriorates over time. In this century, unlike some earlier times, most engineering is limited to equipment lifetimes of a few decades and dependent on continual maintenance. A starship might require near-perfect performance for a thousand years. The longer the mission the more chances for failures at all system levels. Most deterioration has a systemic aspect. Something fails a little, the failure provides a route or feedback for more failure, and so on until catastrophe occurs." Several examples are given notably familiar corrosion and oxidation (inside the starship with oxygen/water vapour atmosphere for the crew), worn bearings, leaking fluids: "once a small flow starts, ions in the water attack the metal parts of the valve, eating it away and increasing the flow until the washer can no longer seal the channel that has formed." and behaviour of metals in a strong electric field.

Wherever there is rubbing of surfaces (lubricated or not), interaction of matter, flow of fluids, collision of particles with matter as in vacuum tubes (older TVs, laser tubes etc), deterioration over time occurs. It mentions that vacuum tubes last decades but working for centuries is unlikely however the use of solid state devices is recommended wherever possible.

The vacuum of space also causes some deterioration and the author points out that some atoms in materials can acquire Kinetic Energy to jump from surfaces and are forever lost in space and there's no atmospheric pressure to stop them: "Many organic solid materials (polymers or plastics) cannot be used long times in vacuum, as their solvents "evaporate", leaving them brittle or without strength. (Same solvents would make an unhealthy atmosphere inside). Most lubricants cannot be used, as most known fluids are not stable in vacuum." [CI: all this reminds me of a recent materials space exposure experiment NASA has done on the International Space Station to study all this, see below:]

Photo: Various materials exposed outside the International Space Station for one year
to determine how they behave long term in the vacuum of space (NASA)

In an attempt to engineer complex systems to be fail-safe, the author points out as discussed in Chapter 5 that systems are made with several layers or levels of self-correction. A good example given of a semi-complex system is an engine with moving parts and discusses the lubrication and cooling systems, the safeguards in place and points out that the example given is relevant because: "rotating systems are likely to be put in starships". Temperature also plays a major role in deteriorating systems such as solar sails and propulsion units because this causes misfits and stress on the various components of the system and long term deterioration, it is pointed out that temperature differences on a starship should be kept minimal wherever possible and: "The warmer a material is, the more agitated its molecules are and the more likely the weaker types of molecular bonds will randomly break." Back to the general engineering rule: the general failure rate doubles with each 10°C increase in temperature and also drives unwanted diffusion of atoms within materials. All this presents problems for example for solar sails approaching relatively close to the Sun to get a boost and a discussion is given on the reliability on semiconductor electronics with temperature. It's also pointed out that some metals and most plastics become brittle when very cold (3K) however gives a reference from which it is found that Titanium alloys, aluminium, stainless steel, copper and nickel keep their strength and toughness at these very cold temperatures, these metals are relatively cheap and easily obtainable which is good news for starship Engineers.

The question of reliability is a major field of engineering: "Reliability results from any or all of several approaches: multiple (redundant) backups, long-term testing during development to establish failure rate and mode, full understanding of the science and multiple-level engineering involved in a device, and identification of all operating conditions in which the device must operate." A discussion is also given with regards to the 1986 Space Shuttle Columbia accident and past nuclear power plant incidents. An interesting case scenario is given for a starship: "If, in a system like a starship with perhaps a million critical parts, each part has one chance in 1000 of failing in 1 year, then about 3 failures per day can be expected. A series of these can constitute catastrophe; for example, the water pump fails, then the power for the shop that fixes pumps fails, then a leak develops in the hull requiring the attention of the same people trained to repair the preceding." and gives another example to bring the point that: "several failures can occur in a short time which are independant initially but afterwards are interactive, making catastrophe out of smaller crises. A longer chain of failures could destroy the starship as a viable system." The Apollo moon program is mentioned to have attempted and achieved failure rates of 1 in 10,000 for all parts for a 10 day mission while the Daedalus study with 100,000 parts required an overall failure rate of 1 in 10,000 over 50 years.

With the possible loss of material from the ship over time, several options are outlined how to recover the material back and discusses the importance of recycling and repairability / maintainability issues including the possible use of maintenance robots and their pros and cons compared to human crew. [CI: The Daedalus study has "Wardens" or mobile maintenance robots able to undertake various tasks on the ship during the voyage.]

Image: Adapted image showing one of the Wardens of Daedalus (This is Rocket Science)

A good deal is covered on the dangers of radiation to the ship and crew, this refers to charged and uncharged high speed particles, energetic photons from material hitting the starship at high speed, interstellar particles and photons, particles from any particle or nuclear drive and solar flares: "At 0.3c, protons carry about 50 MeV and are very dangerous; at 0.1c, 5 MeV and still bad; at 0.01c, 50 KeV and easily shielded. At 0.1c, interstellar protons are encountered at the rate of at least 3*10^12 particles/m^2, bringing energy at a flux (or intensity) of about 10^13 MeV/m^2 or about 1 J/m^2s. The intensity of radiation is approximately proportional to starship speed cubed." It is noted that a shield of a given thickness doesn't absorb all the radiation but as the thickness increases, the amount that gets through exponentially decreases. Radiation from "cosmic rays" in the form mainly of energetic protons and other heavier particles can be a problem as they can range from 10 MeV or lower to 10 GeV or more and can come from any direction but notes that: "radiation from starship motion is much more serious than cosmic background by a factor of about a billion at 0.1c and still a million times worse at 0.01c."

It's also pointed out that radiation from the propulsion can also be a serious problem, in the case of the anti-matter drive, this can put out dangerous 200 MeV gamma rays in all directions for eg. Radiation from a fusion drive might be mainly 14 MeV neutrons depending on the fuel used and would need a 1 m thick shield, for cosmic particles, 2 m of dense material would be required to stop them below 10 GeV. For the Daedalus above, the shield used is made of 50 tonnes of Beryllium only 2 cm thick mainly for dust erosion however 10 MeV protons are expected to be stopped 1 mm into shield at an erosion rate of 1 tonne per year. However this shield isn't thick enough to stop some of the more energetic cosmic radiation. Radiation shielding is important because computers and electronics, data storage devices, sensors etc are sensitive to all this and not to mention the crew. It is also noted that "the Voyager spacecraft were designed to work at the level of 150 kilorads per year (about 1 million rems, depending on the particles), as encountered near the orbit of Io around Jupiter, and did not suffer significant electronic failures" and these days more progress as been made with more durable electronics.

Several active shield systems using electric, magnetic or plasma are then described which could deflect ionized interstellar gas (not neutral gas) and cosmic rays, one system described: "If protons and other nuclei upto 10 GeV in cosmic rays (which inevitably are ions) are the primary radiation problem, then a potential of negative 10 billion volt (10 GV) would be needed to repel those protons." Another option outlined is to try to imitate the Earth's dipole magnetic field (which protects us Earthlings) by creating one around the whole starship using superconductor coils carrying a billion amps. This would deflect some particles and collect others at the "poles". One concern pointed out though is how much of this strong field would penetrate the habitat volume and ship design would have to take this into account. Dust and neutral gas cannot be deflected by fields but a: "foil sail ahead would ionize both as they pass through, so that fields could prevent erosion." Another shield system described also good for larger size dust grains or even small rocks was studied for Daedalus and involved using a "shield cloud" of dust that would be placed ahead of the ship to vaporize these larger objects. 10 Kg of fine dust would be used to create a cloud 100 m in diameter several hundred kilometers ahead of the ship. The dust packet would have its own propulsion system for delivery to the target and this would convert any object less than 0.5 tonne to plasma. Anything larger than small rocks such as comets need to be avoided.

Image: Earth's magnetosphere shields us from the harmful charged particles

from the Sun, good idea for part of a starship's active shield system? (Wikipedia)

In the next several pages, the author discusses various issues regarding computers and long term data storage for use onboard starships. These can handle large numerical calculations associated with ongoing operations and model complex systems. An example is given in the case for navigation on the way out of the Solar System where the computer can predict the effects of non-spherically symmetric gravitating bodies on the starship, effects of dust, solar wind, photon pressure and fluctuations in the drive itself. Reliability and self-diagnosis is emphasised together with backup systems. Self-reproducing systems might be a long way off but mentions that a mathematical theorem has been proved on paper that systems could be organized to reproduce [b-von Neumann; b-Dyson] and that "This abstract work is not purely speculative because biological systems using information coded in DNA follow a similar process. Indeed, the minimum number of coding elements (base pairs) needed by a virus that has its own reproduction machinery is about 200,000." If a system can reproduce itself, it could also fix itself. There would have to be two computers working together so one can turn off/on the other computer to be fixed while it replaces or fixes its parts. 

Also mentioned is the use of Artificial Intelligence (AI) especially for interstellar probes on their own: "Probes without human guidance must be adaptable and make good decisions from vague information, even evolve to improve themselves. This capability is widely proposed as necessary for long-term missions that must do new things at new planets, even "talk" with new civilizations encountered, decide what to tell "them", and determine whether they are any danger to us." I like the last sentence of this paragraph: "There are limits to what we can do with logic systems (real computers or software) but no limits to the variety of situations in the Universe that those systems must cope with."

Data storage issues is then discussed and according to the author we would need to load the starship with at least the amount of information found at a major university library or 10 Terabytes plus a wide sample of Earth arts and other material. Various storage media are compared comparing their long term reliability however the last part was particularly interesting: "If a cubic millimeter (about 10 million atoms thick) is used for information storage with each layer of atoms laid down as one kind or another to encode information, it can store 10 million bits. A cubic meter (mass about 3 tonnes) could store on this basis 10^16 bits or 100 times the above requirement. If information could be stored and retrieved atom by atom in the cubic millimeter, it could hold about 10^21 binary bits, enough to satisfy thousands of high-tech civilisations. These are the nearly ultimate limits of information storage."

Possible breakdowns are discussed and points out that at least three or more large power sources should be available at all times onboard the ship, some backup options given include hydrogen/oxygen fuel cells and Radioisotope Thermoelectric Generators. Other ideas have been suggested for emergency power, one of them is the use of a 1 Km coil in the weak interstellar field which would only produce 10 V per turn at 0.1c via magnetic induction.

The chapter concludes with a discussion on transportation issues between a fleet of starships (send several ships instead of only one makes sense safety wise but costs more) and transportation between the starship and moons or planets. Radiation can be a serious problem to the shuttles travelling between the starships at high speeds as pointed out earlier and storage, fuel, maintenance, protection from erosion are also issues that will need to be addressed. Shuttle transport between the starship and the planet is a BIG problem: "Solutions for transport between starship and an Earth-size planet elsewhere can only be speculative." because using chemical fuel rockets to depart larger size planets is out of the question and there are no launch pads with extensive shuttle facilities on the ground at the new destination and unlikely to have nice long flat hard surfaces for the shuttle to glide and land on. The crew also need to get back to the ship even if they landed on the surface. Other ideas suggested include using extremely high strength long cables to lift items from the ground from orbit (space elevator) which maybe feasible depending if the cable made of high strength carbon nanotubes can be yarned into a large and (very long!) cable with similar strength. Other ideas mentioned include using a laser-heated rocket which requires a very large ground based laser with power supply and isn't feasible and an electromagnetic launcher has limited capability. [CI: this problem also demands Engineering Rule 6]

In the next part of this book review series we'll look into the next two chapters of the book namely more biological issues in a starship and personal, social and political considerations related to interstellar travel.

Part 4: Book Review - Prospects for Interstellar Travel

We move on to Chapter 7 of the book which I found particularly very interesting. There doesn't seem to be much written concerning Interstellar Navigation and the various position fixing methods interstellar voyagers could use (the author names this as "astrogation") so let's have a close look.

An explanation is given on the basics of the celestial sphere and how conventional star map coordinates are setup from our Earth based vantage. It is natural to use angular positions of stars (ie Right Ascension and Declination) however for navigation purposes in interstellar space we may need a rectilinear coordinate system. It is noted that no standard 3D coordinate system in meter or Ly has been established for star listing positions because astronomers haven't planned on interstellar travel but this isn't a problem because spherical coordinates can be easily converted to x,y,z rectilinear sun centered coordinates [CI: for those curious read Spherical Astronomy]. This also applies to the starship's "celestial sphere" when the navigator measures star angular positions and needs to convert these to rectilinear x,y,z coordinates for a position fix on the chart. Moving to the case of the starship in interstellar space: "No absolute reference or coordinate system exists for starship travel or anything else, with the partial exception of the uniform (to 1 part per million or better) background microwave radiation.", in other words we can use any coordinate system we like, Earth centered, Sun centered, Galactic coordinates or other (such as pulsar grids) however as the author points out it makes sense to use a Sun centered system because our Sun is moving 22 Km/s (that's Kilometers per second) in a specific direction compared to the other stars and gas and the travelling ship will somehwat share this motion.

Image: Our galaxy with Sun centered galactic coordinates. (Caltech)

Typical motion of other stars is given at 10 Km/s relative to us or a 0.003 Ly shift in 100 years so we cannot assume they are fixed with respect to the Sun. To give us a perspective on things, it is mentioned that the Sun along with local stars orbit around our galaxy's center at approximately 300 Km/s and our galaxy is also moving about [CI: at 552 Km/s relative to the photons of the background microwave radiation towards the Great Attractor]. Back to our little corner of our galaxy, as the author explains, these previous motions don't affect local interstellar navigation for the starship and we don't have to measure these [CI: however measuring these would be necessary for intergalactic travel, one does wonder sometimes if out of all those 100 billion or so galaxies in our part of the observable universe if there are any beings travelling between these galaxies, will humans one day be able to venture to the Andromeda Galaxy for eg? highly unlikely]. Some further issues outlined include the accuracy of star positions as seen in the sky and their stellar distances. These should be updated wherever possible using probes for interstellar scout missions and send over the data back to Earth as distances to most cataloged stars are unknown or rough figures. Star positions must have an accuracy of at least 0.0001 Ly before a probe or starship is sent on a major mission. The starship "can update its map with new observations, calculate new star positions according to their known motions, update those motions, calculate starship motion in real time (proper and Earth), and use relativistic mathematics for greatest accuracy." Most of these tasks are ideally suited for a navigational computer linked to telescopes/spectrometers and sensors throughout the ship coupled with a 3D virtual star map showing to the crew position, heading and other navigational data. An interesting point made here is: "The first big starship should not be required to acquire accurate information along the way as it may find too late that a major course correction is needed. Course corrections of the order of 1 part in 10,000 can be made as the starship proceeds, but larger corrections are costly at high speed."

It is also noted that as far as getting lost, human voyagers would be able to monitor starship progress and recheck the position of the Sun and destination however for a probe that depends on computers and sensors: "any probe which has an error in orientation due to malfunction of steering jets or gyro, or another failure, must acquire data on bright stars and sort out which ones are to be used for guidance before correcting its orientation. Probes must use an assortment of stars for fixes and might need to measure brightnesses and spectra and compare with prepared desciptions to identify them. Getting lost is unlikely for human missions closer than 100 Ly but always a serious problem for probes."

The faster the starship is travelling, the more pronounced is starlight aberration (apparent position change of the stars due to the finite speed of light c). This must be calculated from the speed of the starship or if the aberration is known, the speed of the starship can be calculated: "At 0.1c aberration causes an apparent shift forward of stars by about 6° for those located to the sides, and less for stars toward front and rear. The apparent brightness is affected by high speed. At 0.1c intensity is increased about 20% front and decreased by 20% behind". [CI: Checkout What would a relativistic interstellar traveller see?]

Image: our view of the stars changes the faster we go due to relativistic effects which will need to be calculated, apart from the position shifts of the stars, note the brightness changes (Physics FAQ)

The author moves on to discuss starship speed measurements which can be made by doppler shift in the spectral lines from any stars or even better using the doppler shifts for known pulsars which send highly regular (millisecond to second range) radio pulses unique to each pulsar and have been accurately measured by radio astronomers (to 6 digit accuracy or better). A pair of radio telescopes on a starship could also be used for position fixing. [CI: Recent studies for a GPS-like position fixing method for use in our solar system and beyond using X-ray pulsars for even greater accuracy is ongoing]. Other navigational instruments mentioned include inertial guidance devices and sophisticated 3-axes gyroscopes using ring lasers: "if accurate measures of acceleration are fed to a computer from the gyro in all three dimensions, it can calculate from prior information on position and velocity the present location without needing outside measurements (relativistic also)." and of course the obligatory atomic clock with an accuracy of 1 part in a trillion or better. [CI: a three axis magnetometer would also be handy for interstellar magnetic field measurements but also could be used for orientation of the starship as a backup system to using stars or pulsars if the magnetic fields are well charted in the space that's navigated ie a sophisticated ship's compass. Checkout this paper: The Orientation of the Local Interstellar Magnetic Field although magnetic deviations onboard the starship may be quite large because of the high energy devices that would be found onboard.]

Another important point mentioned by the author is on the assumption: "that the propulsion force is applied in the desired direction of travel. If force direction differs from the intended direction by a small amount, an increasing error in direction occurs. For example, misalignment by 1" arc results in 100 million Km error after 10 Ly." and goes on to mention that fusion exhaust or photon reflection with large energies aren't perfectly aligned systems just like chemical rockets where the ejected material isn't exactly centered on axis which results in off axis propulsion and these are corrected for by the guidance system or in the case of the starship, telescopes locked on the structure which can detect any changes in direction from a set of star positions. "The average long-term error in direction can be corrected, but the short-term fluctuations should only be measured, not corrected". [CI: similar to the autopilot on boats, everytime the boat goes through a wave, if the autopilot moved the rudder, the steering ram would be working overtime unnecessarily so there's a "wait and see" delay setting.]

Moving on from navigation issues, the author looks at starship manoeuvers that may be required to dodge large micrometer size dust particles along the way for eg and significant changes in course headings will require substantial propulsion energy: "A 6 degrees change requires about 10% additional speed (and about 20% more energy). No way is known to recover momentum from one direction and apply it to another." and points out that changing direction isn't just a matter of rotating the starship by a small angle as it would still continue in the same direction as before until the main drive is used substantially. For minor course corrections mention is made of an inertial wheel and possibly of chemical steering jets. As it was pointed out in Chapter 4, the possible use of interstellar magnetic fields to change course has been discussed using the Lorentz force: "For a radius of turn of 1 Ly at 0.03c, slower than earlier examples yet still very fast, a 1000 tonne starship must use 1 million coulombs." with the mentioned wire requiring to be over 1 million Km long, this whole approach doesn't seem feasible compared to carrying more propulsion onboard.

The author then looks at active detection methods for detecting what's ahead in the first place. We need this information early to give enough time for the ship to alter its course. Just like radar systems, the system would transmit signals ahead and wait for reflected pulses to deduce distance, direction and size of the object however it's pointed out that the best detection is done by observing information coming from distant objects as there is no waiting time however cold dark matter emits only very small amounts of radiation not enough to be easily detectable. Some gases do not emit radio or light waves even if UV light strikes their atoms or molecules and radar cannot make matter respond, it only relflects or scatters.

Fortunetly lasers here come in handy: "Energetic laser light, x-rays, electrons, and neutrons can cause response from distant material to help identify it and determine its composition, density, speed, and temperature." however getting enough intensity for x-rays, electrons and neutrons is much more difficult compared to radio, radar and light. In order to detect sizes ranging from large dust particles to large rocks, radio waves with a wavelength of around 1mm is best and if we use two or more transmitting antennas we get better resolution, for 1Km separation we would get a beam size of 1" arc. The author points out that we still have the problem of dust erosion of the small antennas and these are even more difficult to protect compared to a single 100m dish. Another system example is given: "a system with 1 Km effective aperture can detect objects smaller than 1 mm and locate to an accuracy of 10 m in 10,000 Km. At 10,000 Km/s, there would be a 1 second warning to shift the direction of travel to miss an object. Two gees sideways would be required to shift 10 m in this time, but this amount of drive is probably not available". One option offered as an early warning system is the use of a probe which is travelling far ahead of the starship pushed by an ion drive and powered by the ship via a long cable. Once detected though, the feasible option given by the author is to demolish it by "zapping it" with a high-power laser. There is time for the laser system to confirm the target before main zap, if the starship motion is nonrelativistic. "At relativistic speeds there may not be time for detection and response to objects ahead". [CI: for relativistic speeds, it's not suggested by the author but use of an expendable detachable shield far ahead of the ship may be an option to "clean the way ahead" for the ship (still keeping its main erosion shield on the ship). Let the expendable shield take the damage rather than the vital ship itself]

It's pointed out that lasers would be very useful in detection and possibly zapping objects ahead. An infrared laser is better at detecting finer dust than millimeter radar can due to Rayleigh scattering and because of the typical sizes of dust grains the best wavelength is infrared, larger wavelengths tend to diffract around these objects. The laser light also needs to arrive at the object with enough intensity to excite the atoms in the object for a detectable energy signature for spectrographic analysis.

Image: Porous chondrite interplanetary dust particle, running into one of these at relativistic speeds will cause erosion or some damage to the starship (Institut für Planetologie and University of Washington).

The following paragraph deals with passive observations and makes the case that apart from the need to make accurate observations for interstellar navigation, interstellar and planetary studies, the sensitive scientific instruments mentioned in the previous chapter would also be useful for searching for any signs of Extra-Terrestrial (ET) Intelligence (SETI) however as pointed out: "Observation enroute will be difficult at high speeds because of interference from impinging interstellar hydrogen and dust. Most data must be collected from behind the front shielding." Several frequencies are outlined for observations in the radio spectrum and points out that at a frequency of 10 GHz, the natural noise is the lowest. Bandwidth used (the range of frequency used for one signal) is also important. The narrower the bandwitdth of the signal, the more it stands out from the background noise and the further it can be detected. We can achieve 1 Hz bandwidth or better and so could the ETs. However if broadband receivers are used, the signal could get lost if it doesn't have enough resolution in the spectrum. "Noise in deep space is due to synchroton radiation from electrons all over the galaxy at low frequencies, to background radiation at intermediate radio frequencies, and to quantum noise at high frequencies." and anything warmer than 3 Kelvin emits more radiation than the background. Hyrdrogen atoms emit radiation at 1420 Mhz (21cm) weakly and the author points out that it was first thought that ETs would choose this frequency to broadcast as it is also relatively queit. Extensive searches for unnatural signals haven't been found. Signal leakage from ET civilisations from ordinary activities is more difficult because the fequencies may be in a noisier band: "It has been estimated that if another civilization leaked TV carrier (1 MW typical, 0.1 Hz bandwidth) and radar pulses like ours does, then our astronomers could detect their leakage at about 30 Ly with our largest radio dish, 300 meters at Arecibo, Puerto Rico. Starship radio dishes limited to 100 m diameter might be able to find similar radio leakage from civilizations just a few lightyears away, a marginal capability."

Photo: Arecibo Observatory (NAIC)

The possibility of ETs using laser pulses to send signals is also mentioned and studies have shown that these can be picked out of the starlight if they are distinct from the background light. Further considerations are then given on to communications between the starship and base: "A transmitter with a 100 m dish must put out about 4000 W per cycle of bandwidth at 30 cm to be barely detected 10 Ly away in a 1 m^2 receiver. Only about 0.5 W of transmission in a 1 Hz channel is needed for threshold detection in a receiver at the focus of a 100 m dish across 10 Ly!". For the Daedalus project, the study specified a 2.6 MW transmitter for data transmission at 1 Megabit per second at 2 or 3 Ghz from 6 Ly with a 40 m dish. A 100 m dish is quite large for a starship and we have the dust damage issue to contend with, the Daedalus plan was to use the dead fusion chamber as the main dish to get around this problem. Some studies have also looked into using lasers for optical communications as well.

I'll leave out Chapter 8 "Technological Requirements and Hazards" for the next part of this book review as we have covered quite a lot of material in this part alone.

Captain InterStellar's Big Day Out: walk to the lighthouses

With the busy season here on Sydney Harbour finished, I'm getting more days off to relax or spend more time with my other Physics and Compass Adjuster studies. Went for a big walk today at South Head (the southern side of the entrance to Sydney Harbour), gave my new DSLR camera a workout and did some peaceful study with the ocean for inspiration and a nice cool sea breeze :-) Caught the ferry from Circular Quay to Watsons Bay and first headed off to checkout Macquarie Lighthouse. This is the first lighthouse you'll see approaching Sydney from sea with the light at 105m above sea level and also is the brightest. The other one I checked out was Hornby Lighthouse at South Head with its nice red stripe colour scheme:

 

 The walk along the cliffs is spectacular and you'll see many remains of the fortifications that were placed with heavy artillery for defense. The canons where no longer needed when the missile era came to age. At the Gap, the only remains of the famous shipwreck of the Dunbar was the recovered admiralty anchor: 

As I was contemplating the ocean, a quote from Darwin came to mind:

Part 3: Book Review - Prospects for Interstellar Travel

Continuing with this book review we move onto the fifth chapter where the author gives a rundown on the various subsystems that a starship could have. It is not about designing a starship but "recognising it as a complex system" with or without people onboard. Placing propulsion temporarily aside the chapter assumes that the ship has 0.01 to 0.1c speed capability and that the human social system is in place to support mission design, construction and long term operation.

Section 5.1: Engineering rules, was a gem: "A number of empirical rules for designing complex systems have emerged from centuries of practice with simpler systems.", this list has sound advice for Engineers working on any complex project:

Engineering Rules

• Rule 0: Engineering is setting and making systems that work.
• Rule 1: If there is a way for a system to fail, it will happen (Murphy's law). Failure analysis can never be complete, either. For example, some nuclear reactor accidents have involved a chain of 4 improbable failures.
• Rule 1a: Design fail-safe. That is, if something fails, try to prevent anything else going down with it, or possibly arrange for a beneficial failure or a self-fixing one.
• Rule 2: Humans, at least, tend to build whole from parts. Since a whole system cannot be deduced entirely from its parts, induction is part of the design process.
• Rule 2a: The whole can be greater than the parts. That is, the function of the whole system can be qualitatively, not just quantitativly, different from the functions of its parts. (Note latter comments on systems.
• Rule 3: Define a problem such as propulsion as broadly as possible. Even more broadly, ask: What is the purpose of interstellar travel? Identify unnecessary assumptions and biases which limit the problem artificially and prevent many kinds of solutions. Do not assume the solution (e.g. a rocket), find it. Avoid vested interest until the final design is very clear.
• Rule 4: Once primary and subsidiary problems are well-defined in detail, the solutions may be half accomplished.
• Rule 5: Keep everything as simple as possible.
• Rule 6: Know the limits set by known science. Engineering cannot exceed known science but must wait (and ask for) further science. This does not mean that engineering cannot find new solutions to problems. The realm of engineering creativity, a result of intelligence working against entropy, is probably far larger than the realm of scientific models and laws.
• Rule 6a: Be conservative with technology. This contradicts the wide-eyed technical ideas in most interstellar studies, but this rule is for using established technology at the working hardware stage, not the exploratory stage.
• Rule 6b: Work from the known to the unknown. Start with experience, but take early risks to expand that experience.
• Rule 6c: Models do not scale up linearly (e.g. strength-mass ratio, fluid flows).
• Rule 7: Watch out for interactions. Every functioning part can foul up other needed functions by many parts.
• Rule 8: Most systems involve the need to optimize conflicting functions, and trade-offs are needed where each function is less than ideal.
• Rule 9: If a system of mass m needs parts, each of which masses 0.1m (or even 1m as often occurs in propulsion), stop and redesign until each part does not call for more total mass than intended.
• Rule 9a: If other considerations are equal, the masses of parts of a system (and some other characteristics) might be equal in an optimum design. This rule derives from impedance matching.
• Rule 10: A practical system must be reliable (to an extreme and well-defined degree for interstellar travel), operable (reliably controllable), and repairable (or better, easily maintained so breakdown is unlikely).
• Rule 11: Use multiple backups for subsystems (3 or 4 for interstellar travel). Each backup must be designed as a main system, not an inferior secondary version. Use a different proven method for each system rather than achieve redundancy by making the same system three times. Hint: use several different propulsion methods. (The Voyager probes did not have backups for many components because Voyager was expected to function less than 50 years. Some of the few backups did need to be used.)
• Rule 11a: Where substantial failure rate is unavoidable, use tens or hundreds of identical copies of a subsystem in parallel. (Big example: why rely on one huge tricky drive when banks of identical small independant ones are better. If a few fail, 90% of drive capability should remain.)
• Rule 11b: In counterpoint to multiple backups is multiple use of the same system to save mass. This is gambling. (It is better to have 3 sails, each of which can also be an antenna.)
• Rule 12: Temperature is a major confounding factor. Beyond "room" temperature (20°C) the failure rate generally doubles with each 10°C increase in temperature, the old Arrhenius rule. The size of materials changes at different rates with temperature, causing misfits and stress.
• Rule 13: Systems are rarely self-stabilizing and usually have numerous ways to destabilize themselves. Know the system so well that stability can be made inherent.
• Rule 14: Use self-repairing subsystems and hope that this repairs the whole system. (This rule spins off from the space age).
• Rule 15: Use monitoring sensors more reliable than system components, else they are worse than worthless, they mislead during breakdowns. (This rule spins off the nuclear age.)
• Rule 16: Avoid designs by committee and layers of bureaucratic review. Find the top experts who will live for the project, give them unlimited support, and leave them alone.
• Rule 17: Use the same parts in as many different places as possible. (Reduce the number of different parts needed.)
• Rule 18 (especially for starships): No repair services are available enroute, and no radio or rescue help after leaving the Solar System.

[CI: I'd like to comment on Rule 6 because this is highly relevant to our current state of affairs regarding our current known science and what this allows for our prospects for interstellar travel. My view on this is that the prospects are not very good with known physics. Despite all the great ideas put forth, current known physics does not allow practical interstellar travel within reasonable earth timeframes. Our known physics doesn't prevent us to venture to the stars but it makes the prospect a considerably difficult and slow undertaking and if there is no new enabling physics, this will not change even if we have a vast space infrastructure centuries from now. This is a problem because our long term viability as a somewhat successful and prosperous civilisation is directly linked to our capability of interstellar travel to colonise other earth like planets in other star systems (for breeding room and safety against extinction: don't have your eggs all in one basket ie Earth). The important question is: does Nature allow for a more practical approach to interstellar travel? Is there hidden physics that would allow this? One needs to ask for more science and question everything. Understanding the physics of the vacuum could be one step forward to answer these questions.]

A general discussion is given on possible starship subsystems subdivided into the big heavy duty systems (structure, propulsion system, powerplant, shield system, ecosystem, electic power supply etc), fine subsystems (sensors, communications, computers, science instruments etc), subsystems for human needs (storage tanks, water, food, atmosphere processing, artificial gee system, work, private and leisure areas, waste processing systems etc) and extra equipment (landing craft etc). Starship size is dependent on a number of factors however how many people it carries is one of them: "One hundred people need at least 300m^2 per person (100m^2 with 3m overhead) in the closed ecosystem habitat for enough room to enjoy being with each for decades or centuries." The author emphasises the importance of the shield which inhibits the need to look ahead: "Ineed at higher speeds, the hail of radiation would prohibit anyone or most instruments from peering around the shield to look ahead. At 0.1c hydrogen gas arrives like 5 MeV radiation."

Another good point mentioned is the importance of redundancy built into the ship systems ie have backup systems to the backup systems and the possiblity of having maintenance robots onboard for maintenance tasks of the various systems together with maintenance and power facilities for the robots themselves [CI: sounds like a good job for R2-D2? ;-)].

The sixth chapter deals with possible mission scenarios, where we might go, who and what we might send over. The author runs through our stellar neighborhood outlining the characteristics of various close stars, their spectral classes, planet formation theory and the possiblity of exoplanets in their habitable zones. Since the author wrote the book, 429 exoplanets have been confirmed by astronomers (as of January 2010), most being gas giants Jupiter-like planets) and the next several years could look promising finding more exoplanets with masses down similar to Earth. Finding an earth-like planet would be a major find for us and this would set the star system as a serious possible candidate as a destination for an interstellar journey with a precursor probe followed by a crewed starship if viable. An Earth-size planet at say 10 Ly would have an angular size about 10^-5 arc seconds requiring multiple large telescopes tied in together as an interferometer array possibly on the Moon for effective observation. Hopefully this earth-like planet will be within 10 Ly from us and not too distant. The author on p146 gives a 3D perspective star map of the forty closest stars within 16 Ly. These days there are freely available excellent 3D interactive star maps such as the flash based Exosolar (screenshot below) or try Celestia.

Image: Screenshot from Exosolar showing some of our nearby stars

Stars just like planets are also in motion, these move around in our galaxy and their stellar motion can also be observed, p147: "For almost any mission, stars move fast enough that accurate astrogation [CI: Interstellar Navigation] requires knowing and accounting for their motion. As mentioned earlier, the present Voyager and Pioneer missions will first pass some stars that will have approached closer to the Solar System at that time [a-Cesarone]. However no stars are moving so fast as to provide special close (less than 1 Ly) interstellar mission opportunity in the next hundred thousand years. Humans must wait about 815,000 years for star DM+61 366, type K and now at 33 Ly, to pass about 0.3 Ly from the Solar System."

In the scientific opportunities section, the author gives a rundown on what kind of science would be done on an interstellar mission which could include observations of stars and their planets, composition of the interstellar medium, measurements of fields and high energy particles and accurate measurements for stellar cartography. Close observation of more interesting objects such as white and brown dwarfs, neutron stars, star forming clouds, pulsars and black holes will probably be out of our capabilities as the known ones are too far away. The Crab Nebula for eg, a supernova remnant, is 6500 Ly away (not wise to get too close though because of the excessive X-ray and Gamma ray radiation levels from the pulsar):

   

Photo: The Crab Nebula 6500 Ly away (Hubble Space Telescope)

A general discussion is then given on several mission scenarios including those with crew onboard, simple, advanced and self replicating probes and large colony ships and outlines the benefits and disadvantages in each case. The most likely scenario that will happen as the author points out is that before a human expedition, one or more high speed probes will be sent over to the star for science data return and propulsion proof of concept. Following this preliminary scout mission, the importance of sending over people is emphasised on p152: "Once a propulsion method is shown workable locally, there will be interest in a human expedition however risky. Humans carry the ability to monitor a starship carefully, process and study much relevant science along the way, repair many kinds of failures, overcome many kinds of emergencies, and generally may justify the expense of sending them to obtain a successful mission." If viable, the case of large colony ships is discussed however "Interstellar travel will not be able to reduce population pressure because of the cost of travel" but they would be useful for small population transport for new-earth colonisation purposes. For all these mission scenarios, propulsion will be one of the biggest factors for a go or no-go decision.

In Part 4 of this book review we'll look at Chapters 7&8 which touch on Interstellar Navigation, Observation and Communication methods followed by further discussion on starship technological requirements and hazards.

Part 2: Book Review - Prospects for Interstellar Travel

Continuing on with this book review, we move on to the third chapter which gives an introduction to relativity and what effects this causes to interstellar travellers when one travels at speeds greater than 0.2c (0.2 times the speed of light) this is when one starts to notice relativity effects, the faster one goes from here on, the more pronounced the effects. For different observers mass, length and time differ and another consequence is that it becomes more and more difficult for objects with mass to approach c. At 0.2c the mass of a starship for eg is increased by about 2%, and at 0.9c the increase is a factor of about 2.3. This factor which describes the mass increase is called "gamma" which appears throughout relativity:
\[\ {\gamma &= \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}}\]
Where v is the velocity of the object. Particles at accelerators are routinely accelerated to 0.99c but never at c or beyond. Time dilation is well confirmed and is related to gamma. The particle accelerators in use today would not work if these effects aren't taken into account for eg. [CI: length contraction in practice will probably be unobservable for a long time]. On p75 for eg it mentions "A journey occuring at 0.866c (gamma of 2) results in the flow of time at half rate during the journey. A twin on Earth might have aged 60 years while the voyager aged 30 years.", the famous "twin paradox" which isn't a paradox as the author explains. Hint: it is the starship that accelerates, the Earthlings go about their usual business. One consequence of relativity for interstellar travellers voyaging at high gammas is somewhat troubling, p77:

"Once the regime of large gamma is entered, their Earth is gone forever. There is no way to return to their decade or their century."

In other words fast starships voyaging to distant parts of the galaxy may not return to an Earth they once knew. Assuming interstellar travel at high gammas turns out to be feasible in practice this can be a problem depending on the purpose of the mission. A rundown is also given on how relativity affects the rocket equation and relativistic energy and momentum.  The last paragraph of this chapter is worth quoting here:

"Relativity makes energy a serious problem through the limits imposed to prevent speeds greater than light. Relativity also offers tantalizing solutions: the slowing of time and Total Conversion of mass to energy. How closely propulsion might approach TC is explored in Chapter 4. One could hope to find a way to travel without the action-reaction rocket method--no exhaust, no acceleration, little travel time, no deadly beams, no titanic low-mass energy source--but these are still mostly dreams from sf. Thus far it is not surprising that "visitors" from other stars have not appeared recently nor left their garbage laying about. They also must contend with what their Einsteins discover about interstellar travel. If visitors were to arrive, one of the first facts we would want to know is "how did they do it?"." 

[CI: This paragraph somewhat deals with the 3 goals of the Breakthrough Propulsion Physics Project and touches on SETI issues as well. Quick BPP recap:

1. Mass: Discover new propulsion methods that eliminate (or dramatically reduce) the need for propellant.

2. Speed: Discover how to circumvent existing limits (light-speed) to dramatically reduce transit times.

3. Energy: Discover new energy methods to power these propulsion devices.]
 
Following relativity, the author takes us through several drives that would allow one to travel at relativistic speeds greater than 0.2c. This in turn brings in new problems such as possible hazards encountered by the ship at these high speeds.

Earlier the solar sail case was mentioned with the benefit of using sunlight to accelerate the starship while close to the Sun or star. However if the sunlight can be collected and focused by a giant focusing mirror then this could be beamed towards the sail ship over a longer period of time, this is the concept of beamed power propulsion. Not only could sunlight be beamed over but also light from a powerful laser or maser (microwaves), all this circumvents the low photon intensity past Jupiter's orbit. However not only is this system big (to provide a useful beam at a distance of 1 Ly, the mirror and starship sails described are 100km in diameter) there is the problem of stopping the starship at the destination however this wouldn't be a problem is this was for a fast flyby probe mission. Another option described is the photon drive: generate your own photons to accelerate the ship however this is shown to be highly inefficient.

Enter the anti-matter drive: "This drive determines the prospects for interstellar travel for the future as best known science can predict". Compared to fusion drives, anti-matter produces particles with much higher speeds, the author gives a table describing the outcome particles after the annihilation process if we bring together hydrogen and anti-hydrogen. [CI: In the recent Avatar movie the ISV Venture Star has a hybrid anti-matter / beamed power sail drive and a fusion powerplant, the movie people consulted some knowlegeable people in the field for a realistic starship design for the movie plot, note the red hot glowing radiators for excess heat dissipation after the decceleration phase].

Photo: The InterStellar Vehicle Venture Star from the recent Avatar movie.

In the mixed bag of high energy particles we also obtain after the reaction lots of high energy photons (highly penetrating gamma rays) and this is a problem because there are no known ways to deflect them towards the exhaust in one direction so heavy shielding is required for critical areas of the ship such as crew areas. Another problem with anti-matter is that this form of matter is almost never found in nature and currently extremely expensive to make at particle accelerators. Those that are created have limited storage time due to the imperfect vacuums used to store them here on Earth's surface. Highly reliable magnetic bottles would also be required even if a way is found for mass production because no contact can be allowed to normal matter without loosing the anti-matter fuel. In the rocket drive described by the author, high magnetic fields are used to direct the heavy charged particles (pions, muons) towards the rear to provide momentum transfer to the ship:

Several mission scenarios are described and the extremely high cost of anti-hydrogen production is mentioned, one should note that particle accelerators weren't designed to be anti-matter factories so things could look optimistic if more efficient ways are found however "clearly a very rich civilization is needed to produce this most compact fuel for starship propulsion".

Any venture to the stars at high speeds will have to deal with the possiblity of colliding with (hopefully tiny) particles along the way: gas&dust from the interplanetary medium and the interstellar medium, the author gives a rundown on the interstellar medium (ISM) which is mostly vacuum but still has gases and dust dispersed throughout the galaxy with an average density of mostly neutral/ionized 1 hydrogen atom / cm^3, some helium and traces of other elements, in our neighbourhood these particles are moving towards our Sun from Alpha Centauri at 20Km/s (from the reference the author gives). 1% of the ISM is made up of interstellar dust grains of carbon, nitrogen, oxygen, compounds of silicon, magnesium, iron covered with water, methane, ammonia, organic ices and other compounds, dust sizes vary from 0.1 to 0.01 micrometers. [CI: Visit this website for more info on the ISM].

This interstellar gas will produce slight drag and erosion on the forward surfaces of the ship, the dust could cause severe erosion as the ship is rushing at say 0.5c towards gas&dust. The effects of these collisions on the ship material is debated and needs experimental testing however various possible outcomes are described together with protection methods, some outcomes could be localised heat due to the impact and smoothing of the forward surfaces of the ship over time. In the previous photo shown of Daedalus, note the erosion shield on the forward part of the ship. The chapter finishes off by looking at interstellar electric and magnetic fields and a description of how a starship could use this magnetic field with charged wires for a round trip around a star. Several pages are devoted to interstellar ramjets and the Bussard ramjet which collects material (hydrogen) from space as it moves along for use in a fusion reactor for propulsion. The prospects for this method have shown this to be unviable: the scoop for eg would have to be 10000Km in diameter to collect enough hydrogen to get up to 0.1c. Another study has shown that the hydrogen atoms would also simply bounce back from the scoop and mostly not enter the collection point which defeats to whole purpose of the scoop.

In Part 3 of this book review, we'll look at the author's description of starship subsystems and possible mission scenarios (Chapters 5&6).

Part 1: Book Review - Prospects for Interstellar Travel

Last week I received a copy in the mail of Prospects for Interstellar Travel by John H. Mauldin, 1992. So far I've finished reading the first four chapters and I'm impressed by the amount of thought that went into this book and I'm studying it in detail so decided to write up a comprehensive book review as it seems there aren't that many copies around these days available and helps me digest this book anyway. Although somewhat dated, most of the material is still relevant and covers the prospects and problems of interstellar travel and is highly readable with next to no maths in the main text, for those who like to see what the numbers have to say, there's a comprehensive Appendix as well.

This book is a good read for those who have wondered if it is feasible one day in the not so distant future for us to venture to nearby star systems and their exoplanets. John has done his homework in writing this book with many references along the way from earlier work by Forward, Johnson, Matloff to name a few. According to the short blurb about the author, John has worked at NASA in electronic power engineering for the Voyager missions among other things and has an engineering physics background. Let's get into it. Wherever I put in [CI: this means these are my own thoughts not the author's from the book.]

The book starts off with an introduction to general concepts dealing with interstellar travel explaining what destinations we might want to goto and within what Earth timeframe. This determines mission parameters in the first place (acceleration, speed, time, force, mass etc). Although Proxima Centauri at 4.2 Ly is our closest star, the author points out that "planning for a 10 Ly mission is more realistic" p6, as there are a dozen stars within this distance that could have habitable planets. It would be difficult to justify a mission to the Alpha Centauri system if we are just going to observe and do some scientific sightseeing because of the expected high cost of such a mission. [CI: I read a US magazine article that there's some chance NASA's funding to goto the moon might be scrapped. If the astronomers get lucky and confirm an Earth like planet in another star system within 10 Ly this would be one of the good reasons to justify the cost of such a mission]. There's a table where the author outlines 4 model missions on p9 which shows some of the problems especially the timeframes involved. Note that it makes sense to talk about mission timeframes in Earth years and not the relativistic dilated time for the travellers because we are (presumably) interested in science or material return to Earth.


Moving on to the first chapter, the author explores the basics of space travel explaining such concepts as force, thrust, acceleration, gees etc and newtonian orbital mechanics, the concepts of kinetic and potential energy with rocket propulsion as the focus. The author points out the inefficiencies of using chemical rockets but notes that "chemical rockets handle the most mass per unit of energy making high thrust good for liftoff (and not much else)" [CI: unfortunetly so far we have no other option that will provide this high thrust required to escape Earth's gravity well, more on this later]. I like rockets myself, there're big, they make lots of noise and they go fast ;-) however as the author points they are out of the question for interstellar travel due to the distances involved and the fuel/mass problem required by chemical rockets that they need to carry. It's pointed out that if a starship was 1000 tonnes, it would require at least 50 shuttle missions for the construction parts alone, in other words starships will not be built deep inside Earth's gravity well but in orbit or elsewhere in our solar system, unfortunetly this means having in place an extensive space infrastructure.

An outline is given on planetary gravitational sling shot mechanics and how this can be used to boost a starship's escape velocity to leave the solar system and also dicusses starship course corrections using stars: "If the speed is 1000Km/s (0.0033c), a starship aimed a close 10 million Km from the center of the star would be deflected about 2° from its original course" p32, and also mentions the interesting case of binary star flyby for speed reduction.

In the second chapter the author looks at advanced propulsion methods which carry more energy per kilogram than chemical fuels can or those that leave the fuel behind such as solar powered missions, nuclear fission/fusion, electric ion propulsion and solar sails. Past Jupiter's orbit the intensity of sunlight becomes too low to produce useful power for propulsion. Nuclear fission rockets have more than a million times more energy per kilogram that can be extracted from nuclear fuels such as uranium than from chemical fuels however for interstellar missions this still appears inadequate but looks useful for planetary missions in our solar system. Launching nuclear powered rockets from Earth's surface is not a good idea because of the problems dealing with radioactive waste and potential pollution hazards.

Fusion makes energy production 10 times better than nuclear fission making it a possible candidate for a starship powerplant and propulsion with less problems with radioactive byproducts. Fusion reactor fuel such as hydrogen, deuterium and helium-3 are available in low density in interstellar space and for any long interstellar mission living off the land makes sense. The author describes a fusion drive and how it could work and describes the Daedalus [CI: see Project Icarus] and Orion projects as case studies. A description is given for electric ion propulsion and mass ejector systems however these don't look promising for interstellar missions.

Photos: Right photo, bottom right is Daedalus.

Next we have the solar sail concept described in detail together with several references made to Gregory Matloff's earlier 80's work in this area. With solar sails the big advantage is that one doesn't have to carry fuel and we use sunlight's momentum for propulsion. Some of the problems outlined include the mass problem of the sail (Kg/m^2), the need to bring the solar sail very close to the Sun (to get the boost required to make interstellar trips viable) and issues with the structural fragility of the sail and connecting the sail to the starship. The mentioned designs so far are big (100Km diameter sail). As mentioned earlier past Jupiter's orbit the sunlight's intensity starts to become weak so everything needs to go just right when grazing the Sun's furnace. Towards the end of the chapter on p65, one sentence stood out which I'd like to quote:

"Like other missions involving long-term Earth support of a starship,

they require an extraordinary amount of social commitment."

In Part 2 of this book review, we'll look at Chapters 3 and 4 on relativity and more advanced propulsion systems described by the author.