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.