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.