Thursday, February 4, 2010

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.

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