Saturday, January 30, 2010

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.

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