Saturday, January 30, 2010

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).

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