Part 4: Book Review - Prospects for Interstellar Travel

We move on to Chapter 7 of the book which I found particularly very interesting. There doesn't seem to be much written concerning Interstellar Navigation and the various position fixing methods interstellar voyagers could use (the author names this as "astrogation") so let's have a close look.

An explanation is given on the basics of the celestial sphere and how conventional star map coordinates are setup from our Earth based vantage. It is natural to use angular positions of stars (ie Right Ascension and Declination) however for navigation purposes in interstellar space we may need a rectilinear coordinate system. It is noted that no standard 3D coordinate system in meter or Ly has been established for star listing positions because astronomers haven't planned on interstellar travel but this isn't a problem because spherical coordinates can be easily converted to x,y,z rectilinear sun centered coordinates [CI: for those curious read Spherical Astronomy]. This also applies to the starship's "celestial sphere" when the navigator measures star angular positions and needs to convert these to rectilinear x,y,z coordinates for a position fix on the chart. Moving to the case of the starship in interstellar space: "No absolute reference or coordinate system exists for starship travel or anything else, with the partial exception of the uniform (to 1 part per million or better) background microwave radiation.", in other words we can use any coordinate system we like, Earth centered, Sun centered, Galactic coordinates or other (such as pulsar grids) however as the author points out it makes sense to use a Sun centered system because our Sun is moving 22 Km/s (that's Kilometers per second) in a specific direction compared to the other stars and gas and the travelling ship will somehwat share this motion.

Image: Our galaxy with Sun centered galactic coordinates. (Caltech)

Typical motion of other stars is given at 10 Km/s relative to us or a 0.003 Ly shift in 100 years so we cannot assume they are fixed with respect to the Sun. To give us a perspective on things, it is mentioned that the Sun along with local stars orbit around our galaxy's center at approximately 300 Km/s and our galaxy is also moving about [CI: at 552 Km/s relative to the photons of the background microwave radiation towards the Great Attractor]. Back to our little corner of our galaxy, as the author explains, these previous motions don't affect local interstellar navigation for the starship and we don't have to measure these [CI: however measuring these would be necessary for intergalactic travel, one does wonder sometimes if out of all those 100 billion or so galaxies in our part of the observable universe if there are any beings travelling between these galaxies, will humans one day be able to venture to the Andromeda Galaxy for eg? highly unlikely]. Some further issues outlined include the accuracy of star positions as seen in the sky and their stellar distances. These should be updated wherever possible using probes for interstellar scout missions and send over the data back to Earth as distances to most cataloged stars are unknown or rough figures. Star positions must have an accuracy of at least 0.0001 Ly before a probe or starship is sent on a major mission. The starship "can update its map with new observations, calculate new star positions according to their known motions, update those motions, calculate starship motion in real time (proper and Earth), and use relativistic mathematics for greatest accuracy." Most of these tasks are ideally suited for a navigational computer linked to telescopes/spectrometers and sensors throughout the ship coupled with a 3D virtual star map showing to the crew position, heading and other navigational data. An interesting point made here is: "The first big starship should not be required to acquire accurate information along the way as it may find too late that a major course correction is needed. Course corrections of the order of 1 part in 10,000 can be made as the starship proceeds, but larger corrections are costly at high speed."

It is also noted that as far as getting lost, human voyagers would be able to monitor starship progress and recheck the position of the Sun and destination however for a probe that depends on computers and sensors: "any probe which has an error in orientation due to malfunction of steering jets or gyro, or another failure, must acquire data on bright stars and sort out which ones are to be used for guidance before correcting its orientation. Probes must use an assortment of stars for fixes and might need to measure brightnesses and spectra and compare with prepared desciptions to identify them. Getting lost is unlikely for human missions closer than 100 Ly but always a serious problem for probes."

The faster the starship is travelling, the more pronounced is starlight aberration (apparent position change of the stars due to the finite speed of light c). This must be calculated from the speed of the starship or if the aberration is known, the speed of the starship can be calculated: "At 0.1c aberration causes an apparent shift forward of stars by about 6° for those located to the sides, and less for stars toward front and rear. The apparent brightness is affected by high speed. At 0.1c intensity is increased about 20% front and decreased by 20% behind". [CI: Checkout What would a relativistic interstellar traveller see?]

Image: our view of the stars changes the faster we go due to relativistic effects which will need to be calculated, apart from the position shifts of the stars, note the brightness changes (Physics FAQ)

The author moves on to discuss starship speed measurements which can be made by doppler shift in the spectral lines from any stars or even better using the doppler shifts for known pulsars which send highly regular (millisecond to second range) radio pulses unique to each pulsar and have been accurately measured by radio astronomers (to 6 digit accuracy or better). A pair of radio telescopes on a starship could also be used for position fixing. [CI: Recent studies for a GPS-like position fixing method for use in our solar system and beyond using X-ray pulsars for even greater accuracy is ongoing]. Other navigational instruments mentioned include inertial guidance devices and sophisticated 3-axes gyroscopes using ring lasers: "if accurate measures of acceleration are fed to a computer from the gyro in all three dimensions, it can calculate from prior information on position and velocity the present location without needing outside measurements (relativistic also)." and of course the obligatory atomic clock with an accuracy of 1 part in a trillion or better. [CI: a three axis magnetometer would also be handy for interstellar magnetic field measurements but also could be used for orientation of the starship as a backup system to using stars or pulsars if the magnetic fields are well charted in the space that's navigated ie a sophisticated ship's compass. Checkout this paper: The Orientation of the Local Interstellar Magnetic Field although magnetic deviations onboard the starship may be quite large because of the high energy devices that would be found onboard.]

Another important point mentioned by the author is on the assumption: "that the propulsion force is applied in the desired direction of travel. If force direction differs from the intended direction by a small amount, an increasing error in direction occurs. For example, misalignment by 1" arc results in 100 million Km error after 10 Ly." and goes on to mention that fusion exhaust or photon reflection with large energies aren't perfectly aligned systems just like chemical rockets where the ejected material isn't exactly centered on axis which results in off axis propulsion and these are corrected for by the guidance system or in the case of the starship, telescopes locked on the structure which can detect any changes in direction from a set of star positions. "The average long-term error in direction can be corrected, but the short-term fluctuations should only be measured, not corrected". [CI: similar to the autopilot on boats, everytime the boat goes through a wave, if the autopilot moved the rudder, the steering ram would be working overtime unnecessarily so there's a "wait and see" delay setting.]

Moving on from navigation issues, the author looks at starship manoeuvers that may be required to dodge large micrometer size dust particles along the way for eg and significant changes in course headings will require substantial propulsion energy: "A 6 degrees change requires about 10% additional speed (and about 20% more energy). No way is known to recover momentum from one direction and apply it to another." and points out that changing direction isn't just a matter of rotating the starship by a small angle as it would still continue in the same direction as before until the main drive is used substantially. For minor course corrections mention is made of an inertial wheel and possibly of chemical steering jets. As it was pointed out in Chapter 4, the possible use of interstellar magnetic fields to change course has been discussed using the Lorentz force: "For a radius of turn of 1 Ly at 0.03c, slower than earlier examples yet still very fast, a 1000 tonne starship must use 1 million coulombs." with the mentioned wire requiring to be over 1 million Km long, this whole approach doesn't seem feasible compared to carrying more propulsion onboard.

The author then looks at active detection methods for detecting what's ahead in the first place. We need this information early to give enough time for the ship to alter its course. Just like radar systems, the system would transmit signals ahead and wait for reflected pulses to deduce distance, direction and size of the object however it's pointed out that the best detection is done by observing information coming from distant objects as there is no waiting time however cold dark matter emits only very small amounts of radiation not enough to be easily detectable. Some gases do not emit radio or light waves even if UV light strikes their atoms or molecules and radar cannot make matter respond, it only relflects or scatters.

Fortunetly lasers here come in handy: "Energetic laser light, x-rays, electrons, and neutrons can cause response from distant material to help identify it and determine its composition, density, speed, and temperature." however getting enough intensity for x-rays, electrons and neutrons is much more difficult compared to radio, radar and light. In order to detect sizes ranging from large dust particles to large rocks, radio waves with a wavelength of around 1mm is best and if we use two or more transmitting antennas we get better resolution, for 1Km separation we would get a beam size of 1" arc. The author points out that we still have the problem of dust erosion of the small antennas and these are even more difficult to protect compared to a single 100m dish. Another system example is given: "a system with 1 Km effective aperture can detect objects smaller than 1 mm and locate to an accuracy of 10 m in 10,000 Km. At 10,000 Km/s, there would be a 1 second warning to shift the direction of travel to miss an object. Two gees sideways would be required to shift 10 m in this time, but this amount of drive is probably not available". One option offered as an early warning system is the use of a probe which is travelling far ahead of the starship pushed by an ion drive and powered by the ship via a long cable. Once detected though, the feasible option given by the author is to demolish it by "zapping it" with a high-power laser. There is time for the laser system to confirm the target before main zap, if the starship motion is nonrelativistic. "At relativistic speeds there may not be time for detection and response to objects ahead". [CI: for relativistic speeds, it's not suggested by the author but use of an expendable detachable shield far ahead of the ship may be an option to "clean the way ahead" for the ship (still keeping its main erosion shield on the ship). Let the expendable shield take the damage rather than the vital ship itself]

It's pointed out that lasers would be very useful in detection and possibly zapping objects ahead. An infrared laser is better at detecting finer dust than millimeter radar can due to Rayleigh scattering and because of the typical sizes of dust grains the best wavelength is infrared, larger wavelengths tend to diffract around these objects. The laser light also needs to arrive at the object with enough intensity to excite the atoms in the object for a detectable energy signature for spectrographic analysis.

Image: Porous chondrite interplanetary dust particle, running into one of these at relativistic speeds will cause erosion or some damage to the starship (Institut für Planetologie and University of Washington).

The following paragraph deals with passive observations and makes the case that apart from the need to make accurate observations for interstellar navigation, interstellar and planetary studies, the sensitive scientific instruments mentioned in the previous chapter would also be useful for searching for any signs of Extra-Terrestrial (ET) Intelligence (SETI) however as pointed out: "Observation enroute will be difficult at high speeds because of interference from impinging interstellar hydrogen and dust. Most data must be collected from behind the front shielding." Several frequencies are outlined for observations in the radio spectrum and points out that at a frequency of 10 GHz, the natural noise is the lowest. Bandwidth used (the range of frequency used for one signal) is also important. The narrower the bandwitdth of the signal, the more it stands out from the background noise and the further it can be detected. We can achieve 1 Hz bandwidth or better and so could the ETs. However if broadband receivers are used, the signal could get lost if it doesn't have enough resolution in the spectrum. "Noise in deep space is due to synchroton radiation from electrons all over the galaxy at low frequencies, to background radiation at intermediate radio frequencies, and to quantum noise at high frequencies." and anything warmer than 3 Kelvin emits more radiation than the background. Hyrdrogen atoms emit radiation at 1420 Mhz (21cm) weakly and the author points out that it was first thought that ETs would choose this frequency to broadcast as it is also relatively queit. Extensive searches for unnatural signals haven't been found. Signal leakage from ET civilisations from ordinary activities is more difficult because the fequencies may be in a noisier band: "It has been estimated that if another civilization leaked TV carrier (1 MW typical, 0.1 Hz bandwidth) and radar pulses like ours does, then our astronomers could detect their leakage at about 30 Ly with our largest radio dish, 300 meters at Arecibo, Puerto Rico. Starship radio dishes limited to 100 m diameter might be able to find similar radio leakage from civilizations just a few lightyears away, a marginal capability."

Photo: Arecibo Observatory (NAIC)

The possibility of ETs using laser pulses to send signals is also mentioned and studies have shown that these can be picked out of the starlight if they are distinct from the background light. Further considerations are then given on to communications between the starship and base: "A transmitter with a 100 m dish must put out about 4000 W per cycle of bandwidth at 30 cm to be barely detected 10 Ly away in a 1 m^2 receiver. Only about 0.5 W of transmission in a 1 Hz channel is needed for threshold detection in a receiver at the focus of a 100 m dish across 10 Ly!". For the Daedalus project, the study specified a 2.6 MW transmitter for data transmission at 1 Megabit per second at 2 or 3 Ghz from 6 Ly with a 40 m dish. A 100 m dish is quite large for a starship and we have the dust damage issue to contend with, the Daedalus plan was to use the dead fusion chamber as the main dish to get around this problem. Some studies have also looked into using lasers for optical communications as well.

I'll leave out Chapter 8 "Technological Requirements and Hazards" for the next part of this book review as we have covered quite a lot of material in this part alone.

Captain InterStellar's Big Day Out: walk to the lighthouses

With the busy season here on Sydney Harbour finished, I'm getting more days off to relax or spend more time with my other Physics and Compass Adjuster studies. Went for a big walk today at South Head (the southern side of the entrance to Sydney Harbour), gave my new DSLR camera a workout and did some peaceful study with the ocean for inspiration and a nice cool sea breeze :-) Caught the ferry from Circular Quay to Watsons Bay and first headed off to checkout Macquarie Lighthouse. This is the first lighthouse you'll see approaching Sydney from sea with the light at 105m above sea level and also is the brightest. The other one I checked out was Hornby Lighthouse at South Head with its nice red stripe colour scheme:

 

 The walk along the cliffs is spectacular and you'll see many remains of the fortifications that were placed with heavy artillery for defense. The canons where no longer needed when the missile era came to age. At the Gap, the only remains of the famous shipwreck of the Dunbar was the recovered admiralty anchor: 

As I was contemplating the ocean, a quote from Darwin came to mind:

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.

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).

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.

Australia Day 2010

It was another great day to be on the water on Australia Day with just about anything that floats on Sydney Harbour out for the day. Several flying machines turned up as well. Some shots from my camera (click on photos for bigger version):

The Known Universe

Nuts and bolts (15/12/09)

Some interesting items I came across recently worth a look at:

Interstellar Propulsion Research: Realistic Possibilities and Idealistic Dreams by Les Johnson gives an introductory rundown on the current status of possible interstellar missions and propulsion options. Apart from the mentioned warp drive (update: read this and this), all of the options are based on sound physics however all of them have severe technical and engineering difficulties to overcome if they are going to happen one day. Ironically getting the hardware required into orbit for these big spacecraft could be the biggest hurdle to overcome as using chemical rockets is expensive. The above options are based on our current understanding of Physics and we still have a lot to learn how nature really works.

The Final Frontier: The Science of Star Trek, interview with Lawrence Krauss. I read the first version of his book years ago and found it quite stimulating reading should interest all you Star Trek fans out there. I saw the world premiere opening of the movie at the Sydney Opera House and got a few autographs myself from the (new) Spock, Sulu and JJ Abrams,  I liked the movie.

A Blueprint for a Quantum Propulsion Machine, here's also a review from Paul Gilster on this paper by Alexander Feigel. It remains to be seen if these so called "magneto-electric particles" do change the momentum of the quantum vacuum and thereby provide a means to change the orientation of a spacecraft without using propellant although many do this already using gyroscopes for eg however the physics if sound would be very interesting as this would provide another avenue to study the Quantum Vacuum. Another paper on this worth reading is here.

Been busy working on Sydney Harbour most days day and night as it is the busy season here. Some recent photos: Nice sailing boat in Farm Cove and checkout this sailing boat skipper going under the Anzac bridge, got his air draft right! Big tow job Bradley's Head outbound (click on photos for larger version).