Wednesday, February 3, 2010

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.

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