17.6 - Interstellar Navigation

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

Without some means of navigational control, any interstellar transport system is useless. As we shall see presently, relativistic starship navigation is hardly a trivial affair.

At only 1-10%c there are few problems. Just set the crosshairs on the target star, the home star, and three reference stars to either side, and the ship’s navigator can calculate velocity and heading fairly exactly. Problems begin to crop up at higher speeds, however. Two distorting effects begin to dominate: Aberration and Doppler Shift.^2745,1160

Aberration causes stars to appear to be displaced forward into the direction of flight. The situation is analogous to raindrops streaking the windows of a speeding train. Although we know the rain is falling straight down, the streaks on the window run diagonally, slanting downward from the front as if the source was ahead rather than above. Aberration of starlight, similarly, causes stars to appear farther forward than they really are. At relativistic velocities the effect can be extreme. As the speed of light is approached, stars will appear to move to the front and huddle together in a small patch directly in the line of flight. The rest of the sky is black.

Doppler Shift applies to light as well as sound. The changing pitch of a moving siren as it passes the listener is an example of this effect. On board a starship, Doppler Shift will blue-shift light from approaching stars (looking forward) and red-shift light from receding stars (looking astern). So suns ahead of the vessel in the line of flight will become bluer in color; those behind will become redder.

At 37%c, a starship leaving Sol would no longer be able to see it. Sol’s light, severely red-shifted, would have moved into the infrared and would be invisible to human eyes. If the destination is Alpha Centauri that star would also be invisible, having been blue-shifted up into the ultraviolet range.

As velocity increases still more, a growing zone of darkness appears directly sternward. It grows larger as the ship picks up speed. A similar patch of starless blackness develops toward the bow. At 50%c, the cone of invisibility distends an angle of 30° forward and more than 60° astern. The only stars that are still visible are crammed into a "barrel" surrounding the starcraft. The forward rim of the Star Barrel is seemingly dominated by brilliant blue-white stars. Sweeping the eye upwards and rearward, the hue of star light changes from blue to green to yellow to orange to red, then to blackness. All the familiar constellations are compressed and distorted beyond recognition.

Mounting speed forces the Barrel slightly backwards, then forward again, compacting still narrower with even more vivid coloration. The Barrel has now become what Eugene Sänger once called the Starbow.²⁷⁸³ At 99%c the Starbow, now an annular rainbow-hued ribbon of color leading the spacecraft, is 12° wide with its forward edge raised up 23° from the line of flight. The rest of the sky it jet black. Precise navigation by external fixes has become utterly impossible, and the starship pilot must rely on a system of dead reckoning or inertial guidance.

Communications between starship and home planet become problemmatical as the vessel moves off at relativistic speeds. Not only will there be a growing time delay due to rapidly increasing distance,¹⁰⁹¹ but the frequency of the signals received will be altered. If the communication system uses laser transmitters tuned, say, to monochromatic green light at exactly 5000 Angstroms, then the changes in frequency at the receiver are as shown in Table 17.5. A receding starship sees the green light as infrared for speeds above 50%c, and at excessive suboptic velocities as microwaves. Conversely, an approaching vessel sees ultraviolet signals above 50%c and x-rays above 99.9%c.

Table 17.5 Wavelength of 5000 Angstrom Laser Light Communication Signals Received by a Relativistic Starship
Starship Velocity	Redshifted Signal (Starship Receding)	Blueshifted Signal (Starship Approaching)	Starship Velocity	Redshifted Signal (Starship Receding)	Blueshifted Signal (Starship Approaching)
(psol)	(Angstroms)	(Angstroms)	(psol)	(Angstroms)	(Angstroms)
0%c	5000	5000	99.9%c	223,500	110.
10%c	5500	4500	99.99%c	707,000	35.5
50%c	8500	2900	99.999%c	2,235,000	11.0
90%c	22,000	1150	99.9999%c	7,070,000	3.55
99%c	70,500	355	99.99999%c	22,350,000	1.10

There are many other hazards to interstellar navigation which we can only briefly mention here. Relativistic starcraft will be subject to radiation damage and erosion caused by the impact of interstellar dust and hydrogen atoms.²⁷⁶¹ Besides irradiation of the crew, there will be extreme heating effects on the starship forebody at near optic speeds -- without magnetic shielding, forebody surface temperature could reach 3010 K at 99.9%c in a 1 atom/cm³ interstellar medium.¹¹¹⁶ Besides the possibly devastating effect of even grain-sized meteorites, Oort Belts of from 10¹²-10¹⁵ cometary objects in the plane of the planetary system must be avoided by choosing superecliptic approach trajectories when entering alien stellar systems.²⁰³⁸ There is also the possible problem of encountering unnavigable "starfog" in dense galactic gas clouds,²⁸⁸⁵ and the danger of running into unbuoyed free-wandering black holes and neutron stars is ever-present.²²