Future deep space missions are expected to require extremely high data
transmission rates (on the order of a million bits[16] per second) to
relay the huge quantities of scientific and engineering information
gathered by the spacecraft. Higher data rates are necessary to increase
both the total capacity and the speed of transmission. By comparison,
the Mariner-4 spacecraft that sent back TV pictures of Mars had a data
rate of only eight bits per second—a hundred thousand times too small
for future missions. The use of lasers would mean that results could be
transmitted to earth in seconds instead of the 8 hours it took for the
photos to be sent from Mariner-4.
One of the problems to be solved in using lasers for deep space
communication, oddly enough, is that of pointing accuracy. Since the
beam of laser energy is narrow, it would be possible for the radiation
to miss the earth altogether and be lost entirely unless the laser were
pointed at the receiver with extreme precision. Aiming a gun at a target
50 yards away is one thing; aiming a laser from an unmanned spacecraft
100 million miles away is quite another. It is believed, however, that
present techniques can cope with the problem.
Another peculiarity of laser communication is that it will probably be
accomplished faster and more readily in space than here on earth.
Powerful though laser light may be, it is light and is therefore impeded
to some extent by our atmosphere even under good conditions. Data
transmissions of 20 and 30 miles have already been accomplished in good
weather with lasers.
But if you have ever tried to force a searchlight beam or shine
automobile headlights through heavy fog, rain, or snow, you will
appreciate the magnitude of the problem under these conditions. The use
of infrared frequencies helps to some extent, since infrared is somewhat
more penetrating, but the poor-weather problem is a serious one.
A possible solution is the use of “light pipes”, similar to the wave
guides already in use for certain microwave applications over short
distances. But as often happens, new developments create new needs; how,
for example, can we get the laser beam to stay centered in the pipe and
follow curves? A series of closely spaced lenses, about 1000 per mile,
probably would accomplish this, but too much light would be lost by
scattering from the many lens surfaces.
Scientists are experimenting with a new kind of “lens”, one that uses
variations in the density of gases to focus and guide the beam
automatically. Since there are no surfaces in the path of the light
beam, and since the gas is transparent and free of turbulence, the laser
beam is not appreciably weakened or scattered as it travels through the
pipe.
[Illustration: Figure 31 _Laser light beam being guided through a
“light pipe” by a gas “lens”. Heating coil (lower left) or mixture
of gases (lower right) are two possible ways of maintaining proper
density gradient in the gas._]
Figure 31 shows how the gas focusing principle might be used to guide a
beam through a curving pipe. The shading represents the density of the
gas. Several means have been developed to keep the gas denser in the
center than around the outside. When the pipe curves, the light beam
starts moving off the axis of the pipe. The gas then acts like a prism,
deflecting the light beam in the direction of the curvature of the
“prism”.
In communication between distant space and earth, a light pipe might be
a little cumbersome; hence it may prove necessary to set up an
intermediate orbiting relay station that will, particularly in cases of
poor weather, intercept the incoming laser beam and convert it to radio
frequencies that can penetrate our atmosphere with greater reliability.
Powering space-borne lasers will, of course, be a problem. Indeed one of
the major unsolved problems in production of spacecraft and long-term
satellites is the provision of an adequate supply of power. Fuel cells
and solar cells have helped but do not give the whole answer.[17]
One other approach has already been developed: a sun-pumped laser.
Sunlight focused onto the side of the laser (see Figure 32) provides the
pumping power, enabling the device to put out 1 watt of continuous
infrared radiation, enough for special space applications. Descendents
of this device could produce visible light if this is deemed desirable.
Another approach, using _chemical lasers_, is even more intriguing and
may have greater consequences. Chemical lasers will derive their energy
from their internal chemistry rather than from the outside. A mixture of
two chemicals may be all that is needed to initiate laser action aboard
a spacecraft or satellite. (Chemical lasers also offer the promise of
even greater concentrations of power than have been achieved heretofore,
which may make them useful in plasma research.)
With all these possibilities, it may still be that spacecraft will need
more power than is available on board. The narrow beam of the laser
offers one more fascinating possibility, especially in the case of
satellites relatively near earth. The light of a laser might actually be
used to beam energy to a receiver, either for immediate use or storage.
It would then become possible to “refuel” satellites at will, giving
them much greater capabilities.
If available laser power is great enough, laser beams might even be used
to push satellites back into their proper orbits when they begin to
wander off course, as they almost invariably do after a while.
[Illustration: Figure 32 _Artist’s rendering of sun-pumped laser as
it would operate in space. The sun’s rays are collected by a
parabolic reflector and are focused on the laser’s surface by two
cylindrical mirrors._]
Sun
Parabolic Collector
Hyperbolic-cylindric secondary mirror
Semi-circular-cylindric tertiary mirror
Laser beam