Project Cyclops, A Design... - Department of Earth and Planetary ...

Butdoesbyourtechnology andourassessment ofthe
joint search strategy permit us to make these assumptions?
We think so. We believe that we can pinpoint the
likely stars both as to spectral type and range with a
reasonable optical search program concurrent with the
radio search. We believe we can construct receivers that
will display power spectra over 100 MHz of bandwidth
with a resolution better than 1 Hz. Finally, we believe
that leakage radiation is most likely to be detected at the
low end of the microwave window, and that beacon
signals are most likely to be found in a relatively narrow
range at the low end of the window, for reasons that are
given below.
LEAKAGE
SIGNALS
Electromagnetically, Earth is at present a noisy
planet. We radiate hundreds of megawatts and much of
this power is at frequencies for which the ionosphere is
quite transparent. This, of course, raises the question of
whether or not we might eavesdrop on the signals
another race transmits for its own purposes. There is
then no need to assume the existence of intentional
beacons and at first glance the probability of detection
appears to be greatly increased.
Our interstellar radiation became significant about 20
years ago with the advent of VHF TV broadcasting and
more recently has increased with the expansion of TV
allocations into the UHF band. Today we are surrounded
in space with a sphere of radiation some 20
light-years in radius, and the energy density in this
sphere is growing annually. How long this buildup will
continue is anyone's guess. Cable TV is replacing direct
reception in metropolitan areas where bad reflections
and shadowing exist, and in many rural areas shadowed
by mountains. However, the economics do not favor
cable TV in the normal service areas of broadcast
stations, where good reception exists. On this basis, one
might conclude that powerful TV radiation would be an
enduring phenomenon.
Satellite broadcasting appears to be a greater longterm
threat to our TV leakage than cable TV. A UHF
transmitter in synchronous orbit using present transmission
standards need only radiate a few kilowatts to cover
the entire United States. A fair fraction of this power
would be reflected back into space by the Earth but,
because far fewer stations of lower power would be
needed, the resulting leakage would be negligible compared
with our present leakage.
Nevertheless, it is of interest to calculate how far into
space the present radiation level of Earth might be
detectable. Our TV stations radiate about 50 kW.
Assuming a grey field (signal amplitude halfway between
black level and white level) the effective carrier power is
about 20 kW. The antennas typically have 13 dB of gain
as a result of vertical directivity, so the radiation is
confined between a plane tangent to the Earth and a
cone whose elements have an elevation angle of about
6° . As the earth rotates at 15°/hour this sheet of
radiation sweeps across the celestial sphere. For a station
at latitude 0 the beam would take a time (6/15) sec 0 hours
to scan a given star on the celestial equator. Thus 20 min
is a reasonable average time. Using a receiver with a 20 ° K
total noise temperature, an antenna 5 km in diameter,
a bandwidth of 0.1 HzJand integrating for 1200 sec, we
find from equation (11) that the range limit is 50
light-years. Actually, this is a somewhat pessimistic
figure since many stations, and often several on each
channel, could be received at the same time and proper
data processing could make use of the total power. We
conclude that if we keep on broadcasting TV for another
century, Earth will be visible out to something on the
order of 100 light-years, which could announce our
existence to beings on any of the 1000 or more likely
stellar systems within that range.
To beings that detected us, there would not be doubt
for very long that the signal was the work of man, not
nature. They would observe that the signals (1) had
highly monochromatic components, (2)were distributed
systematically in slots across the spectrum, (3) appeared
and disappeared with great regularity (in particular,
a 24-hour cycle would stand out) and (4) exhibited
a sinusoidal frequency modulation whose period was
proper for the annual motion of a minor planet, and
whose fractional frequency variation Af/f was the same
for all signals. The annual Doppler and daily periodicity
would identify the signals as being of planetary origin
while the monochromaticity and regularity of spacing
would identify them as artificial in origin.
We conclude that leakage signals are a possible means
for the detection of other life. However, the longevity of
their emission is very uncertain, and their low power
compared with an intentional beacon restricts their
detection
BEACONS
range significantly.
One can imagine several reasons why an intelligent
race might construct a beacon (or even many beacons)
but perhaps the strongest reason is to facilitate the
'The Doppler shift would be less than 0.01 Hz during the
observing period, but the frequency instability of the source
might cause larger drifts.
59