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

starsfrom/'18 to G5 could support advanced life, our
target list would be reduced to about one-third the
length we have been assuming. We expect the cutoff of
f(S) at the high temperature end of the main sequence to
occur at early type F stars since larger (and hotter) stars
have lifetimes less than 3 billion years we assume are
needed for the genesis and evolution of advanced life. At
the lower end, the tidal braking of planetary rotation
(because of the closeness to the primary star) may be the
limiting factor and, according to Dole (ref. 1), would set
the lower limit at about type KI or K2 stars. We feel this
lower limit is somewhat uncertain because tidal braking
is enormously dependent on the extent, shape, and
depth of ocean basins. It is extremely important to
establish this low end cutoff more precisely, for 90% of
all main sequence stars are type G5 and later. Since the
frequency of stars by mass goes as M-z -3the position of
the low end cutoff will greatly affect the length of our
target
list.
Another factor that may influence )'(S) considerably
is the variation of coronal activity with spectral class. As
noted in Chapter 2, the X-ray and UV flux of a star,
which depends on coronal activity, is an important
factor in atmospheric evolution. At present we have
good knowledge of the coronal activity of only one G
type star: our Sun. The visibility of normal stars with the
Cyclops array is examined in Appendix R, where it is
concluded that, if the microwave radiation of other stars
is increased over their black-body radiation in the same
ratio as for the quiet Sun, with a 3-kin effective antenna
diameter, the bandwidth and noise temperature of
Cyclops, and a 1-min integration time, about a thousand
stars could be detected at X = 10 cm and about four
times this many at X = 3 cm. Thus, Cyclops should
permit observations of the coronal activity of several
thousand nearby stars on a time scale short enough to
follow bursts and flares with good fidelity. This information
would be a valuable contribution to astronomy
and especially to the understanding of the evolution of
planetary
atmospheres.
There is also a possibility that with two large arrays,
one the size of Cyclops, separated by several thousand
kilometers, very long baseline interferometry could be
done on normal stars and their positions determined to
within ]0 -3 sec of arc or less. It is not clear that the
absolute positions could be determined to this accuracy,
but it might be possible to measure the relative positions
of several
stars this closely.
If so, Cyclops and its companion array would provide
a tool for studying the architecture of planetary systems.
At present this has been done optically only for
Barnard's star. (See Chap. 2.) It would be of tremendous
interest and importance if Cyclops could be used to
obtain the data for enough other stars to begin to form a
picture of the statistics of the architecture of planetary
systems. Such a study would require decades of observing,
because many planetary revolutions must be
observed and the periods of major planets are apt to be
quite long. Thus, we might make contact with intelligent
life before such a study could be completed. This would
give us far more direct and detailed information about at
least one other planetary system than we could hope to
get in any other
THE GALACTIC
way!
CENTER
If, for the reasons given in Chapter 2, interstellar
communication is already a reality, the question arises:
ls there any particularly likely direction in which to
search for beacons? In this connection, Joshua Lederberg
(private communication) has made an interesting
suggestion. In a way it is much like Cocconi and
Morrison's suggestion to use the hydrogen line in the
spectrum. Lederberg asks: Is there any single point in
the galaxy that is unique that might be a natural
cynosure for all races in the galaxy wherever they may
be? He suggests the galactic center is that point.
Following this clue we might conjecture that either
1. The galactic superculture has constructed a powerful
beacon at the galactic center (if indeed the
center is accessible), or
2. Participating members of the galactic community
are expected to radiate beacons not omnidirectionally
but rather in beams directed away from
(or toward) the galactic center. This greatly
reduces the required beacon power or increases
the range, or both.
We consider this suggestion to have merit and, while we
have ruled out a blind search of the entire sky, we do
recommend an area search over perhaps a one or two
degree cone about the galactic center. (Such a search
might also yield unexpected astronomical information.)
We also recommend a similar search about the antipodal
direction in case the strategy is to radiate toward the
center so that the receiver will not be bothered by the
high sky noise near the galactic center.
The difficulty with 2 is that it would tend to confine
contact to a spoke or spokes radiating from the center
since contact in a tangential direction would be difficult.
Nevertheless it would be an appropriate and cheap
strategy to attract new races, once interstellar communication
had already spread around the galaxy through the
use of omnidirectional beacons.
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