Prom:

To:

September 29, 1962

4L

Van Alien
Lapp -~ for further distribution

Jim-= T wonder if I can interest you in taking a close look at the problem of
detecting interstellar commnication (as a Q/S venture). It might be difficult
to find another group that does as well to pull together the very wide range

of interests and competence needed to inspire and overszee the progran. I have
no brilliant ideas about implementation, though I imagine we could set up a joint
venture with one of the aeroapace laboratories to engineer any pronosals we
might develop. Meanwhile, if you would also give it some push, T would hope that
the group of us might cxome up with some concrete ideas.

(Quite apart from Q/S, Carl Sagan is visiting Skanford for 4 months, taxakk starting
next week, with an intention, among others, of our surveying some of the fundamen-
tal issues in this field. He might be an excellent person to pull in as an additional.
consultant if we did decide to go more deeply into this vroject. )

Let us take it that the 0ZMA approach, groundbased detection of raiio-microwave
signals is already in hand by another group «~ though not necessarily beyond the
utility of further impetus.

I have had only two concrete ideas so far:

1). The *others*® (what should we call them?) have reached a standoff not too different
from our own. It is mich cheaper to listen than to send, so everyone is listening,
no one sending. This is an interesting barg aining situation; it may call for |

our invehhing the first kaar alerting signal, just to provoke a response (hopefully
less expensive for them than for us just now.) The cheapest alert I can think of

4s a string of nuclear detonations (thus wideband, pulse signals) fired from a
heliocentric spacecraft, as far from the sun a3 vossible to facilitate thetr
resolution from the solar background. Some of the necessary data may be classified
but Carl will be calculating the interstellar detectivity of such signals in various
spectral regions. We might well think how to dove a detonation to accentuate a
Signature; apart from this, I do not now propose this item for special 9/S con-
sideration.

2). The solar-terrestrial noise background is no help in searching for other sig-
nals, in any wavelength band. We may want 60 put our detectors in a well shielded
site -- the other side of the moon is an obvious sakemx choice. Now we need a
detector of very large aperture, not necessarily of very high res#lution (as these
desiderata tend to conflict with one another for very large systems). When we get
to the point of lunar surface exploration, a large lunar crater may have a suitable
figure for a reflector or antenna. Before then (and probably within the range of
bresent day technique) inflatable, orbiting structures may give us the best opportunity
for mounting very large detectors. I have in mind something like the 100-foot jor
larger) Echo balloon, but this would now be partially silvered to serve as a condensing
mirror.

Am I wrong in thinking that a figure accurate enough to be useful could be construc-
ted this way, if one did not ‘umtetak insist on the kind of angular resolution which
is requisite for the conventional aims of astronomy? As first choice, I have in mind
a scan for a laser type signal, though perhaps at shorter wavelengths than are accessible
to our own lasers just now (karxfextkkteke so that the stellar background would be cro-
portionally less).

Needless to say, I would not be forwarding these tentative suggestions if I did not
take the problem (and likelihood) of commnication with other intelligent life as
ax very serious challenge. When we get over the first hurdle of taking the problen
seriously, then just such groups as we constitute ought to make a systematic study of
imaginative thoughts how to go about it.

(P.S. A large collectors in earth orbit might not be useless for terrestrial
reconnaissance, say for UV~signatures of missile exhausts; whether a loweresolution
system would be beaten by the solar background, you will have to tell ne.)

SO