Defense Intelligence Reference Document An Introduction To The Statistical Drake Equation

UNCLASSIFIED//FOR OFFICIO k YEE a••k>/
An Introduction to the Statistical Drake Equation
1. Introduction
SETI (an acronym for "Search for Extraterrestrial Intelligence") is a relatively
new branch of scientific research, having begun only in 1959. Its goal is to
ascertain whether alien civilizations exist in the universe, how far from us
they exist, and possibly how much more advanced than us they may be.
As of 2009, the only physical tools we know that could help us get in touch
with aliens are the electromagnetic waves an alien civilization could emit and
we could detect. This forces us to use the largest radiotelescopes on Earth for
SETI research, because the higher our collecting area of electromagnetic
radiation is, the higher our sensitivity is (that is, the farther in space we can
probe). Yet, even by using the largest radiotelescopes on Earth (the 310-meter
dish at Arecibo, for instance), we cannot search for aliens beyond, say, a few
hundred light years away. This is a very, very small amount of space around us
within our galaxy, the Milky Way, that is about 100,000 light years in diameter.
Thus, current SETI can cover only a very tiny fraction of the galaxy, and it is
not surprising that in the past 50 years of SETI searches, NO extraterrestrial
civilization was discovered. Quite simply, we did not get far enough!
This demands the construction of much more powerful and radically new
radiotelescopes. Rather than big and heavy metal dishes, whose mechanical
problems hamper SETI research too much, we are now turning to "software
radiotelescopes," where a large number of small dishes (ATA = Allen
Telescope Array, and ALMA= Atacama Large Millimeter/submillimeter Array)
or even just of simple dipoles (LOFAR = Low Frequency Array) using state-of-
the-art electronics and very-high-speed computing can outperform the
classical radiotelescopes in many regards. The final dream in this field is the
SKA(= Square Kilometer Array), currently being designed and expected to be
completed around 2020.
2. The Key Question: How Far are They?
But still, the key question remains: how far are they?
Or, more correctly, how far do we expect the NEAREST extraterrestrial civilization to be
from the Solar System in the galaxy?
This question was first faced in a scientific manner back in 1961 by the same scientist
who also was the first experimental SETI radio astronomer ever: the American, Frank
Donald Drake (born 1930). He first considered the shape and size of the galaxy where
we are living: the Milky Way. This is a spiral galaxy measuring some 100,000 light
years in diameter and some 16,000 light years in thickness of the Galactic Disk at half-
way from its center. That is:
The diameter of the galaxy is (about) 100,000 light years, (abbreviated ly) i.e., its
radius, Rcutu\\'' is about 50,000 ly.
iv
UNCLASSIFIED//EiOAt OEiEil&l11J.k WliF &••LY