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This Defense Intelligence Reference Document, dated 11 March 2010, was prepared by the Defense Intelligence Agency's Defense Warning Office under the Advanced Aerospace Weapon System Applications Program. It introduces the Statistical Drake Equation, which treats each Drake factor as a random variable with a mean value and a standard deviation. Using its example inputs, the paper estimates that the nearest extraterrestrial civilization lies between 1,361 and 3,979 light years away with 75% probability. The author's 2008 International Astronautical Congress paper is attached as an appendix.
From the source: Release of 2026-09-18 Incident: 3/11/10, Las Vegas, Nevada. Released with redactions. This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD introduces the Drake Equation, a well-known thought framework for estimating how many communicative extraterrestrial civilizations might exist in the galaxy. It reformulates the equation in statistical terms, arguing that the usual approach of assigning fixed values to its variables is too simplistic because major inputs are uncertain and are better modeled as probability distributions. Using that approach, it concludes that, if one accepts the underlying logic of the Drake Equation, the estimated number of communicating civilizations should be treated as a range of possible values, and that the likely distance between neighboring civilizations can likewise be expressed statistically rather than as a single figure. The document is primarily a mathematical and methodological exercise, and its worked examples rely on assumed values to illustrate the framework rather than to establish a firm astrophysical estimate. Overall, it is an attempt to formalize uncertainty within the Drake framework rather than an attempt to bound the actual likelihood, prevalence, or proximity of extraterrestrial civilizations.
UNCLASSIFIED/ /fOR OFFI&IAk Wlii QPU,¥ We know Ns, the number of stars in the Milky Way galaxy, fairly well, by careful counts of stars in a small but representative region of the sky. It is a few hundred billion; some recent estimates place it at 4 x 1011 . Very few of these stars are of the massive short lived variety that squander their reserves of thermonuclear fuel. The great majority have lifetimes of billions or more years in which they are shining stably, providing a suitable energy source for the energy and evolution of life on nearby planets. There is evidence that planets are a frequent accompaniment of star formation: in the satellite systems of Jupiter, Saturn and Uranus, which are like miniature solar systems; in theories of the origin of the planets; in studies of double stars; in observations of accretion disks around stars; and is some preliminary investigations of gravitational perturbations of nearby stars. 1 Many, perhaps even most, stars may have planets. We take the fraction of stars that have planets, fp, as roughly equa l to 1/3. Then the total number of planetary systems in the galaxy would be Ns fp ~ 1.3 x 10 11 (the symbol ~ means "approximately equal to"). If each system were to have about ten planets, as ours does, the tota l number of worlds in the galaxy would be more than a trillion, a vast arena for the cosmic drama. In our own solar system there are several bodies t hat may be suitab le for life of some sort: the Earth certainly, and perhaps Mars, Titan and Jupiter. Once life originates, it tends to be very adaptable and tenacious. There must be many different environments suitable for life in a given planetary system. But conservatively we choose ne=2. Then the number of planets in the galaxy su itable for life becomes Ns fp ne ~3 x 1011 . Experiments show that under the most common cosmic conditions the molecular basis of life is read ily made, the building blocks of molecules able to make copies of themselves. We are now on less certain grounds; there may, for example, be impediments in the evolution of the genetic code, although I think this is unlikely over billions of years of primeval chemistry . We choose fl~ 1/3, implying a total number of planets in the Milky Way on which life has arisen at least once as Ns fp ne fl~ 1 x 1011 , a hundred billion inhabited worlds . That in itself is a remarkable conclusion. But we are not yet fin ished. The choices of fi and fc are more difficult. On the one hand, many individually unlikely steps had to occur in biologica l evolution and human history for our present intelligence and technology to develop. On the other hand, there must be quite different pathways to an advanced civilization of specified capab ilities. Considering the apparent difficulty in the evolution of large organisms, represented by the Cambrian explosion, let us choose fix fc = 1/100, meaning that only 1 per cent of planets on wh ich life arises actually produce a technical civilization. This estimate represents some middle ground among the varying scientific options. Some think that the equivalent of the step from the emergence of trilobites to the domestication of fire goes like a shot in all planetary systems; others th ink t hat, even given ten or fifteen billion years, the evolution of a technical civi lization is unlikely. This is not a subject on wh ich we can do much experimentation as long as our investigations are limited to a single planet. Multiplying 1 Carl Sagan was writ ings these lines back in the 1970's, when no extrasolar planets had been discovered yet. The first such discovery occurred in 1995, wh en Michel Mayor and Didier Queloz, working at the "Observatoire de Haute Provence" in France, discovered the first extrasolar planet orbiting the nearby star 51 Peg. This first extrasolar planet was hence named 51 Peg B. Many more extrasolar planets were discovered around nearby stars ever since . As of Apri l 2009, 347 extrasolar planets (exoplanets) are listed in the Extrasolar Planets Encyclopaed ia. UNCLASSIFIED/ /POil Offl@IAL W&i 9Nls¥ 9
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Official release, from the pursue collection. The PDF is mirrored here; the original link is above. 55 pages are in the text index: search them above, or from the library's search.