Associated Press, November 26, 1962
LOS ANGELES. (AP) - Some of the best scientific minds in the country were stumped when a slender, dark-haired young man chalked on the blackboard this equation:
N equals R FP NE FL FI FC L.
The speaker was Dr. Carl Sagan, a 28 year-old assistant professor of astronomy at Harvard University.
His audience consisted of several hundred members of the American Rocket Society, gathered for his luncheon address.
The equation was his way of expressing the mathematical probability that intelligent beings from outer space have visited earth.
Sagan soberly explained that in his equation N Stands for the number of advanced technical civilizations in the universe possessing the capability of interstellar communication.
R is the mean rate of star formation averaged over the lifetime of the galaxy.
FP is the fraction of stars with planetary systems.
NE is the mean number of planets in each system with environments favorable for the origin of life.
FL is the fraction of such planets on which life does develop.
FI is the fraction of such inhabited planets on which intelligent life with manipulative abilities rises during the lifetime of the local sun.
FC is the fraction of planets populated by intelligent beings on which advanced technical civilizations rises.
And L is the lifetime of this technical civilization.
Sagan said information in his formula is based on current estimates by astronomers. In making calculations, he assigned each symbol an arbitrary numerical value.
As expressed in numbers, Sagan said, the formula means that at least 1 million of the 100 billion stars in our Milky Way galaxy have planets which have developed civilizations capable of travel between the stars.
"Let's say that each of these civilizations sends out one interstellar expedition per year," he said.
"That means that every star, such as our sun, would be visited at least once every million years. In some systems where these beings found life, they would make more frequent visits. There's a strong probability, then, that they have visited earth every few thousand years.
"It is not out of the question that artifacts of these visits still exist or even that some kind of base is maintained, possibly automatically, within the solar system, to provide continuity for successive expeditions.
"Because of weathering and the possibility of detection and interference by the inhabitants of earth it would be preferable not to erect such a base on the earth's surface. The moon seems one reasonable alternative."
"Forthcoming photographic reconnaissance of the moon from space vehicles - particularly of the back - might bear these possibilities in mind."
At a news conference Sagan predicted man himself would be capable of interstellar flight at close to the speed of light "within a century or two."
Asked if he believed in flying saucers, he said: "I do believe there are objects which have hot be identified."
