How can we estimate the number of technological civilizations that might exist among the stars?

How can we estimate the number of technological civilizations that might exist among the stars?

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PHYS 105
Unit 4

Application 4 – SETI and You
Part 1: The Drake Equation

As our technology and searching got more intense in the 20th Century, Frank Drake designed what
became the “Drake Equation” to begin to explore what the odds of discovering alien civilizations
might be based on a series of assumptions.

How can we estimate the number of
technological civilizations that might exist
among the stars? While working as a radio
astronomer at the National Radio
Astronomy Observatory in Green Bank,
West Virginia, Dr. Frank Drake (now
Chairman of the Board of the SETI
Institute) conceived an approach to bound
the terms involved in estimating the
number of technological civilizations that
may exist in our galaxy. The Drake
Equation, as it has become known, was
first presented by Drake in 1961 and
identifies specific factors thought to play a
role in the development of such

civilizations. Although there is no unique solution to this equation, it is a generally accepted tool used
by the scientific community to examine these factors.

That these numbers are assumptions and not hard facts is important, because the value that each
person receives when they calculate the equation is different. Coming up with numbers for
civilizations, especially at different technological stages is extremely dicey. We are the only case we
know of – are we typical? Kinda hard to say.


• Watch this video – – Carl Sagan
explains the Drake Equation and why it is useful.

• Are we alone? Calculate the odds using this Drake Equation applet. The suggested values
are listed under the “more” at the end of each sentence.


1. Using the calculator above, make 5 different predictions of N. Vary only 2 of the 7 variables
(use some of the recommended values for the other 5, either using ones from the book or from
another website). Report which variables you changed, why you chose those particular values
for them, and your 5 values of N here (a table would be nice).

2. What pattern did you see in the variation of N based on your changing values?

3. Can you think of another way to calculate the odds of finding a civilization other than the Drake
Equation? Find one on the Net, maybe? How is it different from the Drake Equation? In your
opinion, which is more valid?

4. Of the 7 values, which 2 do you think are the most up in the air in terms of what we think they
should be? Why?

5. Based on these values for the Drake Equation, do you think it likely that we will find another
civilization? Support your argument with data from this calculator.

Part 2: Project Phoenix – Signals From Space

Where would YOU look for signs of extraterrestrial intelligence (SETI)? This is the question
astronomers involved in the selection of target stars for Project Phoenix have been asking.

Project Phoenix was the world’s most sensitive and comprehensive search for extraterrestrial
intelligence. It was an effort to detect extraterrestrial civilizations by listening for radio signals that are
either being deliberately beamed our way, or are inadvertently transmitted from another planet.
Phoenix is the successor to the ambitious NASA SETI program that was cancelled by a budget-
conscious Congress in 1993. Phoenix began observations in February, 1995 using the Parkes 210
foot radio telescope in New South Wales, Australia. This is the largest radio telescope in the Southern

Following the southern observing campaign, the project turned its attention to northern stars.
Appropriately, this phase brought the search back to its roots at the National Radio Astronomy
Observatory in Green Bank, West Virginia. The 140 Foot Telescope is only a short distance from the
antenna used by Frank Drake in Project Ozma. Project Phoenix operated in Green Bank from
September 1996 through April 1998, using the telescope about 50% of the time. As the primary
instrument in Green Bank, the antenna was shared with other astronomers.

Phoenix doesn’t scan the whole sky. Rather, it scrutinizes the vicinities of nearby, sun-like stars. Such
stars are most likely to host long-lived planets capable of supporting life. We naturally include stars
that are known to have planets. There are about one thousand stars targeted for observation by
Project Phoenix. All are within 200 light-years distance.

Because millions of radio channels are simultaneously monitored by Phoenix, most of the “listening”
is done by computers. Nonetheless, astronomers are required to make critical decisions about signals
that look intriguing.

Phoenix looks for signals between 1,000 and 3,000 MHz. Signals that are at only one spot on the
radio dial (narrow-band signals) are the “signature” of an intelligent transmission. The spectrum
searched by Phoenix is broken into very narrow 1 Hz-wide channels, so two billion channels are
examined for each target star.

Observations are currently being made during two three-week sessions each year using the 1,000
foot radio telescope at Arecibo, in Puerto Rico. During the observing sessions, the astronomer on
duty post reports.

When the project concluded in 2004, Phoenix had examined about all of the stars on its “hit list.” So
far, no clearly extraterrestrial transmissions have been found. But the faint whine that would betray an
alien civilization might be heard tomorrow.

Read more about Project Phoenix here.

Questions (search for this with Google or something like it. Don’t plagiarize the sites – say it in your
own words)

1. How many stars are part of Project Phoenix?
2. What types of stars are we looking at? Why those and not others?
3. Why did Phoenix look at signals in the range that were changing quickly? Why would living

sources make those signals?
4. Project Phoenix concentrates on nearby stars, but some astronomers argue that we are more

likely to find distant civilizations, no matter what the Drake Equation says. How should this
change how we search for SETI signals? What assumptions about location, stars, and signal
are different in this theory?

5. Project Phoenix is fairly limited by money and equipment, but here is the future of SETI. What
will change from Project Phoenix? Spectral types that are searched?
Distance? Directions? Frequencies?

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