[sci.astro] ET Life (Astronomy Frequently Asked Questions) (6/8)
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Subject: [sci.astro] ET Life (Astronomy Frequently Asked Questions) (6/8)
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Summary: This posting addresses frequently asked questions about
extraterrestrial life and the search for it.
Last-modified: $Date: 1997/08/07 00:35:52 $
Version: $Revision: 2.4 $
URL: http://astrosun.tn.cornell.edu/students/lazio/sci.astro.html
Posting-frequency: semi-monthly (Wednesday)
Archive-name: astronomy/faq/part6
------------------------------
Subject: Introduction
sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.
However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.
This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is also available via anonymous
ftp in the directory <URL:ftp://seti.tn.cornell.edu/pub/lazio/> and it
is on the World Wide Web at
<URL:http://astrosun.tn.cornell.edu/students/lazio/sci.astro.html>.
Like many other FAQs it is also available from
<URL:ftp://rtfm.mit.edu/pub/usenet/news.answers>,
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and their worldwide mirrors.
Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio (lazio@spacenet.tn.cornell.edu).
------------------------------
Subject: Copyright
This document, as a collection, is Copyright 1995, 1996, 1997 by
T. Joseph W. Lazio (lazio@spacenet.tn.cornell.edu). The individual
articles are copyright by the individual authors listed. All rights
are reserved. Permission to use, copy and distribute this unmodified
document by any means and for any purpose EXCEPT PROFIT PURPOSES is
hereby granted, provided that both the above Copyright notice and this
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this FAQ by any means, included, but not limited to, printing, copying
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This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.
------------------------------
Subject: F.00 Extraterrestrial Life
[Dates in brackets are last edit.]
F.01 What is life?
F.02 Life in the Solar System
02.1 Is there life on Mars? [96-09-03]
02.2 Is there life in Jupiter (or Saturn)? [96-09-03]
02.3 Is there life on Jupiter's moon Europa? [96-09-03]
02.4 Is there life on Saturn's moon Titan? [97-08-05]
F.03 What is the Drake equation? [95-10-04]
F.04 What is the Fermi paradox? [95-12-28]
F.05 Could we detect extraterrestrial life? [96-11-20]
F.06 How far away could we detect radio transmissions? [96-09-03]
F.07 What's a Dyson sphere? [97-06-04]
F.08 What is happening with SETI now? [96-04-20]
See also the entry in Section G of the FAQ on the detection of
extrasolar planets.
------------------------------
Subject: F.01 What is life?
Author: none yet
------------------------------
Subject: F.02 Life in the Solar System
Within the past 100--150 years, the conventional wisdom regarding life
in the solar system (beside the Earth) has been on a roller coaster
ride. Life on other planets used to be considered likely.
Suggestions for sending messages to other planets included cutting
down huge tracts in the Siberian forests or filling and setting afire
trenches of kerosene in the Sahara. Lowell believed that he could see
evidence for a civilization on Mars.
During the Space Age the planets were explored with robotic craft.
The images and other measurements sent back by these craft convinced
most scientists that only the Earth harbored life.
With even more recent findings, the possibility of life that life
exists or existed elsewhere in the solar system is now being
re-examined.
------------------------------
Subject: F.02.1 Is there life on Mars?
Author: Steve Willner <swillner@cfa.harvard.edu>
The Viking landers found conditions on the surface of Mars unlikely to
support life as we know it. The mass spectrometer found too little
carbon, which is the basis for organic molecules. The chemistry is
apparently highly oxidizing as well. Some optimists have nevertheless
argued that there still might be life on Mars, either below the
surface or in surface regions not sampled by the landers, but most
scientists consider life on Mars quite unlikely. Evidence of surface
water suggests, however, that Mars had a wetter and possibly warmer
climate in the past, and life might have existed then. If so, there
might still be remnants (either living or fossil) today, but close
examination will be necessary to find out.
More recently, McKay et al. have invoked biological activity to
explain a number of features detected in a meteorite from Mars. See
<URL:http://www.fas.org/mars/> for additional information.
------------------------------
Subject: F.02.2 Is there life in Jupiter (or Saturn)?
Jupiter (and Saturn) has no solid surface, like the Earth. Rather the
density and temperature increase with depth. The lack of solid
surface need not be a deterrent to life, though, as many aquatic
animals (e.g., fish, jellyfish) never touch a solid surface.
There has been speculation that massive gas-bag organisms could exist
in Jupiter's atmosphere. These organisms might be something like
jellyfish, floating upon the atmospheric currents and eating either
each other or the organic materials formed in Jupiter's atmosphere.
------------------------------
Subject: F.02.3 Is there life on Jupiter's moon, Europa?
This article is adapted from NASA Press Releases.
In the late 1970's, NASA Voyager spacecraft imaged Europa. Its
surface was marked by complicated linear features, appearing like
cracks or huge fractures in the surface. No large craters (more than
five kilometers in diameter) were easily identifiable. One
explanation for this appearance is that the surface is a thin ice
crust overlying water or softer ice and that the linear features are
fractures in that crust. Galileo images have reinforced the idea that
Europa's surface is an ice-crust, showing places on Europa that
resemble ice floes in Earth's polar regions, along with suggestions of
geyser-like eruptions.
Europa's appearance could result from the stresses of the contorting
tidal effects of Jupiter's strong gravity (possibly combined with some
internal heat from decay of radioactive elements). If the warmth
generated by tidal heating is (or has been) enough to liquefy some
portion of Europa, then the moon may have environmental niches warm
and wet enough to host life. These niches might be similar to those
found near ocean-floor vents on the Earth.
------------------------------
Subject: F.02.4 Is there life on Saturn's moon Titan?
Author: T. Joseph W. Lazio <jlazio@patriot.net>
This material is extracted from the review article by Chyba &
McDonald (1995, Annual Review of Earth and Planetary Science).
Titan's atmosphere is a rich mix of nitrogen and methane, from which
organic molecules (i.e., those containing carbon, not necessarily
molecules in living organisms) can be formed. Indeed, there has been
speculation that Titan's atmosphere resembles that of Earth some 4
billion years ago. Complex organic chemistry can result from the
ultraviolet light from the Sun or from charged particle impacts on the
upper atmosphere. Unfortunately, Titan's great distance from the Sun
means that the surface temperature is so low that liquid water is
probably not present globally. Since we believe that liquid water is
probably necessary for the emergence of life, Titan is unlikely to
harbor any life. The impact of comets or asteroids on Titan may,
however, warm the surface enough that any water ice could melt. Such
"impact pools" could persist for as long as 1 thousand years,
potentially allowing life-like chemical reactions to occur.
------------------------------
Subject: F.03 What is the Drake equation?
Author: John Pike <johnpike@fas.org>, Bill Arnett <billa@znet.com>,
Steve Willner <swillner@cfa.harvard.edu>
There are various forms of it, but basically it is a means of doing
boundary calculations for the prevalence of intelligent life in the
universe. It might take the form of saying that if there are:
X stars in the Galaxy, of which
Y % have planets, of which
Z % can support life, on which
A % intelligent life has arisen, with
B representing the average duration of civilizations
then you fool around with the numbers to figure out how close on average
the nearest civilization is. There are various mathematical expressions
for this formula (see below), and there are variations on how many terms
the equations include.
The problem, of course, is that some of the variables are easy to pick
(e.g., stars in the Galaxy), some are under study (e.g., how many
stars have terrestrial-like planets), and others are just flat-out
wild guesses (e.g., duration of civilization, where we are currently
running an experiment to test this here on Terra of Sol).
One useful form says the number of detectable civilizations is:
N = R * fp * ne * fl * fi * fc * L
where
R = "the average rate of star formation in the region in question",
fp = "the fraction of stars that form planets"
ne = "the average number of planets hospitable to life per star"
fl = "the fraction of those planets where life actually emerges"
fi = "the fraction of life-bearing planets where life evolves into
intelligent beings"
fc = "the fraction of planets with intelligent creatures capable
of interstellar communication"
L = "the length of time that such a civilization remains
detectable".
(If you want some definition of civilization other than detectability,
just change your definition of fc and L accordingly.)
Can we provide reasonable estimates for any of the above numbers? The
"social/biological" quantities are at best speculative and aren't
appropriate for this newsgroup anyway. (See _Bioastronomy News_, Third
Quarter 1995 for biologist Ernst Mayr's rather negative view of these.
A copy of the article is at http://planetary.org/tps/mayr.html .) Even
the "astronomical" numbers, though determinable in principle, have
considerable uncertainty. Nevertheless, I will attempt to provide
reasonable estimates. I'll take the "region in question" to be the
Milky Way Galaxy and consider only cases "similar to" our solar system.
For R, I'm going to use only stars with luminosities between half and
double that of the Sun. Dimmer stars have a very small zone where
Earth-like temperatures will be found, and more luminous stars have
relatively short lifetimes. Near the Sun, there are about 4.5E-3 such
stars in a cubic parsec. I'm only going to consider stars in the
Galactic disk, which I take to have a scale height of 660 pc and scale
length of between 5 and 8 kpc. (Stars outside the disk either have
lower metallicity than the Sun or live in a very different environment
and may have formed in a different way.) The Sun is about 8 kpc from
the Galactic center, and thus in a region of lower than maximum star
density. Putting everything together, there ought to be around 1.4E9
stars in the class defined. This represents about 1% of the total mass
of the Galaxy. The age of the Sun is about 4.5E9 years, so the average
rate of formation R is about 0.3 "solar like stars" per year.
Planets are more problematic, since extrasolar planets cannot generally
be detected, but it is thought that their formation is a natural and
indeed inevitable part of star formation. For stars like the Sun, in
fact, there is either observational evidence or clear theoretical
justification for every stage of the planet formation process as it is
currently understood. We might therefore be tempted to take fp=1 (for
stars in the luminosity range defined), but we have to consider binary
stars. A second star may disrupt planetary orbits or may somehow
prevent planets forming in the first place. Because about 2/3 of the
relevant stars are in binary systems, I'm going to take fp=1/3.
Now we are pretty much out of the range of observation and into
speculation. It seems reasonable to take ne=1 or even 1.5 on the basis
of the Solar system (Earth and Mars), but a pessimist could surely take
a smaller number. You can insert your own values for the probabilities,
but if we arbitrarily set all of them equal to one
N <= 0.1 L
seems consistent with all known data.
A more detailed discussion of interpretation of the Drake equation and
the factors in it can be found in Issue 5 of SETIQuest.
------------------------------
Subject: F.04 What is the Fermi paradox?
Author: John Pike <johnpike@fas.org>,
Steve Willner <swillner@cfa.harvard.edu>
One of the problems that the Drake Equation produces is that if you take
reasonable (some would say optimistic) numbers for everything up to the
average duration of technological civilizations, then you are left with
three possibilities:
1. If such civilizations last a long time, "They" should be _here_
(leading either the the Flying Saucer hypothesis---they are here and
we are seeing them, or the Zoo Hypothesis---they are here and are
hiding in obedience to the Prime Directive, which they observe with
far greater fiqdelity than Captain Kirk could ever muster). -or-
2. If such civilizations last a long time, and "They" are not "here"
then it becomes necessary to explain why each and every technological
civilization has consistently chosen not to build starships. The
first civilization to build starships would spread across the entire
Galaxy on a timescale that is short relative to the age of the Galaxy.
Perhaps they lose interest in space flight and building starships
because they are spending all their time surfing the net. (Think about
it---the whole point of space flight is the proposition that there are
privileged spatial locations, and the whole point of the net is that
physical location is more or less irrelevant.) -or-
3. Such civilizations do not last a long time, and blow themselves up
or otherwise fall apart pretty quickly (... film at 11).
Thus the Drake Equation produces what is called the Fermi Paradox
(i.e., "Where are They?"), in that the implications of #3 and #2 are
not terribly encouraging to some folks, but the two flavors of #1 are
kinda hard to come to grips with.
An alternate version of 2 is that interstellar travel is far more
difficult than we think it is. Right now, it doesn't seem much beyond
the boundaries of current technology to launch "generation ships," which
amount to an O'Neill colony plus propulsion and power systems. An
alternative is robot probes with artificial intelligence; these don't
seem so difficult either. The Milky Way galaxy is well under 10^5 light
years in diameter and over 10^9 years old, so even travel beginning
fairly recently in Galactic history and proceeding well under the speed
of light ought to have filled the Galaxy by now. (Travel very near the
speed of light still seems very hard, but such high speed isn't
necessary to fill the Galaxy with life.) The paradox, then, is that we
don't observe evidence of anybody besides us.
------------------------------
Subject: F.05 Could we detect extraterrestrial life?
Author: Steve Willner <swillner@cfa.harvard.edu>
Yes, although present observations can do so only under optimistic
assumptions. Radio and optical searches currently underway are aimed
at detecting "beacons" built by putative advanced civilizations and
intended to attract attention. More sensitive searches (e.g., Project
Cyclops) that might detect normal activities of an advanced
civilization (similar for example to our military radars or TV
stations) have been proposed but so far not funded. No funding of
these is likely until the search for beacons is far closer to being
complete. Why get involved with the difficult until you are done with
the easy?
Ordinary astronomical observations are most unlikely to detect life.
The kinds of life we speculate about would be near stars, and the
light from the star would conceal most signs of life unless a special
effort is made to look for them.
Within the solar system, the Viking landers found conditions on the
surface of Mars unlikely to support life as we know it. The mass
spectrometer found too little carbon, which is the basis for organic
molecules. The chemistry is apparently highly oxidizing as well.
Some optimists have nevertheless argued that there still might be
life on Mars, either below the surface or in surface regions not
sampled by the landers, but most scientists consider life on Mars
quite unlikely. Evidence of surface water suggests, however, that
Mars had a wetter and possibly warmer climate in the past, and life
might have existed then. If so, there might still be remnants
(either living or fossil) today, but close examination will be
necessary to find out.
Other sites that conceivably could have life include the atmosphere
of Jupiter (and perhaps Saturn) and the presumed liquid water under
the surface ice of Jupiter's satellite Europa. Organisms living in
either place would have to be very different from anything we know on
Earth, and it's hard to know how one would even start to look for
them.
Concepts for specialized space missions that could detect Earth-like
planets and return spectral information on their atmospheres have been
suggested, and either NASA or ESA may launch such a mission some time
in the next two decades (see
<URL:http://techinfo.jpl.nasa.gov/www/ExNPS/HomePage.html> and
<URL:http://ast.star.rl.ac.uk/darwin/>). The evidence for life would
be detection of ozone (implying oxygen) in the planet's atmosphere.
While this would be strong evidence for life---oxygen in Earth's
atmosphere is thought to have come from life---it would not be
ironclad proof, as there may be some way an oxygen atmosphere could
form without life.
For more information, see references at the end of F.06. Also, check
out the SETI Institute Web site at <URL:http://www.seti-inst.edu>. A
long article on detectability of various signals and more is at
<http://www.setiquest.com/article1.htm>.
------------------------------
Subject: F.06 How far away could we detect radio transmissions?
Author: Al Aburto <aburto@nosc.mil>
Representative results are presented in Tables 1 and 2. The short answer is
(1) Detection of broadband signals from Earth such as AM radio,
FM radio, and television picture and sound would be extremely
difficult even at a fraction of a Light-Year distant from the
Sun. For example, a TV picture having 5 MHz of bandwidth and
5 MWatts of power could not be detected beyond 0.01 Light-Years
of the Sun even with a radio telescope with 100 times the
sensitivity of the 305 meter diameter Arecibo telescope.
(2) Detection of narrowband signals is more resonable out to
thousands of Light-Years distance from the Sun depending on the
transmitter's EIRP and the receiving antenna size.
(3) Instruments such as the Arecibo radio telescope could detect
narrowband signals originating thousands of Light-Years from the
Sun.
(4) A well designed 12 ft diameter amateur radio telescope could
detect narrowband signals from 30 to 300 Light-Years distance
assuming the EIRP of the transmitter is in the terawatt range.
What follows is a basic example for the estimation of radio and
microwave detection ranges of interest to SETI. Minimum signal
processing is assumed. For example an FFT can be used in the
narrowband case and a bandpass filter in the broadband case (with
center frequency at the right place of course). In addition it is
assumed that the bandwidth of the receiver (Br) is constrained such
that it is greater than or equal to the bandwidth of the transmitted
signal (Bt) (that is, Br >= Bt).
Assume a power Pt (watts) in bandwidth Bt (Hz) radiated isotropically.
At a distance of R (meters), this power will be uniformly distributed
(reduced) over a sphere of area: 4 * pi * R^2. The amount of this
power received by an antenna of effective area Aer with bandwidth Br
(Hz), where Br >= Bt, is therefore:
Pr = Aer * (Pt / (4 * pi * R^2))
If the transmitting antenna is directive (that is, most of the
available isotropic power is concentrated into a narrow beam) with
power gain Gt in the desired direction then:
Pr = Aer * ((Pt * Gt) / (4 * pi * R^2))
The transmit antenna gain, Gt, is given by the following expression:
Gt = Aet * (4 * pi / (w^2)), where
Aet = effective area of the transmitting antenna (m^2), and
w = wavelength (m) the antenna is tuned to.
f = c / w, where f is the frequency and c is the speed of light.
c = 2.99793E+08 (m/sec)
pi = 3.141592654
For a parabolic "dish" transmit or receive antenna:
Aer = nr * pi * dr^2 / 4, and
Aet = nt * pi * dt^2 / 4, where
nt = efficiency of the transmit antenna
nr = efficiency of the receive antenna
dt = diameter (m) of the transmit "dish" antenna.
dr = diameter (m) of the receive "dish" antenna.
Similarly, the receiver gain Gr is given by:
Gr = Aer * (4 * pi / (w^2)),
but it is not used explicitly in the range equation. Only the
effective area (Aer) intercepting the radiated energy at range R is
required.
The Nyquist noise, Pn, is given by:
Pn = k * Tsys * Br, where
k = Boltzmann's constant = 1.38054E-23 (joule/kelvin)
Tsys = is the system temperature (kelvins), and
Br = the receiver bandwidth (hertz).
The signal-to-noise ratio, snr, is thus given by:
snr = Pr / Pn.
If we average the output for a time t, in order to reduce the variance
of the noise, then one can improve the snr by a factor of
sqrt(Br * t). Thus:
snr = Pr * sqrt(Br * t) / Pn.
The factor Br*t is called the "time bandwidth product," of the receive
processing in this case, which we'll designate as:
twp = Br * t.
We'll designate the integration or averaging gain as:
twc = sqrt(twp).
Integration of the data (which means: twp = Br * t > 1, or
t > (1 / Br) ) makes sense for unmodulated "CW" signals that are
relatively stable over time in a relatively stationary (steady) noise
field. On the other hand, integration of the data does not make
sense for time-varying signals since this would distroy the
information content of the signal. Thus for a modulated signal
twp = Br * t = 1 is appropriate.
In any case the snr can be rewritten as:
snr = (Pt * Gt) * Aer * twc / (4 * pi * R^2 * Br * k * Tsys)
Pt * Gt is called the Effective Isotropic Radiated Power (EIRP) in
the transmitted signal of bandwidth Bt. So:
EIRP = Pt * Gt, and
snr = EIRP * Aer * twc / (4 * pi * R^2 * Br * k * Tsys)
This is a basic equation that one can use to estimate SETI detection
ranges.
#######################################################################
# If Rl is the number of meters in a Light-Year #
# (9.46055E+15 (m/LY)) then the detection range in Light-Years #
# is given by: #
# #
# R = sqrt[ EIRP * Aer * twc / (4 * pi * snr * Br * k * Tsys) ] / Rl #
# #
# If we wanted the range in Astronomical Units then replace Rl #
# with Ra = 1.496E+11 (m/AU). #
#######################################################################
Note that for maximum detection range (R) one would want the transmit
power (EIRP), the area of the receive antenna (Aer), and the time
bandwidth product (twp) to be as big as possible. In addition one
would want the snr, the receiver bandwidth (Br), and thus transmit
signal bandwidth (Bt), and the receive system temperature (Tsys) to be
as small as possible.
Now we are in a position to carry out some simple estimates of
detection range. These are shown in Table 1 for a variety of radio
transmitters. We'll assume the receiver is a parabolic type ("dish")
antenna, similar to Arecibo, with diameter dr = 305 m and an
efficiency of 70% (nr = 0.7). We'll also assume snr = 3 is required
for detection and that twp = Br * Tr = 1. Note that with more refined
signal processing, the detection ranges could perhaps be increased by
a factor of 2 to 3 over those shown in the table. An "educated" guess
for some of the parameter values, Tsys in particular, was taken as
indicated by the question marks in the table. As a reference note
that Jupiter is 5.2 AU from the Sun and Pluto 39.4 AU, while the
nearest star to the Sun is 4.3 LY away. Also note that signal
attenuation due to the Earth's atmosphere and ionosphere have been
ignored. AM radio for example, from Earth, is trapped within the
ionosphere.
The receive antenna area, Aer, is thus:
Aer = nr * pi * dr^2 / 4 = 51,143.2 m^2
Hence the detection range (Light Years) becomes:
R = 1.0478E-03 * sqrt[ EIRP / (Br * Tsys) ].
Table 1 Detection ranges of various EM emissions from Earth and the
Pioneer spacecraft assuming a 305 meter diameter parabolic
("dish") receive antenna, similar to the Arecibo radio
telescope. Assuming snr = 3, twp = Br * Tr = 1, nr = 0.7, and
dr = 305 meters.
-------------+--------------+-----------+--------+--------+-----------+
Source | Frequency | Bandwidth | Tsys | EIRP | Detection |
| Range | (Br) |(Kelvin)| | Range (R) |
-------------+--------------+-----------+--------+--------+-----------+
AM Radio | 530-1605 kHz | 10 kHz | 300 ? | 100 KW | 12 AU |
-------------+--------------+-----------+--------+--------+-----------+
FM Radio | 88-108 MHz | 150 kHz | 100 ? | 5 MW | 38 AU |
-------------+--------------+-----------+--------+--------+-----------+
UHF TV | 470-806 MHz | 6 MHz | 50 ? | 5 MW | 9 AU |
Picture | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
UHF TV | 470-806 MHz | 0.1 Hz | 50 ? | 5 MW | 1.0 LY |
Carrier | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
WSR-88D | 2.8 GHz | 0.63 MHz | 20 | 32 GW | 0.05 LY |
Weather Radar| | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo | 2.380 GHz | 0.01 Hz | 20 | 22 TW | 10,990 LY |
S-Band (CW) | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo | 2.380 GHz | 0.01 Hz | 20 | 1 TW | 2,343 LY |
S-Band (CW) | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo | 2.380 GHz | 0.01 Hz | 20 | 1 GW | 74 LY |
S-Band (CW) | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Pioneer 10 | 2.295 GHz | 1.0 Hz | 20 | 1.6 kW | 593 AU |
Carrier | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
It should be apparent then from these results that the detection of AM
radio, FM radio, or TV pictures much beyond the orbit of Pluto will be
extremely difficult even for an Arecibo like 305 meter diameter Radio
Telescope! Even a 3000 meter diameter Radio Telescope could not
detect the "I Love Lucy" TV show (re-runs) at a distance of 0.01
Light-Years!
It is only the narrowband high intensity emissions from Earth
(narrowband radar generally) that will be detectable at significant
ranges (greater than 1 LY). Perhaps they'll show up very much like
the narrowband, short duration, and non-repeating, signals observed by
our SETI telescopes. Perhaps we should document all these
"non-repeating" detections very carefully to see if any long term
spatial detection patterns show up.
Another question to consider is what an Amateur SETI radio telescope
might achieve in terms of detection ranges using narrowband FFT
processing. Detection ranges (LY) are given in Table 2 assuming a
12 ft (3.6576 m) dish antenna operating at 1.420 GHz, for various FFT
binwidths (Br), Tsys, snr, time bandwidth products (twp = Br*t), and
EIRP values. It appears from the table that effective amateur SETI
explorations can be conducted out beyond approximately 30 Light-years
provided the processing bandwidth is near the minimum (approximately
0.01 Hz), the system temperature is minimal (20 to 50 Degrees Kelvin),
and the EIRP of the source (transmitter) is greater than approximately
25 terawatts.
Table 2 Detection ranges (LY) for a 12 foot diameter amateur
radio telescope SETI system, operating at 1.420 GHz.
+-------------------------------+
| EIRP |
+-------+--------+------+-------+
| 100TW | 25TW | 1TW | 100GW |
-------+-------+----------+------+-------+--------+------+-------+
Br | Br*t | Tsys | snr | Detection Range |
(Hz) | | (kelvin) | | (LY) |
-------+-------+----------+------+-------+--------+------+-------+
0.01 | 2 | 20 | 3 | 334 | 168 | 33 | 11 |
-------+-------+----------+------+-------+--------+------+-------+
0.01 | 1 | 20 | 3 | 281 | 141 | 28 | 9 |
-------+-------+----------+------+-------+--------+------+-------+
0.01 | 2 | 50 | 3 | 211 | 106 | 21 | 7 |
-------+-------+----------+------+-------+--------+------+-------+
0.01 | 1 | 50 | 3 | 178 | 89 | 18 | 6 |
-------+-------+----------+------+-------+--------+------+-------+
0.05 | 2 | 20 | 3 | 150 | 75 | 15 | 5 |
-------+-------+----------+------+-------+--------+------+-------+
0.05 | 1 | 20 | 3 | 126 | 63 | 13 | 4 |
-------+-------+----------+------+-------+--------+------+-------+
0.01 | 1 | 20 | 16 | 122 | 61 | 12 | 4 |
-------+-------+----------+------+-------+--------+------+-------+
0.1 | 20 | 50 | 3 | 119 | 59 | 12 | 4 |
-------+-------+----------+------+-------+--------+------+-------+
0.01 | 1 | 50 | 16 | 77 | 39 | 8 | 2 |
-------+-------+----------+------+-------+--------+------+-------+
1.0 | 200 | 50 | 3 | 67 | 33 | 7 | 2 |
-------+-------+----------+------+-------+--------+------+-------+
0.05 | 1 | 50 | 16 | 34 | 17 | 3 | 1 |
-------+-------+----------+------+-------+--------+------+-------+
REFERENCES:
Radio Astronomy, John D. Kraus, 2nd edition, Cygnus-Quasar
Books, 1986, P.O. Box 85, Powell, Ohio, 43065.
Radio Astronomy, J. L. Steinberg, J. Lequeux, McGraw-Hill
Electronic Science Series, McGraw-Hill Book Company, Inc,
1963.
Project Cyclops, ISBN 0-9650707-0-0, Reprinted 1996, by the
SETI League and SETI Institute.
Extraterrestrial Civilizations, Problems of Interstellar
Communication, S. A. Kaplan, editor, 1971, NASA TT F-631
(TT 70-50081), page 88.
------------------------------
Subject: F.07 What's a Dyson spheres?
Author: Anders Sandberg <nv91-asa@nada.kth.se>
Freeman Dyson noted that one of the limiting resources for
civilizations is the amount of energy they can harness. He proposed
that an advanced civilization could harness a substantial fraction of
its sun's energy by enclosing the star in a shell which would capture
most of the radiation emitted by the star. That energy could then be
used to do work.
As originally proposed a Dyson sphere consisted of many solar
collectors in independent orbits. Many science fiction writers have
modified the idea to make a Dyson sphere one complete shell. In
addition to capturing all of the available energy from the star, such
a shell would have a huge surface area for living space. While
Dyson's original proposal of a number of solar collectors is stable,
this later idea of a complete shell is not stable. Without some
stablizing mechanism, even small forces, e.g., a meteor hit, would
cause the shell to drift and eventually hit the star. Also, the
stresses on a complete shell Dyson sphere are huge and no known
material has enough strength to be used in the construction of such a
shell.
There have been searches for Dyson spheres. Such searches typically
take place in the infrared. Because the shell is trapping energy from
the star, it will begin to heat up. At some point it will radiate as
much energy as it receives from the star. For a Dyson sphere with a
radius about the radius of Earth's orbit, most of the radiation
emitted by the shell should be in the infrared. Thus far, no search
has been successful.
Considerably more discussion of Dyson spheres is in the Dyson sphere
FAQ, <URL:http://www.student.nada.kth.se/~nv91-asa/dysonFAQ.html>.
------------------------------
Subject: F.08 What is happening with SETI now?
Author: Larry Klaes <larryk@cambridge.village.com>
Some of the following material is from SETIQuest Magazine, copyright
Helmers Publishing, and used by permission. For subscription or other
information, contact Helmers Publishing, 174 Concord Street,
Peterborough, NH 03458-0874. Phone (603) 924-9631, FAX (603) 924-7408,
Internet: sqinqnet@pixelacres.mv.com or see
http://www.setiquest.com/ .
Project BETA (Billion-channel ExtraTerrestrial Assay) is a radio search
begun 1995 October 30. It is sponsored by the Planetary Society and is
an upgraded version of Project META (Million...). (Actually META I; see
below for META II.) META I/BETA's observatory is the 26-meter radio
antenna at Harvard, Massachusetts. Information at
http://planetary.org/tps/meta_be.html seems to be old, but there are
some nice pictures at http://www.setiquest.com/beta.htm .
META II uses a 30-meter antenna at the Argentine Institute for Radio
Astronomy, near Buenos Aires, Argentina.
META I/II monitored 8.4 million channels at once with a spectral
resolution of 0.05 Hz, an instantaneous bandwidth of 0.4 MHz, a total
frequency coverage of 1.2 MHz, a maximum sensitivity of 7x10^-24 W m^-2,
and a combined sky coverage of 93 percent. After five years of
observations from the northern hemisphere and observing 6x10^13
different signals, META I found 34 candidates, or "alerts".
Unfortunately, the data are insufficient to determine their real origin.
Interestingly, the observed signals seem to cluster near the galactic
plane, where the major density of Milky Way stars dwell. META II, after
three years of observations and surveying the southern hemisphere sky
almost three times, found nineteen signals with similar characteristics
to the META I results. META II has also observed eighty nearby, main
sequence stars (less than fifty light years from the Sun) that have the
same physical characteristics as Earth's star. These observations were
performed using the tracking mode for periods of one hour each at two
different epochs.
On 1992 October 12, NASA began its first SETI program called
HRMS---High-Resolution Microwave Survey. Unfortunately for all,
Congress decided the project was spending way too much money---even
though it received less funds per year than your average big league
sports star or film actor---and cut all money to NASA for SETI work.
This act saved our national deficit by all of 0.0002 percent.
Fortunately, NASA SETI was saved as a private venture called Project
Phoenix and run by The SETI Institute. It operates between 1.0 and 3.2
GHz with 1 Hz resolution and 2.8E7 channels at a time. Earlier this
year they completed a six-month survey of the Southern sky from
Australia (no confirmed ETI signals) and are now trying to find another
radio observatory which will help them scan the Northern skies. More
details are in SETIQuest issue 3. The Project Phoenix home page is:
http://www.seti-inst.edu/phoenix/Welcome.html . They have lots of
general information about SETI as well as details of the survey.
Since 1973, Ohio State University has conducted a radio search with a
telescope consisting of a fixed parabolic reflector and a tiltable flat
reflector, each about 110 m wide and 30 m high. Information is
available at http://everest.eng.ohio-state.edu/~klein/ro/ or a longer
version in SETIQuest issue 3 (also at
http://www.setiquest.com/ohio.htm). The "wow" signal, detected in 1977,
had the appearance of an extraterrestrial signal but was seen only
briefly and never repeated. The latest news is bad, though. The
University administration has decided to let the landlord who owns the
property on which Big Ear resides tear down the radio telescopes and put
up condos and a golf course instead, starting in 1998. Ironically it
will cost more to tear the dishes down than to pay for their upkeep.
OSU SETI is considering its next step, Project Argus, at an undetermined
location.
The UC Berkeley SETI Program, SERENDIP (Search for Extraterrestrial
Radio Emissions from Nearby Developed Intelligent Populations) is an
ongoing scientific research effort aimed at detecting radio signals from
extraterrestrial civilizations. The project is the world's only
"piggyback" SETI system, operating alongside simultaneously conducted
conventional radio astronomy observations. SERENDIP is currently
piggybacking on the 1,000-foot dish at Arecibo Observatory in Puerto
Rico, the largest radio telescope in the world. Information at
http://albert.ssl.berkeley.edu/serendip/ , from which this paragraph was
extracted. SERENDIP operates at 430 MHz; more information is given in
SETIQuest issue 3.
Project BAMBI is an amateur SETI effort operating at a radio frequency
of 4 GHz. See SETIQuest issue 5 for a status report.
The Columbus Optical SETI Observatory uses visible light instead of
radio waves. More information in SETIQuest issue 4 and at
<URL:http://ourworld.compuserve.com/homepages/coseti_observatory/>.
Much of the work on "Optical SETI" comes from Dr. Stuart A. Kingsley
<skingsle@magnus.acs.ohio-state.edu>, who also maintains BBS on
Optical SETI.
The Planetary Society maintains a list of online SETI-related material
at http://planetary.org/tps/seti.html .
And of course SETIQuest magazine, Larry Klaes, Editor. See subscription
information above.
Other references:
Frank Drake, Dava Sobel, Is Anyone Out There: The Scientific
Search For Extraterrestrial Intelligence, 1992, Delacorte
Press, ISBN 0-385-30532-X.
Frank White, The SETI Factor, 1990, Walker Publishing Company,
Inc., ISBN 0-8027-1105-7.
Donald Goldsmith and Tobias Owen, The Search For Life in the
Universe, Second Edition, 1992, Addison-Wesley Publishing
Company, Inc., ISBN 0-201-56949-3.
Walter Sullivan, We Are Not Alone: The Continuing Search for
Extraterrestrial Intelligence, 1993, Dutton, ISBN
0-525-93674-2.
G. Seth Shostak, Editor, Progress In The Search For
Extraterrestrial Life, 1993 Bioastronomy Symposium, Santa
Cruz, California, 16--20 August 1993. Published in 1995 by The
Astronomical Society of the Pacific (ASP). ISBN 0-937707-93-7.
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