Re: Planetary Defense


Alan Lothian wrote:
On Jan 10, 7:49 pm, a...@xxxxxxxxxx (Andrew Robert Breen) wrote:
In article <1168444395.476326.173...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,


The most detectable things would be big scientific radars

<snippaggio, and thanks for that, Andy>

I still want to know what the plausible limit of detection of such
things might be. A multi-megawatt radar may not be something you want
to stand in front of (IIRC there wasn't a lot of green growing grass at
the pointy end of Fylingdales) but is its yoctowatt[1] or wattever is
left at 100 LY going to show up on screens and make aliens light up and
say tilt?


[1] I believe I may be the first person to introduce the term yoctowatt
to smn. It's a stone legit SI unit, amounting to some (indeed, quite
precisely) 10 ^-24 watts. Yes, I know you knew that, Andy. I've only
just discovered that a femtowatt isn't the energy used by a woman
fighting a chap off. (For this particular erroneous value of femtowatt
(glorious typos quite apparent), a kiloton is a fair equivalent, but
Our Lady of the Hotdesking may be able to farther enlighten me.)

But I'm not at all happy at the idea of Nasty Reptilian Aliens from
Zeta Reticulae tuning in their yoctawatt receivers and picking up my
mobile phone calls. I am reasonably certain that the Laws of Physics,
and especially the Second Law of Thermodynamics, make this most
unlikely, but a chap has to be careful these days. I mean, all I said
was "Nice ass, babe" and the next thing you know, 250 years later, a
giant Conquest Fleet is on the way. Doubtless preceded by a tiresome
flurry of .99c rocks. Which is where we get into yottawatts. I'll just
get me coat, no need for unseemly violence.

http://www.computing.edu.au/~bvk/astronomy/HET608/essay/

Armed with Equation 2, and knowing the typical transmitting power of
terrestrial signals, we're now in a position to determine how far into
deep space Earth's leaked electromagnetic radiation could be detected,
assuming that it was possible to estimate the basic characteristics of
the receiving instrument. Typical results are shown in Table 1 and
Figure 3 for an Arecibo-like detector. In keeping with the estimates of
Aburto and Woolley, the Signal to Noise Ratio for all calculations was
taken to be 25 since this is between the SNR of SETI@home (SNR=22) and
the maximum SNR used by Project META (SNR=33).

Data table unreproducable


TABLE 1.
The detection range of various terrestrial signals, calculated using
Equation 2 and assuming a remote Arecibo-like detector. Data has been
adapted from Aburto and Woolley

FIGURE 3.
Log plot showing the detection range results for an Arecibo like-dish
and various terrestrial transmission types assuming an Arecibo-like
detector. The minimum detection range to receive a signal on Alpha
Centauri and Vega is indicated with arrows.

Hypothetically, assume that the Arecibo telescope was put on the back
of a starship making its way into deep space. If it was possible to
point the telescope back towards Earth, how far could the starship
travel and still be able to detect terrestrial electromagnetic
radiation leaking into space? If we were planning on catching up on our
favorite soap, cheering for the home footie team, or grooving to the
top ten, the answer is not very far.

Neglecting atmospheric effects, Table 1 and Figure 3 show the
following:

* An AM radio broadcast could only be detected out to 0.0074
Astronomical Units (AU).
* FM Radio could be detected out to 5.4 AU.
* A 5 Megawatt UHF television picture could be detected out to 2.5
AU, although the carrier wave could be detected much further; out to
0.3 light years.
* The Pioneer 11 can only be detected to 120 AU.

To put this into perspective, consider the following:

* 1 light year is equivalent to 63,240 AU.
* Jupiter is 5.2 AU from the Sun.
* Alpha Centauri is 4.3 light years away.
* Vega is 26.3 light years from Earth.

On our hypothetical starship cruise, we'd be hard pressed to enjoy
entertainment leaked from Earth while cruising though the outer reaches
of our own Solar System, much less from in the vicinity of the Sun's
nearest stellar neighbor.

We'd have better luck detecting occasional military or range finding
radar with characteristics more like those shown in the Arecibo S-Band
columns of Table 1. These signals are of significantly higher power and
transmitted in a manner that concentrates the energy into a relatively
narrow directional beam. Even if our hypothetical starship was lucky
enough to be along the beam's line of sight, current SETI protocols
would probably reject such a signal since it would be of short duration
and not likely to be verified by other observers.

In 1977, SETI researchers operating the Big Ear Radio Observatory at
the University of Ohio recorded a Gaussian signal of great intensity.
The researcher that inspected the computer output from the observation
was so impressed that he wrote "WOW!" in the printout margin. Despite
over 50 repeat attempts, the signal was never observed again. One can
only speculate about the source of the mystery signal. Perhaps it was a
leaked radar signal from an extraterrestrial source. Even in the
absence of any other evidence to the contrary, however, scientists are
forced to invoke the principle of Occam's razor. This asserts that all
things being equal, the simplest explanation is usually the correct
one. Without other supporting evidence, the simplest explanation is
that the "WOW!" signal was not produced by an alien civilization, but
was merely a chance reflection from a source of terrestrial origin.
<more>

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