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94.6 FM: radio meteor |
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Informations |
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Project:
94.6 FM: radio meteor |
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Developed By:
Francis Boulva |
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Type of Project:
Experiment |
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Category:
Physical science |
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Class:
Intermediate |
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Age of Participant:
15 |
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School:
Collège Jean-de-Brébeuf |
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Teacher:
Martine Brousseau |
Project presented at the 1998 Montreal regional
final of the Bell Science Fair
Selected for the 1998 Quebec final (Montreal) of the Bell Super Science
Fair
Selected for the 1998 Pan-Canadian Science Fair in Timmins, where the
participant won:
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a silver medal for a project in physical
science, intermediate level |
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a scholarship for best project communication,
intermediate level |
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outstanding project in physics |
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E.T. phone home? |
Many scientists, such as Carl Sagan (1934-1996),
have been obsessed for decades with the idea of making contact with another
intelligent life form. In an effort to meet extraterrestrials, they have
been searching through space with the help of powerful radio telescopes.
So far, they have had no luck, but maybe someday...
In the meantime, closer to home, amateurs can take part in radioastronomy
projects. One such project consists of detecting the passage of meteoroids
in the atmosphere through the reflection of FM radio waves on their meteoritic
trails. Essentially, you can listen to shooting stars! This project captured
my attention. I wondered whether I would be able to hear these particles
of dust and whether I could find a reliable way of calculating the number
of meteors that fall to the ground each day.
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Detecting meteors...
In theory |
Tens of millions of meteoroids are vaporized each day in our atmosphere.
They are vaporized as they travel through our atmosphere at an average
speed of 100 000 km/hr. An in-depth study has shown that the smallest
ones dissipate at an average altitude of 70 km, while the larger ones
tend to vaporize higher up. Air compression causes the atoms that come
into contact with the meteoroids to ionize. As these atoms return to
an unexcited state, light is produced. The column of air, often several
kilometres long, that has been ionized has the ability to reflect radio
waves. It is this ionization that makes it possible to detect meteors.
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The radio method |
Simply tune in to a faraway radio station (between 800 and 1600 km away)
whose frequency is not used by any local station. Usually, it is impossible
to detect a radio signal from a distant transmitter. When a meteor appears
(i.e. when a meteoroid passes through the upper atmosphere), however,
the radio waves emitted by the radio station are reflected by the ionized
column of air. As a result, the signal is detected for a brief moment.
There are, however, other atmospheric phenomena, such as polar auroras
and certain types of clouds, that cause signals to be detected. Radio
waves can also be detected during temperature inversions, when they become
trapped between masses of cold and warm air. In such cases, the wave
can travel several hundred kilometres.
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Experimenting...
Equipment |
The paraphernalia needed to listen to meteors is
not as complex as you might imagine. I acquired a Yagi antenna (an outdoor
FM antenna often used in rural areas). I connected it to a conventional
analog FM receiver using a simple two-conductor cable. In order to analyze
my results in a reliable manner, I acquired a graphing apparatus. The
tracing mechanism is connected to the sound system in place of the speaker.
Instead of causing a membrane to vibrate to produce sound, the electrical
impulses cause a needle to move over a piece of graph paper at a speed
of 25.4 cm (10 in.) per hour.
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Syntonization |
Choosing a frequency used outside of Montreal, at
some 1000 km from home, was not an easy task. I obtained a list of all
the radio stations in Canada from the Toronto Radio Marketing Bureau.
This helped me choose from among a wide range of frequencies. The signal
emitted had to be very powerful, that is, around 100 kW. It was a difficult
task because practically the entire FM band is used in Montreal. I finally
opted for two different stations, both in Halifax. I was finally ready
and was all ears!
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Listening sessions |
I chose three nights in early December to listen
for sporadic meteors. I chose nighttime for two reasons:
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There are more meteors at night, between 2:00 a.m. and 10:00
a.m., since this part of the earth faces forward (like a car windshield).
As a result, meteors fall directly on us, as opposed to during the evening,
when they have to “catch up” to the earth. |
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| (b) |
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Then, I wanted to try to define a constant to represent increases
in meteor occurrences in the atmosphere, with respect to time.
I listened during two major meteor showers, the Geminids and Ursids,
and one last time (for sporadic meteors) in mid-January. Each time, I
listened between 7:00 p.m. and 7:00 a.m.
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Technical difficulties |
I encountered a few problems during my observation
periods. First, the receiver (because it was analog) tended to lose the
correct frequency. One way of correcting the problem was to connect my
antenna to a digital radio. Unfortunately, for reasons still unknown,
the tracer was not compatible with this receiver. I was therefore obliged
to use the analog radio. The second problem occurred on a commercial
level. Since the FM band is extremely popular, it was very hard to find
an unused frequency. I was forced to use the “FM mute” function
to eliminate unwanted background noise. During my first night of observation,
the neighbouring frequency was being taken up by a local university radio
station. As a result of interference, my listening was limited to the
period from 11:00 p.m. to 6:00 a.m. I also tried changing frequencies,
but without luck.
I admit that I had pretty high expectations as far as results are concerned.
I was convinced that it would be easy to distinguish meteors from other
atmospheric phenomena and to count them. I was wrong!
Results: successes, failures and improvements
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The signal |
Firstly, it is important to know how to perceive
the signal received and what it looks like on paper. When passing through
the atmosphere, a meteor sounds like a “ping” that lasts
a fraction of a second. Sometimes, if the meteor is larger, it is possible
to hear (for 15 to 30 seconds) a faint piece of music or the voice of
a radio announcer on the air at the time. On paper, the trace appears
as a peak with a 1-cm amplitude. It may be somewhat shorter or longer.
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A successful listening session (Dec. 6-7, 1998) |
My first listening session was by far the most successful.
You could clearly distinguish meteors from other atmospheric phenomena.
The auroras and ionized clouds were easy to identify; the needle jumped
up rapidly and dropped a long time after, creating a trace that resembled
a bridge. There was a period of low meteor activity between 11:00 p.m.
and 2:00 a.m. and a period of higher activity between 3:00 a.m. and 5:00
a.m. This partially confirms the theoretical data that indicate that
many meteors travel through the atmosphere in the early hours of the
morning.
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A difficult analysis |
Unfortunately, I was unable to quantify all my data.
In some places, the traces indicating meteors were so numerous and clustered
together that they were impossible to count. After some thought, I even
concluded that not all the lines represented meteors; some were the result
of a weak signal from a neighbouring frequency that created interference.
It is entirely possible and quite probable, however, that hidden among
this confusion of lines were traces revealing the occurrence of dozens
of meteors. This is why, given the tremendous number of lines, I illustrated
my results using two graphs. To try to remedy the problem of overcrowded
traces, I decided to increase the speed of the paper feed. This did not
result in creating more space between traces; on the contrary, there
were more. It did, however, enable me to count smaller isolated traces,
which had a typical meteor profile.
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Possible improvements |
In order to improve my experiment, it would have
been preferable to find a way of connecting my tracer to a digitial receiver.
It also would have been better to install my equipment outside the city,
far from the electromagnetic pollution of the FM band. Furthermore, it
would have been interesting to calculate the duration of each meteor.
The American Meteor Society noted that a signal lasting more than five
seconds is caused by a meteor that is visually bright (magnitude of -1
or more).
It would be rather bold of me to say that my results correspond to the
established average, since the curves that I obtained do not reveal any
notable constants to represent increases in the average hourly rate of
meteors. Usually, when the earth is not being bombarded with meteors,
the radio method makes it possible to detect six or seven meteors an
hour around 6:00 p.m.
Obviously, this number can reach 50 to 200 meteors an hour during meteor
showers.
This being said, rarely do meteor showers enable people to see as many
shooting stars as were observed on the night of November 12, 1833. That
night, observers saw up to 550 meteors per minute!
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The role of amateurs |
When compared with the visual observation method,
the radio “listening” method offers several advantages. Firstly,
you can listen day or night. Radio observation can even be done when
it rains or snows. On the coldest winter nights, observers can remain
snug in their homes. It can also be done during gibbous or full moons,
when visual observation is either difficult or impossible. These advantages
have resulted in groups of amateur radioastronomers listening more often
for space particles.
These types of projects enable amateur astronomers to make serious contributions
to the acquisition and expansion of scientific knowledge. Did you know,
for example, that amateur astronomers David Levy and Thomas Bopp were
responsible for making professional scientists aware of new comets? Large
networks of radio telescopes, such as the Arecibo radio telescope in
Puerto Rico and others in New Mexico, do not have their antennas pointed
toward such minuscule dust particles. That’s why amateurs have
to lend an ear.
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© 2002, Conseil de développement du loisir scientifique (CDLS). This
document is distributed by the Conseil de développement du loisir scientifique.
For more information, visit our Web site at www.cdls.qc.ca. |
The opinions expressed
in this section are those of the authors and do not necessarily
reflect the opinions of Merck Frosst or its employees. |
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