Why count 'em? Basically, asteroids leave a trail of rubbish behind them. This rubbish, if it encounters the Earth's atmosphere, becomes meteors. If you're looking for asteroids, look for a pattern of meteor showers over a period of years and eventually you'll get an idea of the orbits of some of the asteriods that cross the orbit of the Earth. That's the theory, anyway!    NASA is conducting at least one program to do exactly this, which (amongst many other things) uses observations from volunteers who operate simple meteor counting equipment around the world. NASA also uses a specially modified jumbo jet from time to time, one flight of which would probably cost more than the money and time spent by all volunteers over the past decade and maybe the next one, but if you've got the funding and the expertise, then why not flaunt it? Off-topic personal observation: Some people get all excited in a ghoulish sort of way about the possibility of the Earth being surprised by an asteroid collision. It's happened before and it'll happen again. Can we do much about it? Probably not, but it looks to me like a few people have identified a potential career path in Asteroid Management. Call me a cynic, but there's nothing like a good collision with an asteroid to jolt the average species into an awareness of what's important. Ask any dinosaur. Maybe it's time the Earth got rid of Homo sapiens in order to tidy up the mess that species has got it into. In the meantime, I'm having a nice time counting meteors. Detecting meteors Meteors that get into the Earth's path appear to be moving pretty fast compared to the speeds at which humans operate. The Earth wobbles along around the Sun at something like 100 000 km/h, so anything it happens to bump into is going to suffer. A grain of space dust suddenly finds itself rubbing against an increasingly dense collection of molecules around 100 km above the ground. This friction quickly gets converted to so much heat that electrons get stripped off some of the molecules in the vicinity and thus ionise a small patch of sky until things cool down. The cooling process may take only a fraction of a second if the dust particle is small, but larger ones generate more energy so it may take a few seconds before life's back to normal up there. Not just heat is generated: many particles get hot enough to produce light, which is why we can see the familiar streak when a meteor hits.    Each vaporising meteor leaves this brief signature high in the atmosphere. We could detect that light using our eyes or a camera, techniques which are fine as long as it's dark and not cloudy, and as long as the observer is looking at the right part of the sky and hasn't fallen asleep or frozen to death. Infrared detectors might also work, but are subject to the same limitations as visible light detectors in cloud or sunlight. Further down the electromagnetic spectrum things get more promising. Longer wavelengths aren't swamped by solar interference during the daytime and can penetrate cloud. It turns out that frequencies in the low VHF band, between 40 and 150 MHz are strongly reflected. The lower the frequency, the longer and stronger are the reflections. For decades radio amateurs have bounced short messages off meteor trails, but not everyone has the financial or technical resources to use specialised transmitters and receivers to detect meteor trails. What we really need is a reliable and powerful source of VHF signals and a simple receiving setup that can look after itself. No problem! All around the world there are thousands of 50-100 kW VHF transmitters pumping out TV and FM radio signals 24 hours a day between 60 and 108 MHz. Despite the best efforts of antenna designers, not all of the signal transmitted by these stations travel near the ground. Some of it goes up and, unless something gets in the way, continues off into space to become interstellar electronic pollution. Occasionally an aircraft reflects it back to the ground, causing ghosting on TV screens and flutter in FM receivers' sound.   Meteor reflecting FM signal to a receiver beyond the horizon Aircraft are seldom more than 10 km above the ground, so the signals aren't reflected very far. The normal range for a TV or FM station is a couple of hundred kilometres at best, and an aircraft reflection may double this. But a meteor trail is anywhere from 60 to 90 km above the ground, so can reflect signals up to 2000 km. There's little chance of direct reception by a receiver to the frequency of a transmitter more than 500 km away, or even reflections from aircraft. Any signals received would have to come either from a meteor trail reflection or from sporadic ionisation of the ionosphere's "E" layer due to solar activity. It's easy to tell the difference: sporadic E reception lasts for minutes or even hours, whereas meteor reflections seldom last longer than a couple of seconds. What about interference? TV video signals are AM, so are subject to interference from electrical noise. This can be really bad in the lower VHF bands, especially if there's a busy road or industrial complex nearby. FM receivers don't have this problems because just about all AM interference is removed by their limiter stage. In practical terms perhaps the most difficult source of interference to eradicate is cross-modulation in the receiver's front end from nearby powerful stations. In cities this can be a serious challenge to overcome. Some observers use preamps with band-pass filters, while others opt for more subtle approaches. Mine was to set up the observation site at the bottom of a valley about 80 km from the nearest powerful transmitter. Although not necessarily a cheap or convenient solution, it sure worked. Choosing a transmitter to monitor Listings of all Australian radio and TV stations are available for download in Excel or Access format from the ACMA. These listings are updated monthly and include latitude and longitude for each transmitter. Now it's time to take out your Jacaranda Atlas and try to find a transmitter between 750 and 1500 km from your location, ensuring it's using a frequency well clear of local stations. This is not a trivial task, but remember that in places like the USA and Europe it's almost impossible to find clear channels so consider yourself lucky. You need to step through every channel on your receiver (all 200 of them) and make a note of the ones that seem to be free from interference. I eventually opted for ABC FM on 88.3 MHz, transmitting from near Cootamundra, NSW with a power of 80 kW ERP. Its transmitter at Mt Ulandra is about 980 km from my observing location just north of Brisbane. Using a nationally networked station had the advantage in the early stages of setting up that its signal can be compared with the same content coming from a local transmitter. Otherwise it's pretty hard to identify a station when the bursts last less than a second, spaced minutes apart!
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