Meteors
From MariachiWiki
Contents |
Introduction
(by Mike Inglis)
A brilliant meteor, called a fireball, may weigh many kilograms, but even a meteor weighing less than a gram can produce a beautiful trail. Some of these visitors from space are large enough to survive (at least partially) their trip through the atmosphere and impact the ground as meteorites. Fireballs are sometimes followed by trails of light that persist for up to 30 minutes; some, called bolides, explode with a loud thunderous sound.
Question: how can a particle the size of a grain of sand produce such a spectacular sight?
Answer: the speed at which the meteoroid enters the Earth's atmosphere.
Many meteoroids travel at 60-70 kilometers per second. For comparison, the Space Shuttle moves around the earth at about 8 kilometers per second.
During its trip through the atmosphere, meteoroids collide with air molecules, knocking away meteor material [or ablating, as it is called] and ripping electrons from the meteor. When these same atoms recapture electrons, they emit light, the color depending on both the temperature and material of the meteor.
One can usually see a few meteors on any given night, at any location, at any hour. These are sporadic meteors. However, periodically there occur meteor showers, with hundreds of meteors emanating from the same apparent spot in the sky – the Radiant. These showers may last from a few hours to several days and are usually associated with comet paths, and the debris expelled by the comet.
One can make predictions as to the dates and times of many meteor showers, as they repeat every year when the earth passes through the path of a comet
Meteor observing is easy and great fun. Just choose a dark location, take something comfortable to sit on, lie back, and watch the show! Keen meteor observers go a step further and often use many different techniques to capture these swift moving objects: cameras to record the meteor trails, telescopes to find faint meteors, and radio telescopes to detect showers that may occur during the day!
Every day as many as 4 billion meteors, most miniscule in size, fall to Earth. Their total mass may be several tons, a large amount, but this is negligible when compared to the Earth's total mass of 6,600,000,000,000,000,000,000 tons.
July 2007
Meteors For July
Meteor Activity for July 2007
The midsummer overnight twilight persists for most of the month, but by mid to late July, the nights are showing signs of growing darker again. Twilight and exams often deter watchers from observing till late July, by when many school and college long summer vacations are underway of course. This year, July's full Moon ruins the interesting late month near-ecliptic shower peaks, though this does augur well for the Perseid maximum a fortnight later, in August. Some early Perseid activity should be seen in less moonlit skies by mid to late July, however. Included among this month's notes are two radiant drift charts, one showing the two main southern-sky showers and the nearby Antihelion Source, the other the Perseids.
Shower of the Month: The δ Aquarids (SDA)
July-August brings some of the stronger near-ecliptic meteor showers during the year. The strongest of these is the δ Aquarids (previously called the "Southern δ Aquarids", hence the continued use of the abbreviation "SDA" for them). Their maximum Zenithal Hourly Rates (ZHRs) of about 15-20 are usually achieved for a day or two around July 28. Like the α Capricornids and Antihelion Source, the SDA radiant is above the horizon throughout the short summer nights, but full Moon on July 30 creates the worst possible conditions for covering them. Lower activity from this medium-speed shower is usually seen from July 12 into August.
Other Active Showers
The Antihelion Source (ANT) has this year replaced many of the minor near-ecliptic showers, which previously used to chase one another around the sky through the year (as it sometimes seemed), with a single source that remains roughly opposite the Sun in the sky, hence the name. The actual Source's radiant centre is about 12 degrees east of this point. Video results suggest it is active virtually throughout the year, but it produces only low rates from an oval region, about 30 degrees in RA (two hours) by 15 degrees in Dec, centred on the radiant location for any given date.
The other leading, if much less active, late July shower peak is that of the α Capricornids (CAP). The shower lasts from July 3 to mid August. As the accompanying radiant drift chart suggests, their radiant actually overlaps that of the large ANT area's, and we are not yet sure it will still be sensibly observable as separate from the ANT as a result. Unfortunately, and as with the SDA, because their maximum around July 30-31 is wrecked by full Moon, 2007 will not be a good year to test for this. Peak ZHRs are usually only ~4-5, though a minor enhancement to ~10, with several fireballs, was observed from Europe - including Britain - in 1995. The shower is noted for occasionally producing wonderfully bright, slow meteors.
Always a favourite with observers is the swift-moving Perseids begin their annual activity towards July 17. For most of the month, their radiant lies below the "W" of Cassiopeia. Even by the end of July, Perseid rates will still be low (their ZHRs usually do not rise above ~10 until the first week of August), but it's always exciting to spot the first few during July, whenever the Moon allows in late month at least, and we know too that the Moon's presence then means skies will be beautifully Moon-free for the shower's mid August peak.
Happy Observing!
Radio detection of meteors
There is an extensive scientific and engineering literature dealing with the subject of ionization trails produced by the passing of meteors through our atmosphere. The purpose of this summary is not to be as complete as possible, but to pinpoint several aspects that the writer thinks can be of use to the MARIACHI project. A pertinent study of bibliographical references has been made in the presentation of the MARIACHI project.......read the whole summary
Useful References for Radio Detection of Meteors
- FM Detection of Meteors by Marianne Gualtieri.
- International Meteor Organization
- American Meteor Society
- Nippon Meteor Society - Japanese Meteor Society.
Radar Meteor Date - Live!
This web site provides real time results from many commercial radar sites used to detect metors. The information is then used to provide data on atmospheric conditions.
