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By Robert L. Hawkes
The term "meteor" applies to the streak of light (and related phenomena such as ionization) produced when an interplanetary particle (a meteoroid) enters Earth's upper atmosphere and the meteoroid material ablates (evaporates). Heights of ablation for typical meteors are 65 to 135 km above Earth's surface (higher-speed meteors ablate at greater heights). Evaporated meteoroid atoms undergo high-speed collisions with atmospheric constituents, and it is the decay of these excited states that produces the meteor luminosity. Most meteoroids are very small-a bright visual meteor is typically produced by a grain smaller than a pea. Very bright meteors are termed fireballs. If a solid object reaches the ground, it is termed a meteorite.
The meteoroid complex can be conveniently divided into two parts: a stream component, made up of particles in highly correlated orbits, and a more or less random sporadic component. About 5 to 10 sporadic meteors per hour are visible (by a single observer with dark-adapted eyes) from a dark-sky location on any given night. When Earth's orbit intersects a meteor stream, an enhancement of the meteor rate-a meteor shower-is observed. Perspective causes the parallel paths of shower meteors to appear to radiate from a point when plotted on a star map. The constellation in which this point of divergence, or radiant, seems to lie is used in naming the shower. When two or more showers have radiants in the same constellation, a nearby bright star is used in the designation (e.g. the Eta-Aquarids). The radiant of the Quadrantid shower, named after the obsolete constellation Quadrans Muralis, is in Boötes.
The sporadic meteor component is derived from the stream component by dispersion due to collisions with smaller meteoroids, radiative effects, and differential gravitational perturbations. The division between shower and sporadic meteors is not precise, and there continues to be debate regarding the validity of some minor meteor showers. A detailed list of showers, including the minor ones, is available in the annual meteor shower calendar published by the International Meteor Organization (IMO) (see www.amsmeteors.org/imo-mirror).
The activity of a meteor shower is usually specified by the zenithal hourly rate (ZHR). The ZHR is the number of meteors a single perfect observer would see per hour from a shower with a radiant directly overhead in a dark-sky location under exceptional conditions, where +6.5 magnitude stars are visible. Even from dark sky locations an observer will observe fainter meteors effectively over a much smaller field of view. Thus the ZHR figures are not expected visual meteor rates, which will always be significantly less than the ZHR. They do, however, provide a widely accepted standardized method to report meteor rates.
Several research groups have produced independent models of the Leonid stream that predict strong activity again in 2002 (for the last time in the current series of Leonid outbursts). In 2002, Earth will pass close to the dust ejected from P/Comet Tempel-Tuttle on three different perihelion passages, with the two most significant peaks shown in the table. The times of maxima are believed to be well determined, but the strengths at maximum are much less certain. This Handbook goes to press prior to the 2001 Leonid shower, and it is expected that the predictions will be refined based on the 2001 results, so other references should be consulted. Extensive information on the Leonid shower is available at leonid.arc.nasa.gov.
In a number of cases orbital similarity permits a link between a meteor shower and a parent object. For example, the Eta-Aquarids and Orionids are derived from P/Comet Halley, the Leonids from P/Comet Tempel-Tuttle, the Perseids from P/Comet Swift-Tuttle, the Taurid complex from P/Comet Encke, and the Geminids from Asteroid Phaethon.
Dedicated amateur meteor astronomers contribute the majority of visual observations. For visual observations it is critical to choose a dark sky location free from obstructions and to allow at least 20 min for dark adaptation. In general, one should not observe near the horizon (at least 40° elevation) and should look roughly 40° away from the radiant of the shower. Carefully estimate your limiting magnitude to the nearest tenth of a magnitude, and record it, the UT of the observing session, and observing direction (centre of field of view). The simplest observation method involves making an instantaneous decision regarding whether each individual meteor belongs to a shower (from the radiant and apparent speed). The shower association, brightness, and time for each meteor can be recorded orally on a tape recorder for uninterrupted observing periods. Visual observations of this type (after application of correction factors) are valuable for determining rate profiles and population indices of showers. Extensive information on observing meteors by visual and other methods is available from the IMO, the North American Meteor Network (www.namnmeteors.org), and the American Meteor Society (www.amsmeteors.org). These organizations also collect and analyze visual observations from amateurs.
Telescopic observations of meteors (usually employing binoculars) allow the plotting of trails with more precision and emphasize those showers rich in faint meteors (although generally at faint magnitudes the meteor population is dominated by sporadic meteors). Photographic observations can be made with normal or wide-angle lenses, typically using exposures of 5 to 30 min. A rotating shutter permits determination of apparent angular velocity. Increasingly amateurs are using video equipment for meteor observations. A standard home camcorder is limited to about magnitude +2 and will lead to very low rates (a meteor every several hours) except during a strong meteor shower. Image intensifiers (now available as reasonably priced second-hand units) coupled to video cameras (one can simply use the macro lens of the camcorder to image the output phosphor of the image intensifier) can extend the sensitivity to magnitude +9, with sporadic meteor rates of 15/hour. Intermediate sensitivity can be achieved using specialized unintensified, low light level monochrome CCD cameras.
Any of these techniques can be extended to simultaneous observations from two stations, and triangulation will yield the precise trajectory and orbital information. Baselines of 20 to 150 km are appropriate, with intersection heights at about 90 km. Addition of a diffraction grating (preferably a blazed grating, which concentrates visible light in the first order spectrum) in front of the objective lens can be used to make meteor spectra with the photographic or video observation techniques. Typically gratings with 200 to 800 lines per millimetre are used for this work. A meteor produces an ionization trail in the atmosphere, which can be used to reflect electromagnetic waves from distant television or radio transmitters. Meteors will be detected using this forward scatter radio technique as brief (usually a few tenths of a second) reception of the signal from the distant station, which is normally blocked by the curvature of Earth. See RADIO DETECTION OF METEORS for more details.
In addition to the URLs listed above, The Handbook for Visual Meteor Observers is available from the International Meteor Organization, and Neil Bone's book Meteors (Sky Publishing, 1993) provides an excellent introduction to visual, telescopic, and photographic meteor observation.
Excerpted from the Observer's Handbook 2002 © The Royal Astronomical Society of Canada, 2001. Used with permission of The Royal Astronomical Society of Canada. The Observer's Handbook can be ordered at http://www.rasc.ca/publications.htm.