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 The Giacobinid (or October Draconid) meteor showers were among the most spectacular of the last century. Two outstanding Giacobinid meteor storms were seen on 9 October 1933 and, thirteen years later, on 10 October 1946 when visual rates were greater than one meteor every second, but the total duration of each outburst was only about an hour. Lesser Giacobinid showers were also observed in 1926, 1952, 1972 (very weak), 1985, 1998 and 2005, but none in other years. All the observed showers occurred when the parent comet, 21P/ Giacobini-Zinner passed close to the orbit of the Earth. This has lead to the suggestion that the stream is young with meteoroids mostly located close to the parent comet. Now there is the possibility of another fine Giacobinid display on 8 October 2011 – not at the level of the great storms of the past, but still expected to produce a noteworthy shower.
Studies of all the significant Giacobinid displays have been carried out by Jeremie Vaubaillon of the Institut de Mecanique Celeste et de Calcul des Ephemerides (IMCCE), Paris and a number of co-workers using the dust trail model. Each time that an active comet returns to perihelion, meteoroids are released into a range of orbits with parameters close to, but not identical to, those of the parent comet. The range in orbital periods of the ejected dust particles soon causes them to stretch out into a long, narrow dust trail, successive trails being created at each perihelion passage of the comet. Consequently, after a comet has returned many times to perihelion, the meteoroid stream will consists of a large number of separate individual dust trails. It depends upon which (if any) of these trails the Earth intersects, near to the nodal crossing point, as to the strength of the outburst that will be observed. It should also be noted that older dust trails have a tendency to be broader, more diffuse and consequently yielding lower meteor rates than younger, narrower and more concentrated trails.
The Meteor Section of the British Astronomical Association (BAA) is seeking the help of all observers to gain an accurate picture of Draconid meteor activity this year. Your assistance will be greatly appreciated.
Circumstances of the October 2011 shower
In October 2011 there is, once again, the likelihood of a Giacobinid shower. The parent comet returns to perihelion on 11 February 2012 and reaches the descending node six days later on 17 February. On 8 October 2011 the Earth will pass close to the node 132 days before the comet and on the face of it the chances of a great shower do not seem particularly high. However, calculations using the dust trail model, by many different researchers, have indicated that there is a very good chance of a significant outburst that day, with the Earth coming close to a group of rather old trails. The closest encounters will be with the 1880, 1887, 1894 and 1900 dust trails, with miss distances of between 0.0009 AU (1887) and 0.00136 AU (1900).
Overall, Draconid activity is likely to peak somewhere between 16h and 22h UT on 8 October 2011, with the period between 18h and 21h most likely to yield the greatest activity. This means that longitudes in Eastern Europe and the Middle East will be favoured, although observations across all longitudes are important, and it will be essential for would-be observers to be far enough north to ensure that the radiant is at a respectable altitude above the horizon during the period of potential activity.
From the British Isles, observers should go out as soon as twilight falls on the evening of Saturday, 8 October. Observations will also be of considerable value on the evenings immediately before and after the predicted peak to provide a check on background meteor rates at this time. Let us hope for clear skies everywhere on the evenings of 7, 8 and 9 October 2011, but particularly on the 8th!
Interestingly, there is a very considerable spread in the predicted peak rates for the October 2011 Draconids spanning over an order of magnitude, with the maximum ZHR ranging from a few tens of meteors per hour to several hundred, as viewed by a single observer. The only way to find out what happens is to go out and look for yourself!
Effect of moonlight
Unfortunately, there will be a waxing gibbous Moon in Aquarius, less than four days from full at the time of peak Draconid activity, so there will some interference from moonlight. Observers are therefore advised to direct their gaze to the northern half of the sky, keeping the Moon behind them.
Location of Draconid shower radiant
The radiant of the shower will be centred on RA 17h 32m, Dec + 55.5o, not far from the star Nu Draconis in the ‘head’of Draco. Throughout the period of likely activity of the shower (16h to 22h UT), the radiant will be at a respectable altitude above the north-western horizon – and it is on a Saturday evening! Draconid meteors are typically very slow moving, in marked contrast to members of showers such as the Perseids or Leonids.
Visual observing
By far the most widely practised method of meteor observing is naked eye visual work. Visual meteor watches can be carried out by individuals, or by groups of observers working together. The element of teamwork makes meteor observing particularly attractive to local astronomical societies, and it is hoped that many local society groups will make an effort to cover the likely period of Draconid meteor activity this year. Notes on Group Visual Meteor Observing and downloadable Visual Meteor Report Forms are available on the BAA website.
A naked eye visual meteor watch requires the minimum of equipment. The observer should find a dark site away from interfering lights, and recline on a deckchair or similar support such that the sky at 50° altitude can be watched in comfort. Warm clothing is essential. On this point, several thin pullovers are better than one thick one; the head should be covered on a cold night, and heat can also be lost through the soles of thin shoes, or from unprotected ankles and hands! An accurate timepiece, a dim red torch, notepad and several pens and/or pencils are the only other needs. Before starting a watch, the position of the Draconid radiant should be plotted on a star atlas. Most shower meteors will be seen if the field of view is directed some 40° in azimuth from the radiant.
To be of value, a naked eye meteor watch should be of at least one hour's duration. Watches lasting several hours are possible on good nights, but occasional breaks are advisable to maintain alertness. Apart from shower activity, which may vary rapidly on the evening of 8 October, the main influences on observed meteor rates will be moonlight and weather. Haze coupled with the bright Moon will unfortunately reduce the numbers of faint meteors visible. Shower meteor rates are usually best when the radiant is higher in the sky.
During the watch, the observer records various details for each meteor. These include: time of appearance (UT); magnitude; type (shower member, or random, ‘background’ sporadic); constellation in which seen; presence and duration of any persistent train. Other notes may mention flaring or fragmentation in flight, or marked colour.
Meteor imaging
In its simplest form, capturing meteors with a film camera or digital SLR entails taking time exposures with a tripod-mounted camera in the hope of recording events on the film or CCD. With film cameras, at ISO 800, exposures were typically of 10-15 minutes’ duration, and with an unguided camera the stars produced trailed images: if a sufficiently bright meteor appeared in the camera's field of view, it was often recorded as a longer trail cutting across those of the background stars. With equatorially-mounted cameras, guided at sidereal rate, any meteor trails are accurately recorded against the background stars. Although meteors may appear in any part of the sky, it is generally considered best to centre your camera’s field of view at about 50° elevation above the horizon and, say, 20° to 30° elongation from the shower radiant.
Digital SLRs (DSLRs) are very efficient at collecting background light from the sky, particularly at a setting of ISO 800 or 1600. So, exposures with DSLRs should generally be kept relatively short – no more than a few minutes’ duration in a really dark, rural location, and probably only 30 seconds to one minute in duration from a more typical observing site. With some DSLRs, the camera can be operated using a programmable timer attached to the shutter control to take repeated exposures one after the other for as long as required, provided the battery is fully-charged beforehand.
Wide-angle lenses of 18-28 mm focal length, with their correspondingly larger fields of view, may yield a higher meteor capture rate, but they do give a reduced image scale. It is recommended that the observer carry out a visual watch in parallel with camera exposures, noting details of all meteors seen. Satellites and aircraft crossing the field can leave meteor-like trails, and should also be noted. As a general rule of thumb only meteors brighter than about magnitude zero are likely to register on the image.
A recent significant shift in observing techniques for meteors has involved the increasing use of new highly sensitive, high resolution, black and white CCTV video cameras for the recording of brighter meteors. It is hoped that any observers who have such equipment at their disposal will use it to monitor the activity of the Drcaonid meteor shower this year.
Radio observing
Previous maxima of the Draconids (e.g. in 1985) have been monitored effectively using radio methods. The forward scatter radio technique has found favour with a small number of amateur observers, and has the advantage of being possible during daylight and under cloudy conditions. The principle is simple: ionisation produced by meteors provides a ‘surface’ in the high atmosphere from which radio signals can be reflected (or ‘scattered’). By tuning to a station lying over the radio horizon, the number of scattering events can be used to assess activity. Even very small meteoroids produce sufficient ionisation to allow scatter, so radio meteor rates are usually much higher than those observed visually.
Any radio recording apparatus should be operated not only on October 8, but also on the days immediately before and after the expected peak to provide a check on background radio rates at the time. It is very important that receiver settings and antenna alignment are kept constant throughout the period of observation.
The BAA Meteor Section would like to receive tabulated counts of meteorscatter events on the three days, 7, 8 and 9 October 2011, preferably round-the clock, but if this is not possible between 15h UT and 23h UT on the three days. Counts in 15-minute bins will be appropriate for most of the time but on the evening of 8 October – most likely between 18h and 21h UT - rates may be changing very quickly and so it would be advantageous to have the counts in 5-minute bins. During the Draconid outburst in October 1985, the peak observed forward scatter radio rate approached 30 counts per minute in the 4-metre band.
For each radio observer, we shall need the locations of the transmitter and receiver, the
frequency used, and brief details of the receiving equipment and method of recording. Observers have found that radio wavelengths in the range 2-6 metres have proved most effective in forward scatter studies of meteor activity, because this ties in best with the width of the ionised trail.
If the antenna is a Yagi type it would be helpful to have details of the antenna, i.e. the number of elements and the beam width. Also, if possible, the beam of the antenna should be directed towards the mid-point of the great circle path joining the transmitter and receiver.
Submission of observations
The BAA Meteor Section would welcome any observations of the Draconid meteor shower this year from individuals or local society groups, using any of the observing methods outlined above. Even simple counts of meteors seen within given time periods will be welcome. It is intended that a summary of all the observations received, crediting all of the individual observers and society groups, will be published in the BAA Journal as soon as possible after the event.
Observations may be submitted via email to:
This e-mail address is being protected from spambots. You need JavaScript enabled to view it
and by post to:
Draconid Meteor Project 2011
British Astronomical Association
Burlington House
Piccadilly
London W1J 0DU
These notes were issued by Dr John Mason, Director, BAA Meteor Section
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