The Draconid meteors--will there be an outburst this October?
All the observed showers occurred when the parent comet, 21P/ Giacobini-Zinner, passed close to the orbit of the Earth. This has led to the suggestion that the stream is young, with meteoroids mostly located close to the comet. Now there is the possibility of another fine Giacobinid display on 2011 October 8--not at the level of the great storms of the past, but still expected to produce a noteworthy shower.
The table below (reproduced courtesy of Nick James) summarises the eight Giacobinid showers observed to date, C-E (AU) being the minimum distance between the comet's orbit and the Earth's orbit at the descending node (- denoting the Earth outside, and + inside the comet's orbit), and Earth at Node being the number of days before or after the comet that the Earth passed the node. The values given for the peak ZHR (meteors per hour) are only approximate, particularly for the great Draconid storms of 1933 and 1946.
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 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. The strength of the meteor outburst that is observed depends upon which (if any) of these trails the Earth intersects near to the nodal crossing point. 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.
Jeremie Vaubaillon et al. found that it is essential to include a large enough number of old dust trails in the model to be able to explain all the observed Giacobinid showers. In their studies they include all trails going back to the 1596 return of the parent comet. The 1900 and 1907 dust trails were responsible for the 1933 and 1946 Draconid outbursts, each of which is a narrow expanse of dust now displaced from the current position of the comet. The 1887 and 1894 trails (and maybe even older filaments) mainly caused the 1985 outburst, with a contribution from the very thin 1946 trail also detected. Most of the activity in 1998 was due to dust released at the 1926 return of the comet, and the 2005 Draconid shower was due to the 1946 dust trail, the shower being rich in faint meteors producing higher activity in the radar data than in visual observations.
In 2011 October there is, once again, the likelihood of a Giacobinid shower. Comet Giacobini-Zinner returns to perihelion on 2012 February 11 and reaches the descending node 6.06 days later on February 17. On 2011 October 8 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 2011 October 8, with the period between 19h and 21h most likely to yield the greatest activity. This means that longitudes in Eastern Europe and the Middle East will be favoured, although 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 UK, observers should go out as soon as twilight falls that evening. 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.
Interestingly, there is a very considerable spread in the predicted peak rates for the 2011 October Draconids, 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! Unfortunately, there will be a waxing gibbous Moon in Aquarius, less than four days from full, 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.
The radiant of the shower will be centred on RA 17h 32m, Dec +55.5[degrees], 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 conveniently on a Saturday evening! Draconid meteors are typically very slow moving, in marked contrast to members of showers such as the Perseids or Leonids.
Let us hope for clear skies across Northern and Eastern Europe on 2011 October 8. Please send your observations (even if you don't see many meteors) as soon as possible to me at the address below.
John W. Mason, Director
Dr John Mason explains the historical background to the Draconid meteor shower, which may--or may not!--give us a fine display in the early evening of October 8 this year.
Table 1. Observed Draconid showers or storms Date C-E (AU) Earth at node ZHR (m/h) 1926 Oct 09 +0.0005 69.1 before ~17 1933 Oct 09 +0.0054 80.2 after ~6,000 1946 Oct 10 +0.0015 15.4 after ~5,000 1952 Oct 09 -0.0057 195.5 before ~180 1972 Oct 08 -0.00074 58.5 after very low!! 1985 Oct 08 +0.0329 27.2 after ~700 1998 Oct 08 +0.0383 49.5 before ~500 2005 Oct 08 +0.043 91.8 after ~160