The 2011 October Draconids outburst - I. Orbital elements, meteoroid fluxes and 21P/Giacobini-Zinner delivered mass to Earth

On 2011 October 8, the Earth crossed the dust trails left by comet 21P/Giacobini-Zinner during its 19th and 20th century perihelion approaches with the comet being close to perihelion. The geometric circumstances of that encounter were thus favourable to produce a meteor storm, but the trails were much older than in the 1933 and 1946 historical encounters. As a consequence the 2011 October Draconid display exhibited several activity peaks with Zenithal Hourly Rates of about 400 meteors h(-1). In fact, if the display had not been forecasted, it could have passed almost unnoticed as was strongly attenuated for visual observers due to the Moon. This suggests that most meteor storms of a similar nature could have passed historically unnoticed under unfavourable weather and Moon observing conditions. The possibility of obtaining information on the physical properties of cometary meteoroids penetrating the atmosphere under low geocentric velocity encounter circumstances motivated us to set up a special observing campaign. Added to the Spanish Fireball Network wide-field all-sky and CCD video monitoring, other high-sensitivity 1/2 arcsec black and white CCD video cameras were attached to the modified medium-field lenses for obtaining high-resolution orbital information. The trajectory, radiant and orbital data of October 16 Draconid meteors observed at multiple stations are presented. The results show that the meteors appeared from a geocentric radiant located at alpha = 263.0 +/- 0 degrees.4 and delta = +55.3 +/- 0 degrees..3 that is in close agreement with the radiant predicted for the 1873-1894 and the 1900 dust trails. The estimated mass of material from 21P/Giacobini-Zinner delivered to Earth during the 6 h outburst was around 950 +/- 150 kg.