Atmospheric deceleration and light curves of Draconid meteors and implications for the structure of cometary dust

Aims.The observation of Draconid meteors was used to infer information on the structure, porosity, strength, and composition of the dust of comet 21P/Giacobini-Zinner. Methods.Stereoscopic video and photographic observations of six faint and one bright Draconid meteors provided meteor morphologies, heights, light curves, and atmospheric decelerations. The spectrum of the bright meteor was also obtained. We developed a simple model of meteoroid ablation and fragmentation. The model assumes that cometary meteoroids are composed of constituent grains. Results.By fitting the observed decelerations and light curves, we have found that the grain mass range was relatively narrow in all meteoroids but differed from case to case. Some meteoroids were coarse grained with grain masses 10-9 to 10-10 kg, others were fine grained with grain masses one order of magnitude lower. Individual mm-sized meteoroids contained tens of thousands to almost a million grains (assuming grain density close to 3000 kg m-3). The meteoroids were porous aggregates of grains, having porosities of about 90% and bulk densities of 300 kg m-3. Grain separation started after the surface of the meteoroid received energy of 106 J m-2. The separation continued during the first half of meteor trajectories. We call this phase erosion. The energy needed for grain erosion was 15-30$\times$ lower than the energy of vaporization. However, 30% of the largest meteoroid was resistant to thermal erosion; this part disrupted later mechanically under a very low dynamic pressure of 5 kPa. The relative abundances of Na, Mg, and Fe were nearly chondritic, but differential ablation caused preferential loss of sodium at the beginning of the trajectory