THERMAL STRESS IN METEOROIDS BY AERODYNAMIC HEATING

1　IntroductionMostmeteoroidsbreakupduringtheirpassagethroughtheearth’satmosphere .Suchbreakupsareusuallyattributedtothecompressiveaerodynamicstress[1 - 5] .Thepresentpapergivesapreliminarydiscussionaboutthepossibilityoffractureofmete oroidsbythermalstressinducedbythenon uniformtemperaturedistributioninthemeteoroidresultingfromaerodynamicheatingatthesurface[6- 1 1 ] .Whenameteoroid ,whichhasapositivethermal expansioncoefficientandisinitiallycold ,isheatedbyaerody namicheating ,asurfacelayeris…     

A search for interstellar meteoroids using the Canadian Meteor Orbit Radar (CMOR)

Using the CMOR system, a search was conducted through 2.5 years (more than 1.5 million orbits) of archived data for meteoroids having unbound hyperbolic orbits around the Sun. Making use of the fact that each echo has an individually measured error, we were able to apply a cut-off for heliocentric speeds both more than two, and three standard deviations above the parabolic limit as our main selection criterion. CMOR has a minimum detectable particle radius near 100 μm for interstellar meteoroids. While these sizes are much larger than reported by the radar detections of extrasolar meteoroids by AMOR or Arecibo, the interstellar meteoroid population at these sizes would be of great astrophysical interest as such particles are more likely to remain unperturbed by external forces found in the interstellar medium, and thus, more likely to be traceable to their original source regions. It was found that a lower limit of approximately 0.0008% of the echoes (for the 3σ case) were of possible interstellar origin. For our effective limiting mass of 1×10⁻⁸ kg, this represents a flux of meteoroids arriving at the Earth of 6×10⁻⁶ meteoroids/km²/h. For our 2σ results, the lower limit was 0.003%, with a flux of 2×10⁻⁵ meteoroids/km²/h. The total number of events was too low to be statistically meaningful in determining any temporal or directional variations.     

Meteoroid Engineering Model (MEM): A Meteoroid Model For The Inner Solar System

In an attempt to overcome some of the deficiencies of existing meteoroid models, NASA’s Space Environments and Effects (SEE) Program sponsored a 3 year research effort at the University of Western Ontario. The resulting understanding of the sporadic meteoroid environment – particularly the nature and distribution of the sporadic sources – were then incorporated into a new Meteoroid Engineering Model (MEM) by members of the Space Environments Team at NASA’s Marshall Space Flight Center. This paper discusses some of the revolutionary aspects of MEM which include (a) identification of the sporadic radiants with real sources of meteoroids, such as comets, (b) a physics-based approach which yields accurate fluxes and directionality for interplanetary spacecraft anywhere from 0.2 to 2.0 astronomical units (AU), and (c) velocity distributions obtained from theory and validated against observation. Use of the model, which gives penetrating fluxes and average impact speeds on the surfaces of a cube-like structure, is also described along with its current limitations and plans for future improvements.     

The activity of the 2004 Geminid meteor shower from global visual observations

A comprehensive set of 612 h of visual meteor observations with a total of 29 077 Geminid meteors detected was analysed. The shower activity is measured in terms of the Zenithal Hourly Rate (ZHR). Two peaks are found at solar longitudes     and     with  ZHR = 126 ± 4  and  ZHR = 134 ± 4  , respectively. The physical quantities of the Geminid meteoroid stream are the mass index and the spatial number density of particles. We find a mass index of   s ≈ 1.7  and two peaks of spatial number density  234 ± 36  and  220 ± 31  particles causing meteors of magnitude +6.5 and brighter in a volume of 10⁹ km³, for the two corresponding ZHR maxima. There were  0.88 ± 0.08  and  0.98 ± 0.08  particles with masses of 1 g or more in the same volume during the two ZHR peaks. The second of the two maxima was populated by larger particles than the first one. We compare the activity and mass index profiles with recent Geminid stream modelling. The comparison may be useful to calibrate the numerical models.