Physical and Dynamical L (Studies of Meteors
Total Page:16
File Type:pdf, Size:1020Kb
I I # 7L3- 32723 NASA CONTRACTOR NhSA CR-2316 REPORT \D r- c3 PHYSICAL AND DYNAMICAL L(STUDIES OF METEORS by Richurd B. South worth md Zdenek Sekmimt Preparea’ by SMITHSONIAN INSTITUTION ASTROPHYSICAL OBSERVATORY Cambridge, Mass. 02 138 for Langley Research Center. I4ATIONA,L AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. (:. OCTOBER 1973 I. Report No. 2. Government Accession No. 3. Recipient's Catalog No. NASA CR-2316 4. Title and Subtitle 5. Report Date October 1973 PHYSICAL AND DYNAMICAL STUDIES OF METEORS 6. Performing Organization Code 7. Author(s) 8. Performing Organization Report No, Richard B. Southworth and Zdenek Sekanina 10. Work Unit No. 9. Performing Organization Name and Address 188-45-52-03 Smithsonian Institution Astrophysical Observatory 11. Contract or Grant No. Cambridge, Massachusetts 02138 I NAS1-11204 13. Type of Report and Period Covered 2. Sponsoring Agency Name and Address Contractor Report National Aeronautics and Space Administration 14. Sponsoring Agency Code Washington, DC 20546 I 5. Supplementary Notes This is a topical report. 6. Abstract Interplanetary distributions from a sample of 20,000 radar meteor observations are presented. These distributions are freed from all known selection effects with the exception of a possible bias against fragmenting meteors which has not yet been adequately assessed. These data thus represent the largest and most accurate collection of radar meteor distributions. Both general average dis- tribution and the distribution of meteor streams with their comet and asteroid associations are presented. Sporadic space density and space density of meteor streams are presented. ~ ~ ~~ 17. Key Words (Suggested by Author(s)) meteor; radar obser- 18. Distribution Statement vation; distributions; selection effects; space density; fragmentation; meteor streams; luminosi. Unclassified - Unlimited ty; recombination; diffusion; meteor masses; orbital parameters; simultaneous observation; image orthicon 19. Security aasif. (of this report) 20. Security Classif. (of this page) 21. NO. of pages 22. Rice' Domestic, $4.21 Unclassified Unclassified I 108 t Foreign, $6.71 *For sale by the National Technical Information Service, Springfield, Virginia 22151 TABLE OF CONTENTS Section Page 1 SUMMARY ........................................ 1 1.1 Aims ......................................... 1 1.2 Data ......................................... 1 1.3 Orbital Distribution ............................... 1 1.4 Space Density ................................... 2 1.5 Collisions...................................... 2 1.6 Streams ....................................... 2 1.7 Associations with Comets and Asteroids ................. 2 1.8 Height-Velocity Diagram ........................... 3 1.9 Space Density in Streams ........................... 3 2 INTRODUCTION ..................................... 5 3 OBSERVATIONS ..................................... 7 3.1 Radar Meteors .................................. 7 3.2 Synoptic Year ................................... 7 3.3 Simultaneous Radar-Television Observations .............. 8 4 SELECTIONEFFECTS ................................ 9 4.1 Radar Biases ................................... 9 4.2 Observational Geometry ............................ 9 4.3 Ionizing Probability ............................... 10 4.4 Diffusion ...................................... 10 4.5 Initial Radius ................................... 10 4.6 Recombination ................................... 12 4.7 Fragmentation ................................... 13 4.8 Mass Distribution ................................ 13 5 ORBITAL DISTNBUTIONS.............................. 15 5.1 Synoptic-Year Sample .............................. 15 5.2 Velocity ....................................... 16 5.3 Radiants ....................................... 16 5.4 Orbital Elements ................................. 20 6 SPACE DENSITY .................................... 29 6.1 Observed Orbits ................................. 29 6.2 Unobservable Orbits ............................... 29 6.3 Densities ...................................... 31 6.4 Comparisons with Other Data ......................... 33 iii TABLE OF CONTENTS (Cont.) Section Page 7 COLLISIONSINSPACE ................................ 37 7.1 Significance of Collisions ........................... 37 7.2 Theoretical Collision Model .......................... 37 7.3 Comparison with Observation ......................... 42 7.4 Sources of Meteors ............................... 43 7.5 Further Research ................................ 43 8 COMPUTERIZED SEARCH FOR RADIO METEOR STREAMS ....... 47 8.1 Two-Phase Computerized Stream Search in the Synoptic-Year Sample ........................................ 47 8.2 Phase I: Major Source of Initial Orbits .................. 48 8.3 Phase I: Other Sources of Initial Orbits ................. 50 8.4 Phase TI: The Weighting System ....................... 51 9 RESULTS OF THE STREAM SEARCH ...................... 57 9.1 List of the Detected Radio Streams ..................... 57 9.2 Identification of Streams Detected in the 1961- 1965 Sample .... 58 9.3 New Streams: Toroidal Group, Short- and Long-Period Streams. Twin Showers. and the "Cyclid" System .................. 76 9.4 Distribution of Radiants ............................ 77 9.5 Comparison with CookTsWorking List of Meteor Streams ...... 79 10 COMET-METEOR AND ASTEROID-METEOR ASSOCIATIONS ...... 85 10.1 Associations with Periodic Comets ..................... 85 10.2 Adonis Meteor Streams ............................ 85 10.3 Possible Associations with Other Minor Planets of the Apollo and Amor Types and with Meteorites and Bright Fireballs ..... 87 11 HEIGHT-VELOCITY DIAGRAM ........................... 89 11.1 Introduction .................................... 89 11.2 Height-Velocity Plots for Individual Radio Meteors .......... 89 11.3 A Height-Velocity Plot for Radio Streams ................ 97 12 SPACE DENSITY IN RADIO METEOR STREAMS ............... 99 12.1 Mean Space Density in a Stream in Terms of the Sporadic Den- sity .......................................... 99 12.2 Space-Density Gradient in a Stream .................... 100 12.3 Numerical Results ................................ 101 12.4 Space Density from Cometary Production Rates of Solid Material; Comparison of the Two Methods ....................... 103 13 REFERENCES ...................................... 105 iv PHYSICAL AND DYNAMICAL STUDIES OF METEORS Special Progress Report 1. SUMMARY 1.1 Aims This research focused on the interplanetary distribution of radar meteors, freed from observational bias. We sought both the general average distribution and the distribution of meteor streams with their comet and asteroid associations. 1.2 Data The Havana, Illinois, 77synoptic-year*7radar observations and the Havana-Side11 simultaneous radar-television observations constituted the data. In addition to being the largest and most accurate collection, these data have two unique and crucial advan- tages: A. There is an adequate sample of very slow meteors, which recombination has eliminated from all other surveys. B. There is a reliable calibration of the meteor masses. The only known selection effect not yet adequately assessed is a possible bias against fragmenting mete or s . 1.3 Orbital Distribution Low-velocity meteors predominate; the velocity mode in the atmosphere is under 15 km sec-l, and the mode in space at 1 a.u. from the sun is under 20 km sec-l. Radiants cluster to the sunward and antisun directions. Moderate eccentricities and low inclinations predominate; retrograde orbits are very rare. 1 1.4 Space Density We extrapolated the orbital distribution observed at the earth to include unobserv- able orbits and used the resultant value to correct the space density of observed orbits. The result is meaningful between about 0.4 and 2.0 a. u., and uncertain but swggestive farther away. Meteors are concentrated to a relatively thin layer centered on the ecliptic plane. In that plane, there is a minimum of density at about 0.7 a. u., and a maximum beyond 2 a. u. 1. 5 Collisions If this space-density distribution is in equilibrium, then most meteors must end their lives in collisions with each other. Moreover, a simplified theoretical model incorporating collisions and the Poynting-Robertson effect agrees with observation. Nonetheless, collisions need further study. I 1.6 Streams With the use of the most powerful stream-search technique in existence, we found 200 new streams in the synoptic-year data and confirmed the existence of two-thirds of the 83 streams detected previously in the 1961- 1965 sample of radio meteors. Some of the new streams have most uncommon orbits. A new, rich stream with a revolution period of more than 30 years has been discovered. Streams of low inclination are often detected at both nodes. 1.7 Associations with Comets and Asteroids A number of known comet- meteor associations are confirmed, and a few new possible associations detected. The previously detected associations with the minor planet Adonis are strongly reinforced (including a major, broad stream), and several possible associations with other asteroids, meteorite Psfbram, and a Prairie Network fireball are suggested. 2 2 1.8 Height-Velocity Diagram Plots of both the individual radio meteors and the radio streams fail to exhibit the discrete-level structure known to exist for photographic meteors. Some features of the height-velocity diagram are, however, detected. 1.9 Space Density in Streams The mean space