Meteors 1 



By Fred L. Whipple 



Smithsonian Astrophysical Observatory 



and 



Harvard College Observatory 



[With six plates] 



Solid bodies from space continuously bombard the earth at a rate 

 of thousands of tons per day. Fortunately for the life forms on 

 the earth, the earth's atmosphere serves as an admirable buffer to 

 protect them from this constant astronomical shellfire. 



The slowest meteorites strike at a speed of 7.0 miles per second, 

 the speed with which the earth attracts particles that fall from rest 

 at great distances. The speed of faster ones depends upon their 

 origin and direction of motion. Bodies belonging to the solar system 

 travel in closed orbits around the sun at velocities up to the par- 

 abolic limit of 26.3 miles per second at the earth's distance from the 

 sun, while the earth itself moves about the sun at a speed of 18.5 

 miles per second. The highest velocity of impact occurs, of course, 

 when one of these nearly parabolic particles strikes the earth head-on, 

 so that the total velocity reaches a maximum of 45 miles per second. 



Figure 1 shows how the collisions occur. The fastest meteorites 

 tend to strike on the morning side of the earth and the slow ones 

 catch up on the evening side. 



These meteoritic projectiles vary in size from minute particles to 

 very large ones, and are classed as follows: Meteoritic dust, tele- 

 scopic and radio meteors, photographic and visual meteors, fireballs, 

 detonating bolides, meteorite falls, and, finally, crater-producing 

 meteorites. 



Meteoritic dust (see Buddhue, 1950) ranges from barely visible 

 specks down to microscopic objects, limits in size being set by the 

 sun's ability to blow away particles about 1 micron (0.00004 inch) in 



1 Revision of an article in Publ. Astron. Soc. Pacific, vol. 67, pp. 367-386, 

 1955. Published by permission of the Society. 



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