THE SOLAR SYSTEM — LOVELL 287 



enter the atmosphere is radiated away and the flight of the dust gi-ain 

 is stopped before evaporation occurs. These are the micrometeorites 

 which eventually fall to earth as dust. From a study of the deposits 

 on the ocean bed it has been estimated that the earth collects some- 

 thing like a million tons per annum in tliis way. 



The micrometeorites are now the subject of investigations using 

 satellites and space probes, and many space veliicles launched by the 

 Americans and Russians have been equipped with some form of 

 micrometeorite detector. In principle, the detection of these micro- 

 meteorites in space should be simple — by allowing them to collide with 

 a diaphragm which is equipped with a microphone : when a dust gram 

 hits the diaphragm it will make a sound in the microphone and be 

 telemetered back to earth. In practice these unpact methods have 

 proved to be difficult because the microphones record noises other than 

 the impact of the dust grains ; the calibration, too, is micertain. The 

 techniques have now been refined and we have some idea of the amount 

 of dust of this extraordinarily small size which exists in space. For 

 particles which weigh a hundred-millionth of a gram the rate of im- 

 pact is found to be equivalent to one particle per 1,000 second over 

 a surface of area 1 square inch. For particles wliich weigh a thou- 

 sand-millionth of a gram the rate is found to be 1 every 100 second. 

 The quantity of this dust is 1,000 to 10,000 times greater than the 

 particles which are big enough to burn up in the atmosphere. 



From some of the recent analyses of the micrometeorite recordings 

 in the American satellites, Whipple of the Smithsonian Astrophysical 

 Observatory has concluded that a large quantity of this small dust ap- 

 pears to be traveling in an orbit around the earth. It appears that in 

 some circumstances which are not yet understood some of this fine dust 

 gets trapped in gravitational orbits around the earth. So we seem to 

 have two new situations arising. We have the trapped radiation, the 

 protons and electrons in the Van Allen belts (that is a magnetic trap- 

 ping) , and also a gravitational trapping of fine dust in the vicinity of 

 the earth. 



At the other end of the scale of size, as the particles become bigger 

 their numbers decrease. Objects which we see in the sky as bright 

 fireballs probably weigh about a gram, and there may be a million 

 of these entering the earth's atmosphere every day. If the meteor 

 is much larger than this it will not be completely evaporated in its 

 journey through the atmosphere and some part of it will fall to earth 

 as a solid body. Something like 500 kilograms of this material per 

 year fall to earth in this way as meteorites. Occasionally these 

 meteorites are extremely big and there are classic examples such as 

 the meteor crater in Arizona and the Siberian meteorite which fell 

 in 1908 and devastated 100 square miles of countryside. If ever a 

 meteorite of this size fell on a populated area then there would 



