The Solar System' 



By Sir Bernard Lovell 



Professor of Radioastronomy, University of Manchester, England 



During the last few years there has been a renewal of interest in 

 the problems of the solar system. This interest has been stimulated 

 by the discoveries made by using space probes and by the results 

 of research programs of the radio telescopes, which have revealed many 

 new facts about our immediate environment in space, the explanations 

 for which are not yet understood. 



The basic astronomical data about the solar system are well known. 

 The earth, moving in a nearly circular orbit 93 million miles from the 

 sun, is a member of the sun's family of planets. Mercury, at a mean 

 distance of 36 million miles from the sun, and Venus, at a mean dis- 

 tance of 67 million miles, are in orbits closer to the sun. The orbit 

 of Mars lies outside that of the earth at a mean distance of 141 million 

 miles. Then comes the outer planetary system of giants: Jupiter, 

 483 million miles from the sun, Saturn (886 million miles), Uranus 

 (1,783 million miles), Neptune (2,793 million miles), and Pluto 

 (3,666 million miles). 



The inferior planets — Mercury, Venus, Earth, and Mars — are dis- 

 tinguishable from the giants by the fact that they have approximately 

 the same size and density. Compared with the earth as unity, the 

 densities range from 0.69 for Mars to 1.1 for Mercury, and the diam- 

 eters from 0.37 for Mercury to 0.97 for Venus. The giant planets 

 are in a different category, with densities much smaller than the earth 

 (from 0.13 for Saturn to 0.25 for Jupiter), but they are of enormous 

 size, ranging from Uranus which is 4 times the diameter of the earth, 

 to Jupiter, over 11 times the earth's diameter. The outermost planet 

 Pluto is exceptional: although its dimensions are not accurately 

 known, it must be small, with the highest known density in the solar 

 system. Between the inferior and giant planets, that is, between the 

 orbits of Mars and Jupiter, there is a swarm of minor planets, or 

 asteroids, the largest of which is Ceres with a diameter of 400-500 

 miles, but many are probably only a few miles in diameter. The 



1 This article appeared as Cliapter II of "The Exploration of Outer Space," a collection of 

 five lectures by Sir Bernard Lovell, published in 1962 by Harper and Row, 49 East 3od 

 Street, New Yorli, N.Y. 



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