of both Phobos and Deimos. The peculiar shapes of 

 these moons led to speculation they could be two aster- 

 oids that passed close to Mars at some times in the dis- 

 tant past and were captured by its gravity field to be- 

 come permanent companions. Mars is, after all, the 

 outermost planet of the Inner Solar System and close to 

 the belts of asteroids. 



There has long been keen interest among space sci- 

 entists to send a rocket to collect data on an asteroid. 

 We know that most, but not all, of the meteorites that 

 fall on the Earth are bits of asteroids. Altogether there 

 are over two dozen kinds of them. It is not known 

 whether this variety requires separate asteroids for each 

 kind or whether some asteroids are compound types. 

 Meteorites are very ancient matter — the oldest and 

 most primitive objects we have from the Solar System 

 — 4,560,000,000 years old. Everything we know about 

 the history of the Solar System we have learned from 

 them. 



Targeting an asteroid for a collecting mission is ex- 

 tremely difficult. First off, even a large asteroid is a very 

 small target. Second, asteroids in the asteroid belts 

 move along in orbits that contain thousands of them. 



In attempting to rendezvous with one asteroid, a space 

 vehicle could be battered and damaged by other tiny 

 asteroids. So, if one wants to land on an asteroid the 

 simplest way is to go after one of the moons of Mars. 

 Mars is a big target and easy to locate. It is out of the 

 main belts of asteroids, so damage by passing bits of 

 space gravel is minimized. Besides, Mars has been suc- 

 cessfully orbited many times before. We know how to 

 get there. 



It has been known for a long time that 1988-89 is a 

 prime time slot for missions to Mars. Because of the 

 relative orbital motions of the Earth and Mars, the dis- 

 tance of approaches between them varies from as much 

 as 63 million to as little as 35 million miles over a per- 

 iod of 1 5 to 1 7 years. 1988 was one of those years when 

 Mars and Earth were only a little over 35 million miles 

 apart, and now in early 1989 they are still fairly close 

 together. All you have to do is look up into the south- 

 ern sky on a clear night to appreciate this. Mars stands 

 out as a bright disc with its characteristic reddish glow, 

 almost as bright as the brilliant gleam of Jupiter. 



Because the administration of the United States has 

 decided to limit its space efforts to Space Shuttle pro- 



External view ot the alpha-backscatter spectrometer, which will make chemical measurements on the surface of Phobos. The device was 

 developed by Prof. Anthony Turkevich, of the University of Chicago's Department of Chemistry and built in West Germany. Earlier versions 

 were carried on Surveyor missions that explored Earth's moon in the 1960s. PhoiocoonesvA Turke»icn 



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