summer of 1981, NASA had no ap- 
proved mission to Halley, but authori- 
ties were discussing a possible American 
space probe capable of taking more and 
better photographs than the “foreign” 
craft and equipped to collect cometary 
particles for return to Earth and chemi- 
cal analysis in laboratories on the 
ground. 
Despite all of the above, Belton and 
the other early Halley seekers are not 
jumping the gun. It is important to 
locate and observe the comet as soon as 
possible to determine its “far out” state, 
meaning its physical condition while still 
far from the sun and the solar influence. 
In the next few years, as the nucleus of 
the comet — a ball of frozen water and 
the ices of other substances, mixed up 
with a fine powder of rocklike dust — 
approaches the inner solar system, the 
sun’s rays will warm it. Evaporating 
gases will form the head, or coma, 
dwarfing the tiny nucleus, and will 
stream backward from the coma in the 
direction opposite the sun, to form a 
yellow dust tail and a blue plasma tail. 
The latter will be composed of electri- 
fied gases and can be expected to show 
abrupt changes of form as the onrushing 
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Halley encounters disturbances in the 
solar wind. 
The nucleus is the only part of a 
comet that returns from one passage 
around the sun to the next. All the gas 
and dust of the coma and the tails is lost 
in space. Each time Halley’s comet re- 
turns from the outer reaches of the solar 
system (its aphelion, or point farthest 
from the sun, is beyond the orbit of 
Neptune), its nucleus sheds material to 
form a new coma and new tails. As a 
result, the nucleus is gradually shrink- 
ing, and one day it will no longer exist. It 
may split apart, crumble, or just plain 
vanish. Nobody knows. A few astrono- 
mers, including those mainland China 
researchers who have analyzed ancient 
Chinese records, claim that Halley has 
been gradually fading over the centur- 
ies, perhaps due to the shrinkage effect, 
but this conclusion has not been clearly 
substantiated. 
If the searchers succeed in finding 
Halley’s comet while it is still far from 
the sun and before the coma has formed, 
their observations should reveal just how 
large the nucleus is. It’s a fairly simple 
matter: we know how bright the sun is, 
so the observed amount of reflected 
May >5 May ZJ May 
36 
