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\centerline{Detection of Planetesimals}
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\centerline{by Occultation of Maser Sources}
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\centerline{by}
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\centerline{Mitsumi Fujishita, Naoko Ashikawa, Takahiro Kojima,}
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\centerline{Kei Harada and Yuichi Ichikawa*}
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\centerline{Kyushu Tokai University,}
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\centerline{9-1-1, Toroku, Kumamoto, 862, JAPAN}
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It seems that planetesimals are left outside of orbit of the Pluto.
It is very important to confirm the existence of these planetesimals
for the theory of the Solar System evolution.
However light and radio radiation from
planetesimals seem to be too weak to detect them directly.
Therefor the
authors examined possibility to detect planetesimals by
occultation of water vapor maser sources.
\par
Apparent angular diameter of spots of the water vapor
maser sources are small enough
in comparison with estimated one of planetesimals.
This means large intensity variation of maser spots will occur
during the occultation by planetesimal.  
Frequency of each spot is different by Doppler shift.
Therefor monitoring of spot intensity using a single dish
radio telescope with the spectrometer is enough
to detect this phenomena.
The occultation will be occur on one spot, so it is easy to
distinguish from the noise caused by passage of a airplane
or instantaneous power failure.
However, in the case that the occultation occurs on only one spot,
it is very difficult to separate the speed and
the size of a celestial body which covers the maser spot.
If this celestial body will cover other spots in succession,
it is possible to separate these parameters
using spot positions determined by, for example, VLBI.
\par   
Estimated duration and probability of the occultation are 1 second
and 1 event/year. 
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