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Found at: gopher.quux.org:70/Archives/usenet-a-news/FA.space/82.02.12_ucbvax.6165_fa.space.txt

Aucbvax.6165
fa.space
utcsrgv!utzoo!decvax!ucbvax!space
Fri Feb 12 04:07:37 1982
SPACE Digest V2 #105
>From OTA@S1-A Fri Feb 12 03:06:10 1982
SPACE Digest                                      Volume 2 : Issue 105
Today's Topics:
			      Mooning Around
			 Lunar synchronous sites
			 VP Bush's view on budget
		     Eclipses in geostationary orbit
----------------------------------------------------------------------
Date: 11 February 1982 06:33-EST
From: Robert Elton Maas <REM MIT-MC AT>
Subject: Mooning Around
To: ucbvax!decvax!watmath!pcmcgeer at UCB-C70
cc: SPACE at MIT-MC
What makes you think a satellite in GEO is in the Earth's shadow
half the time?? The shadow is only 8000miles in diameter (the umbra
is slightly smaller, tapering to zero at four times lunar distance,
the penumbra is slightly larger, tapering larger at the same rate,
at geosync it's almost all umbra and rather close to 8000 miles in
diameter), while the circumference of the orbit is 25,000 * 2 * PI
which is about 150,000 miles. Thus the satellite spends only 8/150
of its time in Earth-shadow (about 75 minutes a day) on those days
in March and September when the Sun and equator are nearly inline,
less time in shadow when they aren't in line, zero when the sun
is so far north or south of the equator that the far-night point
of the orbit is more than 4000 miles from the center of the shadow.
If major industries are willing to close down for an hour when the
SPS is in shadow one satellite should do fine. If it's over the USA
(actually over South America, with beam aimed at Arizona/NM), this
time should be near local midnight when most industries are quite
willing to shut down anyway.
------------------------------
Date: 11 February 1982 08:32-EST
From: Hans P. Moravec <HPM MIT-MC AT>
Subject: Lunar synchronous sites
To: SPACE at MIT-MC
Lunar synchronous satellites are possible at L4 and L5, which are
stable equilibria.  They are also possible at L1 between the earth and
the moon, 58,000 km from the moon's surface and at L2 on the lunar
farside, 64,500 km from the farside.  These are unstable equilibria
and would require a small amount of station keeping (so is and
does geosynch orbit).  The station keeping can be done away with
if you use a very thin thread that anchors the satellite to the
lunar surface!   This would be a very minimal lunar skyhook, made of a
small amount of a conventional material like fiberglass.  It would
be just strong enough to exert the tiny force needed to keep
the satellite from flying outwards when the satellite is placed
a (relative) gnat's eyebrow farther from the lunar surface than L1 or L2.
ref -  Jerome Pearson, "Anchored Lunar Satellites for Cislunar
	Transportation and Communication"  J. Astronautical Sciences
	ca. October 1977  (presented at the European Conf. on Space
	Settlements and Space Industries, London, 20 Sept. 1977)
------------------------------
Date: 11 February 1982 21:02-EST
From: Robert Elton Maas <REM MIT-MC AT>
Subject:  VP Bush's view on budget
To: SPACE at MIT-MC
I got a reply to my postcard to VP Bush about space budget. He seems
to think the shuttle is the primary thing to spend money on now
because it's needed before we can build LEO station. I'll have to explain
to him that we need to start designing LEO station now so we can start
building it soon and have it ready to put up when the shuttle is operational.
[My letter was actually signed, and presumably written, by William D. Eckert,
 Lt.Col USAF, Military assistant/aide to the VP.]
Anybody want me to type the full 3.5 paragraphs of the letter and send it
to SPACE?
------------------------------
Date: 11 Feb 1982 18:58:40-PST
From: ihnss!karn at Berkeley
To: ucbvax!space@Berkeley
Subject: Eclipses in geostationary orbit
This is a topic that is well understood in the communications
satellite business.  Solar eclipses for spacecraft in geostationary
orbit occur only during the spring and fall seasons (equinoxes) when
the sun appears to pass behind the earth.  During the summer and
winter seasons (for the Northern hemisphere), the sun appears to
pass above the north and south poles at local midnight,
respectively, and there is no eclipse.
The longest eclipses occur at midnight on the equinoxes, where
they are 72 minutes long.  Eclipse durations on days preceeding and
following equinoxes decrease to zero in about + or - 20 days.
A common practice in communications satellite operations is to place
them to the west of the areas they serve.  This causes eclipses to
occur after midnight in the service area, when the traffic has
fallen to lower levels.
A more serious problem for communications are sun transits, in which
the sun passes behind the satellite.  The radio noise from the sun
blots out the satellite signal.  Of course, this would not be
a problem for the SPS;  the sun would even contribute (very slightly) to
the received microwave energy.
Phil Karn
Bell Labs Indian Hill
------------------------------
End of SPACE Digest
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