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Path: utzoo!watmath!clyde!burl!ulysses!allegra!mouton!karn
From: karn@mouton.UUCP
Newsgroups: net.columbia
Subject: Re: ET barge at VAFB breaks loose
Message-ID: <197@mouton.UUCP>
Date: Sat, 27-Oct-84 15:50:27 EST
Article-I.D.: mouton.197
Posted: Sat Oct 27 15:50:27 1984
Date-Received: Sun, 28-Oct-84 06:25:07 EST
References: <271@haddock.UUCP>
Organization: Bell Communications Research, Inc
Lines: 35

The "sun synchronous" orbit is a special case of a polar orbit, but it
is by far the most popular one.

"Sun synchronous" means that the angle between the orbital plane and the
earth-sun line stays essentially constant over time. This cannot happen
in simple two-body motion, of course, because the orbital plane would remain
fixed in inertial space while the sun appeared to move around it once
per year. However, the equatorial bulge of the earth can be exploited.

It turns out that the angle between the orbital plane and the "fixed"
stars measured in the equatorial plane (and known as the Right Ascension
of the Ascending Node) changes on each orbit due to the tugging of the
earth's equatorial bulge. The rate of change of RAAN depends on the orbital
period (and hence on altitude), and also on the orbital inclination.

It turns out that for a given altitude, there is a "magic" inclination
(always greater than 90 degrees) in which the RAAN increases exactly
360 degrees per year. This means that the orbital plane will precess
eastward, exactly matching the apparent motion of the sun around the earth
during the year.  This orbit is very useful for earth observation satellites
(of all types) as they can monitor the earth at the same local time and
sun angle each day.  You select the local time you want by launching
at a particular time. Typical launch windows for sun-synchronous missions
are 5 MINUTES long, once per day, and should prove to be an interesting
challenge for the shuttle.

Because sun synchronous orbits always have inclinations near 90 degrees
(e.g., Oscar-11: 700 km altitude, 98.2 degree inclination; Oscar-7,
1450 km altitude, 101.4 deg inclination), they must be launched on either
a northwest or southwest trajectory.  This is impossible from Cape Canaveral
without passing over land early in the flight. From Vandenburg AFB, however,
there is nothing but thousands of kilometers of empty ocean to the southwest.
This also presents quite a challenge to an emergency landing, of course.

Phil