Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Why STS-35 is being launched at night.
Message-ID: <1990Jun1.164749.5039@utzoo.uucp>
Organization: U of Toronto Zoology
References: <6282.266023d3@jetson.uh.edu> <1990May28.153022.20010@utzoo.uucp> <2939@rwthinf.UUCP>
Date: Fri, 1 Jun 90 16:47:49 GMT

In article <2939@rwthinf.UUCP> dieter@rwthinf.UUCP (Dieter Kreuer) writes:
>> This fits with the night launch.  From a northern launch site, a night
>> launch into low-inclination orbit gives you an orbit with its northern
>> end on the night side of the Earth and its southern end on the day side.
>
>For how long is this orientation of the orbit stable (it is certainly 
>perturbed by Sun, Moon, atmospheric deceleration, revolution of Earth
>around Sun etc.)? ...

The major perturbing factor, actually, is the Earth's equatorial bulge.
Because of that bulge, the Earth's gravitational field is not (quite) that
of a point mass.  This produces various effects; in particular, the plane
of the orbit rotates slowly around the Earth's axis.  The rate of rotation
depends on orbital altitude and inclination (for circular orbits; I think
it's messier yet for elliptical ones).  This is generally on a time scale
of months, not days.  I can dig up the formula if people are interested.
-- 
As a user I'll take speed over|     Henry Spencer at U of Toronto Zoology
features any day. -A.Tanenbaum| uunet!attcan!utzoo!henry henry@zoo.toronto.edu