Newsgroups: sci.space
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: orbit definitions
Date: Fri, 6 Apr 90 16:41:20 GMT

In article <17382@orstcs.CS.ORST.EDU> belevel@nyevax.cas.orst.edu (Bart_Eleveld) writes:
>Would some kind soul(s) out there post definitions for the various types of 
>orbits that are often talked about on the net; e.g., Clarke, geosynchronous,
>geostationary...

Well, let's see...

LEO		Low Earth Orbit, generally somewhere between 250-300 km
		(where air drag starts to get serious) and 1000 km (where
		the inner Van Allen belt starts to get serious).  Usually
		implicitly at a modest inclination to the equator, i.e. the
		lowest achievable from the launch site.

Polar orbit	Technically, an orbit with an inclination of 90 degrees.
		More usually, LEO with an inclination near 90 degrees.

Retrograde	Technically, an orbit with an inclination over 90 degrees.
orbit		More usually, an orbit with an inclination a lot over 90
		degrees.  Rare; pretty useless.

Geosynchronous	Any orbit synchronized with the rotation of the Earth, i.e.
orbit		with a period which is some multiple or divisor of 24 hours.
		Often used sloppily to mean geostationary.

Geostationary	The 24-hour equatorial orbit, where a satellite appears to
orbit		hang motionless in the sky.  Most comsats are found here,
		as are an assortment of others that want a constant view
		of the Earth (early-warning satellites, some weather sats)
		or just easy communications in high orbit (some astronomy
		satellites).

Clarke orbit	Some people prefer this to "geostationary", given that
		Arthur C. Clarke was the first person to realize how useful
		this orbit would be for comsats.

HEO		High Earth Orbit.  Rather vaguely defined.  Usually means
		anything from Clarke orbit up; the region between LEO and
		Clarke orbit is very unhealthy due to the Van Allen belts.

GTO		Geostationary Transfer Orbit, an orbit at modest inclination
		with perigee at LEO and apogee at Clarke orbit.  The usual
		intermediate step en route to Clarke orbit; Ariane launches
		directly into GTO, some other launchers launch into LEO and
		then boost into GTO.

Molniya orbit	Elliptical orbit at a specific inclination, 60-odd degrees,
		usually with apogee above the Northern Hemisphere.  The
		Earth's equatorial bulge normally causes the position of
		apogee&perigee to rotate in the plane of an elliptical
		orbit, but at the particular inclination of the Molniya
		orbits, this effect is zero and the apogee stays where
		it's put.  The Soviets use it for their Molniya comsats
		(whence the name) because it makes them more visible from
		very high latitudes than Clarke orbit.  The inclination is
		high enough to miss the worst part of the inner Van Allen
		belt, which is near the equator.

Sun-synchronous	Another effect of the Earth's bulge is rotation of the plane
orbit		of the orbit.  With the right combination of altitude and
		inclination, the rotation can be set to 360 degrees/year,
		keeping the orbital plane in a roughly constant relation to
		the Earth-Sun line.  For low orbits, the inclination turns
		out to be slightly over 90 degrees.  Very popular for remote
		sensing, weather, and spy satellites that want to view the
		ground at constant Sun angle.

>... Also, how much more 
>energy (in relative terms) does it take to launch a payload to the west, or
>to the poles (N or S) rather than to the east?  

The difference is the Earth's rotation, which is 460m/s times the cosine
of the latitude.  Launching due east from the equator gets you a free
460m/s contribution toward orbital velocity.  Launching due north or south
eliminates that freebie.  Launching due west adds 460m/s to the necessary
velocity.  Orbital velocity is about 8km/s, so the difference is not huge
but is quite noticeable.  This is why spaceports are at the lowest possible
latitudes and maximum payload is had by launching due east.
-- 
Life is too short to spend    |     Henry Spencer at U of Toronto Zoology
debugging Intel parts. -Van J.| uunet!attcan!utzoo!henry henry@zoo.toronto.edu