From: email@example.com (Henry Spencer)
Subject: Re: orbit definitions
Date: Fri, 6 Apr 90 16:41:20 GMT
In article <17382@orstcs.CS.ORST.EDU> firstname.lastname@example.org (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,
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
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 email@example.com