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Date: 8 Oct 92 20:26:38 GMT
From: Henry Spencer <>
Subject: orbital lifetimes

In article <28396@scicom.AlphaCDC.COM> wats@scicom.AlphaCDC.COM (Bruce Watson) writes:
>|How many of these moons have stable orbits?  In this case, I define stable
>|to mean that they will stay in orbit for 100+ years...
>Anything above 1000 km will stay up for 100+ years...

At lower altitudes there's a nice set of rules of thumb...

At 100km your orbit lasts about an hour.
At 150km your orbit lasts about a day.
At 200km your orbit lasts about a week.
At 250km your orbit lasts about a month.
At 300km your orbit lasts about a quarter.
At 350km your orbit lasts a bit under a year.

Don't plan missions on this basis. :-)  Furthermore, don't try to extrapolate
it further:  the more-or-less semilog relationship you would infer from it
is visibly failing on both ends.  It's actually more like log-log, and the
constants vary a lot with how dense your satellite is.
There is nothing wrong with making      | Henry Spencer @ U of Toronto Zoology
mistakes, but... make *new* ones. -D.Sim|  utzoo!henry

From: Henry Spencer <>
Subject: Re: Atmospheric drag question
Date: Wed, 26 Feb 1997 14:38:54 GMT

In article <>, Michael Feher  <> wrote:
> exactly does atmospheric drag come
>into play with respect to orbit decay when most, if not all, satellites
>are in space, above entry interface?  The reason I ask is because there is
>no appreciable atmosphere beyond entry interface... (EI occurs at 121.9 km...

Entry interface is an arbitrary height with no physical significance; it's
just a convenient dividing line for procedures and calculations.  The
atmosphere does not stop there.  The only real end of the atmosphere is
the magnetopause, where Earth's atmosphere and magnetic field stop and the
Sun's atmosphere and field begin, and that's out around 40-50,000km.

What's "appreciable" depends on why you are asking.  Air friction is still
quite substantial at 150km:  spacecraft which use parking orbits at that
altitude need to allow for friction heating in their thermal calculations.
The atmosphere supplies useful radiation shielding out to 500km or so.
Atmospheric drag effects are significant out to about 1000km:  Earth-
observation satellites at 800-900km need regular orbit corrections to
maintain their intended pattern of ground coverage.  (Drag effects
actually remain detectable at much higher altitudes, but become
insignificant compared to other perturbations.)  Electrical effects due
to ionized atmosphere remain a significant design issue all the way out.

Entry interface is just a somewhat arbitrary definition of the altitude
where air drag stops being an ongoing nuisance and starts becoming an
immediate threat.
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

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