# [amsat-bb] Re: ARISSat-1 - Dumb Question

Bob Bruninga bruninga at usna.edu
Thu Feb 10 16:17:13 PST 2011

```> a retrograde maneuver will remove ARISSsat from
> the proximity of ISS very very quickly.

It is interesting that any such one-thrust (arm throw) maneuver will then
intersect the ISS exactly one orbit later. In theory that is.  But the
difference in drag at that low altitude will usually be enough to have a
safe miss distance on the next and subsequent orbits.

I think that is why most space maneuvers require two burns.  One to start a
new orbit (but it will still intersect the original orbit on every orbit.
Then a second burn somewhere else in the orbit to get rid of that
intersecting point?

Bob, WB4APR

> > >>  ... Clint.  When ARISSAT is released it will stay in the "Plane" of
ISS
> >  orbit...they will toss it retrograde meaning in the opposite direction
of
> > the  velocity vector and with its slightly lower velocity the orbit will
> > start to  decrease...this is done so that very quickly the orbits will
stop
> > being "prox  ops" reasonably fast.
> >
> > If so, then I believe as the orbit altitude is reduced, the apparent
> > velocity increases.....(??) which will cause ARISsat-1 to 'move ahead'
of the
> > ISS over a few hours
> >
> > But didn't we say the velocity would be less than the ISS due to the
method
> >  of deploying it against the velocity vector ?
> >
>
> Interesting puzzler, eh?  From what I have read in the past, I think this
their logic.
>
> What they are trying to do is to separate the orbits of the ISS and
ARISSat as quickly as possible, to avoid the potential for a collision.
Consider the options:
>
> 1.  Throw it sideways to the ISS orbit.  The result is that twice per
orbit the two spacecraft's paths will cross, side to side.  Bad idea.
>
> 2.  Throw it ahead of the ISS (faster orbit speed).  This will raise the
orbit, slightly, and also make it a bit elliptical (up and down).  The
higher orbit makes the satellite go behind the ISS, but the elliptical shape
also means that the orbits will cross every orbit (but out of phase, so they
won't be at the same place when they do).  But, then as the ARISSat orbit
decays, they will get closer and closer, potentially getting back to the
same place.  Not good, either.
>
> 3.  Throw it behind the ISS (slower orbit).  As you note, this will lower
the orbit (and make it a bit elliptical), and initially the apogee of the
orbit will intersect that of the ISS.  Being in a lower orbit, ARISSat will
move ahead of the ISS, and over time, as the ARISSat orbit decays, the two
will diverge even farther.  So, this is the safest.
>
> At least, I think that's the logic.  If not, pass me some of that tuna...
>
> Greg  KO6TH

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