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

[Rick Tejera]

Sent from my iPod
Rick Tejera
Editor, SACnews
Saguaro Astronomy Club
www.saguaroastro.org
K7TEJ

On Feb 10, 2011, at 17:17, "Bob Bruninga" <bruninga at usna.edu> wrote:

>> 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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