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

[G0MRF@aol.com]

Thank you Greg.
 
Makes sense to me now.  Also, just calculated the velocity of a 90  minute 
LEO orbit and the velocity of a geostationary 24 hr orbit using  
circumference and time.  Found that the velocity at LEO is higher, so  presumably the 
decrease in orbit altitude of Arissat due to decay must also add  
acceleration. (Gravity)
 
73
 
David  G0MRF

 
 
In a message dated 10/02/2011 05:26:00 GMT Standard Time,  
ko6th_greg at hotmail.com writes:

From:  G0MRF at aol.com
> Date: Wed, 9 Feb 2011 15:45:36 -0500
> To:  clintbradford at mac.com; amsat-bb at amsat.org
> Subject: [amsat-bb] Re:  ARISSat-1 - Dumb Question
 

>  
> Hmm.  This is a bit of a brain teaser...
>  
>  So if the satellite is deployed towards the rear of the ISS, it's   
velocity 
> will be slightly lower. 
> So does that mean it will  go to a lower orbit- (Same as firing retros  
to 
>  reenter)?
>  
> 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 ?
>  
> Time for me to have a Tuna sandwich. We all  know it's probably full of  
> Dolphin...and they are really  clever.
>  
> David G0MRF
>   


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