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@hotmail.com writes:

From: G0MRF@aol.com

Date: Wed, 9 Feb 2011 15:45:36 -0500 To: clintbradford@mac.com; amsat-bb@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