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amstrad
2002-Jun-16, 03:16 AM
I just watched the the shuttle and ISS go over houston, 12 and a half hours after undocking. The shuttle lead the station by 3 minutes 41 seconds (I'm taking NASA's JTrack 3D's word for which flew over first). So my question is: What does the shuttle do after it undocks and does its fly-around?

Based on the fact that it is leading the station, I would guess that it slows down to drop to a slightly lower orbit, then speeds back up to re-attain circular orbit. This lower orbit causes the shuttle to slowly outrun the station.

Am I thinking correctly?

roidspop
2002-Jun-16, 03:57 AM
I watched the station go over tonight at 8:55 MST, but I failed to see the shuttle. What visual magnitude would it have had? The sky was bright enough that it may have swamped the light from anything dimmer than around 1st mag.

...besides, this is a good excuse to post my way out of badnewbiehood.

amstrad
2002-Jun-16, 04:28 AM
Heavens-Above (http://www.heavens-above.com) put my ISS pass at about -0.5, but at zenith it compared to Venus (maybe because there is less light polution at zenith, so the satellites appeared bright, while venus, on the horizon, a little dimmer than normal). The shuttle seemed a little brighter than ISS. They both passed close to Arcturus, which is zero magnitude, and both outshone it.

Silas
2002-Jun-16, 07:32 PM
On 2002-06-15 23:16, amstrad wrote:
I just watched the the shuttle and ISS go over houston, 12 and a half hours after undocking. The shuttle lead the station by 3 minutes 41 seconds (I'm taking NASA's JTrack 3D's word for which flew over first). So my question is: What does the shuttle do after it undocks and does its fly-around?

Based on the fact that it is leading the station, I would guess that it slows down to drop to a slightly lower orbit, then speeds back up to re-attain circular orbit. This lower orbit causes the shuttle to slowly outrun the station.

Am I thinking correctly?


Orbital mechanics are weird... Larry Niven covers this in some cute detail in "The Integral Trees." Forward equals up; Up equals backwards; Backwards equals down; Down equals forwards."

I don't know if the shuttle does exactly what you described....but what you described would work.

Silas

Wally
2002-Jun-17, 01:37 PM
Based on the fact that it is leading the station, I would guess that it slows down to drop to a slightly lower orbit, then speeds back up to re-attain circular orbit. This lower orbit causes the shuttle to slowly outrun the station.

Am I thinking correctly?


Ok. This gets back to a question I had on another forum. Isn't it the actually the opposite? If the shuttle slows down, wouldn't it in fact move into a higher orbit and fall behind the station???

amstrad
2002-Jun-17, 02:32 PM
This site has some nice explainations of orbital maneuvers:

Orbital Mechanics 101 (http://aerospacescholars.org/Cirr/SS/L2/orb1.htm) [aerospacescholars.org]

traztx
2002-Jun-17, 02:38 PM
"If the shuttle slows down, wouldn't it in fact move into a higher orbit and fall behind the station???"

"Slowing down" means firing a rocket into the direction of motion. This causes the shuttle to go from a circular orbit to an elliptical orbit with an apogee (high point) at the radius where the rocket is shut off and a perigee (low point) closer to the planet.

The period (how long it takes to complete an orbit) will be shorter.

To a ground obserber, objects with shorter periods move ahead of objects with longer periods, even though the shorter period object is moving at a slower velocity. It seems this way only becasue they are closer. You can make your hand move ahead of an airplane in your line of sight, but your hand is certainly not moving faster than that airplane.

Simon
2002-Jun-17, 06:30 PM
This (http://www.medphys.ucl.ac.uk/~martins/orbit/orbit.html) nifty little game gave me a better feel for orbital mechanics than any text ever did. It's a big download though.

traztx
2002-Jun-17, 08:07 PM
I should correct/clarify this...

"To a ground obserber, objects with shorter periods move ahead of objects with longer periods, even though the shorter period object is moving at a slower velocity. It seems this way only becasue they are closer. You can make your hand move ahead of an airplane in your line of sight, but your hand is certainly not moving faster than that airplane."

The velocity is only slower at the apogee. At that position, I think you would see the ISS moving ahead from your Earth point of view. But as the shuttle falls closer to the Earth, it will increase velocity and move ahead of the ISS. When both objects have completed 1 revolution, the shuttle will be in the lead (since it's elliptical major axis became smaller during the rocket burn it's period is shorter).