STS88 and ISS: Different Behavior

Jim Varney (
Tue, 15 Dec 1998 07:15:47 -0800

Good morning from Sacramento.

My 7 year old daughter and I observed both STS-88 Endeavour and the 
Zarya/Unity space station modules on Dec 15 at 1413 and 1415 UTC, 
respectively.  Endeavour led Zarya/Unity by 108 seconds.

What was striking was how different they appeared as the phase angle 
changed.  It was a northwest to southeast pass, with both objects going 
into the brightening morning twilight.  STS-88 was immune to the effects of 
phase angle.  It stayed bright at about mag -1 the whole pass.  Zarya/Unity 
was also -1 in the northwest but dimmed shortly after culmination and faded 
badly down to 3rd mag as it sunk in the southeast.

The striking difference in behavior showed that magnitude/phase angle 
models based on theoretical spheres don't always hold up that well.  I 
assume the shuttle was flying "white side down" and that helped the 
brightness considerably in the east.

Looked for but did not see the two microsats that were released yesterday.


Jim Varney
Member, Sacramento Valley Astronomical Society
Sacramento Iridiums