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Why The Drift?

Perhaps some knowledgeable person could educate me on something.  When the
last shuttle mission was launched, I loaded fresh elements for the shuttle
and the ISS into my NOVA tracking software.  Of course, the shuttle was
constantly altering it's orbit to catch up with the ISS, so the shuttle's
orbital elements were only accurate for a short time.

But after the mission was over, I noticed something different about the
station's orbit.  When this week's school contact was announced, I noticed
that the announced AOS time was several minutes off from what NOVA was
predicting for the day of the QSO.

How could this be?  I'm guessing that the shuttle imparted some forces into
the station's orbit while the two were docked.  In short, I think the
shuttle "bumped" the station and altered its orbit slightly.  Several days
later after the shuttle was gone, this slight change in the station's orbit
resulted in several minutes difference in AOS / LOS times during the school
contact.  In other words, a slight course change resulted in a major
difference several days later.

Course, as soon as I saw that my predictions were off, I got new elements
from Celestrak.com and that corrected the problem.  With newer elements,
NOVA predicted when the ISS would fly within range of the school in Canada
down to the minute.

Do I just have gremlins in my computer, or is there something to my theory?
Your comments and physics lessons would be appreciated.

73, James Alderman, KF5WT
Dallas, Texas, USA

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