re: Welcome, and STS visits Mir

Philip Chien (
Sun, 7 Apr 1996 12:50:07 -0400

dk058@cleveland.Freenet.Edu (Walter Nissen) stated:

>An STS isn't going to head up to go home, so it will
>always lead on the way home; and conversely, trail on the way up.
>Subject, of course, to any actual operational difference, which we haven't
>seen yet.

Actually there are circumstances where the shuttle will be ahead of Mir on
the way up -- but it will be by half an orbit.  The phase angle between the
two spacecraft determines their targeting.

On the shuttle-Mir missions there are actually two launch windows each day,
of approximately 5-7 minutes in duration.  They overlap, resuting in a
combined window of typically 6 to 10 minutes.  (with a bunch of fancy
factors which can cut off a minute or two depending on other technical

One of the two launch windows assumes a day 3 rendezvous, the other assumes
a day 4 rendezvous.  I haven't worked out all of the math, but it's quite
possible, depending on when the shuttle launched within the window, for Mir
to be in a position where -- at least at first -- the shuttle will _appear_
to be in front of Mir.  But since the shuttle's in a lower orbit (they've
actually set the new low perigee record on a Mir rendezvous flight!) it
will quickly catch up with Mir during the rendezvous sequence.

Reminds me of a very confused NASA public affairs officer when two
spacecraft separated - and half an orbit away 'appeared' to be coming back
together.  Since then the JSC public affairs office has always talked about
the change in miles per orbit (as opposed to the instantaneous distance)
whenever they've discussed rendezvous!!

Philip Chien, Earth News - space writer and consultant  PCHIEN@IDS.NET
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