re: STS-91, Mir manuevers, long term tracking predictions

Philip Chien (
Tue, 28 Apr 1998 22:57:21 -0400

brixham <> said:

>This is the first Shuttle/Mir docking that I will have a chance to see. I
>know its probably the last as well.

see as in see in person, or see as in see as it goes over your location?

Jason Hatton <> said:

>Although this is the last Shuttle to Mir mission

make that last *planned* mission.  It's certainly possible NASA may be
asked to help with Mir's safe deorbit and reentry using the shuttle.  And
if there are more delays to the space station program, it's possible NASA
may be pressured in to continuing the shuttle-Mir program, even though it's
reached the end of diminishing returns in terms of the amount of science.

Don Gardner said:

>Unless you'd like to see the OMS burn you might as well forget about the
>prelaunch elsets and pay close attention to the launch time and
>docking/undocking times.   The pattern for the Mir dockings seems to be that
>the Shuttle launches about 20 minutes after the Mir makes a norhtbound pass
>over the KSC.

No.  The only constraint on the launch time is when Mir's orbital plane
intersects where the shuttle's orbital plane will be after it enters orbit.
(roughly the same as the orbital plane passing over the Cape).

On the earlier shuttle-Mir flights the launch window was roughly 10 minutes
centered around the optimal time.  On the recent missions the first half of
the window is purposely ignored as long as everything's going okay (e.g. no
prediction of lousy weather at the last second of the window or something
which is expected to break down on the ground if you don't get off early -
really unusual circumstances).  Going for the middle of the actual window
at the "in-plane" time permits additional propellant reserves.

Contrary to the myth which some press and "experts" are spreading it does
*NOT* increase the available payload margin.  Think about it - you add the
additional payload which you get if you to launch at the moment of the
in-plane time and there's a technical delay which causes you to have to
launch at the end of the window.  If that occurs you don't have enough
propellant to reach Mir!

What an in-plane launch does offer is additional _margin_.  If you launch
at the in-plane time you've got a bit more propellant in case there's a
problem (e.g. having to back off and try a second rendezvous) or the
propellant can be used for a fly-around for pretty photos after you undock.
If you launch at the end of the window you still have enough propellant to
complete a nominal mission.  This will be the standard approach for the
shuttle to ISS missions.  So if a shuttle launches at the in-plane time
there may be enough propellant for a flyaround, but a delayed launch would
probably result in not enough excess propellant for the flyaround.

The actual position of Mir in its orbital plane, and the sunlight
conditions at the Cape have very little significance in terms of
determining the launch time.

A couple of examples off the top of my head

For the STS-79 and STS-81 launches Mir passed over Florida shortly after
the shuttle launched.  I tried to convince the launch directors to delay
the launches to the in-plane time but they wouldn't listen to me. :-(  If
they had delayed those times then Mir would have been almost exactly
overhead when the shuttle launched!  As it stood both Shannon Lucid and
John Blaha looked for the shuttle launch, with Shannon successfully
observing Atlantis - and listening in on the KSC-area amateur radio

On the STS-84 mission Mir passed over the Cape about an hour after the
shuttle's launch.  It was a visible pass and I got a crowd together at the
Press site to view Mir.  People kept asking "Is that it?  Is that really
Mir?"  I replied "Sure - can't you see the Russian flag on the side? or the
cosmonaut waving at us?"  Okay, people normally give me strange looks ...

About 20 minutes later Mir passed within range of its European ground
stations and commander Vasliy Tsiblev reported that they saw the lighted
launch pad.  Naturally I told folks that he was waving at me while I was
waving at him ...

The shuttle starts playing 'catch up' with Mir and the phase angle between
the shuttle and Mir determines when the shuttle gradually raises its orbit.
So during the ascent phase the shuttle can be behind Mir, or in front of
Mir (which is actually really really far behind Mir from an orbital
mechanics point-of-view).  As the shuttle's orbit is raised it gradually
catches up with Mir until the shuttle reaches Mir's altitude.

During undockings the shuttle is ALWAYS ahead of Mir since it goes in to a
lower orbit.  On the STS-91 mission the shuttle will remain in space for a
couple of days to operate its secondary payload, the Alpha Magnetic
Spectrometer, so it's possible it may 'catch up' with Mir (obviously in a
lower orbit though).  I haven't seen the altitude yet for those days, so I
haven't calculated whether or not the shuttle will actually 'lap' Mir.

The STS-91 mission is currently scheduled for launch on May 28th at 8:05
pm.  If the launch date changes it moves about 22 minutes forward each day
due to Mir's precession.

And just to cross thread a couple of discussions, the more accurate
satellite prediction programs which accurately calculate the various
orbital pertubations will give fairly accurate positional information for
Mir in to the future.  But quick-and-dirty tracking programs lose accuracy
as you try to predict further in to the future.  It's a tradeoff the
programmer has to make - speed vs. accuracy.

NASA and Russia have an agreement which specifies the minimum and maximum
altitudes for Mir during joint shuttle-Mir activities.  Obviously you don't
want Mir to be at a higher altitude than the shuttle can practically reach
(e.g. where you'd need to offload a bunch of cargo) or too low where the
Earth's atmosphere makes it more difficult to calculate the shuttle's orbit
accurately (see - NASA's got the same problems!).  I forget the exact
values, but I seem to recall that a Progress near the early joint flights
(STS-63 STS-71 timeframe) didn't boost Mir as high as it normally would
have because of that agreement.

In addition the Russian Soyuz launch vehicles are now an older less
powerful version which uses ordinary fuel instead of a heavily refined more
efficient fuel which is now too expensive to obtain.  Hence the lower the
altitude for Mir the more cargo which can be carried on a Soyuz or Progress.

Mir is an incredibly large object with a very large surface area in a
fairly low orbit.  So it's got *LOTS* of drag.  Whenever anything's tossed
overboard (old spacesuits, miniature satellites, old spent thruster units,
etc.) they quickly move in to different orbits because of the difference in
drag and there's no practical chance for a recontact.

In fact Mir holds the world record for the longest duration for an object
in a real low orbit (e.g. MM > 15).  And that should be fairly obvious - no
other object in space has had so many resupply ships continiously boosting
them back up as drag reduces their altitude.

Not continiously, but on periodic occasions the Progress spacecraft are
used to boost Mir's altitude to make up for the altitude lost by drag.

Philip Chien, KC4YER
Earth News
world (in)famous writer, science fiction fan, ham radio operator,
all-around nice guy, etc.