Satgen 624  Media Misinformation  by GM4IHJ  (BID SGEN624) 2001-03-10

Several nervous neighbours of GM4IHJ located at 56.125N 3.6W , have
been alarmed by a newspaper illustration accompanying an article on
the proposed deorbit of space station Mir. The illustration shows the
Mir track passing directly over Scotland and Scandinavia. Presumably
to educate those of us unfamiliar with Mirs real track , but actually
adding to the confusion.

It is probably too much to ask of the media that they get their
illustrations right. But as this kind of error is likely to be
repeated elsewhere. It is perhaps of interest to discuss, just
exactly where satellite orbits take them, what regions they overfly,
and what regions they do not.

The important feature constraining the satellites track around the
globe, is the inclination angle its orbit plane makes with the earths
equatorial plane. In Mirs case this is an angle of 51.64 degrees
measured anticlockwise from the equatorial plane to the orbit plane.
All of which means that Mirs track is confined between 51.64 degs of
latitude above the equator , down to 51.64 degrees of latitude south
of the equator. So at some time of day Mir will pass near all
locations between these two latitudes. Which means in the case of the
British isles , Mir can never come higher than a latitude line
circling the earth just north of London, ie nowhere near Scotland or
Scandinavia. Equally important , is the fact that of all the orbital
parameters (period, semi major axis, right ascension of the ascending
node etc ), inclination of the orbit plane is the most resistant to
change. A fact worth bearing in mind , when one considers whether we
will be able to shift AO40 very far from its present , less than
exciting equatorial orbit.

Mir controllers are said to be aiming to put Mir down into the South
Pacific. The natural place to do this if you look at the map of the
world between 51N and 51S. Noting the lack of any major continent in
this region and very few if any inhabited islands. At first glance
the deorbiters of the 80+ Iridium satellites due for extinction end
2004 , have a very different situation . From their near polar
inclination orbits it should be possible to send them straight down
the Pacific , provided the populated islands can be avoided.
Controllers of satellites in equatorial or near equatorial orbits (
ie some of the Orbcom satellites ) have an awful lot of land under
their satellites flight path, but again the favoured spot would seem
to be the wide Pacific rather than the narrow equatorial Atlantic.But
in any situation where the orbital inclination angle is less than
about 50 degrees , some consideration has also to be given to the
safety of shipping in the drop zone. 

Whatever happens to Mir should hopefully be well documented. The
descents of SkyLab and Salyut 7 , were not closely observed. By
contrast several commercial aircraft have been booked to carry
observers high above the clouds of the Pacific to get a ringside view
of Mirs demise. So perhaps we may see the entry on TV shortly after
it happens ,if someone gets good pictures.