
Some Hints For Naked Eye Satellite Viewing
==========================================

   Many satellites currently in orbit, including the MIR space station and the
shuttle when it is in use, can be seen with the naked eye under certain
conditions. First of all, the ground based observer must be in darkness
(obviously, or the sun light would obscure the satellites).  Where possible,
street lights and any other light sources (including the Moon !!) should also
be avoided. (Basically, if you can see lots of stars, then you have a good
chance of being able to see any passing satellites).  The second thing is that
the satellite must NOT be in darkness, i.e. the satellite MUST be lit by the
Sun so that it can reflect light down towards the observer. (Satellites are not
good light sources on their own, unless they are reentering !!). The
consequence of these two points is that the only time that low earth satellites
(below about 1000 km altitude) are going to be visible is either shortly before
dawn, or shortly after dusk.  Most people prefer the latter !!. Higher
satellites are potentially visible for longer, but they are usually too far
away to be seen.

   It is possible to just go and sit outside in the early evening and wait for
satellites to pass over. Although you may well see many satellites this way, it
is usually very difficult to find out later which spacecraft you have seen. A
much more satisfying approach is to go looking for a particular satellite, with
the knowledge of exactly when it will pass over your location (this also saves
a lot of time!), and this is made very easy with !Spotter.

Choosing a satellite
--------------------

   The number of satellites that you will be able to see depends on your
location. Most satellites launched by the shuttle, for example, are put into
orbits with an inclination of 28.5 degrees, which means that if your location
is more than about 40 degrees off the equator, then you are just not going to
be able to see them. The good news for Europeans, though, is that there are
very many satellites in sun-synchrounous orbits (with inclinations > 90 deg),
that are visible from any location. Also, MIR, one of the brightest orbiting
objects, has an inclination of 51.6 deg and is clearly visible from all over 
Europe, and although most shuttle missions operate at 28.5 deg, the occasional
mission does go to 57 deg (for example the STS 56 mission, scheduled for April
6th 1993), and these also can be seen very easily.

   Basically, then the rules are the following: Choose a large, low satellite
that has an inclination higher than, or not much lower than, your current
latitude. Unless you know a lot about satellites, choosing a large satellite
might be difficult, but most low satellites tend to be large enough to be seen,
and so this size criteria can be neglected. If you have never before seen a
satellite, then I suggest that you start with one of the easier ones like MIR,
or the shuttle if it is available. After this, you could try some of the
sun-synchronous satellites, for example ERS-1 or one of the SPOT satellites.
For people in lower latitudes, I would imagine that the Hubble Space Telescope
would make quite a spectacle, but I have not been lucky enough to see that one
myself.


Determining the Visibility Times
--------------------------------

   When you have decided which satellite you wish to try and see, then you can
use !Spotter to find out when the satellite will be visible. First of all, try
and obtain the most recent set of elements available for this satellite (file
'Elements' in this directory explains how to go about doing this), and then
generate a WIMPY, as described in the User Notes. The WIMPY will list all the
times that your chosen satellite is above the horizon at your location, and if
the eclipse entry for any particular time does not contain a '*', then the
satellite should be visible. Simple, isn't it ?

   I wish you success in your endeavours, and, I hope that you find !Spotter a
useful aid.

Stuart Martin, 3rd April 1993.   
 
