February 5, 2002

A BRILLIANT fireball streaks through the sky, breaking up into
pieces.  What was that?!  It was too bright to be a meteor.  
Did I see space junk re-entering?

PEOPLE who see *randomly* re-entering space junk (as opposed 
to the Space Shuttle and a few other deliberately deorbited 
objects) are *lucky*!  To start with (using generalizations),
only:

 50% of re-entries occur at night
 12% occur over *inhabited* land (.3 land * .4 inhabited: source)
 36% occur when/where the sky is clear (source)

That leaves .5 * .12 * .36 = 2.16%, or 46.3-to-one odds against 
a night re-entry over inhabited land.  

Another limiting factor, given a nighttime clear-sky re-entry 
over inhabited land, is how many people are in a location 
where they can possibly see the re-entry.  At night, most 
people are indoors most of the time.

Another factor is that due to the Earth's equatorial bulge, 
the percentage re-entering over the tropics is increased (due 
to more exposure to atmospheric drag).  Relatedly, probably
the oblateness (pear-shape, with the excess bulge in the 
southern hemisphere) of the Earth should be considered.

Another factor is the number of objects in low-inclination 
orbits (roughly 29 degrees from equator or less), all of 
which of course re-enter in or near the tropics.

One other factor is a significant number whose perigees are 
fixed in the far southern hemisphere (Molniya-type orbits, 
all of which are Soviet-Russian or USA military satellites 
and their upper-stage launch vehicles) and which thus are 
most likely to re-enter there.

Other details:  The northern hemisphere is 39% land, 61%
water; the southern hemisphere is 19% land, 81% water.  This
means that re-entries over the southern hemisphere are, in 
the most general sense, only one-half as less likely to occur 
over land.  Beyond that, about 90% of people live in the 
northern hemisphere.

THE RESULT of all of those factors is that it's not very 
likely for anyone to get to see a random re-entry.  So, if 
you have any reason to think that you might have a shot at 
seeing one, give it a try if at all possible!  As has been 
mentioned, that requires knowing about the possibility 
AHEAD OF TIME!

THE SINGLE BEST source of re-entry predictions is Alan Pickup:

 http://www.wingar.demon.co.uk/satevo/#decayPredictions

However, as has been mentioned, the best source of last-hours
information is the SeeSat mailing list, where another re-entry
analyst, Harro Zimmer, frequently publishes predictions as 
well as Alan Pickup.  You don't have to subscribe to SeeSat-L; 
it is archived automatically -- and very well! -- at this 
location:
 
 http://www.satobs.org/seesat/

There also is NASA's Orbital Information Group (OIG), who do 
the previously mentioned Sixty Day Forecast, whose URL is 
horrible -- but here's OIG's home page:

 http://oig1.gsfc.nasa.gov/scripts/foxweb.exe/app01?

Go to "Main Page", "Reports", "Sixty-Day Forecast Report".

For a well-organized explanation of various aspects of
re-entries, see the Aero Corporation's "Spacecraft Reentry Breakup Overview and FAQs":

http://www.aero.org/cords/reentry_overview.html

The Visual Satellite Observers Home Page also has a very good 
page on re-entries:

 http://www.satobs.org/re-entry.html

Thanks to Julian Grammer and Gene Heyler for helpful information
and suggestions.

Good luck, including clear, dark nights with many fireballs!

Ed Cannon -- ecannon@mail.utexas.edu -- Austin, Texas, USA

Satellites:  http://http://webspace.utexas.edu/cannonea/www/satellite.htm
Meteors:  http://http://webspace.utexas.edu/cannonea/www/meteorlinks.html
Home:  http://http://webspace.utexas.edu/cannonea/www/