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(meteorobs) Iridium satellites WRT re-entry
The Iridium satellites are cylinders with length of four meters,
diameter of 1.3 meter, and a mass of 689 kg:
http://www.friends-partners.org/~mwade/craft/lm700.htm
The mass includes of course the three "main mission antennas"
that produce the famous flares, and two solar arrays. I don't
know how much of the mass is propellant that would be spent
before re-entry. (One of the main reasons for deorbiting them
would be to reduce the risk of in-orbit propellant explosion, so
I would assume that almost all of it would be either burned or
vented.)
Given those data and the re-entry velocity of about 8 km/sec at a
very shallow angle, I wonder how much, if any, of the satellites
will survive to reach the surface. I wonder what mass object of
solar system origin would enter the atmosphere with similar
energy? (I don't know the equations for calculating that sort of
thing.)
Presumably the main mission antennas and solar panels (and other
fragments) will be shed along the re-entry path and might make
for an even more interesting display.
From what I've heard so far, it could be as long as two years
before the last ones leave orbit.
The re-entry of Compton Gamma-Ray Observatory (length, 9.1 m;
diameter, 4.6 m; mass 15,620 kg [including 1,900 kg propellant])
would be a truly spectacular show, but my guess is that they
will try to keep observers well clear of the predicted re-entry
field.
Different topic: Regarding flashing geosynchronous satellites
-- some flash brightly (up to at least +2 if not brighter) as
infrequently as once per two or three or more minutes, and
during each minute the field of background stars moves about
.25 degree to the west. For probably most of them as observed
from any given location, there's an episode of bright flashes
each night that may last only a few minutes. Probably most of
the ones that flash like this are actually not quite
geosynchronous and drift a few degrees from night to night.
Many also have a few degrees of north-south motion every 24
hours. Rapidly flashing satellites are obvious, but the ones
with infrequent bright flashes could be deceptive. For one
thing, their flashes are slow -- as long as 0.5 second, if not
longer, due to the slow rotation of the satellite. From
reading this list for a while now, I get the sense that true
point meteors must be fairly rare. I saw a couple that were
pretty close to points during the 1998 Leonids, but they
weren't perfect.
There are any number of other satellites that can flash
infrequently. One example that comes to mind was a solitary
flash someone reported that proved to match perfectly in time
and position by Midas 9, an old "early warning" satellite
orbiting in a high, circular, nearly polar orbit.
Ed Cannon - ecannon@mail.utexasdot edu - Austin, Texas, USA
http://wwwvms.utexasdot edu/~ecannon/meteorlinks.html
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