re: My folks think I'm crazy...
Walter Nissen (wnissen@freenet.tlh.fl.us)
Fri, 15 Oct 1999 11:29:18 -0400 (EDT)
gambaro@citenet.net (Patrice Gambaro) writes:
> I think I saw Iridium 2 go by (could have been something
> else) around +3 or +4 mag but I did not say a word... humiliation was
> already achieved, no need to add to it...
There are some special circumstances here, but your experience illustrates
a general rule: Never assume that the brightness estimate for a satellite
will be accurate. Maybe it will be pretty good; maybe not. Be prepared.
This problem affects far more than the Iridia. Even Mir, which used to be
quite reliable, has become unreliable, principally, it would appear,
because its solar panels are no longer so well collimated on the Sun,
resulting in many more bright, unanticipated glints. On the evening of
October 2nd, Jay Respler, a careful observer in NJ, reported a bright
glint from Mir. On the same evening, 2 careful observers from the DC area
reported a fine pass, but no such luck. Jim Nix the same from even
further southwest. This sort of pattern has become common.
This may well make very low altitude observations of Mir easier to see.
Observers interested in establishing "personal bests", take note.
Astronomers are particularly susceptible to being fooled by satellites.
I have often advised astronomers who don't like the unpredictability of
satellites to hunt more predictable prey, such as comets and meteors.
Satellites may seem to be more predictable than those rogues, like planets
and their natural satellites are, but they aren't; anything but.
Sometimes, you'll see magnitudes for satellites given with 2 decimal
places of inaccuracy. Pay them no mind. Such precision is not practical
at the present state-of-the-art. The first decimal place is often
useless.
Occasionally, I post a table displaying some of the structure of the
Iridial constellation. I felt compelled to construct the table and the
nomenclature used in it for almost precisely this reason, to prevent
misunderstanding and wasted effort and, if it comes to that, humiliation.
See my next message for the table.
Chasing the Iridia
I don't believe that it has been demonstrated that the state of the art
permits a prediction to discriminate reliably between a mag -8 and a mag
-6. I think mag -8 is the theoretical maximum, isn't it? Paul Maley made
some comments here about attitude maintenance (or pointing accuracy) and I
don't believe the needs of the communications operation require attitude
control within bounds which would make such predictions accurate. In any
event, I have made dozens or hundreds of magnitude estimates of Iridial
glints and often find the predictions from IRIDFLAR are a magnitude or so
off. The most recent such I made on Tuesday evening. I believe this is
roughly what would be expected from Paul's comments. You'll note that the
brightness of a glint declines very rapidly as you move away from the
center of the glint path.
We could organize an effort to make careful observations and improve our
predictions. A less organized effort would still permit the recent
experience of one observer to be used by another. If a glint is expected
on Thursday at mag -2 because the path is to the West of the observer, and
if the observed brightness is mag 1, then on Friday another observer at a
roughly similar latitude might look at a prediction for the same object
and seeing that the path also goes to his West might anticipate that the
object will be fainter than predicted, but if the path goes to his East
might think a much brighter event is likely.
I've thought that it would be particularly valuable to chase down the
attitudes of the spares, in the sense that such a determination would make
possible predictions of bright glints. This could begin informally with
OBS being posted here. Maybe if OBS are reported, the analysis will come,
too. My own OBS hint at likely success; i.e., I have suspected some
consistency in observing a couple of spares.
A more advanced result from such an effort might be an understanding of
the maintenance regime used by Iridium in the operation of the craft,
which in turn might lead to rather accurate predictions.
The table may be more than you want, but if you study the table you will
find the Iridia much more predictable. There is a lot of info in the
table. Unless you're smarter than Marilyn Vos Savant, don't expect to
take it all in in a single gulp.
Analysis of the elset for an Iridium can reveal a lot about it. The range
of mean motion for the operating objects is 14.34215 to 14.34220 with a
strong concentration between 14.34217 and 14.34218. At the moment, at
least, there is apparently a perfect correlation; i.e., every operating
Iridium, and no other, has a MM between 14.34215 and 14.34220. Some other
elements are somewhat less strongly correlated. Eccentricity and
inclination, for example.
On Tuesday evening, I ran IRIDFLAR down to 6th magnitude, as I usually do.
It generated the following:
Ir mag
P36 61 no prediction
P37 19 no prediction
P38 34 no prediction
P39 35/37 5.6
P3A 36 3.4
P30 5 2.4
P31 6 5.5
This is what I saw:
P36 61 glint mag 2?
P37 19 could not locate (clouds?)
P38 34 mag 5 or 6
P39 35/37 located late, glinted to mag 3?, possibly brighter
P3A 36 glint mag 2 or 3
P30 5 glint mag -1?? naked eye
P31 6 could not locate (clouds?)
The conditions were less than ideal, and the results also, but it should
be clear that attitude drift can make the predictions only a rough guide.
Which simply illustrates that the Iridia are satellites, not a
particularly special class, just ordinary satellites with ordinary
unpredictability. Though their brightness reaches extraordinary levels;
in most respects, their reputation as exceptional exceeds the reality.
I've previously posted numerous similar OBS; including double, triple and
even quadruple glints, for which there are, as yet, no predictions.
For the last year or more, I haven't really paid very much attention to
the precise magnitudes predicted by IRIDFLAR. If one is predicted to be
about -3 or brighter and at a sufficiently high altitude and sufficiently
late in the evening, I call my brother-in-law so he can look. If not, I
simply start observing the Iridia an hour (or so?) prior to the first
predicted one. Of course, I use the table to figure out which ones to
look for. I try to record whatever I see. Later, I usually look to see
how accurate the magnitudes were. Sometimes pretty bad, like on Tuesday.
Sometimes pretty good.
Cheers.
Walter Nissen wnissen@tfn.net
-81.8637, 41.3735, 256m elevation
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