RE: Iridium visibility? & Vandenberg launches

Ted Molczan (molczan@fox.nstn.ca)
Mon, 20 Jan 1997 01:26:22 -0500

Jake Rees wrote:

[regarding Iridium]

>Does anyone have a notion as to the potential visibility of these
>satellites once in orbit? In browsing the web, I saw a small image
>and it looks like a dark color scheme so that doesn't look too
>favorable for the visual satellite observer. Or am I wrong?

If someone can provide me with the dimensions of the spacecraft's main 
bus, I can provide an estimate of its standard magnitude. For now, my 
best guess, based on its mass of 689 kg, and allowing for an area to 
mass ratio of, say, around 0.004 m^2/kg, is a mean cross-sectional 
area of about 3 m^2.  That would result in a std mag of about 6.8. 

Under favourable illuminating and range conditions, the satellites 
would reach about 5.5 - 6 magnitude. These are mean values, and given
the satellite's bus is elongated, it could be about 0.5 mag brighter,
about 1 mag fainter, depending upon its orientation.

>Also, I'm attempting to be an avid, gung-ho, Vandenberg space launch
>observer. 

We would appreciate your assistance.

>Does anyone know if there are radio frequencies that can be
>monitored to help me keep on top of exactly when launches will occur?
>Are private and commercial pilots given a "heads-up (or heads-down)"
>on radio when a launch is about to happen? I have some web pages 
>bookmarked and some hotline phone numbers but I'm looking for more
>ways to be on top of launch situations.

It seems that you are doing a good job already. Pilots are issued NOTAM
(Notice To Airmen) warnings, advising them of the coordinates that 
outline the area to avoid. They can be obtained a day or two prior to
launch from the U.S. Coast Guard. Presumably, air traffic controllers
would have the information also.

>Incidentally, I saw the Titan 4 launch Dec. 20. 

>It launched at 10:04 AM PST and Ted Molzcan had predicted
>as I remember 10:07 plus or minus 15 min. I was amazed at his 
>accuracy because the launch window was 3.5 hrs long. 

Glad you saw it. As for my prediction accuracy, it is not as 
amazing as it might appear. Ever since the western-plane Keyhole
was deorbited in May'96, the best bet was that the December launch
would be a replacement. The Earth rotates VAFB under the south-bound
portion of the western KeyHole plane once per day, within a few 
minutes of 18:00 UTC.

>I think there is supposed to be another Titan 4 in April. 

The KeyHole constellation appears to be in good shape now, with fresh
spacecraft in both planes, and one back-up (as of fall'96), but the
two Lacrosse spacecraft are now quite old, so I am "betting" it will
be Lacrosse 3, heading into a 68 deg, 680 km circular orbit. I will
try to estimate the time and search orbits, once the date and 
launch period are announced. My success will depend on whether or not
Lacrosse 2 is still operation, and my ability to guess the desired 
orbital plane spacing. Most likely, I will identify several possible
planes, each with different launch time.

>My "heads-up" on the
>Dec. Titan 4 launch was postings in SeeSat. Please keep up any space
>launch postings.

There are quite a few people on SeeSat-L who have the ability to do
this, so in the unlikely event that I am not available, someone is
likely to do the analysis, and post the results.

Clear skies!
Ted