Re: Satellite Intrinsic Magnitude Data

Mike McCants (
Fri, 27 Sep 1996 18:14:24 -0500

Bill Bard writes:

>In Mike McCants QUICKSAT.MAG and Ted Molczan N2L files, the magnitude 
>data for some/all of the sats are different.

>Ted's data is specified for 1000 km 50% illuminated, Mike's 1000 km 100% 
>illuminated. To convert from 50% to 100%, ...

stdmag(50%) - stdmag(100%) = 0.8

I. e., from that difference in definition, one would expect Ted's
intrinsic magnitude to be 0.8 magitudes larger (fainter) than mine.

>Comparing the data between the two files, the differences is greater than
>that. For instance, Mike's file gives Vanguard 1's mag as 8.0, Ted's 12.4.
>I'm just trying to understand the magnitude data. What am I missing? 

You are (and I was too until Ted and I very recently communicated about
this) missing the fact that Ted is defining his intrinsic magnitude
as a "mean" magnitude and I am defining my intrinsic magnitude as a
"maximum" magnitude.

So this means that Ted is "tilting" a cylinder to compute the effective
average area visible and I am reporting a maximum magnitude which would
normally be seen when the cylinder is at its maximum observable size.

This difference in definition can easily explain 1 magnitude of difference
in "intrinsic magnitude".

In the past, Ted's file had only "computed" (from the size values)
intrinsic magnitudes.  Such a computed magnitude also must assume
some sort of reflectance value.  This is normally stated in the form:
a one square meter object with 40% reflectance will be magnitude 6.0
(for example) at a range of 1000Km.

My intrinsic magnitudes are based on my own (admittedly imperfect)
observations.  So if a given object is white or if (like Vanguard I)
it is "shiny" and has a specular reflection that is greater than
its diffuse reflection, then my observed intrinsic magnitude may be
1 or 2 or 3 magnitudes brighter than what would be expected from 
a 40% diffuse reflectance.

Or, if I repeatedly observe that a "J" object has stabilized with
one end pointing toward the earth, I may decide that its intrinsic
magnitude is now 6.0 instead of a more normal 4.0 for a cylinder
that is oriented for maximum reflectance.

Now that Ted has changed a number of objects to "observed mean magnitude",
there are a number of questions about how that value was derived and
what it really means.

But the bottom line is:

If an object is observed to be tumbling and varies by 2 magnitudes,
my intrinsic magnitude is supposed to predict its maximum brightness
and Ted's intrinsic magnitude is supposed to predict its mean
brightness and the latter is 1 magnitude fainter than the former.

Mike McCants