Re: Superbird A from SoCal

Jim Varney (
Tue, 01 Oct 1996 22:10:39 -0700

Rob Matson wrote:

>Now that the initial excitement of seeing this bird has past, I can
>concentrate on recording the exact UTC times of the flashes.  My feeling is
>that this satellite is rotating about an axis roughly parallel to the earth's
>current velocity vector.  This would cause the specular reflection to rapidly
>sweep either NNW to SSE or SSE to NNW over the earth, while slowly walking
>eastward with each sweep.  (At this time of year, from the satellite's
>perspective at the time of flashing, the sun is to the left of the earth, and
>the earth's axis is tilted around 20 degrees counterclockwise from vertical.)

Enjoyed reading your observations and interpretations.

If by "current velocity vector" you mean a vector tangent to the earth's
orbit about the sun, then I would respectfully disagree -- i think the 
rotation axis is closer to (but not equal to) the sun-earth vector.

The most likely rotation axis is

+-------+ +---+ +--------+
|       | |   | |        |
+-------+ +---+ +--------+

like a ceiling fan with blades turned 90 degrees.

A flat spin (like a propeller) is ruled out because it would appear as
a steady glint.

A twisting about the long axis is possible, but not likely for moment of
inertia reasons.

A complex tumble is possible but the regular flash pattern would seem to
preclude that.

I think the axis is more or less in the plane of the orbit, and is pointing
a little to the "left" of the sun to produce the 107.4 deg/2 = 53.7 deg
angle of incidence I calculated earlier.  

>(4.14 x 10^6 watts/sr) / (3.88 x 10^7 m)^2 = 2.75 x 10^-9 W/m^2.  That
>would correspond to a visual magnitude of around +0.4.  This is around 4.4
>times brighter than magnitude 2.0, so perhaps the projected area of the solar
>panel is smaller, or the reflectivity lower, or the quality of the reflection
>is poorer (spread into more than a half-degree cone).  Or perhaps the flashes
>are indeed this bright, but because of their short persistance the eye
>"registers" them dimmer.

Well, we can rule out smaller area of panel.  According to AWST, Superbird
has a wingspan (now is that a fitting metaphor or what?!) of a whopping
70 feet.  The body, judging by a photo with a worker standing next to it,
is a cube 10 ft on a side.  I would put the solar panels on the order 
of 10 sq m each.

West coast folks: if you haven't seen this bird yet, don't wait much

 -- Jim

Jim Varney       | 121^ 23' 54" W,  38^ 27' 28" N   |           Sacramento, CA
Member, SeeSat-L |           Elev. 31 ft.           |                         
Member, Sacramento Valley Astronomical Society      |