Superbird A quarter-period

ROB MATSON (ROBERT.D.MATSON@cpmx.saic.com)
18 Feb 1997 17:47:08 -0800

I've come up with an intriguing possibility to explain Mike McCants' and Ed
Cannon's observation of Superbird A from Austin, Texas, from 02:06-03:58 UT
Feb. 17th.  To recap, they were using a 12.5" telescope (among others) to
maintain a continuous watch on the satellite until shadow entry.  During this
period, they observed two episodes of bright flashes, one centered around
02:35, and the other around 03:40.  But what REALLY interested me were the
following remarks:

"When first acquired and in between the episodes,
Superbird A had a regular tumble from about magnitude
12 to invisible.  The period was 5.85 seconds.

In each episode, flashes were initially noticed at about
10th or 11th magnitude and these flashes gradually
increased in amplitude until they became about 4th
magnitude.  The duration of increase was about 10 to
15 minutes.  The flashes were assiociated with one of
the regular 5.85 second tumbles.  Even the brightest
flashes were preceded (by 5.85 seconds) by a tumble
to about 12th magnitude and occurred during (in the
middle of) such a tumble.

The flashes had a 23.4 second period and there were
never significant flashes seen at the half-way time of
11.7 seconds."

How's this for an explanation:  Mike and Ed have been observing near specular
reflections off the BODY of Superbird A, in addition to specular reflections
off the solar panels at each of their "episodes"! This would explain the
5.85-second period -- a flash off each surface of the satellite.  (The
satellite body is 2.41 x 2.58 x 2.2 meters).  Every 4th flash, the satellite
has done a full rotation.  Since their bright flashes are in phase with this
5.85-second period, the solar panels must be close to (but not quite) parallel
to one pair of the satellite body surfaces.

But Mike and Ed's observation is very valuable for another reason:  if all the
quarter-period flashes are of roughly equal brightness, the satellite spin
axis must be very close to parellel to all four flashing sides.  And that
means the satellite's dynamics are quite regular (i.e. a nice symmetric axis,
with no complex wobble).  However, this axis may still be precessing at a slow
rate -- a rate which can be determined from continued observation.  --Rob