Superbird A is spinning-up

Matson, Robert (ROBERT.D.MATSON@cpmx.saic.com)
Wed, 30 Sep 1998 15:21:17 -0700

Hi All,

Superbird A (#20040, 89-041A) flash measurement data for
Monday night, September 28 and Tuesday night, September 29
from Belmont Shore, CA (33.7602N, 118.1295W, 5m).

89- 41A 98-09-29 03:23:33.1  RM 289.09 0.05  12.5  23.127  mag +2.5 --> inv
[23.127 +/- .004 seconds]

89- 41A 98-09-30 03:25:32.6  RM 404.70 0.04  17.5  23.126  mag +2.5 --> inv
[23.126 +/- .002 seconds]

Now here's the interesting part.  Since Superbird A's
inertial spin axis is nearly parallel to the earth's
equator, it is in theory possible to use flash times on
consecutive days to greatly improve the accuracy of the
measured rotation period.  For example, using these two
measurements, the "uncorrected" rotation period is
86519.5 seconds / 3741 = 23.12737 +/- .00005 (the error
assumes a day-to-day clock accuracy of 0.2 seconds).
I say "uncorrected" because both the sun and satellite
are moving relative to the earth.  However, I'll show
that this isn't a problem for the case of Superbird.

For the 9/29 time, the sun was at declination -2.30
degrees; for the 9/30 time, the sun was at declination
-2.69 degrees.  The difference is 0.39 degrees.  (The
satellite's declination has changed less than .001
degrees).  This means the satellite must rotate either
.195 degrees more or .195 degrees less to complete an
inertial rotation.  This will take only .0125 seconds,
which is insignificant compared to the assumed 0.2-
second day-to-day measurement error.

I can do the same day-to-day calculation for some of my
prior measurements:

89- 41A 98-09-15 03:09:00.7  RM 300.67 0.1  13 23.128  mag +2 --> inv
89- 41A 98-09-16 03:08:44.7  RM 312.16 0.1  13.5 23.123  mag +2 --> inv
89- 41A 98-09-17 03:10:23.9  RM 300.67 0.1  13 23.128  mag +2 --> inv
89- 41A 98-09-18 03:09:43.2  RM 219.67 0.1  9.5 23.123  mag +2 --> inv

15th --> 16th:  86384.0 / 3735 = 23.12825
16th --> 17th:  86499.2 / 3740 = 23.12813
17th --> 18th:  86359.3 / 3734 = 23.12783
28th --> 29th:  86519.5 / 3741 = 23.12737

Clearly, the satellite is very slowly accelerating.  Indeed,
on May 7th I measured the period at 23.236 +/- .016 seconds,
and on April 21st 23.243 +/- .011.  In other words, something
is spinning up this satellite, and I believe I know what it
is...

The sun!  In the vacuum of space, Superbird A and its solar
arrays are acting as a radiometer.  I have already proven
that Superbird's two solar panels are pointed in almost
opposite directions.  Since one side of each solar panel
is more reflective than the other, light bouncing off the
more reflective side transfers more momentum than light
absorbed by the other side.  So Superbird A will continue
to spin faster and faster.  In about 6 months its period
will be 23.0 seconds.  In 14 years it will be down to 20
seconds, and in 50 years it will be down to 15 seconds.
What an interesting satellite this is turning out to be!

Best wishes,

Rob