tumbling vs. spinning - was re: Obs USA 144

Philip Chien (kc4yer@amsat.org)
Wed, 9 Jun 1999 12:29:21 -0400

James Nix <nixj@bellsouth.net>

>	Using Ted's latest on 25744  epoch 98155.487...Observed it about 14
> secs early, maybe .7 degree off track,tumbling and moving pretty quickly.

tumbling implies a satellite which isn't stabilized or is out-of-control
(e.g. a satellite which is no longer under control and its thrusters or
reaction wheels are no longer in use).

For most visible satellites a semi-consistent periodic flashing appearance
does mean tumbling (obvious examples rocket bodies, OAO-3, etc.)

But there are many satellites which spin for stablization.  Most are
earlier high altitude (geo) comsats (most Hughes models before the HS-601,
European Meterosat, etc.)

I suspect it would be difficult to tell the difference visually between a
tumble and a one-axis spin.  Spinning satellites can appear to be steady if
their viewed head-on (vis USA 40 in its parking orbit) but normally flash
at a regular periodic rate.

One of Ted Molczan's key discoveries with USA 40 is it was seen to flash
once every second, which seemed to be more like a figure chosen by an
engineer than a coincidence.  Obviously an out-of-control tumbler would not
be likely to settle in to an obvious rounded flash rate.

So the flash rate for USA 144 is an EXTREMELY important piece of
information in helping determine its identity and whether or not the
spacecraft is healthy or not.

Philip Chien, KC4YER
Earth News
world (in)famous writer, science fiction fan, ham radio operator,
all-around nice guy, etc.