NASA predictions on satellite visibility - was Re: Few viewings

Philip Chien (
Fri, 8 Aug 1997 01:08:27 -0400

Craig Cholar writes:

> Here are the criteria that NASA uses to determine visibility...

> 1) Apparent spacecraft elevation greater than 15 degrees
> 2) Local solar elevation less than -6 degrees
> 3) Spacecraft in sunlight
> 4) Apparent spacecraft solar elongation greater than 90 degrees.

These parameters should seem very conservative for most Seesat readers, and
rightfully so.  Remember that the average 'joe nasa-fan' doesn't have much
skywatching experience, just interest in the shuttle and maybe having the
opportunity to see it.

By using extremely conservative parameters the odds of success are fairly
high if the weather cooperates.  And for unexperienced observers this is
certainly justifyable.  I would suspect that only a very small minority of
the folks who read the NASA predictions have the capability, or inclination
(no pun intended) to view anything dimmer than third magnitude.

Instinctively, based on the launch time, I can give a guess as to when I'll
start to have visible passes.  The opening of the launch vehicle was
constrained by sunrise on landing day at Edwards Air Force Base in
California.  Given the planned duration of the mission and time of year
that resulted in the 10:41 AM EDT launch time.  So since the shuttle
launched at the beginning of the window it's logical to assume that the end
of mission landing time (whether it's Florida or California) will be close
to sunrise.  And the passes before landing will be good visible passes,
including the couple of days before landing.

The SPAS platform aboard Discovery previously flew as the ORFEUS-SPAS on
the STS-80 mission last November.  And two days before landing I had an
*incredible* pre-dawn pass of both the shuttle and SPAS going past the Moon
and Mars (the only two celestial bodies bright enough to be seen that close
to dawn that day).

On the original planned landing day the weather was incredibly clear the
pass before the planned landing and I saw the shuttle go across the sky.
But was incredibly surprised to see it overtake another satellite going the
same direction!  It was obvious from the direction and brightness that it
had to be either Hubble or GRO, and it turned out to be GRO.  But the
theory by one ex-astronaut Dave Leestma, was that it was the alien
spacecraft coming to pick up Story Musgrave so he wouldn't have to return
to Earth!

Philip Chien [M1959.05.31/31.145//]