Re: Few viewings of STS ahead

Walter Nissen (dk058@cleveland.Freenet.Edu)
Thu, 7 Aug 1997 19:23:39 -0400 (EDT)

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. 
 
I don't know if I would hang all of NASA with responsibility for these 
criteria, but bottom line:  These criteria are somewhat goofy. 
 
I will concede that if all of these criteria are met, a pass will likely 
be a good one; however, many good passes will be missed, and this is 
especially true of the more advanced observers prevalent here on SeeSat-L. 
And I will concede further that these criteria are useful. 
 
Let me go one by one, and hope that others with additional insights will 
post their thoughts as well: 
 
> 1) Apparent spacecraft elevation [sic] greater than 15 degrees 
 
This criterion is very arbitrary and pretty much unsatisfactory for both 
groups I focus on.  It is too small for the general urban public, who can 
seldom find objects below about 30 degrees, because they have badly 
obstructed and brightly lit horizons.  It is way too high for experienced, 
enthusiastic observers who can find the shuttle at much lower altitudes, 
certainly as low as 7 degrees and perhaps as low as 4, or even lower. 
(Typically, for literature from the aerospace industry, the term 
"elevation" is misused here, where the term "altitude" would be correct.) 
 
> 2) Local solar elevation [sic] less than -6 degrees 
 
This is about right, but again, arbitrary.  Under a typical hazy DC summer 
atmosphere, it is doubtful that much would be seen at such an altitude, 
but under a clear atmosphere, the shuttle can possibly be seen with the 
Sun above the horizon. 
 
> 3) Spacecraft in sunlight 
 
For the general public, this is the right criterion, but observers have 
reported seeing the shuttle payload bay lights from the ground.  (Set your 
software appropriately! and don't get confused!, easier said than done.) 
 
> 4) Apparent spacecraft solar elongation greater than 90 degrees. 
 
This is very arbitrary.  In fact, the best passes are not characterized by 
the largest elongations from the Sun.  A large elongation implies the 
shuttle is in the shadow.  But large elongation does tend to improve the 
pass in most cases (until the object passes into the shadow).  Some great 
passes, and many of the observable passes, occur at elongations less than 
90. 
 
 
Also, the idea that the criteria are independent is not fully correct. 
One bad parameter can spoil the show, but one or two marginal parameters 
can be compensated by other strong parameters, especially for experienced 
observers. 
 
Also, additional criteria are equally, or more, important.  Weather, 
obviously.  Spacecraft attitude; you don't want to be looking at (a 
reflection off) the dark tiles on the shuttle's underbelly.  Local 
conditions; mountaintops, rural sites and highrise roofs are far better 
than the average urban or suburban neighborhood. 
 
 
Having said all that, the general conclusion is correct for STS-85. 
QuickSat doesn't generate any passes at all from the first OIG elset, 
until Aug 18, for either DC or Cleveland. 
 
 
Cheers. 
 
Walter Nissen           dk058@cleveland.freenet.edu 
-81.8637, 41.3735, 256m elevation

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