Re: Shuttle in Darkness Part 2

Rainer Kracht (R.Kracht@t-online.de)
Fri, 26 Jan 96 20:44 +0100

>Date: 25 Jan 1996 15:54:25 -0500
>From: "Bill Bard" <wtba@eci-esyst.com>
>Subject: Shuttle in Darkness Part 2

>Recently, Paul Maley (and someone else I think) reported seeing the 
>shuttle while it was in the earth's shadow. I think the estimated 
>magnitude was 7 and the shuttle was oriented with the payload bay towards 
>the earth.
>
>There are two possibilities that account for it being seen. One is that 
>moon light (instead of sun light) was illuminating the shuttle. The other 
>is that viewers on the ground were seeing the cargo bay lights. Regarding 
>the cargo bay lights, I found some info on the lights from a couple years 
>ago:
>
>  There are 6 floodlights in the cargo bay.  Each 160-watt light is rated 
>  at 40 Lumens per watt.  A seventh floodlight located on the forward 
>  bulkhead is 200 watts and is rated at 40 Lumens per watt.  The RMS 
>  light is 150 watts and is rated at 12 Lumens per watt.
>
>With the cargo bay oriented towards the earth, I would assume that you 
>would then have 6 floodlights pointing towards you for a total brightness 
>of (6 * 160 W * 40 L) 38400 Lumens. Paul reported that 4 floddlights were 
>on so then the brightness should be 25600 Lumens. Any comments on how to 
>convert from Lumens to magnitude brightness?

I think this can be done by comparing the Shuttle lights with the Sun.
Our Sun has a visual magnitude of -26.8 and provides us with about
140,000 Lumen/qm. A Shuttle with four floodlights at a range of 100 km
delivers 25600/(4*Pi*100000*100000) = 0.0000002037 Lumen/qm.
Thus, the magnitude difference between the Shuttle and the Sun is
2.5 * log10(140000/0.0000002037) = 29.6 magnitudes. Shuttle magnitude
is then -26.8 + 29.6 = 2.8.

This calculation can be done for various ranges and gives:

     Range (km)    Shuttle mag
       100             2.8
       200             4.3
       300             5.2
       400             5.8
       600             6.7
       800             7.3
      1000             7.8

Paul Maley observed STS-72 on 1995 Jan 12 at about 11:34 UTC below
Spica with mag 6.5 at a slant range of 300 km (elevation about 43 deg).
Thus, this observation is easily explained by seeing parts of two 
floodlights!

Other observations of Shuttles in darkness:

1993 Dec 04  Gary Emerson (STS-61)
floodlit payload bay mag 8 at 1500 km (elevation approx. 17 deg)

1994 Sep 10  Russell Eberst
22:38:49  STS-64  mag 6.4 at 275 km (elevation 85 deg)
22:39:13  STS-64  mag 5.9 at 335 km (elevation 54 deg)

1994 Nov 03  Rainer Kracht
17:22:50  STS-66  mag 5   at 375 km (elevation 40 deg)


By expressing magnitudes & ranges in Lumen one gets:

1993 Dec 04   mag 8     1500 km   47600 Lumen
1994 Sep 10   mag 6.4    275 km    7000 Lumen
              mag 5.9    335 km   16400 Lumen
1994 Nov 03   mag 5      375 km   47100 Lumen
1996 Jan 12   mag 6.5    300 km    7600 Lumen

Observations by Gary Emerson and by me seem to be exceptionally bright,
but can be explained with all floodlights turned on:

6 * 160 * 40 L + 200 * 40 L = 46400 Lumen.


Rainer Kracht   1996 Jan 26
0412188960-0001@t-online.de
R.Kracht@t-online.de