Several SeeSat-L contributors have reported that the TIPS tether (96029F
/ 23937) was much fainter in recent years than when it was deployed in
June 1996. I have confirmed and quantified this through analysis of
Russell Eberst's nearly 100 observations from 1996 Jul 07 through 2002
Mar 11.
The main finding is that the fading began soon after deployment and
continued for about 500 days. It was rapid at first and then became more
gradual. There has been little additional fading over the 2.5 years
ending March 2002. It is now nearly three magnitudes fainter than it was
a few weeks after deployment.
Below is TIPS' standard magnitude (1000 km, 90 deg phase angle) during
several periods:
Period Obs Std Mv
------------------- --- ------
Jul 1996 5 4.5
Aug 1996 - Jan 1997 32 6.2
Feb 1997 - Jan 1998 22 6.7
Sep 1998 - Mar 2000 19 7.2
Apr 2000 - Mar 2002 17 7.3
(the gap between Jan-Sep 1998 is due to a lack of observations)
The standard magnitude during each period was determined by linear
regression analysis of the 1000 km equivalent magnitude of the
observations versus phase angle. The regression line was evaluated at 90
deg phase angle to obtain the standard magnitude.
This method of analysis is not entirely satisfactory, because fading was
taking place during several of the periods; therefore, I performed
another analysis in which I subdivided the 1000 km equivalent magnitude
data by phase angle into four groups. Plots of all four groups against
time since deployment reveal the rapid initial fading, which was mostly
complete within about 500 days.
The apparently immediate onset of fading, initially rapid, then more
gradual, suggests to me that the cause was darkening of the tether due
to exposure to sunlight.
According to the paper, ATTITUDE AND ORBIT DETERMINATION OF A TETHERED
SATELLITE SYSTEM, K. T. Alfriend et al, "The tether for TiPS is made of
Spectra-1000 which is 2-3 mm in diameter. Woven in the center of the
tether is a yarn to make the tether puff up to increase its
survivability."
I am not certain which would be more likely to have darkened; the
Spectra-1000 or the yarn. The former is described as having "high
resistance to chemicals, water, and UV light":
http://www.spectrafiber.com/products/spectra_1000.html
Somehow, I doubt that the manufacturer envisioned it being exposed to
the UV of Earth orbit.
I will make the several text and Excel spreadsheet files of the analysis
available upon request.
Ted Molczan
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