Re: Cassini flyby

Jim Varney (sat_watcher@rocketmail.com)
Tue, 29 Jun 1999 07:37:04 -0700 (PDT)

---Randy John <skysat@home.com> wrote:

> Since Cassini will be in a hyperbolic orbit there will be NO TLE.  Standard
> TLE's do NOT support eccentricities greater than or equal to 1 so you must
> use a program like SKYSAT or Horizons.

Never say never :-)  I made a Cassini TLE.  The TLE is only for my local
area and is only for the night of Aug. 18.

You can make your own Cassini TLE by taking a short piece of the hyperbolic
trajectory and fitting an elliptical orbit to it. Download topocentric Right
Ascension and Declination pairs from the JPL Horizons site.  The pairs
should be for a short time span, say 5 or 6 hours. Then use a TLE-creation
tool to fit a TLE to the "observations."  As long as you only use the TLE
within the narrow time period and not try to predict outside of the time
span, the results should be reasonable.  Pretty much the same idea as the
moon TLE's. Just a matter of fooling the prediction software into giving the
numbers you want.

A much more useful approach, which I'm doing for the local astronomy club
newsletter, is to take the RA and Dec pairs and plot them on a Digitized Sky
Survey plate.  You can request a 0.5-degree plot online at

http://skyview.gsfc.nasa.gov/cgi-bin/skvadvanced.pl

I figure the DSS plate is necessary because by the time the moon sets on
Aug. 18 Cassini will be nearly 200,000 miles downrange and going away.  It
might be possible as a visual object in a large telescope, or as a CCD
object in a moderate scope.
__________________________________________________
Jim Varney   121.398W  38.458N 8m   Sacramento, CA
 jamesv@softcom.net, sat_watcher@rocketmail.com






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