D. Shemansky e:\Cassini\titan\occult\Saturn_titan_occ_design_boundaries.doc 2/2/2007

D. Shemansky
Occultation design boundaries:
Saturn and Titan need different planning ground rules because of atmospheric scale. The
basis for the limits I lay-out below are from direct experience and the general need for the
I0 spectrum to have a much higher quality in counting statistics than the extincted
spectra in order not to limit the noise level in the I/I0 data.
Distance limit to planet:
At a S/C-planet range (distance to planet center) (sc_p) of 10RS the diameter of the sun is
630 km. This is OK for exploring the H distribution and also for lower vertical resolution
measurements of the H2 distribution as long as we have other higher resolution
measurements from closer occs as reference standards. The H2 scale height (H) at the top
of Saturn at this time is 140 km (T = 350 K). Evidence indicates T may increase as we
move toward solar max. The atomic hydrogen thermally connected would have H = 280
km. The expectation, however, is that a significant non thermal component of the
population of atomic hydrogen from a source at effective temperature as high as T =
60000 K would also be present at the top of the atmosphere. We can take sc_p=10 RS as
a loose limit for the S/C-planet range for an acceptable occultation. sc_p= 20 RS would
be questionable as a useful experiment.
sc_p <=12 RS.
Impact parameter to planet center start/end of occultation (r_lim):
For the solar occs r_lim >= 2 RS.
For stellar occs r_lim >= 1.2 RS.
For solar occs: sc_p <= 50 RT
For stellar and solar occs r_lim >= 2.75 RT