August 3, 1993
As you undoubtedly know, the next edge-on presentation of Saturn's
ring plane will occur in 1995/96. This apparition will be a fairly
favorable one, with the solar crossing and one of three earth
crossings occuring within two months of opposition in mid-September
1995. The Sun will cross the ring plane on 19 November 1995, and the
Earth on 21 May and 11 August 1995, and again on 11 February 1996 (see
Fig. 19 in Showalter et al.  Icarus 94, 451).
As in 1980, these events will offer unusual opportunities for
observations of the rings and inner satellites, as well as planetary
eclipses and mutual occultations and eclipses by the larger satellites.
In order to stimulate discussion of observational strategies and plans,
we plan to organize a short workshop on this subject at the DPS meeting
in Boulder, Colorado, during October 1993. The workshop will be held on
Wednesday, October 20, from 2:00 - 4:00 pm, and will be listed in the
third DPS mailing. It is intended that a more formal workshop will be
organized under the auspices of the Planetary Data System Ring Node
sometime in 1994, in Tucson.
A brief list of the kind of observations that may be made at this
time follows, though it is undoubtedly not exhaustive. We have also
included some references to reports of similar observations in 1966 and
1980, and would be pleased to hear of additional references on this
subject, or of other types of observations being planned.
(1) Observations of the tenuous E ring will best be made very close to
the moments of the earth's crossing, especially on 11 August 1995, as
well as during the period when the sun and earth are on opposite
sides of the ring plane (see below). During the latter periods the
scattered light from the main rings will be greatly suppressed. Most
of our knowledge of the E ring's vertical and radial structure comes
from the analysis of CCD observations obtained by Baum et al. in
1980. It may even be possible to detect the fainter G ring.
Feibelmann  Nature 214, 793.
Feibelmann & Klinglesmith  Science 209, 277.
Baum et al.  Icarus 47, 84.
Larson et al.  Icarus 47, 288.
Dollfus & Brunier  Icarus 49, 194.
Showalter et al.  Icarus 94, 451 (E ring).
Showalter et al.  Icarus 103, 124 (G ring).
(2) Observations of the photometric thickness of the rings, as seen
edge-on, and of the precise time of the earth's crossing of the ring
plane will also best be made at the 11 Aug. 1995 crossing, although
it may be possible to get data for 21 May 1995 and/or 11 Feb. 1996
also. It is now known that the thickness is probably due to bending
waves and embedded satellites, or even the projected E ring, but it
is definitely of interest to determine the time of ring plane
crossing as accurately as possible. Recent stellar occultations have
led to a tentative detection of the precession of the ring plane due
to solar torques (French et al. 1993), and the observed rate implies
an advance of ~2 hours in the time of ring plane crossing after a 15
yr interval (see Nicholson & French, 1993 DPS Abstract). Accurate
timing of the crossing(s) could conceivably result in a confirmation
of this precession rate, or at least provide an essential zero-point
for future studies. Dollfus  determined the time of the 18
Dec. 1966 earth crossing to +/-2 hrs, but I have been unable to find
such an absolute determination from the 1980 events.
Focas & Dollfus  A & A 2, 251.
Dollfus  A & A 75, 204.
Fountain & Larson  Icarus 36, 92.
Brahic & Sicardy  Nature 289, 447
Sicardy et al.  A & A 108, 296.
French et al.  Icarus 103, 163.
(3) Observations of the 'dark side' of the main rings, and of the C
ring, are possible during two extended periods when the sun and
earth are on opposite sides of the ring plane: 21 May - 11 August
1995, and 19 November 1995 - 11 February 1996. During the latter
period, the earth will be up to 2.5 degrees above the ring plane
while the sun is in or below the plane, offering an excellent
opportunity to observe light transmitted through the C ring and the
Barnard  A.J. 610, 79.
Focas & Dollfus  A & A 2, 251.
(4) During the 1993 - 1996 period there will be a very large number of
eclipses and occultations of the satellites by Saturn (and some by
the rings), as well as an extensive set of mutual satellite
occultations and eclipses in 1995. A listing of predicted eclipses
and mutual events has been submitted by Arlot & Thuillot to Icarus.
A&T give data for 163 selected eclipses by Dione, Rhea, Titan,
Hyperion and Iapetus. Apart from the two Iapetus eclipses by Saturn
in May and July this year (Soma 1992), the eclipses commence in
October 1994 and continue through to August 1996, but are most
readily observed well away from opposition or conjunction. The
mutual events, which occur only near the passages of the earth
through the ring plane, last from January 1995 to September 1996,
with the highest frequency in November 1995 - January 1996.
There are a handful of early visual eclipse and mutual event
observations, but the only published photoelectric observations of
Saturnian satellite mutual events are by Aksnes et al. , who
report times for a total of 14 events involving Enceladus, Tethys,
Dione and Rhea (S2 - S5). They report post-fit residuals of 80 km
both in-orbit and out-of-plane, corresponding to approx. 0.01" in
astrometric accuracy. These data, though few in number, are by the
most accurate of all the earth-based observations of the Saturnian
satellites used by Harper & Taylor  in their recent orbital
fits, and are probably comparable in accuracy to Voyager astrometry. A
larger set of such mutual event observations in 1995 should lead to
significant improvements in the orbits for all the inner satellites
(S1 - S6), and may help, with the 1980 data, to establish better the
amplitudes and frequencies of the resonant librations involving S1 -
S4. Such observations may also permit mapping of albedo patterns on
the satellites, much as has been done recently for Pluto and Charon,
and studies of aerosols on Saturn and Titan (Smith et al. ,
Combes et al. ).
Aitken  Lick Obs. Bull. 172, 169.
Comrie  JBAA 31, 271.
Innes  Circ. Union Obs. Sth Africa 53, 112.
Reitsema  Nature 272, 601.
Combes et al.  Icarus 47, 291.
Smith et al.  Icarus 46, 424.
Dourneau  A & A 112, 73.
Aksnes et al.  A.J. 89, 280.
Soma  A & A 265, L21.
Harper & Taylor  A & A 268, 326.
(5) The small inner satellites, Prometheus, Pandora, Janus and
Epimetheus, were all discovered during the ring plane passages of
1966 and 1980, supported by the Voyager encounters in 1980 and
1981. The only observations of any of these bodies acquired since
then, to our knowledge, are near-IR observations of the coorbital
satellites in July and August 1990 (Nicholson et al. 1992). The ring
plane crossing will provide additional opportunities to observe all
four objects near elongation, enhanced by our present capability to
predict their appearances with reasonable precision. These data,
especially for the F ring shepherds, may provide very important
links between the Voyager observations in 1980/81 and Cassini
observations to be made in 2004-08, from which it is hoped to
measure the secular deceleration of Prometheus due to ring torques.
Dollfus  C. R. Acad. Sci. Paris B 264, 822.
Fountain & Larson  Science 197, 915.
Aksnes & Franklin  Icarus 36, 107.
Fountain & Larson  Icarus 36, 92
Larson et al.  Icarus 46, 175.
Seidelmann et al.  Icarus 47, 282.
Synnott et al.  Science 212, 191.
Yoder et al.  A.J. 98, 1875.
Nicholson et al.  Icarus 100, 464.
P. D. Nicholson, S. M. Larson, M. R. Showalter
3 August 1993.
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