MISSION_DESCRIPTION |
Mission Overview
================
The Earth and Sun cross Saturn's ring plane every 13-15 years.
Because of the orbital characteristics of Saturn and Earth,
these ring plane crossing (RPX) events can be either triple
(with Earth crossing the ring plane three times) or single
(with Earth crossing the ring plane only once). Single
crossings generally occur with Saturn very close to solar
conjunction, and so are difficult to observe from the Earth.
Triple crossings are generally much more favorable to
Earth-based observers.
The period around a Saturn RPX enables Earth-based astronomers to
study a wide range of phenomena not observable at any other
time. Direct measurements of ring thickness and vertical ring
warps become possible. Precise timing of the crossings make it
possible to better determine Saturn's rotation pole. With the
rings becoming very thin and faint, detections of faint outer
rings and tiny ring-moons become possible. Also, satellite
eclipses, occultations and mutual events during the RPX period
can be used to refine satellite orbits throughout the Saturn
system.
A triple RPX occurred in 1995-1996. Earth passed through the
plane of Saturn's rings on May 22, 1995, August 10, 1995 and
February 11, 1996. Of these events, only the third was
difficult to observe due to Saturn's proximity to the Sun. The
Sun also crossed the ring plane as well, November 17-21, 1995.
Astronomers carried out a wide range of Saturn observations
during this period.
The next two Saturn RPXs crossings will be unfavorable from
Earth: they are both single events occurring on 4 September
2009 and 23 March 2025. Earth-based observers will not get a
favorable edge-on view of Saturn's rings until the triple RPX
events of 2038-2039.
Although no formal NASA 'mission' was organized for the 1995
RPX, data acquired for Saturn and its rings and moons were some
of the best obtained since the Voyager encounters and serve as
a useful bridge to the upcoming Cassini mission in 2004-2008.
To foster the combined analysis of data from different sources
and thereby to maximize the scientific return from this
'mission', the PDS Rings Node is archiving data from the
1995-1996 RPX in a series of CDROMs.
Mission Phases
==============
Four major events occurred during the Saturn ring plane
crossing period---three Earth crossings and one Sun crossing.
The mission phases listed below have been chosen somewhat
arbitrarily to be centered on these four events, plus the two
extended periods before and after the crossings.
PRE-RPX
-------
The pre-RPX phase extended up to the time approximately three
weeks before Earth's first ring plane crossing.
Spacecraft Id : 'N/A'
Target Name : S RINGS, SATURN, MIMAS,
ENCELADUS, TETHYS, DIONE,
RHEA, TITAN, HYPERION,
IAPETUS, PAN, ATLAS,
PROMETHEUS, PANDORA, JANUS,
EPIMETHEUS, HELENE, TELESTO,
CALYPSO
Mission Phase Start Time : 1994-01-01
Mission Phase Stop Time : 1995-05-01
Spacecraft Operations Type : 'N/A'
EARTH-RPX-1
-----------
The Earth-RPX-1 phase spanned the period around the first
Earth crossing of Saturn's ring plane, which occurred at
approximately 5:21 UTC on 22 May 1995. This was a crossing
from the north (lit) side to the south (unlit) side.
Spacecraft Id : 'N/A'
Target Name : S RINGS, SATURN, MIMAS,
ENCELADUS, TETHYS, DIONE,
RHEA, TITAN, HYPERION,
IAPETUS, PAN, ATLAS,
PROMETHEUS, PANDORA, JANUS,
EPIMETHEUS, HELENE, TELESTO,
CALYPSO
Mission Phase Start Time : 1995-05-01
Mission Phase Stop Time : 1995-07-01
Spacecraft Operations Type : 'N/A'
EARTH-RPX-2
-----------
The Earth-RPX-2 phase spanned the period around the second
Earth crossing of Saturn's ring plane, which occurred at
approximately 20:55 UTC on 10 August 1995. This was a
crossing from the south (unlit) side to the north (lit) side.
Spacecraft Id : 'N/A'
Target Name : S RINGS, SATURN, MIMAS,
ENCELADUS, TETHYS, DIONE,
RHEA, TITAN, HYPERION,
IAPETUS, PAN, ATLAS,
PROMETHEUS, PANDORA, JANUS,
EPIMETHEUS, HELENE, TELESTO,
CALYPSO
Mission Phase Start Time : 1995-07-01
Mission Phase Stop Time : 1995-10-01
Spacecraft Operations Type : 'N/A'
SUN-RPX
-------
The Sun-RPX phase spanned the period around the Sun's
crossing from the north side to the south side of Saturn's
ring plane, which occurred from approximately 18:15 UTC 17
November 1995 to 12:00 UTC on 21 November 1995. At this time
Earth's viewpoint was approximately 2.6 degrees north of the
rings.
Spacecraft Id : 'N/A'
Target Name : S RINGS, SATURN, MIMAS,
ENCELADUS, TETHYS, DIONE,
RHEA, TITAN, HYPERION,
IAPETUS, PAN, ATLAS,
PROMETHEUS, PANDORA, JANUS,
EPIMETHEUS, HELENE, TELESTO,
CALYPSO
Mission Phase Start Time : 1995-10-01
Mission Phase Stop Time : 1996-01-01
Spacecraft Operations Type : 'N/A'
EARTH-RPX-3
-----------
The Earth-RPX-3 phase spanned the period around the third and
final Earth crossing of Saturn's ring plane, which occurred
at approximately 23:36 UTC on 11 February 1996. This was a
crossing from the north (unlit) side to the south (lit) side.
This period occurred rather close to solar conjunction and so
was not well observed.
Spacecraft Id : 'N/A'
Target Name : S RINGS, SATURN, MIMAS,
ENCELADUS, TETHYS, DIONE,
RHEA, TITAN, HYPERION,
IAPETUS, PAN, ATLAS,
PROMETHEUS, PANDORA, JANUS,
EPIMETHEUS, HELENE, TELESTO,
CALYPSO
Mission Phase Start Time : 1996-01-01
Mission Phase Stop Time : 1996-03-01
Spacecraft Operations Type : 'N/A'
POST-RPX
--------
The post-RPX phase began shortly after the third Earth
crossing of Saturn's ring plane.
Spacecraft Id : 'N/A'
Target Name : S RINGS, SATURN, MIMAS,
ENCELADUS, TETHYS, DIONE,
RHEA, TITAN, HYPERION,
IAPETUS, PAN, ATLAS,
PROMETHEUS, PANDORA, JANUS,
EPIMETHEUS, HELENE, TELESTO,
CALYPSO
Mission Phase Start Time : 1996-03-01
Mission Phase Stop Time : 1997-01-01
Spacecraft Operations Type : 'N/A'
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MISSION_OBJECTIVES_SUMMARY |
Mission Objectives Overview
===========================
Faint Rings
-----------
Observations of Saturn's faint E, F and G Rings are best made
when the main rings are especially dark, such as close to the
moments of the Earth crossings and during the brief periods
when the Sun and Earth are on opposite sides of the ring
plane. Observations reveal radial and vertical structures,
illuminating, for example, the relationship between the E
Ring and the embedded moon Enceladus. Photometry and
spectrophotometry over a range of phase angles can be used to
better define the particle population in each ring.
Ring Thickness and Event Timing
-------------------------------
Observations of the photometric thickness of the rings, as
seen edge-on, may reveal whether this is caused by bending
waves, embedded moons, contributions from the projected E, F,
and G Rings, or some combination of the above.
Stellar occultations have led to a tentative detection of the
precession of the ring plane due to solar torques, and the
observed rate implies an advance of ~2 hours in the time of
RPX during the 15-year interval since the Voyager encounters.
Accurate timing of the crossing(s) could result in a
confirmation of this precession rate, or at least provide an
essential zero-point for future studies.
'Dark Side' Ring Viewing
------------------------
Observations of the 'dark side' of the main rings are
possible during two extended periods when 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, Earth is 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 Cassini Division. Photometry in these unusual
viewing geometries provides a unique set of constraints on
models of light scattering by denser planetary rings.
Detections of Small Satellites
------------------------------
The small Saturnian satellites Pan, Atlas, Prometheus,
Pandora, Helene, Telesto, and Calypso have not been seen
since the Voyager encounters. Janus and Epimetheus have been
seen only rarely. New detections will make it possible to
refine their orbits substantially. In addition, periods of
ring plane crossing are ideal for detecting previously unseen
moons.
Satellite Eclipses and Mutual Events
------------------------------------
During the period 1993-1996 a very large number of eclipses
and occultations of the satellites by Saturn (and some by the
rings) will take place, as well as an extensive set of mutual
satellite occultations and eclipses in 1995. Most 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 Earth through the ring plane, last from January
1995 to September 1996, with the highest frequency in
November 1995-January 1996.
A large set of mutual event observations in 1995 should lead
to significant improvements in the orbits for all the inner
satellites (S1-S6), and may help 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.
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