Mission Information
MISSION_NAME SATURN RING PLANE CROSSING 1995
MISSION_ALIAS N/A
MISSION_START_DATE 1994-01-01T12:00:00.000Z
MISSION_STOP_DATE 1997-01-01T12:00:00.000Z
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'
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.
REFERENCE_DESCRIPTION