PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "M.R. SHOWALTER, 1999-10-05 M.K. GORDON, 2009-04-21" OBJECT = MISSION MISSION_NAME = "SATURN RING PLANE CROSSING 1995" OBJECT = MISSION_INFORMATION MISSION_START_DATE = 1994-01-01 MISSION_STOP_DATE = 1997-01-01 MISSION_ALIAS_NAME = "N/A" MISSION_DESC = " 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." 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