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News Release Archive:

News Release 808 of 967

June 5, 1995 12:00 AM (EDT)

News Release Number: STScI-1995-25

Hubble Views Saturn Ring-Plane Crossing

Background information useful for exploring this news release:

Saturn's Ring Plane Crossings

Abstract

Abstract: Saturn's rings will sweep across the Earth and Sun in 1995 and 1996. Saturn ring plane crossings provide a splendid opportunity to observe Saturn's faintest rings and smallest satellites. The edge-on viewing geometry also permits direct observations of the main rings' thickness, and the timing of satellite eclipses and mutual events can be used to improve existing ephemerides. The Hubble Space Telescope is the unique instrument of choice for detecting structures within the rings and new [previously un- discovered] Saturnian moons before the Cassini spacecraft arrives at Saturn in late 2004. After 1995-96, the next chance to get a good edge-on view of Saturn's ring system won't come again until the triple passage of 2038-39.

In 1995-96, the Earth and Sun will pass through Saturn's ring plane. The Earth will pass through three times (22 May, 10 August 1995, 11 February 1996), and the Sun will pass through once (19 November 1995). All but the 11 February 1996 event will be visible from the Hubble Space Telescope.

Observatories the world over will direct their telescopes to Saturn as its ring system appears to thin and turns edge-on. Observatories employing CCD imaging, in some instances with coronagraphic or adaptive optics, include Kitt Peak near Tucson, Arizona; Pic-du-Midi, France, the Canary Islands; and the European Southern Observatory in Chile. Near-infrared imaging is planned at Palomar Observatory, California; NASA's Infrared Telescope Facility, Hawaii; Pic du Midi; and Calar Alto, Spain.

The Hubble Space Telescope has the capability to obtain the highest resolution of all observations to better observe vertical structure within the rings; to determine the presence of a ring "atmosphere" (should it exist); to search for faint, undiscovered Saturnian moons; to recover the moons the Voyager spacecraft dis- covered; and to observe a stellar occultation through Saturn's rings. HST will be the unique scientific instrument of choice to examine the planet, its ring system and its environs as Saturn appears to go "ringless." As currently planned, HST will observe Saturn with the Wide Field/Planetary Camera 2 (WF/PC2) and the Faint Object Spectrograph (FOS). Nearly 35 orbits of spacecraft time will be dedicated to the ring-plane crossing events.

HST Principal Investigators for the Saturn Ring-Plane Crossing

Principal Investigator Science Instrument Target Science Objectives
Amanda Bosh
(Lowell Obs'y)
WFPC+FOS Saturn, outer rings determine moment of RPX, precession value, determine ring thinkness, stellar occultation (measure of atmosphere, optical depth of rings)
Doyle Hall
(Johns Hopkins)
FOS ring atmosphere define the vertical extent of neutral gas near ring
John Trauger
(JPL)
WFPC ring atmosphere measure extended atmosphere of rings; discriminate dust and OH emission from rings
Phil Nicholson
(Cornell)
WFPC rings, inner moons recover inner satellites, warps within the rings, particle size distribution and structure of outer rings (E and G rings)

Like the 1966 and 1979-80 crossings, the 1995-96 crossing will be triple. The first ring-plane crossing this season will occur on 21-22 May 1995. The planet Saturn presents an unique aspect to Earthbound observers that last occurred before the Voyager encounters with that gas giant world. The second crossing occurs on 10 August 1995; and the last on 11 February 1996. The Sun crosses the ring-plane on 19 November 1995. Using the HST, astronomers will observe the first two Earth ring-plane crossings and the solar crossing.

Saturn ring-plane crossings generally occur on 13 to 16 year intervals. The next two ring-plane crossings are unfavorable from the Earth. As the Earth crosses the ring-plane on 4 September 2009, Saturn is only 11 degrees east of the Sun; on the next occasion, 23 March 2025, Saturn will only be 10 degrees west of the Sun. The impending crossings are nearly a once in a lifetime event. Earthbounders will not get a good edge-on view of Saturn until the triple passage of 2038-39

HST Saturn Ring-Plane Crossing Programs

  • stellar occultation of Saturn and rings
  • determine overall thickness of rings
  • determine particle size and distribution within the faint outer rings
  • better constrain the precession rate of Saturn's pole
  • recover the inner satellites discovered by Voyager
  • possibly discover new moons that may be embedded in the rings
  • spectroscopically observe the rings to determine the vertical extent of ring atmosphere

Some Background on Saturn and Brief History of Telescopic Observations

In comparison to Jupiter, Saturn's meteorology seems bland. The belts and zones on Saturn are much more muted, belying the winds at Saturn's equator of nearly 500 meters per second. The visible "surface" of Saturn is a deck of clouds of predominantly composed of ammonia crystals. The faint colors and variations are probably due to sulfur- or phosphorous-containing compounds, and/or complex hydrocarbon molecules.

The bulk of the atmosphere is made up of molecular hydrogen (H2) and helium (He). There are small amounts of methane (CH4), ammonia (NH3), phosphine (PH3), carbon monoxide (CO), germane (GeH4) and arsine (AsH3). Ethane (C2H6), acetylene (C2H2), and other hydro- carbons are present in the stratosphere where they are produced by photochemistry. Below the cloud decks, there is likely water (H2O) and hydrogen sulfide (H2S). Many other gases may be present in trace quantities such as N2, HF, and HCl, but are not detected yet because their characteristic spectral lines are faint or in difficult portions of Saturn`s spectrum.

Saturn was first telescopically observed by Galileo Galilei in July 1610. To his surprise, he saw the planet to be "triple." Saturn appeared to have two companions, one on either side that almost touched the parent body and were completely immobile with respect to one another. These lateral bodies were very different from Jupiter's orbiting moons; they were large and motionless. Galileo had unwittingly observed Saturn's rings. With his crude 20 power telescope, he saw a triple-bodied image.

So unchanging was the planet's appearance that Galileo lost interest in Saturn and observed it only occasionally. After not observing Saturn for several months, he telescopically examined it in the autumn of 1612. Much to his surprise, he found the planet to be solitary; perfectly round and much like Jupiter. Galileo never solved the puzzle; the truth regarding Saturn's "lateral bodies" eluded astronomers until 14 years after Galileo's death.

Unbeknownst to Galileo in 1612, the rings were near their edge-on aspect; making the planet appear without its companions, or, "ringless." In 1616 he found Saturn's "lateral bodies" to have reappeared and grown so that they appeared quite different. His best sketch of Saturn, drawn that same year, would today be readily interpreted as Saturn and its ring system. The next ring plane crossing in 1626 went virtually unnoticed. Not until 1642 did astronomers take interest in Saturn's disappearing ansae, or "cup handles".

Between 1642 and 1656 a systematic set of observations was accumulated by several astronomers, most notably Pierre Gassendi, Johannes Hevelius, and Christian Huygens. In 1655 Huygens suggested the solution to the mysterious appendages of Saturn. With a much improved telescope Huygens discerned that Saturn's "appendages" were part of a bright, continuous ring that surrounded the planet. Further, he realized that the variation in Saturn's appearance was due to the apparent tilt of the ring. Saturn's rotational axis is inclined 26.7 degrees to its orbital plane. Since the rings girdle the equator, they are also inclined 26.7 degrees. From Earth, the rings appear "open" when Saturn is in the part of its orbit that corresponds to its summer or winter (i.e., when the poles are tilted towards the Sun and Earth). However, as Saturn approaches the equinoxes of its orbit, the rings appear thinner and are more difficult to see. Huygens realized that the disappearance of Saturn's "companions" noted by Galileo in 1612 occurred when the Earth was in Saturn's ring plane; an event that occurs roughly every 15-16 years. Huygens also discovered Titan, Saturn's largest moon, in 1655 and found that it revolved around the planet in the same plane as the ring.

The next ring plane crossing in 1671-72 revealed two more Saturnian satellites -- Iapetus and Rhea. The discoverer, Giovanni Domenico Cassini, noted that while Iapetus was on one side of Saturn, it could be easily seen; while on the other side of its orbit, it was invisible. He correctly deduced that Iapetus always keeps the same side toward Saturn and that the moon's leading hemisphere is much less reflective [than its trailing hemisphere].

A few years later, Cassini observed a hint of structure within Saturn's ring. In 1676 he found the first breach in the supposedly solid, rigid, and opaque ring when he discovered that it was divided into two separate rings. The apparent gap is about two-thirds of the way out in the ring. Today this gap is more commonly called the Cassini Division. The ring outside the division is the A ring, while the brighter ring within is the B ring.

As the next ring plane crossing approached in 1685, Cassini discovered two more moons of Saturn: Tethys and Dione. New satellite discoveries would not be made for over a century.

Willliam Herschel, discoverer of the planet Uranus, was among the best astronomical observers of the late 18th century. During the Saturn ring plane crossing of 1789 - 90, Herchel discovered two more inner moons -- Enceladus and Mimas. Herschel was also one of the first to emphasize the extreme thinness of the rings; estimating that they must be no more than a few hundred kilometers thick.

Herschel studied Saturn closely. He noted faint bands on the planet's disk. Based on his observations he concluded that Saturn, like Jupiter, had a considerable atmosphere. From subtle features in the cloud bands, Herschel computed Saturn's rotational period at 10 hours, 16 minutes.

Satellite number eight -- Hyperion -- was almost simultaneously and independently discovered by William C. Bond of Harvard Observatory and William Lassell in England during the 1848-49 ring plane crossing. In 1846, Lassell had discovered Triton, Neptune's largest moon. He later discovered two Uranian satellites in 1851.

Throughout the 1600 and 1700s, a number of astronomers had occasionally detected and reported a faint, inner ring of Saturn -- just inside the B ring. By the middle of the 19th century, the notion of Saturn being surrounded by a solid ring gave way to the notion of particles comprising the rings. As with the discovery of Hyperion, Saturn's new inner ring was discovered independently in the United States and England. English amateur astronomer William Rutter Dawes and Americans William Bond and George Bond (Harvard Observatory) noted a faint, dusky ring extending inward from the B ring to almost halfway to the apparent surface of Saturn. The new ring was practically translucent as the planet itself could still be seen through it. Lassell christened the new third ring (C ring) as the "Crepe Ring." The discovery of the C ring did not seem consistent with the theory that Saturn's rings were solid.

Seven years later, in 1857, James C. Maxwell mathematically demonstrated that even a thin, solid ring was unstable and would be ripped apart by gravitational forces. He also showed that Saturn's rings could not be fluid and concluded the only reasonable hypothesis was that the rings consisted of countless small satellites and particles too small to be seen individually. However, collectively they gave the appearance of a seemingly continuous ring. Each particle would follow its own orbit around the giant planet; the particles closer to Saturn moving faster, particles further out moving slower.

In 1895, James E. Keeler at the Allegheny Observatory in Pittsburgh, Pennsylvania, spectroscopically measured the rotation speeds at different parts of the rings. Keeler verified the effects mathematically predicted by Maxwell. The velocities of rotation varied from 20 kilometers/second at the inner edge of the B ring to just under 16 kilometers/second at the outer edge of the A ring.

Saturn's ninth satellite, Phoebe, was photographically discovered by William Pickering in 1898 at the Harvard Observatory's southern hemisphere observing site in Peru. Phoebe's discovery was unrelated to a ring plane crossing. It is Saturn's outermost known moon, orbiting retrograde at a distance of 13 million kilometers from the planet. Phoebe may be a captured asteroid.

Despite advances in technology and dramatic increase in telescope apertures, Saturn's rings remained virtually invisible as the Earth passed through the ring plane. The 1907-08 ring plane crossing led to a better estimate of the rings' thickness of no more than 15 kilometers. This suggested that the total volume of particles comprising the ring was minute compared to that of even a single satellite.

The 1966 ring plane crossing revealed two more satellites. Janus was discovered by the French astronomer Audouin Dollfus and Epimetheus by Stephen Larson and John Fountain of the University of Arizona.

Saturn's Moons Found Near Ring-Plane Crossings

Satellite Rings Edge On Year Discoverer(s) Discovered
Titan 1655-56 1655 C. Huygens
Iapetus 1671-72 1671 G.D. Cassini
Rhea 1671-72 1672 G.D. Cassini
Tethys 1685 1684 G.D. Cassini
Dione 1685 1684 G.D. Cassini
Enceladus 1789-90 1789 W. Herschel
Mimas 1789-90 1789 W. Herschel
Hyperion 1848-49 1848 W.Bond, G. Bond, W. Lassell
Janus 1966 1966 A. Dollfus
Epimetheus 1966 1966 J. Fountain, S. Larson, R. Walker
Telesto 1979-80 1980 B. Smith, S. Larson, H. Reitsema
Calypso 1979-80 1980 D. Pascu, P.K. Seidelmann, W. Baum, D. Currie
Helene 1979-80 1980 P. Laques, J. Lecacheux

Note: The discoveries of five moons were unrelated to ring-plane crossings. Phoebe was captured photographically by Wm. Pickering in 1898; while Pan, Atlas, Prometheus, and Pandora were found in Voyager images from 1980 and 1981.

The 1966 event also led to the discovery of another Saturnian ring. W.A. Feibelman of the Allegheny Observatory took a relatively long (30-minute) exposure in which Saturn and the known bright rings were completely overexposed. Following image enhancement and and processing at the Jet Propulsion Laboratory (JPL), a very tenuous ring emerged from the glare of the planet. The new ring extended out 400,000 kilometers from Saturn. The new ring was simultaneously photographed at the University of Arizona. In 1969, yet another faint ring was discovered interior to the C ring by Pierre Gurin of the Pic-du-Midi Observatory in France. The new inner ring was designated the D ring, while the outer ring became known as the E ring. Although seemingly confusing, the rings are named in order of their discovery, not in order of relative distance/positions from Saturn. During the most recent ring plane crossing in 1979-80; teams of astronomers discovered three more moons: Telesto, Calypso, and Helene. New Saturn discoveries rapidly unfolded from 1979 to 1981 with flybys by the Pioneer 11, Voyager 1 and Voyager 2 spacecraft. In the wake of the incredible discoveries made by these pioneering spacecraft, another opportunity to examine Saturn's environs awaits as observatories prepare to catch the planet's unique aspect in 1995. Now, on the verge of the next set of ring plane crossings in 1995, we are presented an opportunity to increase our knowledge of Saturn, its rings, and inner moons. Advances in technology incorporated in ground-based observing and the unique Hubble Space Telescope afford astronomers the opportunity to keenly survey the Saturnian system before the Cassini orbiter arrives in 2004.


Spacecraft Visits to Saturn

In the mid- to late-1960s, scientists and engineers at JPL devised solutions to send spacecraft to the outer planets. The first spacecraft to visit Saturn, Pioneer 11, was launched from Earth on a trajectory towards Jupiter on 6 April 1973. Pioneer 11 provided scientists with their closest view of Jupiter, passing within 43,000 kilometers of the cloud tops on 3 December 1974. Before closest approach to Jupiter, the spacecraft was aimed just ahead of and below Jupiter and the resulting "slingshot" flung Pioneer 11 above the ecliptic and inward [toward the Sun]. The close approach and the spacecraft's speed of 172,000 kilometers per hour hurled Pioneer 11 2.4 billion kilometers across the solar system toward Saturn.

Before reaching Saturn in September 1979, Pioneer 11 reached an inclination of 17 degrees above the solar equatorial plane, high enough to illuminate the true character of the sun's magnetic field. Now 780 million miles above the ecliptic plane the spacecraft approached Saturn from an unique perspective. The rings were not directly illuminated by the Sun; rather, the rings appeared illuminated from beneath.

Pioneer 11 flew to within 24,000 kilometers of Saturn and took the first close-up images of the planet. Instruments detected two previously undiscovered small moons and an additional ring (F ring).

The spacecraft acted as a pathfinder for the Voyager missions that followed in 1980 and 1981. Pioneer 11 crossed the orbit of Neptune in 1990 and became the fourth spacecraft to leave the solar system. It has joined Pioneer 10 and Voyagers 1 and 2 in searching for the heliopause, the point at which the Sun's electromagnetic influence gives way to the interstellar medium.

The twin spacecraft Voyager 1 and Voyager 2 were launched by NASA in late summer 1977 from Cape Canaveral, Florida. As originally designed, the Voyagers were to conduct closeup studies of Jupiter and Saturn, Saturn's rings, and the larger moons of the two planets. Originally intended as a two-planet (Jupiter and Saturn) mission for both spacecraft, Voyager 2's mission was extended from its original five-year plan to a twelve year plan which included encounters at Uranus in 1986 and Neptune in 1989.

The Voyager 1 and 2 Saturn flybys occurred nine months apart, with the closest approaches falling on 12 November 1980 and 25 August 1981. Voyager 1 flew within 64,200 kilometers of the cloud tops, while Voyager 2 came within 41,000 kilometers.

Perhaps the greatest surprises and the most perplexing puzzles were found by the Voyagers in Saturn's rings. It is thought that the rings formed from larger moons that were shattered by impacts of comets and meteoroids. The resulting dust and boulder- to house-size particles have accumulated in a broad plane around the planet varying in density.

Voyager 1 found much structure in the classical A, B and C rings. Some scientists suggest that the structure might be unresolved ringlets and gaps. Voyager 1 images were of lower resolution than those of Voyager 2, and scientists at first believed the gaps might be created by tiny satellites orbiting within the rings and sweeping out bands of particles. One such gap was detected at the inner edge of the Cassini Division. The higher-resolution Voyager 2 images of the inner edge of the Cassini Division showed no sign of satellites larger than five to nine kilometers. No systematic searches were conducted in other ring gaps.

Voyager 2's photopolarimeter instrument measured changes in starlight from Delta Scorpii as the spacecraft flew above the rings and the light passed through them. The photopolarimeter could resolve structure smaller than 300 meters. The star-occultation experiment showed that few clear gaps exist in the rings. The structure in the B ring, instead, appears to be variations in density waves or other, stationary, forms of waves. Density waves are formed by the gravitational effects of Saturn's satellites. (The resonant points are places where a particle would orbit Saturn in one-half or one-third the time needed by a satellite, such as Mimas.) The edges of the rings where the few gaps exist are so sharp that the ring must be less than about 200 meters thick there, and may be only 10 meters thick.

Voyager images showed the F ring, discovered by Pioneer 11, to appear twisted or braided. The twists are believed to originate in gravitational perturbations caused by a shepherding satellite. Clumps in the F ring appear uniformly distributed around the ring every 9,000 kilometers, a spacing that very nearly coincides with the relative motion of F ring particles and the interior shepherding satellite in one orbital period.

Radial, spoke-like features in the broad B ring were found by the Voyagers. As both spacecraft approached Saturn, the spokes appeared dark against a bright ring background. As the Voyagers departed, the spokes appeared brighter than the surrounding ring areas, indicating that the material scatters reflected sunlight more efficiently in a forward direction, a quality that is characteristic of fine, dust-sized particles. Spokes are also visible at high phase angles in light reflected from Saturn on the unilluminated underside of the rings. The spokes were observed to form and dissipate in time-lapse images taken by the Voyagers. While electrostatic charging may create spokes by levitating dust particles above the ring, the exact cause of the formation of the spokes is not well understood. The irregular shapes of Saturn's eight smallest moons indicates that they too are fragments of larger bodies. Unexpected structure s uch as kinks and spokes were found in addition to thin rings and broad, diffuse rings not observed from Earth. Much of the elaborate structure of some of the rings is due to the gravitational effects of nearby satellites. This phenomenon is most obviously demonstrated by the relationship between the F ring and two small moons that "shepherd" the ring material. The variation in the separation of the moons from the ring may the ring's kinked appearance. Shepherding moons were also found by Voyager 2 at Uranus. The Voyagers are now departing the the solar system. Following its encounter at Saturn, Voyager 1's trajectory took it above the plane of the solar system at an angle of 35 degrees; while Voyager 2, after its enounter with Neptune, is diving below the ecliptic at an angle of 48 degrees. The next spacecraft scheduled to encounter Saturn is Cassini. Before arriving at Saturn, Cassini will first execute two gravity assist flybys of Venus, then one of Earth, and then one of Jupiter before arriving at Saturn in June 2004. The Hubble Space Telescope observations of the forthcoming ring-plane crossings are essential to assessing the relative risk to the Cassini orbiter before it arrives.


Geometrical Explanation for Saturn's Ring Plane Crossings

It takes Saturn 29.5 years to complete one revolution around the Sun. As it circles the Sun, the angle of the Saturn's rings relative to the Sun varies by 26.7 degrees. Twice during the 29.5 years, the rings are edge-on to the Sun. Since the Earth is near the Sun and only six degrees from Saturn's ecliptic plane, it also crosses the ring plane at around the same time. Since Saturn's rings are so thin, when they are edge-on to the Earth, they appear to disappear when viewed with a small telescope.

Some additional characteristics and "patterns" of Saturn ring plane crossings:

  • There may be one or three Earth crossings (there must be an odd number) during any half orbit of Saturn (15 years).
  • If there is only one Earth crossing then Saturn and the Earth will be on almost opposite sides of the Sun; hence making observations difficult.
  • If there are three crossings, the middle one is near opposition and the other two are near quadratures.
  • The chance of three intersections is about 53% and the chance of one intersection is about 47%.
  • There is an occasional case where the Earth hangs in the plane without passing through it.

The Saturn ring plane crossings only occur about every 13 to 15 years. Unfortunately, the next two ring plane crossings are unfavorable from Earth. As the Earth crosses through the ring plane on 4 September 2009, Saturn is only 11 degrees east of the Sun; the next occasion, 23 March 2025, Saturn will only be 10 degrees west of the Sun. Earthbound viewers won't get a "ringless" view of Saturn until the triple-passing of 2038-2039. The next favorable ring plane crossings (together with Saturn's elongation from the Sun) are:

  • 15 October 2038 (28 degrees west)
  • 1 April 2039 (163 degrees east)
  • 9 July 2039 (66 degrees east)

Saturn ring plane crossings are worthy of scientifice study because when the rings are nearly edge-on to Earth, the glare from the rings is reduced considerably, and faint objects near Saturn, such as moons, are easier to see. During the period before, during and after the ring plane crossings, unique observations of Saturn, its rings and moons can be made from Earth which are available at no other time. Any new information obtained during the 1995-1996 ring plane crossing may prove to be invaluable for the upcoming Cassini mission to Saturn.

The three other gas giant worlds have rings: Jupiter, Uranus and Neptune. Of course, none of these ring systems are as spectacular as Saturn's, but it appears to be a common phenomenon for large planets.


Where to Find Saturn in the Sky

Through the garden variety backyard telescope, Saturn's rings are much less prominent than usual, sometimes invisible, in 1995-1996. At the beginning of January 1995, the ring system was clearly visible as the ring plane was tilted 7 degrees to our line of sight and its sunlit north face in view.

In May 1995, for the first time since 1980, the angle of inclination of the ring plane diminishes to 0 degrees as the rings present themselves edge-on to Earth on 21 - 21 May. Saturn is low in east-southeast at dawn, so unsteady air near the horizon may interfere with viewing. From 22 May until our second edge-on view on 10 - 11 August, the rings' dark southern face will be tipped slightly to our view.

The edge-on presentation of 10 - 11 August will have Saturn in excellent observing position, high in the southern sky after midnight, providing a clear view that won't be matched until the year 2039. Enjoy the rare view of a "ringless" Saturn and its moons arranged in a straight line! The north face of the rings thereafter in view, sunlit until the night of 18 - 19 November, when the Sun crosses the south side of the ring plane. From then until the final edge-on presentation on 11 February 1996, the rings again appear "dark" with their north face tipped into our view. After 11 February 1996, we'll be seeing the rings lighted south face until their next edge-on presentation, in the year 2009.

These events are part of a long cycle of visibility of the rings, located directly above Saturn's equator. The planet's axis points within 6 degrees of the star Polaris and maintains its orientation as Saturn revolves around the Sun. In 2002 - 03, around the date of Saturn's winter solstice, the planet's north pole will be tipped away from Earth and the Sun; the rings will appear "full-on" (27 degrees from edge-on).


HST's Planned WF/PC Observations of the May Ring Plane Crossing

During the May 1995 crossing of the Earth through Saturn's ring plane, the Hubble Space Telescope will monitor the changing brightness of the rings with the WF/PC2. Amanda Bosh of Lowell Observatory leads the HST investigation of the May ring-plane crossing.

Her group will obtain WF/PC2 images of Saturn and its rings for 7 orbits centered on the predicted time of the ring-plane crossing. Imaging for 7 orbits allows for uncertainty in the exact time of ring-plane crossing due to the unknown magnitude of the precession of Saturn's ring-plane pole (+/-2 hours). These exposures (20 to 30 minutes per orbit) will be obtained with a methane filter in order to minimize the contribution of scattered light from Saturn to background noise. Multiple frames with some shorter exposures will be obtained each orbit to allow for the removal of cosmic rays and photometric calibration using Saturn's satellites.

A measurement of the brightness of the rings in their edge-on configuration, combined with photometric properties of the rings derived from calibration observations will allow astronomers to determine the vertical thickness of the rings. During the last ring-plane crossing (1980) the ring thickness was measured to be approximately 1.1 km by Andre Brahic and others. However, a large uncertainty remains in this value for the rings' thickness due to a lack of suitable calibration targets and the relatively crude detectors then available. With the excellent seeing of the HST and the technological advances incorporated into WF/PC2, Bosh's team should be able to improve the determination of this value by more than a factor of 20. Knowledge of the thickness of the rings (a collisional many-body system) is crucial for several aspects of dynamic models of ring systems: their evolution, particle size distribution, reaction to perturbations, and more.

The time of the ring-plane crossing (the moment of minimum light from the rings) will help researchers determine the precession rate of Saturn's pole. Recent analyses (based on stellar occultation data, including one occultation observed in 1991 with the High Speed Photometer aboard the HST) allow for a wide range of values for the precession rate. The HST observations of the ring-plane crossings may offer the best opportunity for determining this value.

Results from the May 1995 ring-plane crossing will aid observers of the later crossings by providing a more refined determination of the times of these crossings and a measurement of the residual brightness of the rings. Later observations can be designed to optimize the data received in response to information from this first ring-plane crossing.


HST WF/PC2 Observations of the August and November Events

The Earth passes through Saturn's ring plane on 10 August 1995 and the Sun passes through the ring plane on 17 - 21 November 1995, providing a rare opportunity to determine the thickness, vertical distortions, and pole orientation of the rings. Phil Nicholson of Cornell University leads a team of seven co-investigators that will obtain a time series of images using the WF/PC2 (in wide field mode) and methane filter at each crossing time in order to measure the radial profile of the apparent ring thickness, to determine the instant of Earth's crossing to within a few minutes, and to look for the expected warp in the ring plane due to satellite perturbations. In November, with the last rays of sunlight grazing the northern side of the rings, the images may reveal shadowing by "bending waves" with predicted amplitudes of about 500 meters. In addition, the WF/PC2 (in planetary camera, or "high resolution" mode) will be used to recover the small Saturnian moons Pan and Atlas in order to refine their orbital periods, as well as to observe the kinked and multi-stranded F ring. A series of multi-color WFC observations will be used to probe the structure and particle size distribution within the faint E and G rings. These features may constitute a hazard to the Cassini spacecraft headed for Saturn in 2004.


HST Observes Saturn's Ring Atmosphere During the August Event

Just before and after the August 1995 ring-plane crossing event, the Hubble Space Telescope will look for faint ultraviolet light emitted from a thin gaseous envelope that is known to exist around Saturn's rings. The gas is thought to come from the icy ring particles themselves, which produce gas as they are slowly eroded by sunlight, micrometorites, and the energetic particles that zip around within Saturn's magnetic field. Because the ring particles are water-rich like the nuclei of comets, the gas surrounding Saturn's rings may have a composition similar to the coma of an active comet. Measurements made by the Voyager spacecraft indicate that Saturn's ring "atmosphere" is at least partly composed of hydrogen atoms, but the quantity and type of other neutral gases surrounding the densest part of the ring system are unknown. Doyle Hall and colleagues at The Johns Hopkins University and the Space Telescope Science Institute plan to use the Faint Object Spectrograph to observe during a ring-plane crossing event in order to get an edge-on view of Saturn's ring atmosphere -- the best view to determine the composition and quantity of other gases that may be present.

WFPC2 Observations of Saturn's Ring Atmosphere

Approximately six hours after Bosh's team's May observations, the WFPC2 team, led by John Trauger of the Jet Propulsion Laboratory (JPL) and John Clarke of the University of Michigan, will attempt to measure the vertical extent of the Saturninan rings' extended atmosphere. Using WFPC2, Trauger and Clarke's team will discriminate between the elevated dust and hydroxyl (OH) emission.

In Voyager images of Saturn dark bands were occasionally seen in the rings, resembling dark spokes in a wheel. These may be produced when small clouds of dust are knocked loose from Saturn's rings, perhaps by the impact of a meteorite, extending radially from Saturn and absorbing some of the reflected sunlight from the rings. These appear most commonly as the rings leave Saturn's shadow. In addition, there is evidence for water molecules released from the icy covering of the ring particles, from Voyager ultraviolet spectra of hydrogen atom emission surrounding Saturn and from HST / FOS spectra of the OH (hydroxyl) molecule just beyond the extent of the rings. H and OH are the dissociation products of water molecules from the rings, and they are believed to be one of the principal sources of neutral material and plasma in Saturn's magnetosphere.

OH emissions should be detectable from a spectral emission near 3100 Angstroms and extend vertically above and below the ring plane. To see the faint emission above the bright reflected sunlight from the rings, Saturn's rings must be close to edge-on so that they appear dark. When the rings are nearly edge-on it may also be possible to measure the vertical extent of any dust or OH gas above and below the rings. Images will be taken through red, blue, and near-UV filters to try to discriminate the different sources of light. Scattered light from Saturn will have a nearly solar color, while suspended dust should appear somewhat bluer and OH emission should appear only in the near-UV filter.

Using the Planetary Camera (PC) on the rings, Saturn will be deliberately shifted out of the PC aperture. The team will experiment with filter combinations and exposure times during the May event in order to prepare for, and optimize, data collection for the August event. The August observations will be made roughly six hours before the ring-plane crossing and are coordinated around Nicholson's observations. The OH images will also be compared with the FOS spectra taken under Hall's program to deduce the amount and distribution of OH around the rings.

An Occultation by Saturn near the Ring-Plane Crossing of the Sun

On 20 - 21 November 1995, Saturn and its rings will occult GSC5249-01240, a star in the Hubble Space Telescope Guide Star Catalog. Amanda Bosh of Lowell Observatory and co-investigator Jim Elliot of MIT will utilize the Faint Object Spectrograph (FOS) to observe the occultation event. This event will occur near the stationary point in Saturn's orbit, so the apparent velocity of the event will be low. Serendipitously, it also occurs during the crossing of the Sun through Saturn's ring-plane. The Earth and Sun will be on opposite sides of the rings, so the rings will appear much darker than usual. Both of these circumstances contribute to the potential quality of data collected from the event.

During an occultation of a star by a planet, the planet passes between the star and the observer. The star then becomes a "probe" of the planet, allowing scientists to determine the temperature and composition of the occulting planet's atmosphere and the structure of its ring system. Such events led to the discovery of the Uranian ring system in 1977 and revealed Pluto's atmosphere structure in 1988. Because this event occurs near Saturn's stationary point the orbit of the HST will introduce a parallax effect, resulting in multiple passes of the star through Saturn's atmosphere and rings. Observing this event will: 1) establish Saturn's atmospheric structure (in the 1 to 100 microbar region, which was inaccessible to Voyager instruments), 2) determine how Saturn's atmospheric composition and temperature change with latitude, and 3) probe the ring system for features of low optical depth (with spatial resolution better than 1 kilometer).

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Facts at a Glance

Diameter of Saturn: 120,536 km
Diameter of Earth: 12,750 km
Diameter of Saturn's rings: 270,000 km
Earth-Moon distance: 384,000 km
Mass: 5.69 x 10^26 kg (or 95 Earth masses)
Average distance from Sun: 9.539 AU
Rotation period: 10 hours, 39 minutes
Revolution period: 29.46 Earth-years
Obliquity (tilt of axis): 26.7 degrees
Orbit inclination: 2.49 degrees
Atmospheric components: 97% hydrogen,
3% helium,
0.05% methane

The Hubble Space Telescope is the unique instrument of choice for the upcoming Saturn ring-plane crossings. The data gleaned from these events will be invaluable in support of the Cassini mission scheduled to arrive at Saturn in 2004. The next opportunity for Earthbounders to view Saturn "ringless" will not come for another 43 years in 2038-39.


Acknowledgements

This document would not be possible if not for the assistance and collaboration of scientists, science writers, and sky watchers. I am particularly grateful to Joe Tucciarone, astronomical artist for supplying the cover artwork to this primer of Saturn's ring-plane crossings; Bob Victor of Abrams Planetarium for use of the Sky Calendar date block diagrams indicating Saturn's position; and the support staff of the Moving Targets Group at Space Telescope Science Institute. The selection of material and any errors contained within it are the sole responsibility of the editor/author. Gratitude and many thanks are extended to: Ron Baalke (JPL), Amanda Bosh (Lowell Observatory), John Clarke (University of Michigan), Doyle Hall (Johns Hopkins University), Andy Lubenow (STScI), Phil Nicholson (Cornell University), Keith Noll (STScI), Guy Ottewell (Furman University), Alex Storrs (STScI), John Trauger (JPL), Joe Tucciarone (Interstellar Illustrations), Robert Victor (Abrams Planetarium, Michigan State University), and all others who may have been omitted.


All comments should be addressed to: Rob Landis

Space Telescope Science Insitute
3700 San Martin Drive
Baltimore, MD 21218
landis@stsci.edu

August 11, 1995

 
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