Saturn at its magnificent best.

What observers can expect to see as the ringed Planet approaches its two closest oppositions in over 15 years.  

By Damian A. Peach

BAA Saturn Section, Assistant Director.

 

Fig 01: Above is an image of Saturn obtained by the Author in October of 2000 using his 30cm reflector and CCD camera. The Planet was 1.25 billion Km’s distant at the time. The magnificent ring system dominates the view, bisected by the dark Cassini division.

Introduction  

How many times has been said, that the rings of Saturn are one of the most memorable and magnificent telescopic sights in the night sky?. Well, many would agree this is certainly true, and the good news is for observers both old and new, now has never been a better time to study Saturn and its rings as it now approaches its two best oppositions for northern hemisphere observers since 1974. The last time Saturn was tilted at its maximum toward us was back in 1988, though the Planet’s declination during this time was unfavourable for Northern hemisphere observers, as the Planet passed through Scorpius into Sagittarius. Now, once again after a very long wait, the “ringed Planet” is again fast approaching its best for observers across the northern hemisphere.

 

A gaseous world.

Saturn is the Solar System’s second largest Planet with an Equatorial diameter of 120, 536 Km’s, making it some 9.5 x the diameter of the Earth. Its polar diameter is almost 10% smaller at the poles making it the most flatted Planet in the Solar system. It is composed of various gases, primarily hyrdrogen, helium, methane and ammonia. It has no solid surface beneath its thick clouds, which at its equator are swept around by winds of well over 1000kph – far stronger than the winds on Jupiter. However, despite this it is a much less active world than its giant neighbour, often appearing devoid of large storms and spots, unlike Jupiter – though large bright storms do occasionally occur, such as back in 1990, which I will cover in more detail later.

 

Saturn like Jupiter is divided into rotation periods, due to the varying motions of the observed atmosphere at different latitudes. These periods are called System 1  (which is assigned to the equatorial regions) and assumes a drift rate of 844.3 degrees per day (period 10h 14m) and System III (which is assigned to the remainder of the planet) and assumes a drift rate of 810.8 degrees per day (period 10h 39m 22s.4). Most of the transient Saturnian features the amateur will observe occur in the Equatorial Zone of Saturn (System I.) Only very occasionally are small spots followed in areas outside the Equatorial Zone (EZ.)  It takes Saturn over 29 years to orbit the Sun once, meaning its position in the sky changes quite slowly from year to year, meaning it can remain well placed for many years, but for northerly observers, it can of course also remain poorly placed, as was the case during the mid-late 1980s into the early 1990s.

 

The famous rings

The rings are without doubt Saturn’s most well-known, and well-observed feature, ever since Galileo first turned his small telescopes on the Planet back in 1610. The rings are divided into 5 main rings. There are two main rings, called rings A and B, of which B is the brightest. Then ring C or the Crepe ring, which is inside the B ring and appears very dusky. Rings D and F are faint, and not observable to amateurs. Two main divisions dark division are observable to amateurs. The Cassini Division (a 4,700 km gap) which separates the bright A and B rings is easily observed in Telescopes as small as 2.4 inches (60mm) when the rings are well presented, as is currently the case. The other division (which is located near the outer edge of Ring A) is know as the Encke Division, a small 325km gap actually discovered by Astronomer, James Keeler in 1888. The division is much smaller, and will require a telescope of at least 9 inches (235mm) aperture, and very steady atmospheric seeing conditions to resolve. Though the rings our very wide (almost 300,000km) they are extremely thin (just 1 km) making them almost invisible when they are presented edge on to us, as was last the case back in 1995.

 

Fig 01a: A simulated view of Saturn with the major belts, zones, rings and divisions indicated.

 

Saturn’s many Satellites.

Another well known feature of the Saturnian system is its many moons (28 in total.) Several of these small worlds are observable to amateur observers. Equipped with a typical 200mm telescope, the observer will be able to follow Titan, Dione, Enceladus, Iapetus, Mimas, Rhea, and Tethys. A 300mm telescopes will reveal the faint Hyperion. There movements can be easily followed from night to night. Titan is the largest, brightest and most famous moon. It has a thick atmosphere of Nitrogen, giving the moon an orange cast. Telescopes of 200mm or greater aperture under very stable seeing will reveal Titan as a small disk, spanning just 0.9” arc seconds in angular diameter.

 

The coming 2002 and 2003 apparitions.

The coming two apparitions of Saturn, as previously mentioned are the most favourable to occur in many years for those observers located in the Northern hemisphere. The Planet remains in Gemini throughout this period at over +20 declination. The Earth will is now passing over 26 degrees south of the ring plane, presenting us with a fine open view of the Southern hemisphere of the Planet, peaking over the next 18 months. The Earth will be at its greatest distance South of the Ring plane in March of 2003, when it will be just over 27 degrees South. Also, the Planet is passing closest to Earth during the next two apparitions, which will mean the Planet will appear larger in the telescope and brighter than at any other time in the last 25 years. This peaks on Dec 31st, 2003 when Saturn will appear 20.55” arc seconds in diameter and shining at magnitude –0.5.

 

Minimum distance between Earth and Saturn 1988-2005.

 Date (UT)                     Dist (AU)         Declination.                  

     1988 Jun 20                    9.0278            -22° 18'      

          1989 Jul 02                     9.0212            -22° 24'           

1990 Jul 14                     8.9938            -21° 34' 

1991 Jul 27                     8.9466            -19° 50' 

1992 Aug 07                    8.8812            -17° 16' 

1993 Aug 20                    8.8000            -13° 57' 

1994 Sep 01                    8.7062            -10° 01' 

1995 Sep 14                    8.6039            -05° 35' 

1996 Sep 26                    8.4976            -00° 49' 

1997 Oct 10                    8.3924            +04° 05' 

1998 Oct 23                    8.2934            +08° 56' 

1999 Nov 06                    8.2057           +13° 27' 

2000 Nov 19                    8.1333           +17° 20' 

2001 Dec 03                    8.0806           +20° 18' 

2002 Dec 17                    8.0519           +22° 03' 

2003 Dec 31                    8.0501           +22° 24' 

2005 Jan 13                     8.0756           +21° 20'

 

Fig 02: A table listing all Saturn-Earth minimum distances during its last 16 apparitions. Note, not all actually occurred on opposition night during that apparition. Note that the coming 2002 and 2003 oppositions are the closest in many years.

 

 

Fig 03: Simulated views of Saturn on the next two opposition nights in 2002 and 2003. These years will present us all with an excellent opportunity to study the ring system in detail.

 

 

Making worthwhile visual observations.

So what can the observer expect to see during the next two apparitions. Well, or course this will depend upon the telescope he/she is using, and the experience of the observer. In my opinion a 100mm (4 inch) refractor or 150mm (6 inch) reflector is the smallest telescope with which detailed observations can be made. 250mm aperture (10 inch) and larger telescopes will provide glorious views under stable seeing conditions, and very useful, and valuable observations can be made using such instruments.

 

Before the start of making an observation the observer should ensure that there telescope is properly collimated, and they have a pencil and pad at hand to sketch and note the details of there observation. Its best to start with a blank outline of the correct aspect (available from the BAA or ALPO Saturn sections) and slowly fill in the details that can be seen. Points that should be noted by the observer are in general:

 

1) Position and shape of shadows of globe on rings (ShGR)and rings on globe (ShRG).

2) Any change in the position or form of the belts on the globe.

3) Any star that appears likely to be occulted by the globe or rings. These events are very rare and may not be predicted far in advance.

4) Position of any bright or dark spots within the belts and zones which appear to rotate with the planet.

5) Any change in the visibility or any irregularity of the outline of Ring C.

6) Any indications of a faint ring inside or outside the principle rings.

7) Position of any dusky patches or subdivisions on the ansae of the rings.

8) Any brightness or colour asymmetry visible across the ring system.

 

Observers with smaller telescopes should carry out items '1' to '3' with items '4' to '8' being in the remit of larger apertures.

 

It is also worthwhile to make intensity estimates of Belts, Zones and Ring system to help monitor any changes that may occur. Colour estimates are also worthwhile by those using telescopes of 200mm (8”) and larger. Transit estimates of any bright or dark spots seen on the Planet is also very important, but again this is usually only within reach of larger telescopes. Colour filters, as with the other Planets, are of great use on Saturn. I would recommend Wratten #21 (orange), #58 (green) and #38A (blue) filters for use on 200mm – 400mm aperture telescopes. Unfiltered views in larger telescopes are also very worthwhile, as stubble shades of colour can often be noted, both on the Planet itself, and the ring system.

Finally, the observer should note with their observation the date and time (UT), Telescope, magnification, filters used, and the quality of the seeing conditions using either the Pickering or Antonaidi scales.

I would very much encourage observers to submit their work to the Saturn sections of the British Astronomical Association (BAA) and Association of Lunar and Planetary Observers (ALPO.) These organisations specialise in producing detailed reports each Saturn apparition based on the work of amateur observers worldwide.

 

 

Fig 05: A drawing based on an actual visual observation made in 1997 by experienced planetary observer Carlos Hernandez of FL, USA. The drawing accurately represents the visual appearance of Saturn in a typical amateur telescope under good seeing conditions.

 

CCD imaging and photography.

As well as being a rewarding object for the visual observer to study, it is perhaps equally (or perhaps more so) rewarding an object for today’s telescopes equipped with CCD cameras. Though a bright object visually, for CCD cameras, Saturn is perhaps the most challenging Planet to capture high quality images of. Its low surface brightness means that fairly long exposures will be required. In general a telescope of at least 200m (8”) is required to produce high quality results. Those equipped with larger apertures will find it easier as exposure times will be reduced.

 

During a night of CCD imaging I would recommend that the observer use a set of colour filters to image the Planet to produce a set of Red, Green and Blue filtered images – which must be used in conjunction with a filter to block IR light. If these images are obtained correctly within a ~5min window the may be composited to create a true colour image of the Planet. The CCD can record allot more detail on Saturn’s low contrast disk than is visible to the eye, as we can sharpen images to give us a clearer picture of the Planet. Red filtered images are especially useful for viewing the belts and zones of Saturn, and high quality images often reveal many belts and zones are present. High quality true colour images often reveal stubble colour shades of the belts and zones, occasionally ranging from red to green!. CCD images also give us a very clear picture of the ring system. Images of the highest resolution obtained by amateurs reveal the tiny 325km Encke Division in Ring A – which subtends a width of less than 0.1” arc seconds. As many amateurs know, CCD imaging is a very extensive subject, and I would advise the reader to contact the affore mentioned organisations for further advice on this subject.

 

Photography of Saturn through the telescope is a very challenging operation, and is only recommended using telescopes of at least 300mm (12”) aperture. Photograpy has now been almost completely superseded by CCD imaging. During the late 1980’s observers such as Don Parker and Isao Miyazaki produced high quality photographs of Saturn using large telescopes under excellent seeing conditions.

 

As with visual observations, I greatly encourage observers to submit their CCD images of Saturn to the BAA and ALPO Saturn sections for analysis. The great majority of data obtained by amateurs today is by the use of CCD cameras, and they have greatly aided in our study of the ringed Planet.

 

Fig 04: A set of 3 images obtained by the author, showing the different appearance of the Planet in Red (left), Green (centre) and Blue (right) filters.

 

Recent activity on Saturn – what can we expect in 2002-03?

During recent years, the activity on Saturn has been quite low, but there have been some very interesting periods. During 1999 the band surrounding the South Polar Cap exhibited a strong red colouration, discovered by Toshihiko Ikemura and the author. This is the first time such a strong colouration has been noted, and it was followed by amateurs both visually and with CCD cameras during the autumn of 1999. Also, distinct green coloured bands were also observed in high-resolution images. These colours have been present on Saturn every apparition since then, though now not as distinctly. Some transient spots have been followed in recent years, especially during 1998 and 2000.

Another interesting phenomenon noted last year in high quality images, was a distinct colour asymmetry observed in Ring A. This asymmetry is still under investigation by observers, and more observations are needed to fully understand its nature.  

In the coming apparitions I would expect to see a continuation of the present level of activity. Occasionally huge outbursts of activity can occur, known as great white spots (GWS.) Theses spots erupt in Saturn’s equatorial zone (EZ), usually once every thirty years. This last occurred in September of 1990, when observers were treated to such an outburst, lasting several weeks, and encircling the entire EZ. Notable activity also occurred in 1994 and 1996, though not to the degree of 1990. Observers should always be on the lookout for periods of increased activity across the Equatorial Zone of the Planet.

 

 

Fig 05: Images of Saturn obtained in 1990 by the Pic du Midi observatory 1 metre cassegrain, showing the Great White Spot outbreak that occurred that year. This was the biggest such eruption to occur on the Planet in 30 years.

 

A Mission to Saturn.

It is over the next few years that interest, especially from the professional community, will greatly increase as NASA’s Cassini spacecraft finally reaches Saturn in July 2004. It was launched back on October 15th, 1997, and has been en-route to Saturn ever since. It passed Jupiter in late December of 2000 and sent back some of the best images of the Planet to date. We can only hope, (and expect it) to the do the same when it reaches Saturn.

 

 

          Fig 06: NASA’s Cassini spacecraft was launched back in 1997, and is due to spend 4 years studying Saturn from 2004 to 2008. This artists view depicts the release of the Huygens probe toward Titan.

 

The craft itself consists of two main sections. The Cassini orbiter, which will stay in orbit around Saturn to study its atmosphere, rings, and general environment. The second main section (and mission objective) is to deploy the Huygens probe down through the Atmosphere of Saturn’s largest moon, Titan, to the surface. This is perhaps the most fascinating and ambitious part of the whole mission, as astronomers currently do not know what the surface of Titan is like, as it is permanently hidden from view by the thick nitrogen atmosphere that covers Titan. By the time the spacecraft reaches Saturn it will have travelled some 3.2 billion Km’s to reach it. The mission is planned to last until 2008, but this will probably be extended, as was the case with Galileo at Jupiter.

Now has never been a better time for the amateur observer to make a real contribution to the study of Saturn. The novice observer equipped with a typical 150mm telescope can make detailed observations with practice, and today’s advanced amateur, equipped with powerful modern telescopes and CCD cameras, can produce data that is useful not only to amateur organisations, but professionals as well. As Cassini draws ever nearer to its destination in 2004, we come into a very interesting and exciting time as everyone turns there attention to the Solar System’s most beautiful Planet.

 

Damian Peach. July, 2002.