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Saturn satellite observations and orbits from the 1980 ring plane crossing
ICARUS 47, 282--287 (1981)
Saturn Satellite Observations and Orbits from the 1980 Ring Plane
Crossing P. K. S E I D E L M A N N , R. S. HARRINGTON, AND D. PASCU U.S. Naval Observatory, 34th St. and Massachusetts Ave., N.W., Washington, D.C. 20390
W. A. BAUM Lowell Observatory, P.O. Box 1269, Flagstaff, Arizona 86002
D. G. CURRIE Univers#y of Maryland, College Park, Maryland 20742,
AND
J. A. W E S T P H A L AND G. E. D A N IE L SO N California Institute of" Technology, Pasadena, Califi~rnia 91103 Received March 9, 1981; revised July 2, 1981 The g r o u n d - b a s e d observations o f the recently discovered Saturnian satellites, obtained during the 1980 apparition, have been collected from the IAU Circulars and identified with and fit to four orbital groups: (1) the inner pair o f coorbital librating satellites, (2) the satellite k n o w n as " D i o n e B'" near the L 4 point of D i o n e - S a t u r n , (3) the satellites associated with the L4 and Ls points of T e t h y s - S a t u r n or, alternatively, one satellite uncomfortably near the orbit o f T e t h y s , and (4) the Fring satellites o b s e r v e d by Voyager 1.
INTRODUCTION
From February to May 1980, when the rings of Saturn were presented edgewise to the Earth, several observing groups, using special equipment, undertook programs of observation aimed at detecting the faint E ring (Feibelman, 1967) and inner moons reported after the 1966 ring plane crossing (Dollfus, 1968; Fountain and Larson, 1977; Aksnes and Franklin, 1978). Our observations were made on the 155-cm astrometric reflector of the U.S. Naval Observatory, Flagstaff Station. A coronographic system designed by Baum, along with the Space Telescope Widefield/Planetary Camera Instrument Definition Team ground-based CCD camera 282 0019-1035/81/080282-06502.00/0 Copyright ~) 1981by AcademicPress, Inc. All rights of reproduction in any form reserved.
were used. The camera employed a 500 × 500-pixel array detector. The details of the instrumentation and results of the E ring observations are described by Baum e t al. (1981). Our satellite observations have been reported in IAUC numbers 3496, 3534, and 3605. As of IAUC 3574 more than 140 observations of new satellites were reported in the circulars by all observers. Identities between many of these " n e w " satellites were noted in the circulars and eventually three new satellites were recognized. These are 1980S1 (mag. 14) and 1980S3 (mag. 15) which move in nearly the same orbit at a distance of 24'.'5 from Saturn and 1980S6 (mag. 18), known as "Dione B" because it is located at the L4 point of Dione-Saturn.
SATURN SATELLITE OBSERVATIONS Many of the reported observations had not yet been identified with these three satellites and there appeared to be at least two additional new satellites involved. Thus, it was our intention to identify the observations reported in the IAU Circulars with the three recognized satellites and then attempt to sort out the remaining observations and relate them to individual orbits. Initially, numerical integrations were computed for objects in orbits similar to 1980S1, 1980S3, and 1980S6 and also for 2day and 6-day orbits to determine whether perturbations due to the larger classical satellites had a significant effect during the short time span of the observations. The dynamical model was planar and included the J2 and J4 terms of Saturn's potential function and perturbations due to satellites I through VIII, but excluded perturbations due to the Sun. No significant perturbative effects were detected. Next, approximate ephemerides were computed for these three satellites which were used to make a first pass at selecting those observations associated with each orbit. Improved orbits were derived by differential correction to the selected observations and the selection process was repeated. RESULTS AND DISCUSSION The resulting observation identifications for 1980S 1, 1980S3, and 1980S6 are given in Tables I, II, and III, respectively. Listed TABLE I OBSERVATtONS OF 1980S 1 Date
D
TABLE I--Continued
Date
O- C
IAUC
Desig.
MAR
15.98300 15.98510 16.94390 17.03100 17.96610 18.04010 18.04430 18.04940 18.10520 18.10650 23.88900 23.90400 23.90700 23.91500 23.91800 23.93100 23.93500 23.95600 23.96000 23.97100 23.97400 23.98700 23.99000
20.78 -24.00 -24.30 -20.50 - 16.90 24.50 - 25 - 24.00 -22.45 22.00 21.83 - 22.20 -24.64 - 23.90 -23.33 - 17.74 - 17.57 23.39 23.89 24.26 24.48 24.59 24.48 24.48 23.61 23.06 21.86 21.64 20.33 19.89
-0.47 -0.29 -0.84 -0.53 -0.48 0.57 - 2 - i .87 -0.50 -0.64 0.27 - 1.30 -0.97 - 0.39 -0.06 -0.15 -0.17 0.72 0.38 0.63 0.62 0.68 0.54 0.59 0.54 0.24 -0.12 -0.07 -0.04 - 0.13
3463 3496 3496 3496 3496 3496 3463 3463 3491 3463 3483 3463 3463 3463 3463 3463 3463 3497 3497 3497 3497 3497 3497 3497 3497 3497 3497 3497 3497 3497
66S02 80S01 80S01 80S01 80S01 80S01 80S09 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01
APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR
9.92430 9.93330 9.95200 10.29500 10.97220 11.00760 11.01460 11.95900 11.96810 11.97290 11.97850 11.98960 11.99440 11.99860 12.00280 12.00690 12.01110 12.01530 12.01870 12.02220 12.02710
-23.31 -23.79 -23.50 24.65 23.55 22.98 22.44 - 20.15 -20.61 - 21.39 - 22.16 - 22.50 ~22.77 - 22.91 -23.10 -23.27 -23.55 -23.62 -23.71 -23.77 - 23.62
0.64 0.37 0.56 0.50 -0.58 - 0.59 -0.73 0.08 0.45 0.36 0.10 0.62 0.64 0.73 0.73 0.71 0.55 0.56 0.52 0.48 0.62
3470 3470 3470 3484 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470
66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02
MAR MAR MAR
MAR MAR MAR MAR MAR
ORBITAL COMPUTATIONS
O-C
IAUC
Desig.
FEB FEB FEB
19.24020 23.40600 29.02200
-24.63 -25.00 -20.09
-0.14 -0.65 1.15
3454 3456 3463
80S01 80S02 80S01
MAR MAR MAR
1.04880 1.05710 1.05750
22.16 21.76 21.43
-0.43 -0.12 -0.41
3483 3463 3463
66S02 66S02 66S02
283
MAR MAR MAR
MAR MAR
MAR MAR
MAR MAR
MAR MAR MAR MAR MAR MAR
MAR MAR MAR MAR MAR MAR
1.06340 13.17853 13.18492 13.22638 13.25174 14.21100 15.27
D
284
SEIDELMANN TABLE
ET AL.
I1
TABLE
OBSERVATIONS OF 1980S3 Date
D
O-C
III
OBSERVATIONS OF 1980S6 IAUC
Desig.
FEB FEB
26.54 27.6
-ELONG +ELONG
C = 24 C = +22
3457 3457
80S03 80S04
MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR
1.34 9.30567 9.30897 9.30975 14.47 15.19000 16.9156 17.00900 18.04000 23.88600 23.88900 23.90400 23.90700 23.91500 23.93100 23.93500
+ELONG 24.90 - 24.90 - 25.10 +22 24.00 -22.7 21.60 22.30 -24.96 -24.86 -24.26 -24.48 -23.61 22.79 -22.79
C-
+25 0.01 0.08 -0.10 0 -0.07 + 1.0 0.23 -0.50 -0.40 -0.25 0.31 0.03 0.65 0.58 0.28
3457 3496 3496 3496 3466 3470 3574 3483 3483 3497 3497 3497 3497 3497 3497 3497
80S05 80S03 80S03 80S03 80Sll 80S08 80S03 80S03 80S03 80SI9 80S19 80S19 80S19 80S19 80S19 80S19
APR APR APR APR APR APR APR APR APR APR APR APR
9.91320 10.97920 10.98610 10.99300 11.01460 11.97290 11.97850 11.98960 11.99440 11.99860 12.00280 12.00690
23.10 22.10 -21.85 -21.30 - 20.94 24.09 24.26 24.50 24.54 24.60 24.48 24.26
0.96 0.78 0.51 0.44 - 1.66 0.09 0.17 0.40 (I.51 0.67 0.69 0.63
3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470
80S 15 80S16 80SI6 80S16 80S 16 80S17 80S17 80S17 80~17 80S17 80S17 80S17
are the date of observation, distance from the center of Saturn (East being positive), (O - C) (observed minus computed distance), IAU Circular reporting the observation, and the temporary designation. Table IV gives the final orbital elements for these satellites, listing the synodic period (days), the orbital longitude at 12.0 March 1980 = JD2444310.5, which is not corrected for light time (greatest eastern elongation corresponds to zero longitude), and the apparent orbital radius at epoch (derived from the period and oblateness). The values of the orbital radii tabulated were consistent (within the errors) with the semimajor axis derived from the differential correction solutions. The quoted errors are the internal standard errors of the linear corrections. ( I ) 1980S I, 1980S3 Smith (IAUC 3483) had suggested that 1980S1 and 1980S3 were in the same orbit
Date
D
O-C
IAUC
Desig.
MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR
1.03000 1.04900 1.09900 1.13100 9.39498 9.40219 9.42662 10.31414 10.31968 10.32358 10.34718 13.18171 13.18788 13.26449 15.95280 15.98510 16.02360 16.02400 16.08920 16.12600 28.21250 28.24510 28.26920 28.29270 28.31530
62.40 62.20 61.40 59.90 57.90 57.60 56.90 -47.00 -47.50 -47.70 -49.80 -56.60 -56.90 -60.00 -57.24 58.93 -60.40 -61.40 -61.45 -61.65 56.50 57.90 59.00 60.20 61.70
0.70 0.46 0.11 -0.67 -0.46 -0.43 0.13 0.03 0.04 0.19 0.12 0.16 0.19 0.26 1.26 0.83 0.44 -0.55 0.12 0.28 0.67 0.30 0.30 0.60 1.40
3483 3483 3483 3483 3496 3496 3496 3496 3496 3496 3496 3496 3496 3496 3495 3495 3495 3463 3495 3495 3466 3466 3466 3466 3466
80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S10 80S10 80S10 80S10 80S10 80S10 80S12 80S12 80SI2 80S12 80S12
APR APR
8.30500 12.02800
61.00 -34.80
0.19 0.38
3466 3545
80S14 80S06
and we confirm that. Further, work by Harrington and Seidelmann (1981) shows that the satellites never approach each other closer than 6° in longitude, but librate between the limits around the orbit with a period of 3000 days. Attempts to connect 1980S1 with 1966S2 in the IAU Circulars have been premature TABLE
IV
SATELLITE ORBITAL ELEMENTS
Satellite
Synodic period
Orbital longitude
Orbital radius
1980S1 1980S3 1980S6 1980S13
0.%945 0.6942 2.7351 1.8389 1.9985
306 ° 134° 2° 246 ° 313 °
24'.'811 24'.'803 61'!60 47'.'31 50'.'00
± + _+ ± +_
0.0001 0.0001 0.0005 0.0003 0.0012
_+ +_ _+ ÷ +_
1 2 1 2 41
N o t e . E p o c h 12 M a r c h 1980 = J D 2 4 4 4 3 1 0 . 5 .
SATURN SATELLITE OBSERVATIONS
285
and have caused some confusion. The reaTABLE V son for suggesting this identity in the first OBSERVATIONS OF 1980S13 place was that the orbit of 1980S 1 is about the same as that given for 1966S2 by FounDate D O-C IAUC Desig. tain and Larson (1977) and by Aksnes and 0.77 3496 80S25 Franklin (1978). The problem is that there MAR 13.36949 -44.10 MAR 13.38126 -44.60 0.89 3496 80S25 are two satellites in the same orbit and the MAR 15.35059 -46.76 0.62 3605 80S32 present precision of the motions does not MAR 16.01000 36.30 1.39 3549 80S29 permit the identification of 1980S1 or APR 8.31764 -46.50 -1.13 3466 80S13 1980S3 with either 1966S1 or 1966S2. APR 8.35208 -45.20 -2.08 3466 80S13 Even if Voyager II is successful in ob- APR 12.02800 -41.40 1.91 3484 80S24 taining observations of these moons, con- MAY 19.34000 31.50 -0.07 3549 80S30 necting the 1980 and the 1966 observations will not be possible without continued Subsequently, observations of satellites ground-based observations. Barfing that, made on 16 March and 19 May were rewe will have to await the Space Telescope ported (IAUC 3545) and the 8 April obserto resolve this problem. vations were revised (IAUC 3549). A new fit with these additional observations re(2) 1980S6 ("Dione B") sulted in an orbit of 1.84 days. Table V lists The discovery o f this satellite by Laques the residuals from that solution. This orbit and Lecacheux (IAUC 3457) came as a sur- suggests an association with the orbit of prise considering Kuiper's (1961) complete- Tethys which has a period of 1.8878 days. ness limits. Even more surprising was the However, an integration of this provisional finding that it was associated with the L4 orbit shows circulation with respect to point of Dione-Saturn (Lecacheux et al., Tethys but with destabilizing close encoun1980; Reitsema et al., 1980). We confirm ters. that conclusion and report a libration peAssuming that the 19 May observation is riod of approximately 800 days, in agree- not related to 1980S13, we find that a 1.99ment with that reported by Reitsema et al. day period orbit fits the March and April (1980). dates better as indicated by Table VI but a reexamination of our observational mate(3) 1980SI3, 1980S25 (Tethys Librators) rial disclosed three negative observations; At this point in the identification process 1980 March 13.33946, 14.21292, and there still remained at least one and possi14.25936! The orbital elements for both the bly three satellites (outside of Mimas' orbit) 1.84- and 1.99-day period orbits are listed in distinct from Dione B. The observations Table IV. are listed in Table VI (with the exception of Significantly, these orbits show the obthe 15 March observation which was re- servations to be close to the L4 point of ported later and 1980S22, listed in Table TABLE VI IX, which does not fit anything). An atOBSERVATIONS OF 1980SI3 tempt was made to fit these observations with preliminary orbits of 2 days and 6 Date D O-C IAUC Desig. days. Our observational material was reex0.94 3496 80S25 amined for additional or negative observa- MAR 13.36949 -44.10 MAR 13.38126 -44.60 0.32 3496 80S25 tions. A good image was found in a set of MAR 15.35059 -46.76 - 1.39 3605 80S32 exposures taken on 15 March which had 0.26 3466 80S13 been noted before but not measured be- APR 8.31764 - 4 6 . 5 0 3466 80S13 cause of poor reference images. A new APR 8.35208 -45.20 - 0 . 2 0 APR 12.02800 - 41.40 0.08 3484 80S24 solution favored a 2-day period.
286
SEIDELMANN ET AL. T A B L E IX
T A B L E VII OBSERVATIONS OF 1980S13 NEAR TETHYS L 4 POINT Date APR APR APR
8.31764 8.35208 12.02800
D
IAUC
Desig.
- 46.50 - 45.20 -41.40
3466 3466 3484
80S 13 80S 13 80S24
Tethys-Saturn in April (Table VII) and the L5 point in March and May (Table VIII). Thus an alternate interpretation, and the one we prefer, is that two objects are involved; 1980S 13 at the leading triangular libration point (L4) of Tethys (Table VII) and 1980S25 at the following (Ls) (Table
VIII). The objects are either solid bodies or significant accumulations of E ring material. A provisional analysis shows the April observations to be approximately 65 ° ahead of Tethys, while the March and May observations are about 40° behind. Due to the libration motion, the March and May observations should not necessarily be at the same elongation and thus, a solution for a unique angle is probably incomplete. Unfortunately, the limited number of observations now available preclude the exact determination of the orbits and residuals at this time. (4) F Ring Satellites?
Finally, Table IX lists those observations which do not fit any of the previous orbits, nor can they be accounted for by a single orbit. Except for 1980S22, the observed magnitudes and separations correspond
UNIDENTIFIED OBSERVATIONS Date
D
IAUC
Desig.
MAR MAR MAR MA R MA R MAR MAR MA R MAR MAR MAR MA R MA R MAR MAR
13.29000 15.19000 16.12600 20.88500 20.89900 20.90800 20.91500 20.92500 20.92800 20.97800 22.97300 22.97400 22.97700 22.98300 22.98800
23.00 19.00 19.75 -58.05 18.90 20.12 21.89 21.72 21.66 -51.74 -22.44 -22.00 -21.72 -21.67 -21.79
3463 3483 3574 3474 3497 3497 3497 3497 3497 3574 3497 3497 3497 3497 3497
80S07 80S23 80S31 80S22 80S20 80S20 80S20 80S20 80S20 80S22 80S21 80S21 80S21 80S21 80S21
APR APR APR APR APR
12.01530 12.02710 12.02990 12.03330 12.03890
-24.80 -25.60 -26.12 26.35 26.12
3470 3470 3470 3470 3470
80S18 80S18 80S18 80S18 80S 18
roughly to what one would expect for the F ring and its satellites, but the observations do not accurately fit an orbit with the announced periods of the F ring satellites. (5) Orbital Elements
Table X gives a comparison among the orbital periods and longitude at epoch given by Reitsema et al. (1980), Synnott et al. (1981), and this paper. The difference between Synnott et al. and the other determinations should be expected for 1980S6. The TABLE X COMPARISON OF ORBIT DETERMINATIONS
T A B L E VIII
Satellite
OBSERVATIONS OF 1980S25 NEAR TETHYS L5 POINT Date
D
IAUC
Desig.
M AR M AR MAR MAR
13.36949 13.38126 13.35059 16.01000
-44.10 -44.60 -46.76 36.30
3496 3496 3605 3549
80S25 80S25 80S32 80S29
M AY
19.34000
31.50
3549
80S30
Synodic period and Orbital longitude
This paper
Synnott
Reitsema
et al.
et al.
1980S1
P h0
0~1.6945 306°
0~6946 301°
1980S3
P h0
0~6942 134°
0d.6942 1320
1980S6
P h0
2~7363 2°
2~7374 1~
Note. ~o for JD2444310.5.
2~7361 1o
SATURN SATELLITE OBSERVATIONS determinations are based on observations taken at different times and thus different locations in the motion of the satellite around the libration point. In conclusion, we have managed to sort out the 1980 spring observations of new Saturnian satellites into the above four orbital groups and confirm previous orbital work on 1980S1, 1980S3, and 1980S6. In addition, we find evidence to suggest that 1980S 13 and 1980S25 are distinct new satellites and that they are located near the L4 and L5 libration points o f Tethys-Saturn. AC KNOWLEDGMENTS The authors would like to acknowledge the staff of the Flagstaff Station of the U.S. Naval Observatory for their assistance during the observing periods, and Linda Gardner for special computer programming for the measurement of the images. REFERENCES AKSNES, K., AND FRANKLIN, F. A. (1978). The evidence for new faint satellites of Saturn reexamined. Icarus 36, 107-118. BAUM, W. A., KREIDL, T., WESTPHAL, J. A., DAN-
287
IELSON, G. E., SEIDELMANN, P. K., PASCU, D., AND CURRIE, D. G. (1981). Saturn's ring. I. CCD observations of March 1980. Icarus 47, 84-96. DOLLFUS,A. (1968). La decouverte du 10* satellite de Saturne. L'Astronomie 82, 253-262. FEIBELMAN,W. A. (1967). Concerning the 'D' Ring of Saturn. Nature 214, 793-794. FOUNTAIN, J. W., AND LARSON, S. M. (1977). A new satellite of Saturn? Science 197, 915-917. HARRINGTON, R. S., AND SEIDELMANN, P. K. (1981). The dynamics of the Saturnian satellites 1980S1 and 1980S3. Icarus 47, 97-99. KUIPER, G. P. (1961). Limits of completeness. In Planets and Satellites (G. P. Kuiper and B. M. Middlehurst, Eds.), pp. 575-591. Chicago: Univ. of Chicago Press. LARSON, S. M., SMITH, B. A., FOUNTAIN, J. W., AND REITSEMA, H. J. (1981). The 1966 observations of the coorbiting satellites of Saturn, SI0 and S l l . Icarus 46, 175-180. LECACHEUX, J., LAQUES, P., VAPILLON, L., AUGE, A., AND DESPIAU, R. (1980). A new satellite of Saturn: "Dione B." Icarus 43, 111-115. REITSEMA, H. J., SMITH, B. A., AND LARSON, S. M. (1980). A new Saturnian satellite near Dione's L4 point. Icarus 43, 116--119. SYNNOTT, S. P., PETERS, C. F., SMITH, B. A., AND MORABITO, L. A. (1981). Orbits of the small satellites of Saturn. Science 212, 191-192.
Saturn Satellite Observations and Orbits from the 1980 Ring Plane
Crossing P. K. S E I D E L M A N N , R. S. HARRINGTON, AND D. PASCU U.S. Naval Observatory, 34th St. and Massachusetts Ave., N.W., Washington, D.C. 20390
W. A. BAUM Lowell Observatory, P.O. Box 1269, Flagstaff, Arizona 86002
D. G. CURRIE Univers#y of Maryland, College Park, Maryland 20742,
AND
J. A. W E S T P H A L AND G. E. D A N IE L SO N California Institute of" Technology, Pasadena, Califi~rnia 91103 Received March 9, 1981; revised July 2, 1981 The g r o u n d - b a s e d observations o f the recently discovered Saturnian satellites, obtained during the 1980 apparition, have been collected from the IAU Circulars and identified with and fit to four orbital groups: (1) the inner pair o f coorbital librating satellites, (2) the satellite k n o w n as " D i o n e B'" near the L 4 point of D i o n e - S a t u r n , (3) the satellites associated with the L4 and Ls points of T e t h y s - S a t u r n or, alternatively, one satellite uncomfortably near the orbit o f T e t h y s , and (4) the Fring satellites o b s e r v e d by Voyager 1.
INTRODUCTION
From February to May 1980, when the rings of Saturn were presented edgewise to the Earth, several observing groups, using special equipment, undertook programs of observation aimed at detecting the faint E ring (Feibelman, 1967) and inner moons reported after the 1966 ring plane crossing (Dollfus, 1968; Fountain and Larson, 1977; Aksnes and Franklin, 1978). Our observations were made on the 155-cm astrometric reflector of the U.S. Naval Observatory, Flagstaff Station. A coronographic system designed by Baum, along with the Space Telescope Widefield/Planetary Camera Instrument Definition Team ground-based CCD camera 282 0019-1035/81/080282-06502.00/0 Copyright ~) 1981by AcademicPress, Inc. All rights of reproduction in any form reserved.
were used. The camera employed a 500 × 500-pixel array detector. The details of the instrumentation and results of the E ring observations are described by Baum e t al. (1981). Our satellite observations have been reported in IAUC numbers 3496, 3534, and 3605. As of IAUC 3574 more than 140 observations of new satellites were reported in the circulars by all observers. Identities between many of these " n e w " satellites were noted in the circulars and eventually three new satellites were recognized. These are 1980S1 (mag. 14) and 1980S3 (mag. 15) which move in nearly the same orbit at a distance of 24'.'5 from Saturn and 1980S6 (mag. 18), known as "Dione B" because it is located at the L4 point of Dione-Saturn.
SATURN SATELLITE OBSERVATIONS Many of the reported observations had not yet been identified with these three satellites and there appeared to be at least two additional new satellites involved. Thus, it was our intention to identify the observations reported in the IAU Circulars with the three recognized satellites and then attempt to sort out the remaining observations and relate them to individual orbits. Initially, numerical integrations were computed for objects in orbits similar to 1980S1, 1980S3, and 1980S6 and also for 2day and 6-day orbits to determine whether perturbations due to the larger classical satellites had a significant effect during the short time span of the observations. The dynamical model was planar and included the J2 and J4 terms of Saturn's potential function and perturbations due to satellites I through VIII, but excluded perturbations due to the Sun. No significant perturbative effects were detected. Next, approximate ephemerides were computed for these three satellites which were used to make a first pass at selecting those observations associated with each orbit. Improved orbits were derived by differential correction to the selected observations and the selection process was repeated. RESULTS AND DISCUSSION The resulting observation identifications for 1980S 1, 1980S3, and 1980S6 are given in Tables I, II, and III, respectively. Listed TABLE I OBSERVATtONS OF 1980S 1 Date
D
TABLE I--Continued
Date
O- C
IAUC
Desig.
MAR
15.98300 15.98510 16.94390 17.03100 17.96610 18.04010 18.04430 18.04940 18.10520 18.10650 23.88900 23.90400 23.90700 23.91500 23.91800 23.93100 23.93500 23.95600 23.96000 23.97100 23.97400 23.98700 23.99000
20.78 -24.00 -24.30 -20.50 - 16.90 24.50 - 25 - 24.00 -22.45 22.00 21.83 - 22.20 -24.64 - 23.90 -23.33 - 17.74 - 17.57 23.39 23.89 24.26 24.48 24.59 24.48 24.48 23.61 23.06 21.86 21.64 20.33 19.89
-0.47 -0.29 -0.84 -0.53 -0.48 0.57 - 2 - i .87 -0.50 -0.64 0.27 - 1.30 -0.97 - 0.39 -0.06 -0.15 -0.17 0.72 0.38 0.63 0.62 0.68 0.54 0.59 0.54 0.24 -0.12 -0.07 -0.04 - 0.13
3463 3496 3496 3496 3496 3496 3463 3463 3491 3463 3483 3463 3463 3463 3463 3463 3463 3497 3497 3497 3497 3497 3497 3497 3497 3497 3497 3497 3497 3497
66S02 80S01 80S01 80S01 80S01 80S01 80S09 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01 80S01
APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR APR
9.92430 9.93330 9.95200 10.29500 10.97220 11.00760 11.01460 11.95900 11.96810 11.97290 11.97850 11.98960 11.99440 11.99860 12.00280 12.00690 12.01110 12.01530 12.01870 12.02220 12.02710
-23.31 -23.79 -23.50 24.65 23.55 22.98 22.44 - 20.15 -20.61 - 21.39 - 22.16 - 22.50 ~22.77 - 22.91 -23.10 -23.27 -23.55 -23.62 -23.71 -23.77 - 23.62
0.64 0.37 0.56 0.50 -0.58 - 0.59 -0.73 0.08 0.45 0.36 0.10 0.62 0.64 0.73 0.73 0.71 0.55 0.56 0.52 0.48 0.62
3470 3470 3470 3484 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470
66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02 66S02
MAR MAR MAR
MAR MAR MAR MAR MAR
ORBITAL COMPUTATIONS
O-C
IAUC
Desig.
FEB FEB FEB
19.24020 23.40600 29.02200
-24.63 -25.00 -20.09
-0.14 -0.65 1.15
3454 3456 3463
80S01 80S02 80S01
MAR MAR MAR
1.04880 1.05710 1.05750
22.16 21.76 21.43
-0.43 -0.12 -0.41
3483 3463 3463
66S02 66S02 66S02
283
MAR MAR MAR
MAR MAR
MAR MAR
MAR MAR
MAR MAR MAR MAR MAR MAR
MAR MAR MAR MAR MAR MAR
1.06340 13.17853 13.18492 13.22638 13.25174 14.21100 15.27
D
284
SEIDELMANN TABLE
ET AL.
I1
TABLE
OBSERVATIONS OF 1980S3 Date
D
O-C
III
OBSERVATIONS OF 1980S6 IAUC
Desig.
FEB FEB
26.54 27.6
-ELONG +ELONG
C = 24 C = +22
3457 3457
80S03 80S04
MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR
1.34 9.30567 9.30897 9.30975 14.47 15.19000 16.9156 17.00900 18.04000 23.88600 23.88900 23.90400 23.90700 23.91500 23.93100 23.93500
+ELONG 24.90 - 24.90 - 25.10 +22 24.00 -22.7 21.60 22.30 -24.96 -24.86 -24.26 -24.48 -23.61 22.79 -22.79
C-
+25 0.01 0.08 -0.10 0 -0.07 + 1.0 0.23 -0.50 -0.40 -0.25 0.31 0.03 0.65 0.58 0.28
3457 3496 3496 3496 3466 3470 3574 3483 3483 3497 3497 3497 3497 3497 3497 3497
80S05 80S03 80S03 80S03 80Sll 80S08 80S03 80S03 80S03 80SI9 80S19 80S19 80S19 80S19 80S19 80S19
APR APR APR APR APR APR APR APR APR APR APR APR
9.91320 10.97920 10.98610 10.99300 11.01460 11.97290 11.97850 11.98960 11.99440 11.99860 12.00280 12.00690
23.10 22.10 -21.85 -21.30 - 20.94 24.09 24.26 24.50 24.54 24.60 24.48 24.26
0.96 0.78 0.51 0.44 - 1.66 0.09 0.17 0.40 (I.51 0.67 0.69 0.63
3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470 3470
80S 15 80S16 80SI6 80S16 80S 16 80S17 80S17 80S17 80~17 80S17 80S17 80S17
are the date of observation, distance from the center of Saturn (East being positive), (O - C) (observed minus computed distance), IAU Circular reporting the observation, and the temporary designation. Table IV gives the final orbital elements for these satellites, listing the synodic period (days), the orbital longitude at 12.0 March 1980 = JD2444310.5, which is not corrected for light time (greatest eastern elongation corresponds to zero longitude), and the apparent orbital radius at epoch (derived from the period and oblateness). The values of the orbital radii tabulated were consistent (within the errors) with the semimajor axis derived from the differential correction solutions. The quoted errors are the internal standard errors of the linear corrections. ( I ) 1980S I, 1980S3 Smith (IAUC 3483) had suggested that 1980S1 and 1980S3 were in the same orbit
Date
D
O-C
IAUC
Desig.
MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR MAR
1.03000 1.04900 1.09900 1.13100 9.39498 9.40219 9.42662 10.31414 10.31968 10.32358 10.34718 13.18171 13.18788 13.26449 15.95280 15.98510 16.02360 16.02400 16.08920 16.12600 28.21250 28.24510 28.26920 28.29270 28.31530
62.40 62.20 61.40 59.90 57.90 57.60 56.90 -47.00 -47.50 -47.70 -49.80 -56.60 -56.90 -60.00 -57.24 58.93 -60.40 -61.40 -61.45 -61.65 56.50 57.90 59.00 60.20 61.70
0.70 0.46 0.11 -0.67 -0.46 -0.43 0.13 0.03 0.04 0.19 0.12 0.16 0.19 0.26 1.26 0.83 0.44 -0.55 0.12 0.28 0.67 0.30 0.30 0.60 1.40
3483 3483 3483 3483 3496 3496 3496 3496 3496 3496 3496 3496 3496 3496 3495 3495 3495 3463 3495 3495 3466 3466 3466 3466 3466
80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S06 80S10 80S10 80S10 80S10 80S10 80S10 80S12 80S12 80SI2 80S12 80S12
APR APR
8.30500 12.02800
61.00 -34.80
0.19 0.38
3466 3545
80S14 80S06
and we confirm that. Further, work by Harrington and Seidelmann (1981) shows that the satellites never approach each other closer than 6° in longitude, but librate between the limits around the orbit with a period of 3000 days. Attempts to connect 1980S1 with 1966S2 in the IAU Circulars have been premature TABLE
IV
SATELLITE ORBITAL ELEMENTS
Satellite
Synodic period
Orbital longitude
Orbital radius
1980S1 1980S3 1980S6 1980S13
0.%945 0.6942 2.7351 1.8389 1.9985
306 ° 134° 2° 246 ° 313 °
24'.'811 24'.'803 61'!60 47'.'31 50'.'00
± + _+ ± +_
0.0001 0.0001 0.0005 0.0003 0.0012
_+ +_ _+ ÷ +_
1 2 1 2 41
N o t e . E p o c h 12 M a r c h 1980 = J D 2 4 4 4 3 1 0 . 5 .
SATURN SATELLITE OBSERVATIONS
285
and have caused some confusion. The reaTABLE V son for suggesting this identity in the first OBSERVATIONS OF 1980S13 place was that the orbit of 1980S 1 is about the same as that given for 1966S2 by FounDate D O-C IAUC Desig. tain and Larson (1977) and by Aksnes and 0.77 3496 80S25 Franklin (1978). The problem is that there MAR 13.36949 -44.10 MAR 13.38126 -44.60 0.89 3496 80S25 are two satellites in the same orbit and the MAR 15.35059 -46.76 0.62 3605 80S32 present precision of the motions does not MAR 16.01000 36.30 1.39 3549 80S29 permit the identification of 1980S1 or APR 8.31764 -46.50 -1.13 3466 80S13 1980S3 with either 1966S1 or 1966S2. APR 8.35208 -45.20 -2.08 3466 80S13 Even if Voyager II is successful in ob- APR 12.02800 -41.40 1.91 3484 80S24 taining observations of these moons, con- MAY 19.34000 31.50 -0.07 3549 80S30 necting the 1980 and the 1966 observations will not be possible without continued Subsequently, observations of satellites ground-based observations. Barfing that, made on 16 March and 19 May were rewe will have to await the Space Telescope ported (IAUC 3545) and the 8 April obserto resolve this problem. vations were revised (IAUC 3549). A new fit with these additional observations re(2) 1980S6 ("Dione B") sulted in an orbit of 1.84 days. Table V lists The discovery o f this satellite by Laques the residuals from that solution. This orbit and Lecacheux (IAUC 3457) came as a sur- suggests an association with the orbit of prise considering Kuiper's (1961) complete- Tethys which has a period of 1.8878 days. ness limits. Even more surprising was the However, an integration of this provisional finding that it was associated with the L4 orbit shows circulation with respect to point of Dione-Saturn (Lecacheux et al., Tethys but with destabilizing close encoun1980; Reitsema et al., 1980). We confirm ters. that conclusion and report a libration peAssuming that the 19 May observation is riod of approximately 800 days, in agree- not related to 1980S13, we find that a 1.99ment with that reported by Reitsema et al. day period orbit fits the March and April (1980). dates better as indicated by Table VI but a reexamination of our observational mate(3) 1980SI3, 1980S25 (Tethys Librators) rial disclosed three negative observations; At this point in the identification process 1980 March 13.33946, 14.21292, and there still remained at least one and possi14.25936! The orbital elements for both the bly three satellites (outside of Mimas' orbit) 1.84- and 1.99-day period orbits are listed in distinct from Dione B. The observations Table IV. are listed in Table VI (with the exception of Significantly, these orbits show the obthe 15 March observation which was re- servations to be close to the L4 point of ported later and 1980S22, listed in Table TABLE VI IX, which does not fit anything). An atOBSERVATIONS OF 1980SI3 tempt was made to fit these observations with preliminary orbits of 2 days and 6 Date D O-C IAUC Desig. days. Our observational material was reex0.94 3496 80S25 amined for additional or negative observa- MAR 13.36949 -44.10 MAR 13.38126 -44.60 0.32 3496 80S25 tions. A good image was found in a set of MAR 15.35059 -46.76 - 1.39 3605 80S32 exposures taken on 15 March which had 0.26 3466 80S13 been noted before but not measured be- APR 8.31764 - 4 6 . 5 0 3466 80S13 cause of poor reference images. A new APR 8.35208 -45.20 - 0 . 2 0 APR 12.02800 - 41.40 0.08 3484 80S24 solution favored a 2-day period.
286
SEIDELMANN ET AL. T A B L E IX
T A B L E VII OBSERVATIONS OF 1980S13 NEAR TETHYS L 4 POINT Date APR APR APR
8.31764 8.35208 12.02800
D
IAUC
Desig.
- 46.50 - 45.20 -41.40
3466 3466 3484
80S 13 80S 13 80S24
Tethys-Saturn in April (Table VII) and the L5 point in March and May (Table VIII). Thus an alternate interpretation, and the one we prefer, is that two objects are involved; 1980S 13 at the leading triangular libration point (L4) of Tethys (Table VII) and 1980S25 at the following (Ls) (Table
VIII). The objects are either solid bodies or significant accumulations of E ring material. A provisional analysis shows the April observations to be approximately 65 ° ahead of Tethys, while the March and May observations are about 40° behind. Due to the libration motion, the March and May observations should not necessarily be at the same elongation and thus, a solution for a unique angle is probably incomplete. Unfortunately, the limited number of observations now available preclude the exact determination of the orbits and residuals at this time. (4) F Ring Satellites?
Finally, Table IX lists those observations which do not fit any of the previous orbits, nor can they be accounted for by a single orbit. Except for 1980S22, the observed magnitudes and separations correspond
UNIDENTIFIED OBSERVATIONS Date
D
IAUC
Desig.
MAR MAR MAR MA R MA R MAR MAR MA R MAR MAR MAR MA R MA R MAR MAR
13.29000 15.19000 16.12600 20.88500 20.89900 20.90800 20.91500 20.92500 20.92800 20.97800 22.97300 22.97400 22.97700 22.98300 22.98800
23.00 19.00 19.75 -58.05 18.90 20.12 21.89 21.72 21.66 -51.74 -22.44 -22.00 -21.72 -21.67 -21.79
3463 3483 3574 3474 3497 3497 3497 3497 3497 3574 3497 3497 3497 3497 3497
80S07 80S23 80S31 80S22 80S20 80S20 80S20 80S20 80S20 80S22 80S21 80S21 80S21 80S21 80S21
APR APR APR APR APR
12.01530 12.02710 12.02990 12.03330 12.03890
-24.80 -25.60 -26.12 26.35 26.12
3470 3470 3470 3470 3470
80S18 80S18 80S18 80S18 80S 18
roughly to what one would expect for the F ring and its satellites, but the observations do not accurately fit an orbit with the announced periods of the F ring satellites. (5) Orbital Elements
Table X gives a comparison among the orbital periods and longitude at epoch given by Reitsema et al. (1980), Synnott et al. (1981), and this paper. The difference between Synnott et al. and the other determinations should be expected for 1980S6. The TABLE X COMPARISON OF ORBIT DETERMINATIONS
T A B L E VIII
Satellite
OBSERVATIONS OF 1980S25 NEAR TETHYS L5 POINT Date
D
IAUC
Desig.
M AR M AR MAR MAR
13.36949 13.38126 13.35059 16.01000
-44.10 -44.60 -46.76 36.30
3496 3496 3605 3549
80S25 80S25 80S32 80S29
M AY
19.34000
31.50
3549
80S30
Synodic period and Orbital longitude
This paper
Synnott
Reitsema
et al.
et al.
1980S1
P h0
0~1.6945 306°
0~6946 301°
1980S3
P h0
0~6942 134°
0d.6942 1320
1980S6
P h0
2~7363 2°
2~7374 1~
Note. ~o for JD2444310.5.
2~7361 1o
SATURN SATELLITE OBSERVATIONS determinations are based on observations taken at different times and thus different locations in the motion of the satellite around the libration point. In conclusion, we have managed to sort out the 1980 spring observations of new Saturnian satellites into the above four orbital groups and confirm previous orbital work on 1980S1, 1980S3, and 1980S6. In addition, we find evidence to suggest that 1980S 13 and 1980S25 are distinct new satellites and that they are located near the L4 and L5 libration points o f Tethys-Saturn. AC KNOWLEDGMENTS The authors would like to acknowledge the staff of the Flagstaff Station of the U.S. Naval Observatory for their assistance during the observing periods, and Linda Gardner for special computer programming for the measurement of the images. REFERENCES AKSNES, K., AND FRANKLIN, F. A. (1978). The evidence for new faint satellites of Saturn reexamined. Icarus 36, 107-118. BAUM, W. A., KREIDL, T., WESTPHAL, J. A., DAN-
287
IELSON, G. E., SEIDELMANN, P. K., PASCU, D., AND CURRIE, D. G. (1981). Saturn's ring. I. CCD observations of March 1980. Icarus 47, 84-96. DOLLFUS,A. (1968). La decouverte du 10* satellite de Saturne. L'Astronomie 82, 253-262. FEIBELMAN,W. A. (1967). Concerning the 'D' Ring of Saturn. Nature 214, 793-794. FOUNTAIN, J. W., AND LARSON, S. M. (1977). A new satellite of Saturn? Science 197, 915-917. HARRINGTON, R. S., AND SEIDELMANN, P. K. (1981). The dynamics of the Saturnian satellites 1980S1 and 1980S3. Icarus 47, 97-99. KUIPER, G. P. (1961). Limits of completeness. In Planets and Satellites (G. P. Kuiper and B. M. Middlehurst, Eds.), pp. 575-591. Chicago: Univ. of Chicago Press. LARSON, S. M., SMITH, B. A., FOUNTAIN, J. W., AND REITSEMA, H. J. (1981). The 1966 observations of the coorbiting satellites of Saturn, SI0 and S l l . Icarus 46, 175-180. LECACHEUX, J., LAQUES, P., VAPILLON, L., AUGE, A., AND DESPIAU, R. (1980). A new satellite of Saturn: "Dione B." Icarus 43, 111-115. REITSEMA, H. J., SMITH, B. A., AND LARSON, S. M. (1980). A new Saturnian satellite near Dione's L4 point. Icarus 43, 116--119. SYNNOTT, S. P., PETERS, C. F., SMITH, B. A., AND MORABITO, L. A. (1981). Orbits of the small satellites of Saturn. Science 212, 191-192.