J/AJ/149/27 Cassini ISS astrometry of Saturnian satellites (Cooper+, 2015)
Saturn's inner satellites: orbits, masses, and the chaotic motion of Atlas from
new Cassini imaging observations.
Cooper N.J., Renner S., Murray C.D., Evans M.W.
<Astron. J., 149, 27 (2015)>
=2015AJ....149...27C 2015AJ....149...27C
ADC_Keywords: Solar system ; Minor planets ; Positional data
Keywords: astrometry - celestial mechanics - planets and satellites: rings -
planets and satellites: dynamical evolution and stability
Abstract:
We present numerically derived orbits and mass estimates for the inner
Saturnian satellites, Atlas, Prometheus, Pandora, Janus, and
Epimetheus from a fit to 2580 new Cassini Imaging Science Subsystem
astrometric observations spanning 2004 February to 2013 August. The
observations are provided as machine-readable and Virtual Observatory
tables. We estimate GMAtlas=(0.384±0.001)x10-3km3/s2, a
value 13% smaller than the previously published estimate but with an
order of magnitude reduction in the uncertainty. We also find
GMPrometheus=(10.677±0.006)x10-3km3/s2,
GMPandora=(9.133±0.009)x10-3km3/s2,
GMJanus=(126.51±0.03)x10-3km3/s2, and
GMEpimetheus=(35.110±0.009)x10-3km3/s2, consistent with
previously published values, but also with significant reductions in
uncertainties. We show that Atlas is currently librating in both the
54:53 co-rotation-eccentricity resonance (CER) and the 54:53 inner
Lindblad (ILR) resonance with Prometheus, making it the latest example
of a coupled CER-ILR system, in common with the Saturnian satellites
Anthe, Aegaeon, and Methone, and possibly Neptune's ring arcs. We
further demonstrate that Atlas's orbit is chaotic, with a Lyapunov
time of ∼10years, and show that its chaotic behavior is a direct
consequence of the coupled resonant interaction with Prometheus,
rather than being an indirect effect of the known chaotic interaction
between Prometheus and Pandora. We provide an updated analysis of the
second-order resonant perturbations involving Prometheus, Pandora, and
Epimetheus based on the new observations, showing that these resonant
arguments are librating only when Epimetheus is the innermost of the
co-orbital pair, Janus and Epimetheus. We also find evidence that the
known chaotic changes in the orbits of Prometheus and Pandora are not
confined to times of apse anti-alignment.
Description:
We present new astrometric observations spanning 2004 February to 2013
August for the inner Saturnian satellites, Atlas, Prometheus, Pandora,
Janus, and Epimetheus.
Cassini Imaging Science Subsystem (ISS) images used in this work come
mainly from SATELLORB image sequences. These sequences form part of a
campaign of observations of the small inner satellites of Saturn in
progress throughout the Cassini tour, a campaign which by end of
mission will have provided dense coverage between 2004 and 2017. In
addition, we include observations from image sequences designed to
study Saturn's F ring, many of which contain opportunistic sightings
of Atlas, Prometheus, and Pandora, in addition to Pan and Daphnis
(though these satellites do not form part of the current work). Of the
2580 observations presented here, the majority (2567) used narrow
angle camera (NAC) images while the remaining 13 came from wide-angle
camera (WAC) images.
Astrometric measurements were made using the Caviar software package.
Caviar (CAssini Visual Image Analysis Release) was developed at Queen
Mary University of London for the analysis and astrometric reduction
of Cassini ISS images and uses the IDL data language (Exelis Visual
Information Solutions, Boulder, Colorado).
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 47 253 Spacecraft Planet Instrument C-matrix Events (SPICE)
kernels used in orbit determination and numerical
modeling
table2.dat 82 5 Summary of Cassini Imaging Science Subsystem (ISS)
observations
table3.dat 118 2580 Sample of Cassini Imaging Science Subsystem (ISS)
observations
table6.dat 226 5 Solutions for the planetocentric state vectors in
the ICRF at epoch 2007 June 1 00:00:00.0 UTC
(2007 June 1 00:01:05.184 or 2454252.50075446 JD TDB)
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See also:
I/322 : UCAC4 Catalogue (Zacharias+, 2012)
I/289 : UCAC2 Catalogue (Zacharias+ 2004)
I/259 : The Tycho-2 Catalogue (Hog+ 2000)
J/A+A/572/A43 : Saturnian satellites Cassini ISS astrometry (Cooper+, 2014)
J/A+A/551/A129 : Mimas and Enceladus Cassini ISS astrometry (Tajeddine+ 2013)
J/A+A/544/A29 : 2009 Saturnian satellites mutual events (Arlot+, 2012)
J/other/SoSyR/45.523 : Saturnian satellites CCD observations (Grosheva+, 2011)
J/A+A/400/1095 : 1995-99 CCD observation of Saturnian satellite (Veiga+, 2003)
J/A+A/383/296 : Saturnian Satellites positions (1996-2000) (Peng+, 2002)
J/A+A/380/727 : 1995 Saturnian satellite observations (Vienne+, 2001)
J/A+AS/139/47 : 1995 Saturnian satellites mutual events (Emelianov+, 1999)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 47 A47 --- Kernel Name of the latest SPICE Kernel (1)
--------------------------------------------------------------------------------
Note (1): Available by anonymous ftp from
ftp://naif.jpl.nasa.gov/pub/naif/CASSINI/kernels
The principal components of the Spacecraft Planet Instrument C-matrix
Events (SPICE; Acton 1996P&SS...44...65A 1996P&SS...44...65A) system are a set of elemental
data files, called kernels, and NAIF Toolkit software.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Sat Satellite name
12- 22 A11 "Y:M:D" Obs0.date Starting date of observation
24- 35 A12 "h:m:s" Obs0.time Starting time of observation
37- 47 A11 "Y:M:D" Obs1.date Ending date of observation
49- 60 A12 "h:m:s" Obs1.time Ending time of observation
62- 64 I3 --- Nt Total number of observations (1)
66- 68 I3 --- N1 Center-of-light (number of observations
centroided) (2)
70- 72 I3 --- N2 Center-of-figure (number of observations
limb-fitted) (2)
74- 77 F4.1 km O-Cl The rms O-C residual in the line direction (3)
79- 82 F4.1 km O-Cs The rms O-C residual in sample direction (3)
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Note (1): Totals include Narrow Angle Camera (NAC) and Wide-Angle Camera (WAC)
images (the overwhelming majority are NACs). WAC counts are: Atlas (0),
Prometheus (1), Pandora (3), Janus (5), Epimetheus (4).
Note (2): The astrometric positions of unresolved or poorly resolved satellites
were estimated using a centroid-estimation method based on Stetson
(1987PASP...99..191S 1987PASP...99..191S). The measured positions of the center-of-light were
then adjusted to the center-of-figure using a correction depending on the
phase angle between the observer, satellite, and the Sun. For images of
resolved satellites, the center-of-figure was estimated by fitting an
ellipsoidal shape model obtained from the latest Spacecraft Planet
Instrument C-matrix Events (SPICE) kernels (Table 1).
Note (3): The smaller rms residuals for Atlas reflect the fact that most
observations for this satellite used a centroiding rather than a
limb-fitting technique. See Section 3.1 for further discussion.
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 A11 --- Image Image identifier
13- 23 A11 "Y:M:D" Obs.date UTC date of the observation start
25- 36 A12 "h:m:s" Obs.time UTC time of the observation start
38- 47 F10.6 deg RAideg Image Right Ascension in decimal degrees
(J2000) (1)
49- 58 F10.6 deg DEideg Image Declination in decimal degrees
(J2000) (1)
60- 69 F10.6 deg Twist [0/360] Twist angle of the camera optical
axis (1)
71- 77 F7.2 pix Line Measured line value (2)
79- 85 F7.2 pix Sample Measured sample value (2)
87- 96 F10.6 deg RAdeg Object Right Ascension in decimal degrees
(J2000) (1)
98-107 F10.6 deg DEdeg Object Declination in decimal degrees
(J2000) (1)
109-118 A10 --- Sat Satellite name (3)
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Note (1): Angular coordinates are expressed with respect to the International
Celestial Reference Frame (ICRF). Columns IRAdeg, IDEdeg and Twist together
provide the corrected pointing direction and orientation of the camera
optical axis.
Note (2): The origin of the line,sample coordinate system is at the top left of
the image with line, y, increasing downwards and sample, x, to the right
(we use the Cassini convention of referring to the x-coordinate as "sample"
and y-coordinate as "line").
Note (3): Janus, Epimetheus, Atlas, Prometheus, Pandora
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Byte-by-byte Description of file: table6.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Sat Satellite name (4)
12- 33 E22.16 km x Value of the parameter x (6)
35- 46 F12.10 km e_x Uncertainty in x
48- 69 E22.16 km y Value of the parameter y (6)
71- 82 F12.10 km e_y Uncertainty in y
84-105 E22.16 km z Value of the parameter z (6)
107-118 F12.10 km e_z Uncertainty in z
120-141 E22.16 km/s dx/dt Value of the parameter dx/dt (6)
143-154 F12.10 km/s e_dx/dt Uncertainty in dx/dt
156-177 E22.16 km/s dy/dt Value of the parameter dy/dt (6)
179-190 F12.10 km/s e_dy/dt Uncertainty in dy/dt
192-213 E22.16 km/s dz/dt Value of the parameter dz/dt (6)
215-226 F12.10 km/s e_dz/dt Uncertainty in dz/dt
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Note (4): Satellites are Janus, Epimetheus, Atlas, Prometheus, Pandora.
The pixel rms value is 0.855 (Narrow Angle Camera, NAC) and the arcsec
rms is 1.057 (NAC).
Note (6): A numerical integration of the full equations of motion in three
dimensions (3D) was fitted to the astrometric data for Atlas, Prometheus,
Pandora, Janus, and Epimetheus, solving for their state vectors at the
epoch 2007 June 1 00:00:00.0. Solutions for the state vectors were obtained
by solving the variational equations simultaneously with the equations of
motion, minimizing the observed-minus-computed (O-C) residuals iteratively.
All observations were equally weighted. Numerical integration of the
equations of motion and the variational equations was performed using the
12th order Runge-Kutta-Nystroem RKN12(10)17 M algorithm of Brankin et al.
(1989, ACM Trans. Math. Softw., 15, 31), while the equations of condition
were solved using the SVD-based approach of Lawson and Hanson (1975,
Solving Least Squares Problems; Philadelphia: SIAM). For more details
relating to the numerical integration and model fitting scheme used in this
work, see Murray et al. (2005Icar..179..222M 2005Icar..179..222M).
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Sylvain Guehenneux [CDS] 03-Feb-2015