Elsevier

Icarus

Volume 181, Issue 1, March 2006, Pages 223-234
Icarus

Cassini ISS astrometric observations of the inner jovian satellites, Amalthea and Thebe

https://doi.org/10.1016/j.icarus.2005.11.007Get rights and content

Abstract

We present a total of 289 new astrometric observations of the inner jovian satellites, Amalthea and Thebe, obtained using the Cassini ISS narrow angle camera. Observations were made using image sequences from 2000 December 11–12 (inbound) and 2001 January 15–16 (outbound), at phase angles of approximately 2° and 122°, respectively. Target distances were of order 284 RJ, giving a maximum resolution of approximately 100 km/pixel. Centroided line and sample values for 239 observations of Amalthea and 50 of Thebe are provided, together with estimated camera pointing information for each image. Orbit fitting using a uniformly precessing Keplerian ellipse model, taking into account the oblateness of Jupiter up to terms in J6, gave RMS fit residuals of 0.364 and 0.443 pixel for Amalthea and Thebe, respectively (equivalent to 0.450 and 0.547 arcsec). RMS residuals relative to the JPL JUP230 ephemeris were 0.306 and 0.604 pixel (equivalent to 0.378 and 0.746 arcsec), for Amalthea and Thebe. The fitted orbital parameters confirm the relatively high inclinations of these satellites (0.374°±0.002° and 1.076°±0.003°, respectively), equivalent to maximum vertical displacements above Jupiter's equatorial plane of 1188±6 and 4240±12km, respectively, consistent with current estimates of the half-thicknesses of the Amalthea and Thebe gossamer rings [Ockert-Bell, M.E., Burns, J.A., Dauber, I.J., Thomas, P.C., Veverka, J., Belton, M.J.S., Klaasen, K.P., 1999. Icarus 138, 188–213].

Introduction

Approximately 26,000 images were produced by the narrow angle camera (NAC) of the Cassini Imaging Science Subsystem (ISS) during the Cassini spacecraft's Jupiter flyby of late 2000/early 2001, providing a substantial new source of data for the study of the dynamics of the jovian satellites and rings. Porco et al. (2003) independently used ISS images to measure the orbits of Metis and Adrastea, the innermost of the four inner satellites. They empirically demonstrated for the first time that the inclinations of Metis and Adrastea are consistent with the thickness of the jovian main ring, thus supporting the hypothesis that the satellites are themselves the source of the ring material (Burns et al., 1999). This has since been confirmed by Evans et al. (2005), using both Cassini and Galileo data.

The primary purpose of this paper is to make available new Cassini observations of the other two inner satellites, Amalthea and Thebe. The procedures used to reduce and process the observations are described in the main body of the paper and in Appendix A. Orbital solutions are also provided, based on a precessing Keplerian ellipse model, and brief comments are included in Section 4 regarding the implications of the fitted orbital elements, in view of current knowledge of the geometric relationship between the inner satellites and the ring system. A more in-depth examination of these issues is beyond the scope of this paper, and the reader is referred to Porco et al. (2003) and Evans et al. (2005) for further discussion.

The timing of this work was partly dictated by JPL's need for new observations of Amalthea, to help in the planning of the close encounter between the Galileo spacecraft and Amalthea scheduled for November 2002. Cassini observations of Amalthea proved to be important both because of their better accuracy than existing Earth-based data and due to the lack of Galileo optical navigation data for this encounter (Jacobson, R.A., personal communication). Observations of Thebe were made subsequently from the same pool of images used to measure Amalthea. Thus, it should perhaps be emphasised that this work does not by any means represent an exhaustive search for all possible observations of these satellites within the entire Cassini ISS data set.

Section snippets

Satellite astrometry

Image re-pointing and satellite measurements were made using IDL-based procedures within the framework of the MINAS (Modular Image Navigation and Analysis Software) package, developed at Cassini Imaging Central Laboratory for Operations (CICLOPS). Before describing these particular procedures, we firstly outline the technique used to select images for measurement.

Orbital solutions

Orbital elements have been derived for each satellite, based on a uniformly precessing Keplerian ellipse model (Taylor, 1998), incorporating the effects of Jupiter's oblateness up to terms in J6. The expressions for the calculated apsidal and nodal precession rates, ϖ˙calc and Ω˙calc, respectively, were taken from Cooper and Murray (2004). The adopted reference frame is planetocentric, with x-axis corresponding to the position of the ascending node of Jupiter's equatorial plane on the Earth

Discussion

The close dynamical and geometrical relationship between the four inner jovian satellites and the rings is now well-established (see, for example, Burns et al., 1999, Ockert-Bell et al., 1999, Porco et al., 2003). In particular, the correlation between the measured half-widths of the Amalthea and Thebe rings and the maximum vertical extents of the orbits of Amalthea and Thebe above the ring plane, lead Ockert-Bell et al. (1999) to conclude that these satellites are sources of the ring material.

Acknowledgements

The authors gratefully acknowledge the many helpful comments of Bob Jacobson and Bill Owen at JPL, with special thanks to Bill Owen for details of his geometric calibration model, as described in Appendix A. Thanks also to the Cassini ISS team. In addition, Cooper and Murray express their gratitude to the UK Particle Physics and Astronomy Research Council for financial support.

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