Orbit determination of Transneptunian objects and Centaurs for the prediction of stellar occultations

The prediction of stellar occultations by Transneptunian objects and Centaurs is a difficult challenge that requires accuracy both in the occulted star position as for the object ephemeris. Until now, the most used method of prediction involving tens of TNOs/Centaurs was to consider a constant offset for the right ascension and for the declination with respect to a reference ephemeris. This offset is determined as the difference between the most recent observations of the TNO and the reference ephemeris. This method can be successfully applied when the offset remains constant with time. This paper presents an alternative method of prediction based on a new accurate orbit determination procedure, which uses all the available positions of the TNO from the Minor Planet Center database plus sets of new astrometric positions from unpublished observations. The orbit determination is performed through a numerical integration procedure (NIMA), in which we develop a specific weighting scheme. The NIMA method was applied for 51 selected TNOs/Centaurs. For this purpose, we have performed about 2900 new observations during 2007-2014. Using NIMA, we succeed in predicting the stellar occultations of 10 TNOs and 3 Centaurs between 2013 and 2015. By comparing the NIMA and JPL ephemerides, we highlighted the variation of the offset between them with time. Giving examples, we show that the constant offset method could not accurately predict 6 out of the 13 observed positive occultations successfully predicted by NIMA. The results indicate that NIMA is capable of efficiently refine the orbits of these bodies. Finally, we show that the astrometric positions given by positive occultations can help to further refine the orbit of the TNO and consequently the future predictions. We also provide the unpublished observations of the 51 selected TNOs and their ephemeris in a usable format by the SPICE library.Comment: 12 pages, 9 figures, accepted in A&

Astrometry of the main satellites of Uranus: 18 years of observations

We determine accurate positions of the main satellites of Uranus: Miranda, Ariel, Umbriel, Titania, and Oberon. Positions of Uranus, as derived from those of these satellites, are also determined. The observational period spans from 1992 to 2011. All runs were made at the Pico dos Dias Observatory, Brazil. We used the software called Platform for Reduction of Astronomical Images Automatically (PRAIA) to minimise (digital coronography) the influence of the scattered light of Uranus on the astrometric measurements and to determine accurate positions of the main satellites. The positions of Uranus were then indirectly determined by computing the mean differences between the observed and ephemeris positions of these satellites. A series of numerical filters was applied to filter out spurious data. These filters are mostly based on the comparison between the positions of Oberon with those of the other satellites and on the offsets as given by the differences between the observed and ephemeris positions of all satellites. We have, for the overall offsets of the five satellites, -29 (+/-63) mas in right ascension and -27 (+/-46) mas in declination. For the overall difference between the offsets of Oberon and those of the other satellites, we have +3 (+/-30) mas in right ascension and -2 (+/-28) mas in declination. Ephemeris positions for the satellites were determined from DE432+ura111. Comparisons using other modern ephemerides for the solar system -INPOP13c- and for the motion of the satellites -NOE-7-2013- were also made. They confirm that the largest contribution to the offsets we find comes from the motion of the barycenter of the Uranus system around the barycenter of the solar system, as given by the planetary ephemerides. Catalogues with the observed positions are provided.Comment: 13 pages, 21 figure

PRAIA - Platform for Reduction of Astronomical Images Automatically

4 p.International audiencePRAIA performs high precision differential photometry and astrometry on digitized images (CCD frames, Schmidt plate surveys, etc). The package main characteristics are automation, accuracy and processing speed. Written in FORTRAN 77, it can run in scripts and interact with any visualization and analysis software. PRAIA is in cope with the ever growing amount of observational data available from private and public sources, including data mining and next generation fast telescope all sky surveys, like SDSS, Pan-STARRS and others. PRAIA was officially assigned as the astrometric supporting tool for participants in the GAIA-FUNSSO activities and will be freely available for the astronomical community

Triplicity and Physical Characteristics of Asteroid (216) Kleopatra

To take full advantage of the September 2008 opposition passage of the M-type asteroid (216) Kleopatra, we have used near-infrared adaptive optics (AO) imaging with the W.M. Keck II telescope to capture unprecedented high resolution images of this unusual asteroid. Our AO observations with the W.M. Keck II telescope, combined with Spitzer/IRS spectroscopic observations and past stellar occultations, confirm the value of its IRAS radiometric radius of 67.5 km as well as its dog-bone shape suggested by earlier radar observations. Our Keck AO observations revealed the presence of two small satellites in orbit about Kleopatra (see Marchis et al., 2008). Accurate measurements of the satellite orbits over a full month enabled us to determine the total mass of the system to be 4.64+/-0.02 10^18 Kg. This translates into a bulk density of 3.6 +/-0.4 g/cm3, which implies a macroscopic porosity for Kleopatra of ~ 30-50%, typical of a rubble-pile asteroid. From these physical characteristics we measured its specific angular momentum, very close to that of a spinning equilibrium dumbbell.Comment: 35 pages, 3 Tables, 9 Figures. In press to Icaru

A stellar occultation by the transneptunian object (50000) Quaoar observed by CHEOPS

Context. Stellar occultation is a powerful technique that allows the determination of some physical parameters of the occulting object. The result depends on the photometric accuracy, the temporal resolution, and the number of chords obtained. Space telescopes can achieve high photometric accuracy as they are not affected by atmospheric scintillation. Aims. Using ESA’s CHEOPS space telescope, we observed a stellar occultation by the transneptunian object (50000) Quaoar. We compare the obtained chord with previous occultations by this object and determine its astrometry with sub-milliarcsecond precision. Also, we determine upper limits to the presence of a global methane atmosphere on the occulting body. Methods. We predicted and observed a stellar occultation by Quaoar using the CHEOPS space telescope. We measured the occultation light curve from this dataset and determined the dis- and reappearance of the star behind the occulting body. Furthermore, a ground-based telescope in Australia was used to constrain Quaoar’s limb. Combined with results from previous works, these measurements allowed us to obtain a precise position of Quaoar at the occultation time. Results. We present the results obtained from the first stellar occultation by a transneptunian object using a space telescope orbiting Earth; it was the occultation by Quaoar observed on 2020 June 11. We used the CHEOPS light curve to obtain a surface pressure upper limit of 85 nbar for the detection of a global methane atmosphere. Also, combining this observation with a ground-based observation, we fitted Quaoar’s limb to determine its astrometric position with an uncertainty below 1.0 mas. Conclusions. This observation is the first of its kind, and it shall be considered as a proof of concept of stellar occultation observations of transneptunian objects with space telescopes orbiting Earth. Moreover, it shows significant prospects for the James Webb Space Telescope

Astrometric positions for 18 irregular satellites of giant planets from 23 years of observations

The irregular satellites of the giant planets are believed to have been captured during the evolution of the solar system. Knowing their physical parameters, such as size, density, and albedo is important for constraining where they came from and how they were captured. The best way to obtain these parameters are observations in situ by spacecrafts or from stellar occultations by the objects. Both techniques demand that the orbits are well known. We aimed to obtain good astrometric positions of irregular satellites to improve their orbits and ephemeris. We identified and reduced observations of several irregular satellites from three databases containing more than 8000 images obtained between 1992 and 2014 at three sites (Observat\'orio do Pico dos Dias, Observatoire de Haute-Provence, and European Southern Observatory - La Silla). We used the software PRAIA (Platform for Reduction of Astronomical Images Automatically) to make the astrometric reduction of the CCD frames. The UCAC4 catalog represented the International Celestial Reference System in the reductions. Identification of the satellites in the frames was done through their ephemerides as determined from the SPICE/NAIF kernels. Some procedures were followed to overcome missing or incomplete information (coordinates, date), mostly for the older images. We managed to obtain more than 6000 positions for 18 irregular satellites: 12 of Jupiter, 4 of Saturn, 1 of Uranus (Sycorax), and 1 of Neptune (Nereid). For some satellites the number of obtained positions is more than 50\% of what was used in earlier orbital numerical integrations. Comparison of our positions with recent JPL ephemeris suggests there are systematic errors in the orbits for some of the irregular satellites. The most evident case was an error in the inclination of Carme.Comment: 9 pages, with 3 being online materia

JWST observations of stellar occultations by solar system bodies and rings

In this paper we investigate the opportunities provided by the James Webb Space Telescope (JWST) for significant scientific advances in the study of solar system bodies and rings using stellar occultations. The strengths and weaknesses of the stellar occultation technique are evaluated in light of JWST's unique capabilities. We identify several possible JWST occultation events by minor bodies and rings, and evaluate their potential scientific value. These predictions depend critically on accurate a priori knowledge of the orbit of JWST near the Sun-Earth Lagrange-point 2 (L2). We also explore the possibility of serendipitous stellar occultations by very small minor bodies as a by-product of other JWST observing programs. Finally, to optimize the potential scientific return of stellar occultation observations, we identify several characteristics of JWST's orbit and instrumentation that should be taken into account during JWST's development.Comment: This paper is one of a series for a special issue on Solar System observations with JWST in PASP. Accepted 2-Oct-2015. Preprint 30 pages, 5 tables, 8 figure

The 2021 mutual phenomena involving the Galilean satellites of Jupiter and the inner satellite Thebe

Astrometric studies and orbital modeling of planetary moons have contributed significantly to advancing our understanding of their orbital dynamics. These studies require precise positions measured over extended periods. In this paper, we present the results of the 2021 Brazilian Jovian mutual phenomena campaign. The data correspond to eight events between Galilean satellites, in addition to a rare eclipse of Thebe, an inner satellite, totaling nine events. A geometric model along with the DE440/JUP365 ephemerides was used to reproduce the events and simulate the light curves. A Monte Carlo method and chi-squared statistics were used to fit the simulated light curves to the observations. The reflectance model adopted for our simulations was the complete version of the Oren-Nayer model. The average uncertainty of the relative positions of the Galilean satellites was 5 mas (15 km) and for the inner Thebe satellite 32 mas (96 km). The seven mutual events (nine independent observations) here analyzed represent and addition of 17% events (10% light curves) with respect to the PHEMU21 international campaign. Furthermore, our result of Thebe eclipse is only the second measurement published to date. Our results contribute to the ephemeris database, being fundamental to improving satellite orbits and thus minimizing their uncertainties.Comment: 10 pages, 5 figure