First stellar occultation by the Galilean moon Europa and upcoming events between 2019 and 2021

Abstract

Context. Bright stellar positions are now known with an uncertainty below 1 mas thanks to Gaia DR2. Between 2019–2020, the Galactic plane will be the background of Jupiter. The dense stellar background will lead to an increase in the number of occultations, while the Gaia DR2 catalogue will reduce the prediction uncertainties for the shadow path. Aims. We observed a stellar occultation by the Galilean moon Europa (J2) and propose a campaign for observing stellar occultations for all Galilean moons. Methods. During a predicted period of time, we measured the light flux of the occulted star and the object to determine the time when the flux dropped with respect to one or more reference stars, and the time that it rose again for each observational station. The chords obtained from these observations allowed us to determine apparent sizes, oblatness, and positions with kilometre accuracy. Results. We present results obtained from the first stellar occultation by the Galilean moon Europa observed on 2017 March 31. The apparent fitted ellipse presents an equivalent radius of 1561.2 ± 3.6 km and oblatenesses 0.0010 ± 0.0028. A very precise Europa position was determined with an uncertainty of 0.8 mas. We also present prospects for a campaign to observe the future events that will occur between 2019 and 2021 for all Galilean moons. Conclusions. Stellar occultation is a suitable technique for obtaining physical parameters and highly accurate positions of bright satellites close to their primary. A number of successful events can render the 3D shapes of the Galilean moons with high accuracy. We encourage the observational community (amateurs included) to observe the future predicted events.

Arecibo Radar Astrometry of the Galilean Satellites from 1999 to 2016

“…Astrometry of the Galilean satellites was also reported in Morgado et al (2019aMorgado et al ( , 2019b. Morgado et al (2019a) measured instants of apparent encounters of satellites in the plane of sky from 2016 to 2018.…”

“…Morgado et al (2019a) reported 14.4 mas as the rms of the residuals with respect to JUP310 and planetary ephemeris DE435 (Folkner 2016). Additionally, Morgado et al (2019b) reported a very precise measurement of Europa's position, with an uncertainty of 0.8 mas, based on the first stellar occultation. The offsets with respect to JUP310 ephemeris and planetary ephemeris DE438 (Folkner 2018) were −7.94 mas for Δα cos δ and −10.91 mas for Δδ.…”

“…The season of mutual events at Jupiter occurs every six years when the planet passes though the equinox. Morgado et al (2019b) reported on even more precise measurement based on the first recorded stellar occultation of Europa on 2017 March 31. They estimated the position of Europa with 0.8 mas uncertainty.…”

“…Unfortunately, serendipitous stellar occultations of the Galilean moons do not occur often because Jupiter needs to be in a region of sky with a dense star field. Such opportunities occur in 2019, 2020, and 2031(Gomes-Júnior et al 2016Morgado et al 2019b). Harmon et al (1994) reported on the first radar ranging measurements of Ganymede and Callisto from Arecibo in 1992.…”

Milliarcsecond Astrometry for the Galilean Moons Using Stellar Occultations

“…The case of Galilean moons is a critical example because Jupiter's brightness in the Field of View (FoV) would quickly saturate the CCD, thus providing positions with uncertainties that range between 100 and 150 milliarcseconds (mas; Kiseleva et al 2008). This scenario motivates the search for alternative methods for the astrometry of these satellites, such as the mutual phenomena events (Aksnes & Franklin 1976;Aksnes et al 1984;Emelyanov 2009;Arlot et al 2014;Saquet et al 2018;Morgado et al 2019c, and references therein), mutual approximations (Morgado et al 2016(Morgado et al , 2019a, radar astrometry (Brozović et al 2020), and stellar occultations (Morgado et al 2019b), among others.…”