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

## TL;DR

This paper reports the first stellar occultation observation of Europa, demonstrating the technique's effectiveness for precise size and position measurements of Galilean moons, and proposes a campaign for future occultation events between 2019 and 2021.

## Contribution

It presents the first occultation observation of Europa and outlines a campaign to observe all Galilean moons, improving size and position accuracy using stellar occultations.

## Key findings

- Europa's apparent radius measured as 1561.2 ± 3.6 km
- Europa's position determined with 0.8 mas uncertainty
- Successful demonstration of occultation technique for satellite characterization

## 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 $\pm$ 3.6 km and oblatenesses 0.0010 $\pm$ 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.