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Register a new userFirst stellar occultation by the Galilean moon Europa and upcoming events between 2019 and 2021
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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.
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We present a study of an impacting descent probe that increases the science return of spacecraft orbiting or passing an atmosphere-less planetary body of the solar system, such as the Galilean moons of Jupiter. The descent probe is a carry-on small spacecraft (< 100 kg), to be deployed by the mother spacecraft, that brings itself onto a collisional trajectory with the targeted planetary body in a simple manner. A possible science payload includes instruments for surface imaging, characterisation of the neutral exosphere, and magnetic field and plasma measurement near the target body down to very low-altitudes (~1 km), during the probes fast (~km/s) descent to the surface until impact. The science goals and the concept of operation are discussed with particular reference to Europa, including options for flying through water plumes and after-impact retrieval of very-low altitude science data. All in all, it is demonstrated how the descent probe has the potential to provide a high science return to a mission at a low extra level of complexity, engineering effort, and risk. This study builds upon earlier studies for a Callisto Descent Probe (CDP) for the former Europa-Jupiter System Mission (EJSM) of ESA and NASA, and extends them with a detailed assessment of a descent probe designed to be an additional science payload for the NASA Europa Mission.
A new Hubble Space Telescope observation of the 7:4 resonant transneptunian binary system (385446) Manwe has shown that, of two previously reported solutions for the orbit of its satellite Thorondor, the prograde one is correct. The orbit has a period of 110.18 $pm$ 0.02 days, semimajor axis of 6670 $pm$ 40 km, and an eccentricity of 0.563 $pm$ 0.007. It will be viewable edge-on from the inner solar system during 2015-2017, presenting opportunities to observe mutual occultation and eclipse events. However, the number of observable events will be small, owing to the long orbital period and expected small sizes of the bodies relative to their separation. This paper presents predictions for events observable from Earth-based telescopes and discusses the associated uncertainties and challenges.
Plutos atmospheric profiles (temperature and pressure) have been studied for decades from stellar occultation lightcurves. In this paper, we look at recent Pluto Global Climate Model (GCM) results (3D temperature, pressure, and density fields) from Bertrand et al. (2020) and use the results to generate model observers plane intensity fields (OPIF) and lightcurves by using a Fourier optics scheme to model light passing through Plutos atmosphere (Young, 2012). This approach can accommodate arbitrary atmospheric structures and 3D distributions of haze. We compared the GCM model lightcurves with the lightcurves observed during the 15-AUG-2018 Pluto stellar occultation. We find that the climate scenario which best reproduces the observed data includes an N2 ice mid-latitude band in the southern hemisphere. We have also studied different haze and P/T ratio profiles: the haze effectively reduces the central flash strength, and a lower P/T ratio both reduces the central flash strength and incurs anomalies in the shoulders of the central flash.
A stellar occultation by the large trans-Neptunian object (90482) Orcus was predicted to occur on 2017 March 07. Observations were made at five sites in North and South America. High-speed, visible-wavelength images were taken at all sites, in addition to simultaneous K-band images at one location. Solid-body occultations were observed from two sites. Post-event reconstruction suggested an occultation of two different stars observed from two different sites. Follow-up, speckle imaging at Gemini Observatory revealed a second star, which verified that the occulting body in both cases was Orcus` satellite, Vanth. The two single-chord detections, with an anomalously large timing delay in one chord, have lengths of 291+/-125 km and 434.4+/-2.4 km. The observations, combined with a non-detection at a nearby site, allow a tight constraint of 443+/-10 km to be placed on Vanth`s size (assuming it is spherical). A 3-{sigma} upper limit of 1-4 {mu}bar (depending on constituent) is found for a global Vanth atmosphere. The immersion and emersion profiles are slightly different, with atmospheric constraints 40 percent higher on immersion than on emersion. No rings or other material were detected within ten thousand kms of Vanth, and beyond 8010 km from Orcus, to the tightest optical depth limit of approximately 0.1 at 5 km scale. The occultation probed as close as 5040 km from Orcus, placing an optical depth limit of approximately 0.3 at 5 km scale on any encircling material at that distance.
In 2019, the Research and Education Collaborative Occultation Network (RECON) obtained multiple-chord occultation measurements of two centaur objects: 2014 YY$_{49}$ on 2019 January 28 and 2013 NL$_{24}$ on 2019 September 4. RECON is a citizen-science telescope network designed to observe high-uncertainty occultations by outer solar system objects. Adopting circular models for the object profiles, we derive a radius $r=16^{+2}_{-1}$km and a geometric albedo $p_V=0.13^{+0.015}_{-0.024}$ for 2014 YY$_{49}$, and a radius $r=66 ^{+5}_{-5}$km and geometric albedo $p_V = 0.045^{+0.006}_{-0.008}$ for 2013 NL$_{24}$. To the precision of these measurements, no atmosphere or rings are detected for either object. The two objects measured here are among the smallest distant objects measured with the stellar occultation technique. In addition to these geometric constraints, the occultation measurements provide astrometric constraints for these two centaurs at a higher precision than has been feasible by direct imaging. To supplement the occultation results, we also present an analysis of color photometry from the Pan-STARRS surveys to constrain the rotational light curve amplitudes and spectral colors of these two centaurs. We recommend that future work focus on photometry to more deliberately constrain the objects colors and light curve amplitudes, and on follow-on occultation efforts informed by this astrometry.
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