Direct physical and astrometric characterisation of a kilometre-scale Jovian moon for a potential JUICE flyby

Direct physical and astrometric characterisation of a kilometre-scale Jovian moon for a potential JUICE flyby | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Direct physical and astrometric characterisation of a kilometre-scale Jovian moon for a potential JUICE flyby Juan Rizos, Jose María Gómez-Limón, Yücel Kilic, Altair Gomes-Junior, and 50 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8869986/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Jupiter’s irregular satellites are thought to be relics of the early outer Solar System, yet their small sizes and large distances make them challenging to characterise, limiting their feasibility for in situ exploration. Kallichore, a kilometre-scale member of the retrograde Carme group, is currently the only irregular satellite potentially accessible to a close flyby by ESA’s JUICE mission, now en route to Jupiter with arrival scheduled for 2031. However, its poorly constrained orbit and size pose a major challenge for encounter planning and execution. We report Hubble Space Telescope astrometry and photometry, a stellar occultation by Kallichore, and follow-up astrometric and photometric observations with the 10.4-m Gran Telescopio de Canarias. Three positive occultation detections allow us to determine the astrometric position of Kallichore with sub-milliarcsecond precision, corresponding to a sky-projected positional accuracy of about 1 km. From the occultation geometry and complementary photometry, we constrain an area-equivalent diameter of 3.8 (+2.3 / −0.3) km, infer an elongated shape with a minimum semi-axis ratio of a/b = 1.53 ± 0.10, and estimate a geometric albedo of 3.7 (+0.7 / −2.2)%. No close companions were detected. This represents the first direct physical characterisation of the smallest outer Solar System satellite measured from Earth. Our astrometric measurements reduce Kallichore’s orbital uncertainty by up to 80%. These constraints on both orbit and physical properties establish a viable pathway toward a future JUICE flyby. If Kallichore originates from primordial trans-Neptunian populations as a captured fragment, such an encounter would provide access to one of the most pristine remnants of the early outer Solar System. Physical sciences/Astronomy and planetary science/Planetary science/Asteroids, comets and Kuiper belt Physical sciences/Astronomy and planetary science/Planetary science/Rings and moons Jovian Irregular Satellites JUICE Mission Stellar Occultations Full Text Additional Declarations There is NO Competing Interest. Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8869986","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":594514032,"identity":"89dc08e1-f48a-4f95-ba91-ddcdd2cd8e54","order_by":0,"name":"Juan Rizos","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA9klEQVRIiWNgGAWjYBACxgYg8YBBgoGBGcStAGJm5gbCWhLgWs6AtDDi1wIGCXAD2uDG4AbMM5KfPUhss7A3b2d/+PDnvNpo/naglh8V23A7bEaauUFim0TinMMMyca8247nzjjM2MDYc+Y2Hi0JZhJALQkSzAzHpBm3HcttAGphZmzDpyX9G0iLvQRQmeTPOcdy5xPWkgO2hXEGMzObBG9DTe4Gglp63pRJJJyTSJzBzMZszHPsQO5GoJaD+Pxi2J6+TeJDWZ29BP/xhw9/1NTlzjt/+OCDHxV4tDSg8g+DyQM41QOBPBq/Dp/iUTAKRsEoGKEAAJesVWnhEdIRAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-9789-1203","institution":"Instituto de Astrofísica de Andalucía","correspondingAuthor":true,"prefix":"","firstName":"Juan","middleName":"","lastName":"Rizos","suffix":""},{"id":594514033,"identity":"98ee7170-9508-4396-bdcc-ad75c7338ed8","order_by":1,"name":"Jose María Gómez-Limón","email":"","orcid":"https://orcid.org/0009-0006-8584-1416","institution":"Instituto de Astrofísica de Andalucía","correspondingAuthor":false,"prefix":"","firstName":"Jose","middleName":"María","lastName":"Gómez-Limón","suffix":""},{"id":594514034,"identity":"08fcd8e5-7423-4f98-b388-94a53296e94e","order_by":2,"name":"Yücel Kilic","email":"","orcid":"https://orcid.org/0000-0001-8641-0796","institution":"Instituto de Astrofísica de Andalucía","correspondingAuthor":false,"prefix":"","firstName":"Yücel","middleName":"","lastName":"Kilic","suffix":""},{"id":594514035,"identity":"97cb4479-d814-41d9-a840-2da7a62db8a8","order_by":3,"name":"Altair Gomes-Junior","email":"","orcid":"https://orcid.org/0000-0002-3362-2127","institution":"Institute of Physics, Federal University of Uberlândia, Av. 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Kallichore, a kilometre-scale member of the retrograde Carme group, is currently the only irregular satellite potentially accessible to a close flyby by ESA’s JUICE mission, now en route to Jupiter with arrival scheduled for 2031. However, its poorly constrained orbit and size pose a major challenge for encounter planning and execution.\r\n\r\nWe report Hubble Space Telescope astrometry and photometry, a stellar occultation by Kallichore, and follow-up astrometric and photometric observations with the 10.4-m Gran Telescopio de Canarias.\r\n\r\nThree positive occultation detections allow us to determine the astrometric position of Kallichore with sub-milliarcsecond precision, corresponding to a sky-projected positional accuracy of about 1 km. From the occultation geometry and complementary photometry, we constrain an area-equivalent diameter of 3.8 (+2.3 / −0.3) km, infer an elongated shape with a minimum semi-axis ratio of a/b = 1.53 ± 0.10, and estimate a geometric albedo of 3.7 (+0.7 / −2.2)%. No close companions were detected.\r\n\r\nThis represents the first direct physical characterisation of the smallest outer Solar System satellite measured from Earth. Our astrometric measurements reduce Kallichore’s orbital uncertainty by up to 80%. These constraints on both orbit and physical properties establish a viable pathway toward a future JUICE flyby. If Kallichore originates from primordial trans-Neptunian populations as a captured fragment, such an encounter would provide access to one of the most pristine remnants of the early outer Solar System.","manuscriptTitle":"Direct physical and astrometric characterisation of a kilometre-scale Jovian moon for a potential JUICE flyby","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-23 02:43:51","doi":"10.21203/rs.3.rs-8869986/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-astronomy","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"natastron","sideBox":"Learn more about [Nature Astronomy](http://www.nature.com/natastron/)","snPcode":"","submissionUrl":"","title":"Nature Astronomy","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"78dfb751-54a5-493a-afaf-255ea62837f6","owner":[],"postedDate":"February 23rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":63268211,"name":"Physical sciences/Astronomy and planetary science/Planetary science/Asteroids, comets and Kuiper belt"},{"id":63268212,"name":"Physical sciences/Astronomy and planetary science/Planetary science/Rings and moons"}],"tags":[],"updatedAt":"2026-05-07T15:21:53+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-23 02:43:51","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8869986","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8869986","identity":"rs-8869986","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}