{"id":156117,"date":"2025-09-14T12:02:07","date_gmt":"2025-09-14T12:02:07","guid":{"rendered":"https:\/\/www.newsbeep.com\/us\/156117\/"},"modified":"2025-09-14T12:02:07","modified_gmt":"2025-09-14T12:02:07","slug":"is-the-nucleus-of-3i-atlas-15-kilometers-in-diameter-by-avi-loeb-sep-2025","status":"publish","type":"post","link":"https:\/\/www.newsbeep.com\/us\/156117\/","title":{"rendered":"Is the Nucleus of 3I\/ATLAS 15 kilometers in diameter? | by Avi Loeb | Sep, 2025"},"content":{"rendered":"

\"Avi<\/a>Press enter or click to view image in full size\"\"Data from the Zwicky Transient Facility<\/a> (ZTF) camera for the evolution in the reflecting area of 3I\/ATLAS as a function of time, relative to a model in which the nucleus of 3I\/ATLAS is 5.6 kilometers in diameter. An early phase \u2014 marked \u201cplateau\u201d, precedes a late phase \u2014 marked \u201cuniform brightening\u201d. (Credit: Q. Ye et al. 2025<\/a>)<\/p>\n

Newly released data (accessible here<\/a>) from the Zwicky Transient Facility<\/a> (ZTF) camera, identifies the interstellar object 3I\/ATLAS<\/a> starting on May 15, 2025, when its distance from the Sun was a factor of 6.1 bigger than the Earth-Sun separation (AU).<\/p>\n

The ZTF data shows a steady brightening of 3I\/ATLAS relative to a solid object of a fixed size, starting at the beginning of June 2025. This brightening can be explained by the mass loss from 3I\/ATLAS, which was observed to produce a glow of scattered light around it in the Hubble Space Telescope image taken on July 21, 2025 (and reported here<\/a>), when 3I\/ATLAS was at a heliocentric distance of 3.8 AU. Surprisingly, the Hubble glow was extended by a factor of 10 towards the Sun relative to other directions (as discussed here<\/a>), suggesting that the scattered sunlight was produced by ice fragments that get evaporated by sunlight \u2014 as discussed in a paper I co-authored with Eric Keto (available here<\/a>). Refractory dust would have been swept by solar radiation pressure to trail the object as in a typical cometary tail, which is not seen in the Hubble image.<\/p>\n

However, between May 15 and the beginning of June, 2025, the ZTF data shows no growth in the reflecting area of 3I\/ATLAS, suggesting a \u201cplateau\u201d in its effective size as a function of time. The authors calibrate the growth in the effective size of 3I\/ATLAS relative to a model which assigns a diameter of 5.6 kilometers to the nucleus of 3I\/ATLAS based on the Hubble image. This size estimate is model-dependent. In the context of this model, the ZTF data shows a constant enhancement factor of 5\u201310 for the expected flux during the early \u201cplateau\u201d period between May 15 and June 1, 2025. The authors note: \u201cThe nature of this \u201cplateau\u201d is uncertain given the relatively larger error bars during this period, although we re-inspected the images and confirmed that these were not due to star contamination.\u201d<\/p>\n

The simplest interpretation of this early plateau is that it represents the bare nucleus, before substantial mass loss was initiated. In that case, the actual diameter of the nucleus of 3I\/ATLAS is larger than the assumed value of 5.6 kilometers by the square root of the enhancement factor of 5\u201310 during the plateau period. This would mean that the diameter of 3I\/ATLAS is about 15 kilometers, approximately 20 times larger than the estimated diameter of the nucleus of the previous interstellar object, 2I\/Borisov (see Table 2 here<\/a>). This disparity in sizes would imply that the nucleus mass of 3I\/ATLAS is bigger than that of 2I\/Borisov by a factor of 8,000 if both objects have solid density.<\/p>\n

In that case, the obvious question that arises is why have we not detected thousands of objects like 2I\/Borisov before detecting one on the scale of 3I\/ATLAS? A related concern is that the reservoir of rocky material in interstellar space is only able to supply a massive rock of this scale once per several millennia. I discussed both of these issues in the paper I wrote a few days after the discovery of 3I\/ATLAS (accessible here<\/a>). One way out of the mass budget limitation is that 3I\/ATLAS was on a selective plunging trajectory towards the inner solar system, as discussed in my paper (here<\/a>) and analyzed in more detail in the paper I co-authored last week with Oem Trivedi (here<\/a>). Such a trajectory could arise from some unknown astrophysical origin or from a technological fine-tuning of the trajectory. A technological design is consistent with the unlikely alignment of the path of 3I\/ATLAS with the ecliptic plane of the planets around the Sun (as discussed here<\/a>).<\/p>\n

Given the error bars in the early ZTF data, it is also possible that the detected plateau is spurious and the nucleus size of 3I\/ATLAS is much smaller.<\/p>\n

Gladly, the alignment of the path of 3I\/ATLAS with the ecliptic plane brings 3I\/ATLAS to a close proximity relative to Mars, where NASA, ESA and the Chinese Space agency placed orbiters with cameras. In particular, on October 3, 2025, 3I\/ATLAS will pass at a distance of 29 million kilometers from the HiRISE camera<\/a> onboard the Mars Reconnaissance Orbiter. The camera\u2019s 0.5-meter aperture will be able to image 3I\/ATLAS with a resolution of 30 kilometers per pixel. The glowing cloud around 3I\/ATLAS is optically thin (transparent). Hence, the total luminosity emanating from the central pixel in the HiRISE image will provide a strict upper limit on the nucleus brightness and hence its size, better by two orders of magnitude than the Hubble image.<\/p>\n

Engaging in evidence-based science is as much fun as getting involved in a detective story. The high-resolution image from the HiRISE camera will serve as a detective\u2019s magnifying glass in constraining the nucleus diameter of 3I\/ATLAS. The author Arthur Conan Doyle formulated eloquently two important principles through his character, the detective Sherlock Holmes. These guidelines apply well to anomalous interstellar objects like 1I\/`Oumuamua<\/a> and 3I\/ATLAS. The first principle states: \u201cIt is a capital mistake to theorize before you have all the evidence,<\/a>\u201d and the second principle is: \u201cWhen you have eliminated the impossible, whatever remains, however improbable, must be the truth<\/a>.\u201d<\/p>\n

ABOUT THE AUTHOR<\/p>\n

Press enter or click to view image in full size\"\"(Image Credit: Chris Michel, National Academy of Sciences, 2023)<\/p>\n

Avi Loeb is the head of the Galileo Project, founding director of Harvard University\u2019s \u2014 Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011\u20132020). He is a former member of the President\u2019s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of \u201cExtraterrestrial:<\/a> The First Sign of Intelligent Life Beyond Earth<\/a>\u201d and a co-author of the textbook \u201cLife in the Cosmos<\/a>\u201d, both published in 2021. The paperback edition of his new book, titled \u201cInterstellar<\/a>\u201d, was published in August 2024.<\/p>\n","protected":false},"excerpt":{"rendered":"Press enter or click to view image in full sizeData from the Zwicky Transient Facility (ZTF) camera for…\n","protected":false},"author":2,"featured_media":156118,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[49],"tags":[199,79],"class_list":{"0":"post-156117","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-physics","8":"tag-physics","9":"tag-science"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts\/156117","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/comments?post=156117"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts\/156117\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/media\/156118"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/media?parent=156117"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/categories?post=156117"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/tags?post=156117"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}