Follow-up observations of the dust fan ejected from interstellar comet 3I/ATLAS

ATel #17372; Oleksandra Ivanova (AI SAS), Tomasz Kwiatkowski (AOI AMU), Nicolas Erasmus (SAAO), Sofiia Mykhailova (AOI AMU), Dagmara Oszkiewicz (AOI AMU), Tony Santana-Ros (Universidad de Alicante), Thobekile Ngwane (SAAO), Antti Penttila (University of Helsinki), Marek Husaric (AI SAS), Krzysztof Kaminski (AOI AMU)
on 5 Sep 2025; 13:31 UT
Credential Certification: Tomasz Kwiatkowski (tkastr@vesta.astro.amu.edu.pl)

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In ATel #17350 Virginio Oldani et al. report observations of the ‘dust fan’ ejected from 3I/ATLAS. Their main images were acquired on 13 and 14 August 2025, and revealed the presence of a dense fan at PA 280 deg, extending 5 arcsec from the photo-centre of the comet. Here, we report observations of what is presumably the same feature, which remained observable after 18 days On 2025-08-26 (17:41–19:37) UTC, we imaged 3I/ATLAS with the 1.0-m Lesedi telescope located at SAAO (South Africa). We tracked the comet sidereally and obtained a series of 20 images in the SDSS g, r, i, z bands, respectively. The exposure time was 30 seconds. The images were shifted and stacked. Then we processed them with the Larson–Sekanina filter, applied a division by the average and then subtracted the average. In the final images we clearly see that the comet shows a tail in the direction opposite to the Sun, as well as an additional outburst towards the Sun at PA = 288 deg. The dust cloud was located at a distance of 10 arcsec (about 18,500 km) from the photo-centre of the comet (see attached g, r, i, and z composite images). The magnitudes of the comet, within a 5 arcsec radius aperture (which corresponds roughly to 8900 km), were: g = 17.29 ± 0.02 mag r = 16.61 ± 0.02 mag i = 16.33 ± 0.02 mag z = 16.26 ± 0.03 mag and the corresponding colour indices were: g - r = 0.68 ± 0.02 r - i = 0.28 ± 0.02 i - z = 0.07 ± 0.02 A follow-up dataset on 2025-09-01 (17:16–18:02) UTC, obtained with the same telescope setup as on the previous night, includes g, r, i, z series of 9 images each (exp. time = 30 s). After processing them in the same way, we could see the outburst was still present. We assume this is the same feature as the one reported by Virginio Oldani et al. If yes, then it means that after 18 days the dust cloud did not disappear, which suggests it contained larger particles (tens of micrometres in size). This dust-grain size for the 3I/ATLAS ejecta was earlier proposed by Jewitt et al. (arXiv 2508.02934v2, 2025). Similar imaging reported by Bolin et al. (ATel #17363) on 2025 August 27, 23:53 UTC, did not show any such feature. After our finding, Bryce Bolin kindly provided us with his images, and after processing them in the same way as ours, we could easily see a significant active structure in the sunward direction, which looks more like ongoing activity rather than just a fading outburst. This can be potentially associated with a localized active area on the nucleus. We encourage additional follow-up imaging of the comet, which can trace the temporal evolution of the dust fan.

Images of 3I/ATLAS taken in g, r, i, and z bands on 2025 August 26 UTC