The interstellar comet 3I/ATLAS reached its closest approach to Earth on 19th December 2025, at a distance of about 1.8 AU. We conducted low-frequency radio observations of the comet using the upgraded Giant Metrewave Radio Telescope (uGMRT) Band-5 at a central frequency of 1239 MHz on the day of the closest approach and two days before and two days after. The preliminary data analysis suggests the successful detection of comet 3I/ATLAS on 17 and 19 December 2025, with a significance greater than 10σ with flux densities on 17 and 19th December of 3.86 ± 0.38 mJy and 0.81 ± 0.05 mJy, respectively. The comet was not detected on 21 December 2025.
We find that comet 3I/ATLAS was significantly bright before its close approach (17 December) and became considerably faint at the closest approach (19 December), followed by a non-detection afterwards (21 December).
This study represents a key step toward probing cometary radio emissions at meter wavelengths. The detection confirmed the presence of
measurable radio continuum emissions, likely arising from dust-related emissions and/or plasma processes associated with the interactions between the cometary environment and solar wind. Detailed calibration, imaging, time-variability studies, and spectral line analyses, including a search for atomic HI emissions at 1420 MHz, are currently in progress. These analyses will place important constraints on the physical conditions of the cometary coma, including dust content and interaction mechanisms.
Further scientific results will be presented in a follow-up publication.
We thank the GMRT operations team for scheduling and supporting these observations. The GMRT is operated by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research.
The continuum image of 3I/ATLAS observed on 17 December 2025 at a frequency of 1239 MHz is attached at the following
link.
