Interstellar Interloper C/2025 N1 is Active

ATel #17263; David Jewitt (UCLA), Jane Luu (University of Oslo)
on 3 Jul 2025; 13:40 UT
Credential Certification: David Jewitt (djewitt@gmail.com)

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Observations from the Nordic Optical Telescope on UT 2025 July 2 (heliocentric distance 4.5 au) show that interstellar interloper C/2025 N1 ("3I") is clearly active, with a diffuse tail extending towards position angle 280+/-10 degrees. The absolute magnitude, H = 11.9, therefore sets only an upper limit to the size of the nucleus. Assuming a geometric albedo 0.04, the radius of an equal area circle is about 14 km. The radius of the nucleus is likely much smaller, as is expected from a simple scaling of the interstellar interloper number density obtained from the earlier detection of 'Oumuamua (i.e. the big ones should be very rare). A better constraint on the nucleus size might be obtained from high resolution (e.g. HST) observations. A sample image is posted at https://faculty.epss.ucla.edu/~jewitt/C2025N1_NOT.jpg

NOT Image of C/2025 N1