Initial Results from NASA/IRTF Observations of Interstellar Comet 3I/ATLAS

ATel #17283; Chansey Champagne (NAU), Lucas McClure (NAU), Joshua Emery (NAU), James M. Bauer (UMD), Theodore Kareta (Villanova U.), Benjamin N. L. Sharkey (UMD), Michael Connelly (UH/IRTF), John Rayner (UH/IRTF), Cristina Thomas (NAU), Vishnu Reddy (UAz)
on 11 Jul 2025; 23:35 UT
Distributed as an Instant Email Notice Comets
Credential Certification: Theodore Kareta (theodore.kareta@villanova.edu)

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We report two nights of observations of the Interstellar Comet 3I/ATLAS with the NASA Infrared Telescope Facility (IRTF) obtained one and two days after discovery, respectively. Sloan g - i colors derived from the large-FOV guide camera ‘Opihi (Lee+2022) on 2025 July 3 agree with spectra and colors obtained by Seligman+2025 and Opitom+2025 which show the object to reflect light similarly to the surfaces of D-type asteroids and comet nuclei, although a significant fraction of the light we measured must have been from the object’s coma. Qualitatively, the object appeared slightly more extended on the second night than the first. Lightcurves from `Opihi and MORIS (Gulbis+2010) show small variation on July 3 and July 4. The small variation, while potentially linked to an underlying lightcurve, is far more likely to be from background source contamination as the object is within the Galactic Plane. This is consistent with the object’s lightcurve being masked by a small-but-compact dust coma around the object. The lack of an extended tail in our images or those taken by others is at least partially due to the tail being ‘behind’ the comet due to the low phase angle of the observations, but could be enhanced further by the comet having relatively few fine grains which are most susceptible to radiation pressure. Eight frames of SpeX/Prism (Rayner+2003) data were collected on July 3 , and a larger dataset was obtained on July 4th. The two datasets are consistent, showing a red near-infrared spectral slope that is shallower than the optical slope. The higher SNR July 4th dataset shows clear spectral curvature, with a steep red slope below 0.9 microns shallowing to an almost neutral slope beyond 1.2 microns. This short wavelength slope agrees with our `Opihi colors and visually aligns quite well with the Seligman+ and Opitom+ results. The spectrum shows no obvious hints of water ice absorption from the target’s coma, and forthcoming modeling will translate this nondetection into an upper limit of water ice content. Full reduction and analysis of these datasets is being actively worked for a peer-reviewed submission. Discussions are ongoing about the potential for and specifics of a continuing near-infrared monitoring campaign with the IRTF. Suggestions on this campaign are welcome.