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NSF-DOE Vera C. Rubin Observatory Observations of Interstellar Comet 3I/ATLAS (C/2025 N1)

Colin Orion Chandler, Pedro H. Bernardinelli, Mario Jurić, Devanshi Singh, Henry H. Hsieh, Ian Sullivan, R. Lynne Jones, Jacob A. Kurlander, Dmitrii Vavilov, Siegfried Eggl, Matthew Holman, Federica Spoto, Megan E. Schwamb, Lauren A. MacArthur, Rahil Makadia, Marco Micheli, Aren Heinze, Eric J. Christensen, Wilson Beebe, Aaron Roodman, Kian-Tat Lim, Tim Jenness, James Bosch, Brianna Smart, Eric Bellm, Sean MacBride, Meredith L. Rawls, Sarah Greenstreet, Colin Slater, Željko Ivezić, Bob Blum, Andrew Connolly, Gregory Daues, Michelle Gower, J. Bryce Kalmbach, Michele T. Bannister, Luke Dones, Rosemary C. Dorsey, Davide Farnocchia, Wesley C. Fraser, John C. Forbes, Cesar Fuentes, Carrie E. Holt, Laura Inno, Geraint H. Jones, Matthew M. Knight, Chris J. Lintott, Tim Lister, Robert Lupton, Mark Jesus Mendoza Magbanua, Renu Malhotra, Beatrice E. A. Mueller, Joseph Murtagh, Nitya Pandey, William T. Reach, Nalin H. Samarasinha, Darryl Z. Seligman, Colin Snodgrass, Michael Solontoi, Gyula M. Szabó, Peter Vereš, Ellie White, Maria Womack, Leslie A. Young, Russ Allbery, Shreya Anand, Roberto Armellin, Éric Aubourg, Chrysa Avdellidou, Farrukh Azfar, James Bauer, Keith Bechtol, vValerie R Beckerć, Matthew Belyakov, Susan D. Benecchi, Ivano Bertini, Dennis Bodewits, Patricia Boeshaar, Bryce T. Bolin, vMaitrayee Bose, Laura E. Buchanan, Alexandre Boucaud, Rodrigo C. Boufleur, Dominique Boutigny, Andrew Bradshaw, Felipe Braga-Ribas, Johan Bregeon, Daniel Calabrese, J. I. B. Camargo, Neven Caplar, Jeffrey L. Carlin, Benoit Carry, Juan Pablo Carvajal, Ross Ceballo, Hsin-Fang Chiang, Yumi Choi, Laura Toribio San Cipriano, Céline Combet, Luiz da Costa, Preeti Cowan, John Franklin Crenshaw, Steve Croft, Matija Ćuk, Philip N. Daly, Felipe Daruich, Guillaume Daubard, James R. A. Davenport, Tansu Daylan, Jennifer Delgado, Hadrien A. R. Devillepoix, Peter E. Doherty, Abbie Donaldson, Holger Drass, Stephanie JH Deppe, Gregory P. Dubois-Felsmann, Peter S. Ferguson, Frossie Economou, Marielle R. Eduardo, Ioana Sotuela Elorriaga, Anthony Englert, Edward-Karavakis, Kevin Fanning, D'Ammando Filippo, Maxwell K. Frissell, Grigori Fedorets, Maryann Benny Fernandes, Ǎgnès Ferté, Mark L Freytag, Marco Fulle, vJohn Gatesć, David W. Gerdes, Alex R. Gibbs, A. Fraser Gillan, T. Glanzman, Leanne P. Guy, Mark Hammergren, Andrew Hanushevsky, Fabio Hernandez, Ǎdis Herrold, Daniel Hestroffer, Joshua Hoblitt, Guillem Megias Homar, Matthew J. Hopkins, Massimiliano Giordano Orsini, Iain Goodenow, Miranda R. Gorsuch, Mikael Granvik, Wen Guan, Laurent Le Guillou, Simone Ieva, Patrick Ingraham, David H. Irving, Šebag Jacques, Buell T. Jannuzi, M. James Jee, David Jimenez, Altair Ramos Gomes-Júnior, Claire Juramy, Steven M. Kahn, Arun Kannawadi, Yijung Kang, JJ Kavelaars, Michael S. P. Kelley, Kshitija Kelkar, Lee S. Kelvin, Agnieszka Kryszczyńska, Ivan Kotov, Alec Koumjian, Gábor Kovács, K. Simon Krughoff, Petr Kubánek, Craig Lage, Travis J. Lange, Pierre-François Léget, Merlin Fisher-Levine, Benjamin Levine, W. Garrett Levine, Zhuofu Li, Shuang Liang, Javier Licandro, Čarey Lissé, Nate B. Lust, Ryan R. Lyttle, Gabriele Mainetti, Ashish A. Mahabal, Max Mahlke, Andrés A. Plazas Malagón, Rachel Mandelbaum, Luis E. Salazar Manzano, Moniez Marc, Steven J. Margheim, Giuliano Margoti, Morales Marín C. A. L., Dušan Marčeta, Mario D. Melita, Felipe Menanteau, Joshua Meyers, Dave Millsc, Naomi Morato, Surhud More, Christopher B. Morrison, Kris Mortensen, Youssef Moulane, Karlo Mrakovčić, Fritz Mueller, Marco A. Muñoz-Gutiérrez, Newcomer F. M., Homer Neal, Erfan Nourbakhsh, Paul O'Connor, Drew Oldag, William J Oldroyd, William O'Mullane, Cyrielle Opitom, Dagmara Oszkiewicz, Gary L. Page, Jack Patterson, Maria T Patterson, Matthew J. Payne, Julien Peloton, Eske M. Pedersen, Chrystian Luciano Pereira, John R. Peterson, Stephen R. Pietrowicz, Edyta Podlewska-Gaca, Daniel Polin, Hannah Mary Margaret Pollek, Rebekah Polen, Yongqiang Qiu, Bruno Quint, Markus Rabus, Darin Ragozzine, Jayadev Rajagopal, Arianna Ranabhat, vWouter van Reeven, Kevin Reil, Tiago Ribeiro, Malena Rice, Stephen T. Ridgway, Steven M. Ritz, Andrew S. Rivkin, James E. Robinson, Agata Ro{ż}ek, Eli Rykoff, Andrei Salnikov, R. Zanmar Sanchez, Bruno O. Sánchez, David Sanmartim, Gal Sarid, Charles A. Schambeau, Theo Schutt, Samuel J. Schmidt, German Schumacher, Daniel Scolnic, Rafe H. Schindler, Robert Seaman, Nima Sedaghat, Jacqueline Seron, Richard A. Shaw, Alysha Shugart, Jonathan Sick, Jaladh Singhal, Amir Siraj, Michael C. Sitarz, Shahram Sobhani, Christine Soldahl, Dallin Spencer, Brian Stalder, Steven Stetzler, Christopher W. Stubbs, Krzysztof L. Suberlak, John D. Swinbank, Adam Snyder, László Szigeti, Michael Tauraso, Dan S. Taranu, John Gregg Thayer, Sandrine Thomas, Adam Thornton, Luca Tonietti, David E. Trilling, Chadwick A. Trujillo, Te-Wei Tsai, Douglas L. Tucker, Max Turri, Tony Tyson, Elana K. Urbach, Stelios Voutsinas, Christopher W. Walter, Yuankun, Wang, Charlotte Ward, Michael Warner, Maxine West, Ian Wong, W. M. Wood-Vasey, Antonia Sierra Villarreal, Emerson Whittaker, Bin Yang, Quanhzi Ye, Peter Yoachim, Jinshuo Zhang, Conghao Zhou

TL;DR

The study demonstrates that the NSF-DOE Vera C. Rubin Observatory can deliver high-precision astrometry and multi-band photometry of a newly discovered interstellar object during commissioning. By combining bespoke analysis with early Rubin pipelines, the team extracted reliable positions, colors, and morphology of 3I/ATLAS, revealing a developing coma and a sunward-tail geometry. The results place 3I/ATLAS in the context of ISO demographics and show Rubin's potential to constrain the ISO luminosity function and size distribution as the LSST era begins. These findings underscore Rubin's capability to enable rapid, detailed follow-up of interstellar visitors and to drive population-level insights for exoplanetary system architectures.

Abstract

We report on the observation and measurement of astrometry, photometry, morphology, and activityof the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object's region of the sky during routine commissioning. Facilitated by Rubin's high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of about 70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of about 3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at about 0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-section ratio of eta >= 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g - r = (0.657 +/- 0.013) mag, r - i = (0.235 +/- 0.018) mag, i - z = (0.147 +/- 0.042) mag, z - y = (0.047 +/- 0.052) mag. These data represent the earliest observations of this object by a large (>=8-meter class) telescope and illustrate the type of measurements (and discoveries) Rubin's Legacy Survey of Space and Time (LSST) will begin to provide after it begins in early 2026.

NSF-DOE Vera C. Rubin Observatory Observations of Interstellar Comet 3I/ATLAS (C/2025 N1)

TL;DR

The study demonstrates that the NSF-DOE Vera C. Rubin Observatory can deliver high-precision astrometry and multi-band photometry of a newly discovered interstellar object during commissioning. By combining bespoke analysis with early Rubin pipelines, the team extracted reliable positions, colors, and morphology of 3I/ATLAS, revealing a developing coma and a sunward-tail geometry. The results place 3I/ATLAS in the context of ISO demographics and show Rubin's potential to constrain the ISO luminosity function and size distribution as the LSST era begins. These findings underscore Rubin's capability to enable rapid, detailed follow-up of interstellar visitors and to drive population-level insights for exoplanetary system architectures.

Abstract

We report on the observation and measurement of astrometry, photometry, morphology, and activityof the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object's region of the sky during routine commissioning. Facilitated by Rubin's high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of about 70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of about 3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at about 0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-section ratio of eta >= 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g - r = (0.657 +/- 0.013) mag, r - i = (0.235 +/- 0.018) mag, i - z = (0.147 +/- 0.042) mag, z - y = (0.047 +/- 0.052) mag. These data represent the earliest observations of this object by a large (>=8-meter class) telescope and illustrate the type of measurements (and discoveries) Rubin's Legacy Survey of Space and Time (LSST) will begin to provide after it begins in early 2026.

Paper Structure

This paper contains 26 sections, 7 equations, 16 figures, 1 table.

Table of Contents

  1. Introduction
  2. Background
  3. 3I/ATLAS
  4. The Vera C. Rubin Observatory
  5. Observations
  6. Data Processing
  7. Analysis and Results
  8. Astrometry
  9. Rubin Astrometric Calibration
  10. Curated Astrometric Measurements
  11. Orbit Determination
  12. Photometry
  13. Photometric Measurements
  14. Photometric variability
  15. Colors
  16. ...and 11 more sections

Figures (16)

  • Figure 1: 3I/ATLAS minimum co-addition from UT 2025 July 3, 21$\times$30 s $r$-band images acquired by NSF-DOE Vera C. Rubin Observatory. The FOV is $\sim15\arcsec \times 15\arcsec$, with North up and East left. The anti-solar (yellow arrow) and anti-motion (red-outlined black arrow) directions are shown.
  • Figure 2: Gallery of serendipitous observations of 3I/ATLAS from the NSF-DOE Vera C. Rubin Observatory. All images are $30 \arcsec\times 30\arcsec$ and have been reprojected so that North appears up, and East to the left (green arrows). The anti-solar (yellow, black-outlined arrow) and anti-motion (black, red-outlined arrow) directions are indicated. All dates and times are TAI. (a) 2025 June 21 08:11:32. (b) 2025 June 22 02:32:47. An area of roughly vertical saturation masking can be seen near the center of the frame; 3I/ATLAS is not within the masking, but the nearby blended star is. (c) 2025 June 22 03:07:49. (d) 2025 June 24 03:07:46. The image was unintentionally out of focus. (e) 2025 June 30 02:26:26. 3I/ATLAS is adjacent to the saturated star at the center. (f) 2025 July 02 00:44:25. (g) 2025 July 02 01:20:33. (h) 2025 July 02 02:31:16. (i) 2025 July 02 03:33:02.
  • Figure 3: Sky-map of the final SV observations distributions of 3I/ATLAS. The SV here is simulated from recorded pointings already enacted at the time of writing and nominal operations thereafter. The black points represent individual observations of 3I/ATLAS, with the yellow star highlighting its final observation in the SV in UT 2025 July 20. The red solid line marks the ecliptic plane, and the solid and dashed blue lines mark the galactic plane and $\pm10^{\circ}$ from the plane, respectively. (This static figure represents the final frame of an animation available in the online HTML version of the final article, showing the progression of 3I/ATLAS detections during the nominal SV. Each frame of the animation represents one additional night of observations, with the HEALPy sky-map updated accordingly. The plot title updates to reflect the night of each observation, and new black points appear as previous observations and are added to the map. The yellow star moves with each frame, representing the most up-to-date observation of 3I/ATLAS within the SV. The total duration of the animation is $\sim$28 s.)
  • Figure 6: Deblending 3I/ATLAS and background sources in a Rubin calibrated exposure (visit 2025070100200, detector 119) imaged UT 2025 July 2 00:44:25 (TAI). On the left is a cutout from a crowded field centered on 3I/ATLAS. The red ring represents the LSST Science Pipeline's aperture photometry radius (see Section \ref{['subsec:lightcurve']}). In the image, 3I/ATLAS is seen nestled between four sources. The brighter three have been detected as separate and deblended (marked with yellow plus symbols). The fourth source (just to the left of 3I/ATLAS, and inside the red ring) has not. The resulting footprint (the dark gray region in the left panel) -- a set of pixels deemed by the deblender to contain 3I/ATLAS's flux -- is shown on the right. The deblender successfully removed the flux belonging to the three detected stars, preventing major biases in photometry and astrometry. However, the flux due to the fainter undetected star is still present. This illustrates both the power and the caveats of deblender applications: while the worst effects of crowding are mitigated, some low-level residual flux from faint blended sources likely remains.
  • Figure 7: Two-dimensional histograms of the astrometric residuals between Rubin observations of objects matched to the Quaia quasar catalog (left) and Rubin observations of known asteroids with positions derived by the JPL Horizons ephemeris service (right). In both cases, the color scale indicates the number of sources per bin, and the dashed lines intersect at the origin.
  • ...and 11 more figures