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1. Precovery Observations of 3I/ATLAS from TESS Suggest Possible Distant Activity

   https://doi.org/10.3847/2041-8213/adfd4d  

   Feinstein, Adina D; Noonan, John W; Seligman, Darryl Z (September 2025, The Astrophysical Journal Letters)

   Abstract 3I/ATLAS is the third macroscopic interstellar object detected traversing the solar system. Since its initial discovery on UT 2025 July 1, hundreds of hours on a range of observational facilities have been dedicated to measuring the physical properties of this object. These observations have provided astrometry to refine the orbital solution, photometry to measure the color, a rotation period and secular light curve, and spectroscopy to characterize the composition of the coma. Here, we report precovery photometry of 3I/ATLAS as observed with NASA’s Transiting Exoplanet Survey Satellite (TESS). 3I/ATLAS was observed nearly continuously by TESS from UT 2025 May 7 to 2025 June 2. We use the shift-stack method to create deepstack images to recover the object. These composite images reveal that 3I/ATLAS has an average TESS magnitude ofT_mag = 20.83 ± 0.05, 19.28 ± 0.05 and an absolute visual magnitude ofH_V = 13.72 ± 0.35;12.52 ± 0.35, the latter being consistent with magnitudes reported in 2025 July. When coupled with recent Hubble Space Telescope images deriving a nucleus size ofR< 2.8 km (H> 15.4), our measurements suggest that 3I/ATLAS may have been active out at ∼6 au. Additionally, we extract a ∼20 day light curve and find no statistically significant evidence of a nucleus rotation period. Nevertheless, the data presented here are some of the earliest precovery images of 3I/ATLAS and may be used in conjunction with future observations to constrain the properties of our third interstellar interloper. 

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2. Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS

   https://doi.org/10.3847/2041-8213/adf49a  

   Seligman, Darryl Z; Micheli, Marco; Farnocchia, Davide; Denneau, Larry; Noonan, John W; Hsieh, Henry H; Santana-Ros, Toni; Tonry, John; Auchettl, Katie; Conversi, Luca; et al (August 2025, The Astrophysical Journal Letters)

   Abstract We report initial observations aimed at the characterization of a third interstellar object. This object, 3I/ATLAS or C/2025 N1 (ATLAS), was discovered on 2025 July 1 UT and has an orbital eccentricity ofe ∼ 6.1, perihelion ofq ∼ 1.36 au, inclination of ∼175°, and hyperbolic velocity ofV_∞ ∼ 58 km s⁻¹. We report deep stacked images obtained using the Canada–France–Hawaii Telescope and the Very Large Telescope that resolve a compact coma. Using images obtained from several smaller ground-based telescopes, we find minimal light-curve variation for the object over a ∼4 day time span. The visible/near-infrared spectral slope of the object is 17.1% ± 0.2%/100 nm, comparable to other interstellar objects and primitive solar system small bodies (comets and D-type asteroids). Moreover, 3I/ATLAS will be observable through early 2025 September, then unobservable by Earth-based observatories near perihelion due to low solar elongation. It will be observable again from the ground in late 2025 November. Although this limitation unfortunately prohibits detailed Earth-based observations at perihelion when the activity of 3I/ATLAS is likely to peak, spacecraft at Mars could be used to make valuable observations at this time. 

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3. Lightcurves, Rotation Periods, and Colors for Vera C. Rubin Observatory’s First Asteroid Discoveries

   https://doi.org/10.3847/2041-8213/ae2a30  

   Greenstreet, Sarah; Li, Zhuofu Chester; Vavilov, Dmitrii E; Singh, Devanshi; Jurić, Mario; Ivezić, Željko; Eggl, Siegfried; Koumjian, Alec; Moeyens, Joachim; Carruba, Valerio; et al (January 2026, The astrophysical journal Letters)

   Abstract We present lightcurves, rotation periods, and colors for the first asteroid discoveries made with the NSF-DOE Vera C. Rubin Observatory. These are the first science results derived from the 2103 asteroid discoveries released as part of the Rubin First Look (RFL) media event on 2025 June 23, in which the first LSST Camera commissioning images were released. The ∼340,000 observations in which the discoveries were made span nine nights between 2025 April 21 and May 5. With a limiting single-epoch 5σdepth of ∼23–25 mag and dense temporal sampling under an irregular, commissioning-driven cadence, the RFL observations provide an ideal test bed for determination of rotation periods, including sensitivity to rapid rotation. We model lightcurves and derive rotation periods and colors for the ∼2000 objects. We find 75 main-belt asteroids (MBAs) and one near-Earth object (NEO) with reliable rotation periods spanning 0.031–21.3 hr and a photometric precision in the range of 0.05–0.15 mag. We find 19 superfast rotators with periods shorter than the 2.2 hr spin barrier. Rubin-discovered MBA 2025 MN₄₅is the fastest-rotatingd > 0.5 km known asteroid with a rotation period of 1.9 minutes; along with NEO 2025 MJ₇₁(1.9 minutes) and Rubin-discovered MBAs 2025 MK₄₁(3.8 minutes), 2025 MV₇₁(13 minutes), and 2025 MG₅₆(16 minutes), these five super- to ultrafast rotators join a couple of NEOs as the fastest-spinning subkilometer asteroids known. As this study demonstrates, even in early commissioning, Rubin is successfully probing a previously sparsely sampled region of the subkilometer size−spin rate regime for MBAs. 

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4. The Kinematic Age of 3I/ATLAS and Its Implications for Early Planet Formation

   https://doi.org/10.3847/2041-8213/adfa28  

   Taylor, Aster G; Seligman, Darryl Z (August 2025, The Astrophysical Journal Letters)

   Abstract The recent discovery of the third interstellar object (3I/ATLAS) expands the known census from two to three and significantly improves statistical inferences regarding the underlying Galactic population. In this Letter, we argue that cometary activity likely significantly contributes to 3I/ATLAS’s brightness since the nuclear size inferred when assuming an asteroidal reflectance implies an untenable interstellar object mass per star. Relative to the Sun, 3I/ATLAS exhibits a high excess velocity ofv_∞= 58 km s⁻¹, which implies that 3I/ATLAS is relatively old in comparison to previous interstellar objects. Here, we calculate the posterior distribution of ages implied by the kinematics of the interstellar objects and find that 3I/ATLAS is likely ∼3–11 Gyr old, assuming that the interstellar object and stellar age–velocity dispersion relations are equivalent. We also calculate the distribution of host star metallicities and find that 3I/ATLAS has a 12% chance of originating from a star with [Fe/H] ≤ −0.4. These results show that interstellar object formation is likely efficient at low metallicities and early in the history of the Galaxy. Finally, we estimate the interstellar object formation rate throughout Galactic history implied by these three objects. As future interstellar objects are discovered, the framework presented here can be applied to further refine this calculation. Comparison between the interstellar object and stellar formation histories will provide unique insights into the history of stellar system formation in the Galaxy. 

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5. Onset of CN Emission in 3I/ATLAS: Evidence for Strong Carbon-chain Depletion

   https://doi.org/10.3847/2041-8213/ae1232  

   Salazar_Manzano, Luis E; Lin_林, Hsing_Wen 省文; Taylor, Aster G; Seligman, Darryl Z; Adams, Fred C; Gerdes, David W; Ruch, Thomas; Frincke, Tessa T; Napier, Kevin J (October 2025, The Astrophysical Journal Letters)

   Abstract Interstellar objects provide a direct window into the environmental conditions around stars other than the Sun. The recent discovery of 3I/ATLAS, a new interstellar comet, offers a unique opportunity to investigate the physical and chemical properties of interstellar objects and to compare them with those of comets in our own solar system. In this Letter we present the results of a 10 night spectroscopic and photometric monitoring campaign with the 2.4 m Hiltner and 1.3 m McGraw–Hill telescopes at the MDM Observatory. The campaign was conducted between August 8 and 17 while 3I/ATLAS was inbound at heliocentric distances of 3.2–2.9 au. Our observations captured the onset of optical gas activity. Nightly spectra reveal a weak CN emission feature in the coma of 3I/ATLAS, absent during the first nights but steadily strengthening thereafter. We measure a CN production rate ofQ(CN) ∼ 6 × 10²⁴s⁻¹, toward the lower end of activity observed in solar system comets. Simultaneous photometry also indicates a small but measurable increase in the coma’s radial profile and increasingr-bandAfρwith values in the order of ∼300 cm. We derived a gas-to-dust production ratio of $\log Q\left(\mathrm{CN}\right)/Af\rho \sim 22.4$. Our upper limit on the C₂-to-CN ratio ( $\log Q\left({\mathrm{C}}_2\right)/Q\left(\mathrm{CN}\right)\lesssim -0.8$) indicates that 3I/ATLAS is a strongly carbon-chain-depleted comet. Further observations of 3I/ATLAS are required to verify the apparent carbon-chain depletion and to explore whether such composition represents a recurring trait of the interstellar comet population. 

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