We report on the observation and measurement of astrometry, photometry, morphology, and activity of 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 similar to 70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of similar to 3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at similar to 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-sectional ratio of eta greater than or similar to 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, and z - y = (0.047 +/- 0.052) mag. These data represent the earliest observations of this object by a large (greater than or similar to 8 m class) telescope and illustrate the type of measurements (and discoveries) Rubin's Legacy Survey of Space and Time will provide after it begins in early 2026.

There was an unprecedented opportunity to study the inner dust coma environments, where the dust and gas are not entirely decoupled, of comets 45P/Honda–Mrkos–Pajdus̆áková (45P/HMP) from 2016 December 26 to 2017 March 15, and 46P/Wirtanen from 2018 November 10 to 2019 February 13, both in visible wavelengths. The radial profile slopes of these comets were measured in the R and HB-BC filters most representative of dust, and deviations from a radially expanding coma were identified as significant. The azimuthally averaged radial profile slope of comet 45P/HMP gradually changes from −1.81 ± 0.20 at 5.24 days preperihelion to −0.35 ± 0.16 at 74.41 days postperihelion. Contrastingly, the radial profile slope of 46P/Wirtanen stays fairly constant over the observed time period at −1.05 ± 0.05. Additionally, we find that the radial profile of 46P/Wirtanen is azimuthally dependent on the sky-plane-projected solar position angle, while that of 45P/HMP is not. These results suggest that comets 45P/HMP and 46P/Wirtanen have vastly different coma dust environments and that their dust expansion properties are distinct. As evident from these two comets, well-resolved inner comae are vital for detailed characterization of dust environments.

Studying materials released from Jupiter-family comets (JFCs)—as seen in their inner comae, the envelope of gas and dust that forms as the comet approaches the Sun—improves the understanding of their origin and evolutionary history. As part of a coordinated, multiwavelength observing campaign, we observed comet 45P/Honda–Mrkos–Pajdušáková during its close approach to Earth in 2017 February. Narrowband observations were taken using the Bok 90″ telescope at Kitt Peak National Observatory on February 16 and 17 UT, revealing gas and dust structures. We observed different jet directions for different volatile species, implying source region heterogeneity, consistent with other ground-based and in situ observations of other comet nuclei. A repeating feature visible in CN and C2 images on February 16 was also observed on February 17 with an interval of 7.6 ± 0.1 hr, consistent with the rotation period of the comet derived from Arecibo Observatory radar observations. The repeating feature’s projected gas velocity away from the nucleus is 0.8 km s−1, with an outflow velocity of 0.5 km s−1. A bright compact spot adjacent to the nucleus provides a lower limit of the amount of material released in one cycle of ∼9.2 kg, depending on composition—a quantity small enough to be produced by repeated exposure of nucleus ices to sunlight. This repeating CN jet, forming within 400 km of the nucleus, may be typical of inner-coma behavior in JFCs; however, similar features could be obscured by other processes and daughter product species when viewed from distances further than the scale length of CN molecules.

We present observations of five stellar occultations for (11351) Leucus and reports from two efforts on (21900) Orus. Both objects are prime mission candidate targets for the Lucy Discovery mission. Combined results for Leucus indicate a very dark surface with p V = 0.037 ± 0.001, which is derived from the average of the multichord occultations. Our estimate of the triaxial ellipsoidal shape is for axial diameters of 63.8 × 36.6 × 29.6 km assuming that the spin pole is normal to the line of sight. The actual shape of the object is only roughly elliptical in profile at each epoch. Significant topography is seen with horizontal scales up to 30 km and vertical scales up to 5 km. The most significant feature is a large depression on the southern end of the object as seen from a terrestrial viewpoint. For this work we developed a method to correct for differential refraction, accounting for the difference in color between the target object and the reference stars for astrometry derived from ground-based images.

We had a unique opportunity to observe three close approach Jupiter Family comets in the last two and a half years, 41P/Tuttle-Giacobini-Kresàk (41P/TGK), 45P/Honda-Mrkos-Pajdus̆áková (45P/HMP), and 46P/Wirtanen. Investigating at the dust environment of the inner coma of these comets, which can be resolved due to their proximity, dust radial profiles can suggest different environments. Our three comets have appeared to have different dust environment in the past. In our work, we present radial profiles of 46P/Wirtanen and compare them to the radial profiles of the other two comets. In general, if we obtain similar radial profile for two comets, we would expect radar data probing large grains around the nucleus to also be similar. Our radial profiles, when combined with radar data obtained at Arecibo, suggest that even if the radial profiles of 46P/Wirtanen and 41P/TGK are similar, the respective dust environments might still be different.

We show that 'Oumuamua's excited spin could be in a high-energy long axis mode (LAM) state, which implies that its shape could be far from the highly elongated shape found in previous studies. CLEAN and ANOVA algorithms are used to analyze 'Oumuamua's lightcurve using 818 observations over 29.3. days. Two fundamental periodicities are found at frequencies (2.77 +/- 0.11) and (6.42 +/- 0.18). cycles/day, corresponding to (8.67 +/- 0.34) hr and (3.74 +/- 0.11) hr, respectively. The phased data show that the lightcurve does not repeat in a simple manner, but approximately shows a double minimum at 2.77. cycles/day and a single minimum at 6.42. cycles/day. 'Oumuamua could be spinning in either the LAM or short axis mode (SAM). For both, the long axis precesses around the total angular momentum vector with an average period of (8.67 +/- 0.34) hr. For the three LAMs we have found, the possible rotation periods around the long axis are 6.58, 13.15, or 54.48 hr, with 54.48 hr being the most likely. 'Oumuamua may also be nutating with respective periods of half of these values. We have also found two possible SAM states where 'Oumuamua oscillates around the long axis with possible periods at 13.15 and 54.48 hr. In this case any nutation occurs with the same periods. Determination of the spin state, the amplitude of the nutation, the direction of the total angular momentum vector (TAMV), and the average total spin period may be possible with a direct model fit to the lightcurve. We find that 'Oumuamua is "cigar-shaped," if close to its lowest rotational energy, and an extremely oblate spheroid if close to its highest energy state.

Samarasinha & Mueller related changes of cometary rotation to other physical parameters for four Jupiter-family comets defining a parameter X , which is approximately constant within a factor of two irrespective of the active fraction of a comet. In this paper two additional comets are added to the original sample, and the claim of a nearly constant parameter X for these six comets is confirmed, albeit with a larger scatter. Taking the geometric mean of X for all of the comets above excluding 2P/Encke (as X for each comet was determined with respect to that of 2P/Encke), the expected changes in the rotation periods for a sample of 24 periodic comets are derived. From this sample we identify the comets that are most likely to show observationally detectable changes in their rotation periods. Using these 24 comets and including the six comets used to determine X , we find a correlation between the parameter ζ (i.e., the total water production per unit surface area per orbit approximated by that inside of 4 au) and the perihelion distance q ; specifically, we derive ζ ∝ q −0.8 and provide a theoretical basis for this in the Appendix. This relationship between ζ and q enables ready comparisons of activity due to insolation between comets. Additionally, a relationship between the nuclear radius R and the rotation period P is found. We find that on average smaller nuclei have smaller rotation periods compared to the rotation periods of larger nuclei. This is consistent with expectations for the rotational evolution and spin-up of comet nuclei, providing strong observational evidence for sublimation-driven rotational changes in comets.

Introduction: In the three years between 2016 and 2018 three small Jupiter-family comets (JFCs)—45P/ Honda-Mrkos-Pajdušáková (HMP), 41P/TuttleGiacobini-Kresák (TGK), and 46P/Wirtanen—will pass within 0.2 au of Earth, affording a unique opportunity to study the inner coma of these objects at unprescedented resolution. After the 2018 apparition of 46P we expect no similar opportunity to observe JFCs at close range for several decades. Studying the inner comae of these JFCs reveals cometary surface activity patterns, compositional uniformity, and the rotation states (whether rotating uniformly or tumbling). Ground-based telescopic measurements provide information about gas physics processes and interaction between the solid, central nucleus surface and the inner coma. We will present the results of a coordinated multiwavelength observational campaign focused on the inner coma of comet HMP with a 0.08 au in February 2017, close enough to observe structures in the near nucleus region before they merge with the larger coma. From these high resolution observations short lived molecular species can be detected [1] and, from radial distribution models, used to constrain the photochemical evolution of short lived or multiple source molecular species [2]. The observation campaign includes data from a global network of amateur observers, as well as a course at the University of Arizona on handson observational comet astronomy. Our observational campaign includes temporal monitoring of comet volatiles using narrow band imaging (including CN, C2, C3, NH) [3], as well as radio measurements of OH production [4] and planetary radar measurements of shape, rotation, and dust distrubtion [5]. Narrowband temporal monitoring data will be obtained over the course of 60+ nights of observations from telescope facilities of Steward Observatory at the University of Arizona as well as the Vatican Advanced Technology Telescope. Radar and radio measurements will be obtained from the Green Bank Radio Telescope and Arecibo Observatory. In addition to presenting narrowband imaging results from HMP and preliminary OH and radar results from this comet, we will present preliminary narrowband imaging of comet TGK. References: [1] Samarasinha, N. H. et al. (2011) ApJ Lett, 734, L3. [2] Harris, W. M. et al. (1997) Science, 277, 676-681. [3] Samarasinha N., et. al. (2015) Plan. Space Sci., 118, 127. [4] Schloerb F. P., De Vries C. H., Lovell A. J., Irvine W. M., Senay M., and Wooten H. A. (1999) Earth, Moon, and Planets, 78, 45-51. [5] Harmon J. K. et al. (2004) Comets II, 265279. [6] Howell et al. (2014) AAS/DPS meeting 46, abstract #209.24. [7] Lovell A. J. and Howell E. S. (2009) AAS/DPS meeting 41, abstract #15.06. Acknowledgements: This program is supported by NASA Solar System Observations Program grant NNX16AG70G, Steward Observatory at the University of Arizona, and the Vatican Observatory.

We present analysis of five nights of R-band observations of Comet 29P/Schwassmann-Wachmann 1 (SW1) taken on September 2008 which show the comet undergoing an outburst. Coma morphology shows a projected asymmetric shell of material expanding radially and four linear features on the northern side of the coma at position angles 37 degrees, 78 degrees, 300 degrees, and 353 degrees. Using the measured projected radial outflow velocity of 0.11 +/- 0.02 km/s for the shell material, we calculate an outburst time of UT 2008-09-21.03 +/- 10.95 days. By tracking the inner and outer extent of the northern linear features, we found that the features are fully contained within the expanding shell of material. This suggested both shell and linear features originated during the same event and activity originating from different regions on the nuclear surface are not necessary to generate both types of morphological structure observed. A 3-D Monte Carlo coma model was used to model the outburst. Morphological features present in the observations were modeled allowing constraints to be placed on the spin state of SW1's nucleus. The evolution of morphological features allows constraints on the rotation period P assuming an outburst duration At and the spin period constraints are expressed in terms of their ratio P/Delta t. Since the spin-pole orientation could not be constrained, four spin-pole orientations were chosen for modeling the coma. Spin-period constraints for each assumed pole orientation are discussed. Overall, modeling suggested either a spin period on the order of days, a spin-pole orientation nearly along the sub-Earth direction, or a combination of both. To place an independent constraint on the outburst duration, radial surface-brightness profiles of the observations were compared with profiles from synthetic models, giving an upper-limit of Delta t <= 1.5 days. Longer outbursts resulted in a higher number of dust grains in close proximity to the nucleus during the observations and a profile slope too steep to model observations. Lastly, from photometry of the five nights of observation, a lower limit of (1.8 +/- 0.07) x 10(9) kg was estimated for the total amount of dust emitted during the outburst. Assuming the outburst was triggered by either the sublimation of pure CO or CO2 ice and a dust to gas ratio of similar to 4 (Rosetta results for Comet 67P, Rotundi et al. 2015), a lower limit for the outburst duration on the order of hours was obtained. (C) 2016 Elsevier Inc. All rights reserved.