Gal Sarid

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 ∼70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of ∼3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at ∼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 η ≳ 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 (≳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.

With the increasing number of binary asteroid systems being discovered, ejecta studies must expand from solely investigating single-body systems to modeling more complex multiple-body systems. For example, the Double Asteroid Redirection Test provides an opportunity to study the dynamics of a debris cloud around Didymos and Dimorphos, a near-Earth binary asteroid system. Here we simulate 72 variations on the Didymos system in order to categorize types of ejecta outcomes and analyze the influence of the varying system parameters on each outcome. We have varied five parameters: the system separation, the mass ratio between the two bodies, the impact location, the target-body shape, and the target-body rotation period. The resulting provenance maps of the final ejecta distributions were blindly sorted into five categories, while the resulting cumulative distribution functions (CDFs), describing the rate at which particles hit the surface, were blindly sorted into eight categories. We count the occurrences of the parameter values in each of the categories and apply a Cramer’s V statistical test to evaluate the significance of the association between each varied effect and the overall grouping of the provenance maps and CDFs. We conclude that more dominant effects, such as a small rotation period, produce notably similar ejecta distributions that result in being assigned to the same category. Less dominant effects, such as target-body location, are sorted into several categories due to the larger influence of varying dominant effects.

The impact of the Double Asteroid Redirection Test spacecraft into Dimorphos, moon of the asteroid Didymos, changed Dimorphos’s orbit substantially, largely from the ejection of material. We present results from 12 Earth-based facilities involved in a world-wide campaign to monitor the brightness and morphology of the ejecta in the first 35 days after impact. After an initial brightening of ∼1.4 mag, we find consistent dimming rates of 0.11–0.12 mag day⁻¹ in the first week, and 0.08–0.09 mag day⁻¹ over the entire study period. The system returned to its pre-impact brightness 24.3–25.3 days after impact though the primary ejecta tail remained. The dimming paused briefly eight days after impact, near in time to the appearance of the second tail. This was likely due to a secondary release of material after re-impact of a boulder released in the initial impact, though movement of the primary ejecta through the aperture likely played a role.

Conditions in the outer protoplanetary disk during solar system formation were thought to be favorable for the formation of amorphous water ice (AWI), a glassy phase of water ice. However, subsequent collisional processing could have shock-crystallized any AWI present. Here we use the iSALE shock physics hydrocode to simulate impacts between large icy bodies at impact velocities relevant to these collisional environments, and then we feed these results into a custom-built AWI crystallization script, to compute how much AWI crystallizes/survives these impact events. We find that impact speeds between icy bodies after planet migration (i.e., between trans-Neptunian objects) are too slow to crystallize any meaningful fraction of AWI. During planet migration, however, the amount of AWI that crystallizes is highly stochastic: relatively little AWI crystallizes at lower impact velocities (less than ∼2 km s⁻¹), yet most AWI present in the bodies (if equally sized) or impactor and impact site (if different sizes) crystallizes at higher impact velocities (greater than ∼4 km s⁻¹). Given that suspected impact speeds during planet migration were ∼2–4 km s⁻¹, this suggests that primordial AWI’s ability to survive planet migration is highly stochastic. However, if proto-Edgeworth–Kuiper Belt (proto-EKB) objects and their fragments experienced multiple impact events, nearly all primordial AWI could have crystallized; such a highly collisional proto-EKB during planet migration is consistent with the lack of any unambiguous direct detection of AWI on any icy body. Ultimately, primordial AWI’s survival to the present day depends sensitively on the proto-EKB’s size–frequency distribution, which is currently poorly understood.

Centaur 29P/Schwassmann–Wachmann 1 (SW1) is a highly active object orbiting in the transitional “Gateway” region between the Centaur and Jupiter-family comet (JFC) regions. SW1 is unique among the Centaurs in that it experiences quasi-regular major outbursts and produces CO emission continuously; however, the source of the CO is unclear. We argue that, due to its very large size (∼32 km radius), SW1 is likely still responding, via amorphous water ice (AWI) conversion to crystalline water ice (CWI), to the “sudden” change in its external thermal environment produced by its Myrs-long dynamical migration from the Kuiper Belt to its current location at the inner edge of the Centaur region. It is this conversion process that is the source of the abundant CO and dust released from the object during its quiescent and outburst phases. If correct, these arguments have a number of important predictions testable via remote sensing and in situ spacecraft characterization, including the quick release on Myr timescales of CO from AWI conversion for any few kilometer-scale scattered disk Kuiper Belt Objects transiting into the inner system; that to date SW1 has only converted between 50% and 65% of its nuclear AWI to CWI; that volume changes on AWI conversion could have caused subsidence and cave-ins, but not significant mass wasting or crater loss; that SW1's coma should contain abundant amounts of CWI+CO₂ “dust” particles; and that when SW1 transits into the inner system within the next 10,000 yr, it will be a very different kind of JFC.

VIIRS day-night band (DNB) covers a wavelength range from 500 nm to 900 nm, has three gain stages enabling a dynamic range of 7 orders of magnitude, and is calibrated by a solar diffuser. In this paper, the calibration uncertainty of the DNB is analyzed for both SNPP and NOAA-20 VIIRS instruments. It is shown that the uncertainties of the DNB for all gain stages, detectors, half angle mirror sides, and aggregation modes are much smaller than the uncertainty specifications of the band, which is 5%, 10%, and 100% for low, middle, and high gain stage, respectively.

The S-NPP and NOAA-20 VIIRS instruments have successfully operated since their launches on October 28, 2011 and November 18, 2017, respectively. A panchromatic channel onboard VIIRS is referred to as the day-night band (DNB), was designed with a large dynamic range and high sensitivity, such that its detectors can make observations during both daytime and nighttime. The DNB uses an onboard solar diffuser (SD) panel for low gain stage calibration, and the SD observations are also carefully selected to compute gain ratios between low-to-mid and mid-to-high gain stages. In this paper, we present the S-NPP and NOAA-20 VIIRS DNB calibration performed by the NASA VIIRS Characterization Support Team (VCST) to generate the calibration coefficient look up tables (LUTs) for the latest NASA Level 1B Collection 2 products. This activity supports the NASA Earth science community by delivering consistent VIIRS sensor data products via the Land Science Investigator-led Processing Systems. The DNB stray light contamination and its different behavior have been highlighted between two instruments. Its estimate and correction methods as well as performance are illustrated.

MODIS is an imaging spectroradiometer onboard NASA’s Terra and Aqua satellites with visible to short-wave infrared capability enabled through 20 reflective solar bands (RSB) in the wavelength range of 400 to 2200 nm. RSB gain changes are monitored by using observations of a fully sunlit solar diffuser (SD) for the on-orbit radiometric calibration. SD signal changes, due to degradation of the SD surface, are monitored by using the SD Stability Monitor (SDSM), which has 9 detectors in the wavelength range of 400 to 936 nm. These calibration approaches depend on the geometry of the spacecraft’s orbit. Both Terra and Aqua have recently started to exit their respective constellations and are (or soon will be) in the process of drifting from their nominal orbits. These orbital changes will cause changes in the solar diffuser (SD) and solar diffuser stability monitor (SDSM) viewing geometry and calibration conditions. This, in turn, will drive a variation in the calibration parameters used to calculate reference adjustments for the MODIS reflective solar bands (RSB). We examine the expected effect of orbital drift on the reflectance and transmittance functions, the evaluation of on-going SDSM detector response, the variation of SD surface incidence angle ranges, and the estimation of fully-illuminated observing condition for SD signal monitoring.

Aboard the polar-orbiting SNPP satellite, the VIIRS instrument has been in operation since launch in October 2011. It is a visible and infrared radiometer with a unique panchromatic channel capability designated as a day-night band (DNB). This channel covers wavelengths from 0.5 to 0.9 µm and is designed with a near-constant spatial resolution for Earth observations 24 h a day. The DNB operates at 3 gain stages (low, middle, and high) to cover a large dynamic range. An onboard solar diffuser (SD) is used for calibration in the low gain stage, and to enable the derivation of gain ratios between the different stages. In this paper, we present the SNPP VIIRS DNB calibration performed by the NASA VIIRS characterization support team (VCST). The DNB calibration algorithms are described to generate the calibration coefficient look up tables (LUTs) for the latest NASA Level 1B Collection 2 products. We provide an evaluation of DNB on-orbit calibration performance. This activity supports the NASA Earth science community by delivering consistent VIIRS sensor data products via the Land Science Investigator-led Processing Systems, including the SD degradation applied for DNB calibrations in detector gain and gain ratio trending. The DNB stray light contamination and its correction are highlighted. Performance validations are presented using comparisons to the calibration methods employed by NOAA’s operational Interface Data Processing Segment. Further work on stray light corrections is also discussed.

The Gateway Centaur and Jupiter co-orbital P/2019 LD2 (ATLAS) provides the first opportunity to observe the migration of a solar system small body from a Centaur orbit to a Jupiter Family Comet (JFC) four decades from now. The Gateway transition region is beyond where water ice can power cometary activity, and coma production there is as poorly understood as in all Centaurs. We present contemporaneous multiwavelength observations of LD2 from 2020 July 2–4: Gemini North visible imaging, NASA IRTF near-infrared spectroscopy, and ARO Submillimeter Telescope millimeter-wavelength spectroscopy. Precovery DECam images limit the nucleus’s effective radius to ≤1.2 km and no large outbursts were seen in archival Catalina Sky Survey observations. LD2's coma has g ′ − r ′ = 0.70 ± 0.07 , r ′ − i ′ = 0.26 ± 0.07 , a dust-production rate of ∼10–20 kg s⁻¹, and an outflow velocity between v ∼ 0.6–3.3 m s⁻¹. We did not detect CO toward LD2 on 2020 July 2–3, with a 3σ upper limit of Q(CO) < 4.4 × 10²⁷ mol s⁻¹ (⪅ 200 kg s⁻¹). Near-infrared spectra show evidence for water ice at the 1%–10% level depending on grain size. Spatial profiles and archival data are consistent with sustained activity. The evidence supports the hypothesis that LD2 is a typical small Centaur that will become a typical JFC, and thus, it is critical to understanding the transition between these two populations. Finally, we discuss potential strategies for a community-wide, long-baseline monitoring effort.