arXiv subjects

Karri Muinonen

Publications and source records attributed to Karri Muinonen.

17 recordsLinked to original sources

arXiv2026

Nordic-based observations of binary asteroid (163693) Atira reveal a hydrated CM chondrite composition

We present photometric, spectroscopic, and polarimetric observations of (163693) Atira collected on the night of 05 March 2024. A dual-band lightcurve acquired in the V and R_C filters using the Metsähovi Observatory (L08) in Kirkkonummi, Finland spans nearly 8~hours. From these measurements we estimate a V-R_C = 0.43 +/- 0.07 mag. A visible spectrum and linear polarisation measurement were obtained with the Nordic Optical Telescope in La Palma, Spain. We employ a semi-empirical model to recover the spectral flux that was lost due to a mis-aligned spectroscopic slit. The corrected model reflectance spectrum indicates a primitive Cgh taxonomic type with a distinct absorption feature from 0.60 to 0.85 um with a depth of ~7.4 %. Direct comparison with laboratory spectra of meteorites indicates close matches with CM carbonaceous chondrites. We detect a large degree of linear polarisation at a phase angle of 83 degrees which is consistent with (3200) Phaethon and (155140) 2005 UD, but lower than (162173) Ryugu and (152679) 1998 KU2. We use a CM carbonaceous chondrite density to estimate its bulk porosity of 51 +/- 31 % from previously estimated bulk density. The uncertainty in bulk density and bulk porosity is dominated by the volume of the primary. Future near-infrared spectroscopic and thermal infrared observations can be used to refine the composition and bulk density. Finally, we discuss implications for ground-based physical characterisation studies of the Atira population.

astro-ph.EP↗

arXiv2026

Reliability of Zodiacal Background Models at Small Solar Elongations

Present models of the zodiacal light (scattered sunlight from interplanetary dust in the visible) and zodiacal emission (thermal emission in the mid-infrared) are primarily constrained by surface-brightness measurements obtained at relatively large solar elongations (e >~ 60 deg). As a result, their predictive accuracy at smaller elongations remains uncertain. This is particularly relevant for solar elongations of e ~ 30-60 deg, where next-generation near-Earth-object discovery missions such as NEO Surveyor and NEOMIR are designed to operate. To evaluate the reliability of these models at small solar elongations, we compile and reanalyze the available small-elongation zodiacal-light and zodiacal-emission measurements and provide the first systematic validation of modern zodiacal-light models in a regime where they have never been independently tested. We compare two models derived from the COBE/DIRBE survey: one constrained primarily by the spatial and temporal variation of the monochromatic sky brightness, and another that additionally uses interband color information and fixes the absolute normalization by requiring the minimum high-latitude residual at 25um to vanish. We find that the former reproduces the observations significantly better. At wavelengths most relevant to infrared near-Earth-object surveys (8-10 um), it typically agrees with measurements to within ~10% at e ~ 40-60 deg. Although it also performs better at e ~ 30 deg, significant discrepancies remain. These results show that zodiacal emission at small solar elongations is still only weakly constrained and that its absolute brightness remains uncertain. New mid-infrared observations are required to improve background estimates for future infrared survey missions.

astro-ph.EP↗

arXiv2026

Multi-Wavelength Post-Perihelion Polarimetry of Interstellar Comet 3I/ATLAS

We present new post-perihelion polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1; hereafter 3I), obtained using FORS2 at the Very Large Telescope, ALFOSC at the Nordic Optical Telescope, and FoReRo2 at the 2 m Ritchey-Chrétien-Coudé telescope. The observations span phase angles 1-30$^{\circ}$, providing the most extensive polarimetric phase angle coverage obtained for an interstellar object to date. The post-perihelion measurements reveal that the unusual polarimetric properties of 3I persist across perihelion and match recent independent post-perihelion observations, indicating no significant evolution in the polarimetric properties. The increased phase angle coverage allowed us to further constrain the minimum polarisation to $-2.9\%$ at phase angle $5.5^{\circ}$. Multi-band BVRI observations reveal a wavelength dependence of polarisation, with the negative polarisation branch becoming deeper and shifted toward smaller phase angles at longer wavelengths. The polarimetric colour is predominantly red and increases with phase angle, consistent with behaviour observed in other comets. Imaging and polarimetric maps show a substantially more extended coma after perihelion, while the polarisation distribution itself remains smooth and spatially homogeneous. Numerical modelling suggests that the unusually deep polarisation phase curve originates from moderately porous dust aggregates consisting of weakly absorbing sub-micron monomers, resulting in much brighter dust than that in typical solar system comets.

astro-ph.EP↗

arXiv2026

Physical Characteristics of the Asteroid (469219) Kamo'oalewa as a target of the Chinese Tianwen-2 mission

The Near-earth asteroid (469219) Kamo'oalewa, a quasi-satellite of the Earth, is going to be observed in site and sampled by the Chinese space mission Tianwen-2 in near future. Here. we analyze its photometric and spectroscopic data to figure out its basic physical properties, which are very important for the sample return task of the Tianwen-2 mission. With photometry inversion methods, we derived a pole $(276^{o}.79, -21^{o}.43)$ with a spin period of 28.4517 minutes and a slightly flat convex shape. The estimated photometry slope of $0.998 mag/rad$ implies a large albedo of the Kamo'oalewa, i.e. S-type. Using the estimated absolute magnitude of $24.98$ mag, its size could be 27.4m assuming a typical albedo of S-type asteroids. The taxonomy analysis with a constructed ANN tool also supports that the Kamo'oalewa should belong to S-type asreroids, it may be a strong weathering fragment of an A-type or Q-type asteroid. Using derived pole, size and shape information of the target, we estimated its thermal inertia as $163.0 Jm^{-2}K^{-1}s^{-1/2}$ based on the new derived Yarkovski draft $A_2=-13.29349563\times10^{-14}au/day^2$, which means the target has a surface of mixture of grains and small bounds, like the surface of asteroid Bennu.

astro-ph.EP↗

arXiv2025

Extreme Negative Polarisation of New Interstellar Comet 3I/ATLAS

We present the first polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1), obtained pre-perihelion with FORS2/VLT, ALFOSC/NOT, and FoReRo2/RCC, over a phase angle range of 7.7-22.4°. This marks the second ever polarimetric study of an interstellar object, the first distinguishing 2I/Borisov from most Solar System comets by its higher positive polarisation. Our polarimetric measurements as a function of phase angle reveal that 3I is characterised by an deep and narrow negative polarisation branch, reaching a minimum value of -2.7% at phase angle 7°, and an inversion angle of 17° -- a combination unprecedented among asteroids and comets, including 2I/Borisov. At very small phase angles, the extrapolated slope of the polarisation phase curve is consistent with that of certain small trans-Neptunian objects and Centaur Pholus, consistent with independent spectroscopic evidence for a red, possibly water-ice-bearing object. Imaging confirms a diffuse coma present from our earliest observations, though no strong polarimetric features are spatial resolved. These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies.

astro-ph.EP↗

arXiv2025

Photometric analysis of asteroids in the Phocaea region

The Phocaea asteroid family, one of the large ancient families located in the inner main belt, may be the sources of near-Earth asteroids (NEAs) due to the nearby 3:1 mean motion resonance with Jupiter, the v6 secular resonance, and the Yarkovsky and YORP effects. Thus, understanding the influence of the Yarkovsky and YORP effects on the Phocaea family is one of the keys to figuring out the source of NEAs. However, the physical properties of most of the Phocaea family members are unknown at present. We perform a photometric analysis for 44 asteroids in the Phocaea region using photometric data obtained by ground-based and space-based telescopes (i.e., TESS and Gaia). Based on the derived physical properties, we find significant footprints of the Yarkovsky and YORP effects on the Phocaea family members. Selecting five asteroids nearby the inside boundary of the V-shape in the absolute-magnitude semimajor-axis (H, a) space, we estimate their densities considering their migration in semimajor-axis arises from the Yarkovsky effect. The bulk density of (852) Wladilena ({3.54 g/cm3) suggests a link to the H chondrite meteorites. Incorporating the grain density of the H chondrites, we estimate the macroporosities of the asteroids (290) Bruna, (1164) Kobolda, and (587) Hypsipyle, respectively 41%, 47%, and 65%, implying rubble pile structures. Considering the H chondrites link to asteroid (25) Phocaea, we suggest the parent body of the Phocaea family has been composed of H chondrite like material and the Phocaea family may be one of the sources of H chondrite meteorites.

astro-ph.EP↗

arXiv2024

A method of photometric data extraction for asteroids from time-domain surveys

The lightcurves of asteroids are essential for determining their physical characteristics, including shape, spin, size, and surface composition. However, most asteroids are missing some of these basic physical parameters due to lack of photometric data. Although a few telescopes or surveys are specially designed for photometric lightcurve observations of asteroids, many ground-based and space-based sky surveys for hunting new exoplanets, transient events, etc., should capture numerous small Solar System objects. This will benefit the physical studies of these objects. In order to extract data of these moving objects from time-domain photometric surveys, we have developed a new method using the model tree algorithm in the field of machine learning. A dedicated module is built to automatically identify moving objects in dataset, and extract their photometric and astrometric data. As the first application of this novel method, we have analyzed data in five fields of the Yunnan-Hong Kong wide field photometric (YNHK) survey, from which 538 lightcurves of 211 asteroids are successfully extracted. Meanwhile, we also tested the method based on the data from NASA's Transiting Exoplanet Survey Satellite, and the result proves the reliability of our method. With derived lightcurves of 13 asteroids from the YNHK survey, we have determined their synodic spin periods, among which the periods of 4 asteroids are estimated for the first time. In future, we are going to apply this method to search for small objects in the outer part of the Solar System from the Chinese Space Station Telescope survey.

astro-ph.EP↗

arXiv2023

Polarimetry of Didymos-Dimorphos: Unexpected Long-Term Effects of the DART Impact

We have monitored the Didymos-Dimorphos binary system in imaging polarimetric mode before and after the impact from the Double Asteroid Redirection Test (DART) mission. A previous spectropolarimetric study showed that the impact caused a dramatic drop in polarisation. Our longer-term monitoring shows that the polarisation of the post-impact system remains lower than the pre-impact system even months after the impact, suggesting that some fresh ejecta material remains in the system at the time of our observations, either in orbit or settled on the surface. The slope of the post-impact polarimetric curve is shallower than that of the pre-impact system, implying an increase in albedo of the system. This suggests that the ejected material is composed of smaller and possibly brighter particles than those present on the pre-impact surface of the asteroid. Our polarimetric maps show that the dust cloud ejected immediately after the impact polarises light in a spatially uniform manner (and at a lower level than pre-impact). Later maps exhibit a gradient in polarisation between the photocentre (which probes the asteroid surface) and the surrounding cloud and tail. The polarisation occasionally shows some small-scale variations, the source of which is not yet clear. The polarimetric phase curve of Didymos-Dimorphos resembles that of the S-type asteroid class.

astro-ph.EP↗

arXiv2023

Taxonomic analysis of asteroids with artificial neural networks

We study the surface composition of asteroids with visible and/or infrared spectroscopy. For example, asteroid taxonomy is based on the spectral features or multiple color indices in visible and near-infrared wavelengths. The composition of asteroids gives key information to understand their origin and evolution. However, we lack compositional information for faint asteroids due to limits of ground-based observational instruments. In the near future, the Chinese Space Survey telescope (CSST) will provide multiple colors and spectroscopic data for asteroids of apparent magnitude brighter than 25 mag and 23 mag, respectively. For the aim of analysis of the CSST spectroscopic data, we applied an algorithm using artificial neural networks (ANNs) to establish a preliminary classification model for asteroid taxonomy according to the design of the survey module of CSST. Using the SMASS II spectra and the Bus-Binzel taxonomy system, our ANN classification tool composed of 5 individual ANNs is constructed, and the accuracy of this classification system is higher than 92 %. As the first application of our ANN tool, 64 spectra of 42 asteroids obtained in 2006 and 2007 by us with the 2.16-m telescope in the Xinglong station (Observatory Code 327) of National Astronomical Observatory of China are analyzed. The predicted labels of these spectra using our ANN tool are found to be reasonable when compared to their known taxonomic labels. Considering the accuracy and stability, our ANN tool can be applied to analyse the CSST asteroid spectra in the future.

astro-ph.EP↗

arXiv2020

Rough-Surface Shadowing of Self-Affine Random Rough Surfaces

Light scattering from self-affine homogeneous isotropic random rough surfaces is studied using the ray-optics approximation. Numerical methods are developed to accelerate the first-order scattering simulations from surfaces represented as single-connected single-valued random fields, and to store the results of the simulations into a numerical reflectance model. Horizon mapping and marching methods are developed to accelerate the simulation. Emphasis is given to the geometric shadowing and masking effects as a function of surface roughness, especially, to the azimuthal rough-surface shadowing effect.

physics.optics↗

arXiv2019

Phase integral of asteroids

The values of the phase integral q were determined for asteroids using (i) a numerical integration of the brightness phase functions over a wide phase-angle range and (ii) the relations between q and the G parameter of the HG-function and q and the G1, G2 parameters of the HG1G2-function. The phase-integral values for asteroids of different albedo range from 0.34 to 0.54 with an average value of 0.44. These values can be used for the determination of the Bond albedo of asteroids. Estimates for the phase-integral values using the G1 and G2 parameters are in very good agreement with the available observational data. We recommend using the HG1G2-function for the determination of the phase integral. Comparison of the phase integrals of asteroids and planetary satellites shows that asteroids have systematically lower values of q.

astro-ph.EP↗

arXiv2018

Interpretation of the phase functions measured by the OSIRIS instrument for Comet 67P/Churyumov-Gerasimenko

We show that the scattering phase functions of the coma and the nucleus of the comet 67P/Churyumov-Gerasimenko measured by the Rosetta/OSIRIS instrument can be reproduced by a particle model involving clustered densely packed submicrometer-sized grains composed of organic material and larger micrometer-sized silicate grains. The simulated and measured coma phase functions suggest that near the nucleus scattering is dominated by large particles, and the size distribution of dust particles varies with time and/or local coma environment. Further, we show that the measured nucleus phase function is consistent with the coma phase function by modelling a nucleus-sized object consisting of the same particles that explain the coma phase functions.

astro-ph.EP↗

arXiv2018

Radio interferometric observation of an asteroid occultation

The occultation of the radio galaxy 0141+268 by the asteroid (372) Palma on 2017 May 15 was observed using six antennas of the Very Long Baseline Array (VLBA). The shadow of Palma crossed the VLBA station at Brewster, Washington. Owing to the wavelength used, and the size and the distance of the asteroid, a diffraction pattern in the Fraunhofer regime was observed. The measurement retrieves both the amplitude and the phase of the diffracted electromagnetic wave. This is the first astronomical measurement of the phase shift caused by diffraction. The maximum phase shift is sensitive to the effective diameter of the asteroid. The bright spot at the shadow's center, the so called Arago--Poisson spot, is clearly detected in the amplitude time-series, and its strength is a good indicator of the closest angular distance between the center of the asteroid and the radio source. A sample of random shapes constructed using a Markov chain Monte Carlo algorithm suggests that the silhouette of Palma deviates from a perfect circle by 26+-13%. The best-fitting random shapes resemble each other, and we suggest their average approximates the shape of the silhouette at the time of the occultation. The effective diameter obtained for Palma, 192.1+-4.8 km, is in excellent agreement with recent estimates from thermal modeling of mid-infrared photometry. Finally, our computations show that because of the high positional accuracy, a single radio interferometric occultation measurement can reduce the long-term ephemeris uncertainty by an order of magnitude.

astro-ph.EP↗

arXiv2015

The daily processing of asteroid observations by Gaia

The Gaia mission started its regular observing program in the summer of 2014, and since then it is regularly obtaining observations of asteroids. This paper draws the outline of the data processing for Solar System objects, and in particular on the daily "short-term" processing, from the on-board data acquisition to the ground-based processing. We illustrate the tools developed to compute predictions of asteroid observations, we discuss the procedures implemented by the daily processing, and we illustrate some tests and validations of the processing of the asteroid observations. Our findings are overall consistent with the expectations concerning the performances of Gaia and the effectiveness of the developed software for data reduction.

astro-ph.EP↗

arXiv2013

Hubble Space Telescope Pre-Perihelion ACS/WFC Imaging Polarimetry of Comet ISON (C/2012 S1) at 3.81 AU

We present polarization images of Comet ISON (C/2012 S1) taken with the Hubble Space Telescope (HST) on UTC 2013 May 8 (rh = 3.81 AU, Delta = 4.34 AU), when the phase angle was alpha = 12.16 degrees. This phase angle is approximately centered in the negative polarization branch for cometary dust. The region beyond 1000 km from the nucleus shows a negative polarization amplitude of p% -1.6%. Within 1000 km of the nucleus, the polarization position angle rotates to be approximately perpendicular to the scattering plane, with an amplitude p% +2.5%. Such positive polarization has been observed previously as a characteristic feature of cometary jets, and we show that Comet ISON does indeed harbor a jet-like feature. These HST observations of Comet ISON represent the first visible light, imaging polarimetry with sub-arcsecond spatial resolution of a Nearly Isotropic Comet (NIC) beyond 3.8 AU from the Sun at a small phase angle. The observations provide an early glimpse of the properties of the cometary dust preserved in this Oort-cloud comet.

astro-ph.EP↗

arXiv2013

Mineralogy, reflectance spectra, and physical properties of the Chelyabinsk LL5 chondrite, insight into shock induced changes in asteroid regoliths

The mineralogy and physical properties of Chelyabinsk meteorites (fall, February 15, 2013) are presented. Three types of meteorite material are present, described as the light-colored, dark-colored, and impact-melt lithologies. All are of LL5 composition with the impact-melt lithology being close to whole-rock melt and the dark-colored lithology being shock-darkened due to partial melting of iron metal and sulfides. This enables us to study the effect of increasing shock on material with identical composition and origin. Based on the magnetic susceptibility, the Chelyabinsk meteorites are richer in metallic iron as compared to other LL chondrites. The measured bulk and grain densities and the porosity closely resemble other LL chondrites. Shock darkening does not have a significant effect on the material physical properties, but causes a decrease of reflectance and decrease in silicate absorption bands in the reflectance spectra. This is similar to the space weathering effects observed on asteroids. However, compared to space weathered materials, there is a negligible to minor slope change observed in impact-melt and shock-darkened meteorite spectra. Thus, it is possible that some dark asteroids with invisible silicate absorption bands may be composed of relatively fresh shock-darkened chondritic material.

astro-ph.EP↗

arXiv2012

Modeling collision probability for Earth-impactor 2008 TC3

We study the evolution of the Earth collision probability of asteroid 2008 TC3 using a short observational arc and small numbers of observations. To assess impact probability, we use techniques that rely on the orbital-element probability density function characterized using both Markov-chain Monte-Carlo orbital ranging and Monte-Carlo ranging. First, we evaluate the orbital uncertainties for the object from the night of discovery onwards and examine the collapse of the orbital-element distributions in time. Second, we examine the sensitivity of the results to the assumed astrometric noise. Each of the orbits obtained from the MCMC ranging method is propagated into the future (within chosen time bounds of the expected impact), and the collision probability is calculated as a weighted fraction of the orbits leading to a collision from the Earth. We compare the results obtained with both methods.

astro-ph.EP↗