Explore the vast range of titles available.

So far, only two interstellar objects have been observed within our Solar System. While the first one, 1I/‘Oumuamua, had asteroidal characteristics, the second one, 2I/Borisov, showed clear evidence of cometary activity. We performed polarimetric observations of comet 2I/Borisov using the European Southern Observatory Very Large Telescope to derive the physical characteristics of its coma dust particles. Here we show that the polarization of 2I/Borisov is higher than what is typically measured for Solar System comets. This feature distinguishes 2I/Borisov from dynamically evolved objects such as Jupiter-family and all short- and long-period comets in our Solar System. The only object with similar polarimetric properties as 2I/Borisov is comet C/1995 O1 (Hale-Bopp), an object that is believed to have approached the Sun only once before its apparition in 1997. Unlike Hale-Bopp and many other comets, though, comet 2I/Borisov shows a polarimetrically homogeneous coma, suggesting that it is an even more pristine object. Polarimetry provides information about physical characteristics of cometary dust. Here, the authors show that the polarization of interstellar comet 2I/Borisov exceeds the typical values for comets, and this together with its polarimetrically homogenous coma suggests a more pristine nature of the object.

. The large majority of astronomical observations are based on intensity measurements as a function of either wavelength or time, or both. Polarimetry, a technique which measures the way in which the electromagnetic field associated to the radiation oscillates, does provide further information about the objects that have emitted or scattered the observed radiation. For instance, polarimetric measurements can provide important constraints to the characterisation of cosmic dust (be it of interstellar or cometary origin), of the surfaces of the atmosphereless bodies and of planetary atmospheres. This property has been exploited in solar system science to study asteroids, comets, rocky and giant gaseous planets, and their satellites. In this paper we present a review of the polarimetric studies of the small bodies of the Solar System.

According to current plans of the European space agency, Gaia will be launched in 2011. By performing a systematic survey of the whole sky down to magnitude V = 20, this mission will provide a fundamental contribution in practically all branches of modern Astrophysics. Gaia will be able to survey with repeated observations spanning over 5 years several 100,000 s asteroids. It will directly measure sizes of about 1,000 objects, obtain the masses of about 100 of them, derive spin properties and overall shapes of more than 10,000 objects, yield much improved orbits and taxonomic classification for most of the observed sources. The final harvest will very likely include new discoveries of objects orbiting at heliocentric distances less than 1 AU. At the end of the mission, we will know average densities of about 100 objects belonging to all the major taxonomic classes, have a much more precise knowledge of the inventory and size and spin distributions of the population, of the distribution of taxonomic classes as a function of heliocentric distance, and of the dynamical and physical properties of dynamical families.

Asteroid polarimetry has taken profit in recent years of a renewed interest triggered by exciting results from observing campaigns and theoretical studies. One of the most important applications of polarimetry to asteroid studies is the derivation of the geometric albedo and of the typical sizes of the particles forming the regolith layer covering the surface. Moreover, the serendipitous discovery of a new class of asteroids displaying unusual polarimetric properties, the so-called “Barbarians”, has been followed by increasing evidence that these objects can be extremely primitive and may be interpreted as remnants of the very first generation of solid bodies accreted in the inner Solar System. In addition, some results of asteroid polarimetry are going to be interpreted, for the first time, in terms of some “ground truth” evidence, made possible by in situ observations of the surface of the asteroid (4) Vesta by the Dawn space probe. Finally, some preliminary evidence suggests that spectro-polarimetry is going to become a major tool for the physical characterization of the small bodies of the solar system.

The surfaces of the atmosphere-less objects of our solar system are traditionally probed via reflectance measurements and/or broadband linear polarimetry. Little attention has been paid so far to the wavelength dependence of the linear polarization of the scattered light. We decided to explore the potential of spectropolarimetry as a remote sensing tool for asteroids in addition to the more traditional reflectance measurements, and we carried out a spectropolarimetric survey of asteroids – to our best knolwedge, the first of its kind. We observed a sample of asteroids of different albedo and taxonomic classes, as well as a few regions at the limb of the Moon. We show that objects exhibiting similar reflectance spectra may display totally different polarization spectra, and we suggest that both intensity and polarization spectra should be used for asteroid classification. We also found that in some cases the Umov law is violated, that is, in contrast to what is expected from simple physical considerations, the fraction of linear polarization and the reflectance spectra may be correlated positively. We conclude that future modelling attempts of the surface structure of asteroids should be aimed at explaining both reflectance and polarization spectra.

Numerical simulations of particles placed in orbital resonances in the main asteroid belt show that the typical dynamical lifetimes of objects that could become near-Earth asteroids or meteorites are only a few million years, with the majority destroyed by being transferred to Jupiter-crossing orbits or being driven into the sun. Particles that fortuitously migrate to the terrestrial planet region may be pushed to high-inclination orbits by resonances but are still dynamically eliminated on time scales of ∼10 million years. These shorter lifetimes may require a reassessment of our qualitative understanding about near-Earth asteroids and meteorite delivery.

GAIA will likely be a real milestone for asteroid science. Its unprecedented astrometric accuracy can be exploited to compute orbits better than those obtained by using the whole set of available astrometric observations carried out from ground during the last 100 years. GAIA will make it possible to measure asteroid masses through the measurement of the tiny displacements caused by asteroid mutual encounters. This will be a major breakthrough for physical studies of minor planets. Moreover, sizes will be also directly measured for a big sample of more than 1000 objects. From masses and sizes, estimates of the average densities will be also obtained. The spectrophotometric capability of GAIA will be used for purposes of taxonomic classification, with important implications for surface mineralogical characterization. Finally, GAIA will be an ideal platform to detect near-Earth asteroids having orbits interior to that of the Earth.

A novel method for the preliminary identification of asteroids at discovery and a few days thereafter is being developed in Helsinki. Having two different sets of asteroid observations, the goal is to identify all possible pairs of objects between the sets. An arbitrary asteroid can either remain unidentified, or be preliminary linked to one or more asteroids. In the case of ambiguity, the final decision must usually be based on additional observations. We use a multistep approach, during which possible pairs of objects are first selected by comparing ephemerides that have been generated for three common epochs. The method has been successfully tested using both Very Large Telescope observations, and simulated observations of near-Earth and main-belt objects. Identification results of simulated observations indicate that the observing strategy promoted by the Minor Planet Center might not be the best one, at least for the purposes of identification. The ultimate goal is to produce a real-time asteroid identification tool for ESA's astrometric space observatory Gaia, the Lowell Observatory Near-Earth-Object Search, the Near-Earth Space Surveillance mission, and the Nordic Near-Earth Object Network. The tool could also benefit large-scale surveys done with the Large Synoptic Survey Telescope, and the Discovery Channel Telescope.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The observational evidence of a gradient in composition of the solid matter in the Solar System as a function of heliocentric distance, with more volatile elements increasingly abundant at larger heliocentric distances, is in general agreement with commonly accepted ideas about condensation of solid material in the early protoplanetary disk. In this respect, the gradual transition from regions mostly populated by S-type objects in the inner asteroid belt to the C-type dominated outer belt, and to the D-type dominated Trojan clouds has generally been interpreted as diagnostic of a zoning of the asteroid population in terms of mineralogic composition and thermal history. The situation, however, is not completely clear, and several contradictory facts still need an explanation. Another major problem concerns the origin of meteorites. Understanding the history of these bodies is critically important, since most of our ideas about the composition of the original protosolar nebula and protoplanetary matter come from meteoritic data. In this respect, very recent advances have been achieved in the knowledge of dynamical transport mechanisms. This can have important consequences for the characterization of plausible meteorite parent bodies.[PUBLICATION ABSTRACT]

Results of photometric observations of a small sample of selected Main Belt asteroids are presented. The obtained measurements can be used to achieve a better calibration of the asteroid photometric system \\((H, G_1, G_2)\\) adopted by the IAU, and to make comparisons with best-fit curves that can be obtained using different photometric systems. The new data have been obtained as a first feasibility study of a more extensive project planned for the future, aimed at obtaining a reliable calibration of possible relations between some parameters characterizing the phase-magnitude curves and the geometric albedo of asteroids. This has important potential applications to the analysis of asteroid photometric data obtained by the Gaia space mission.