Name: EU-ESA Workshop on Size Determination of Potentially Hazardous Near-Earth Objects
Start: 2024-11-11T14:00:00+01:00
End: 2024-11-13T18:00:00+01:00
Location: ESOC

Monday 11 November
- Welcome
  
  Welcome to the EU-ESA Workshop on size determination of potentially hazardous near-Earth objects
  - 1
    
    Welcome
    
    Welcome and introduction to the workshop
- Photometric observations
  - 2
    
    Scattering and absorption of light by near-Earth-object surfaces: photometric and polarimetric phase effects
    
    We consider scattering and absorption of light in planetary regoliths composed of sparsely or densely packed nonspherical particles. For the particles, we incorporate sizes and refractive indices and generate sample regolith geometries using varying packing algorithms for particles and their clusters. To ensure computational eUiciency, we make use of average elementary scattering and absorption properties of the particles. These can derive from experimental measurements and numerical computations.
    We consider radiative transfer and coherent backscattering (RT-CB) in discrete random media of particles, where the scattering phase matrix has the symmetry corresponding to an ensemble of nonspherical particles and their mirror particles, both in random orientation. Using the Cloude decomposition, we present the ensemble-averaged scattering phase matrices as a linear superposition of four pure Mueller matrices (Muinonen & Penttilä, JQSRT, 2024). The pure Mueller matrices enable RT-CB computations based on the assumption of independence of the four contributing components (Muinonen et al., JQSRT, in revision, 2024). We have validated the RT-CB decomposition method for both sparsely and densely packed random media of particles by using the Fast Superposition T-matrix Method (Markkanen & YuUa, JQSRT, 2017). To facilitate eUicient use of the RT-CB method, we have devised an empirical parameterization of the ensemble-averaged scattering matrix (publication in preparation).
    Fractional Brownian motion statistics (fBm) provide a realistic model for surface roughness, that is, the interface between the regolith and free space (e.g., Björn et al., PSJ, in revision, 2024). The fBm statistics are described by two parameters: the Hurst exponent related to the fractal dimension and describing the horizontal variegation and the amplitude describing the vertical variegation.
    Finally, we discuss the application of RT-CB to the photometric and polarimetric phase curves and to the spectral phase dependencies of near-Earth objects and airless Solar System objects at large.
    
    Speaker: Karri Muinonen (Department of Physics, University of Helsinki, Helsinki, Finland)
    
    01_01_Muinonen.pdf
  - 3
    
    NEO absolute magnitudes with H,G1,G2 photometric function
    
    The photometric observations of an asteroid can be used to derive the reduced magnitudes at different phase angles, V(α). A photometric function can be used to fit the magnitudes at different phase angles and to predict the behavior into the exact backscattering geometry at V(0), giving the absolute magnitude H of the object. In 2012 IAU adopted the H,G1,G2 photometric function which is an improvement to the H,G function, improving especially the behavior close to backscattering by allowing more variability in the phase-magnitude curve with small phase angles [1,2]. Since the absolute magnitude H can be used, with the asteroids’ geometric albedo p, to derive its size, the improved H estimates will produce improved size estimates.
    The improved flexibility in the photometric function comes not for free, and estimating three parameters instead of two will require more observations to reach the same accuracy in the estimated parameters. For this reason, also a two-parameter version of the system was developed. In this H,G12 function the typical relation between the parameters G1 and G2 has been modeled and tight together in one parameter, the G12. In [2], this simplification was taken even further and average G1,G2 parameters were derived for several taxonomic classes of asteroids, simplifying the system even more so that the estimate for H is directly computed from V(α) observations without further parameter estimation. Both the G12 function and the method of fixing G1,G2 if the target taxonomy is known can be compared to previous practice of setting G=0.15 in the H,G function.
    In this presentation, we will present the computational methods to estimate the absolute magnitude H for NEA observations using the H,G1,G2 function and its simplified forms.
    
    References:
    [1] Muinonen K, Belskaya IN, Cellino A, Delbò M, Levasseur-Regourd A-C, Penttilä A, and Tedesco EF (2010). A three-parameter magnitude phase function for asteroids. Icarus 209(2), 542–555. DOI: 10.1016/j.icarus.2010.04.003
    [2] Penttilä A, Shevchenko VG, Wilkman O, and Muinonen K (2024). H,G1,G2 photometric phase function extended to low-accuracy data. Planetary and Space Science 123, 117–125. DOI: 10.1016/j.pss.2015.08.010i
    
    Speakers: Antti Penttilä (Department of Physics, University of Helsinki, Finland), Karri Muinonen (Department of Physics, University of Helsinki, Finland)
    
    01_02_Penttila.pdf
  - 4
    
    Absolute magnitude determination from multi-filter all-sky photometric surveys
    
    The absolute magnitude H of asteroids is a fundamental property.
    It is a proxy to diameter, it is required to predict apparent magnitude, and it is the only way to measure colors whenever filters are not observed (near-)simultaneously.
    
    Major ephemerides computation centers like the Minor Planet Center (MPC), the Jet Propulsion Laboratory (JPL), the Asteroid Dynamical Site (AstDyS), and the ESA near-Earth Objects coordination center (NEOCC) provide the absolute magnitude of each asteroid, computed together with its osculating elements. These centers use the HG'' model by Bowell (1989) to determine the absolute magnitude from the evolution of photometry against the phase angle. This model has clear limitations at small and large phase angles and has been superseded by theHG1G2'' model by Muinonen et al. (2010).
    However, none of these models account for the constant changing geometry
    of asteroids, which shapes are not spherical. This has been reported by many authors,
    as the analysis of different apparitions of the same asteroid may not provide consistent
    absolute magnitudes. Jackson et al. (2022) even recently showed how this affects near-Earth asteroids over a single apparition.
    
    We have recently developed a new model for phase curves, dubbed sHG1G2'', and implemented it within the FINK broker of alerts designed for LSST (Moller et al. 2021). This new model accounts for the changing geometry without adding much complexity, hence without requiring large datasets. It provides are more robust determination of the absolute magnitude, together with the orientation and oblateness of the targets (Carry et al. 2024). After summarizing open issues in the current determination of absolute magnitudes, I will present thesHG1G2'' model and the results already achieved using ZTF photometry.
    
    Speaker: Benoit Carry (Lagrange, Observatoire de la Côte d'Azur)
    
    01_03_Carry.pdf
  - 5
    
    Phase curves and the taxonomy of asteroids
    
    Asteroid phase curves provide insights into their surface properties. In particular, the surge of brightness towards low phase angles (opposition effect) and the change in brightness with change in phase angle (photometric slope) are function of the surface composition and particle properties. This dependence opens up phase curves to taxonomic classification of asteroids, a process that has traditionally relied on spectroscopic data.
    
    In this presentation, I will introduce the fundamentals of asteroid taxonomy, outlining the major taxonomic classes and the criteria used to categorise them.
    I will then explore how phase curve analysis can be integrated into this classification system. Phase curves offer a complementary method to spectral data, akin to albedo observations, while providing an independent and more accessible means of determining asteroid types. This is particularly valuable for asteroids that are too faint to observe spectroscopically or where spectral data may be ambiguous.
    
    I will present recent efforts to model the phase curves of hundreds of thousands of asteroids and the challenges that we still face in order to extract reliable compositional information from these serendipitous observations.
    
    Speaker: Max Mahlke (Institut d'Astrophysique Spatiale)
    
    01_04_Mahlke.pdf
- 15:55
  
  Coffee break
- Photometric observations
  - 6
    
    ESA's PDO Telescope Network: High-Precision Astrometry for Physical Characterization
    
    ESA's Planetary Defence Office has traditionally focused most of its observational activities to astrometry, in order to provide high-precision measurements for the orbit determination and impact monitoring processes that form a significant component of our activities.
    Although most of our observations are still designed to optimize the astrometric output, during the last year we have increased our attention to other types of observations, including some planned specifically to physically characterize NEOs.
    In this talk we will briefly introduce our observational network, and highlight how some of the facilities we use can also be beneficial for physical characterization observations.
    We will then go into more detail on how our usual astrometric observations are performed, highlighting the peculiar observation modes that characterize them, such as the unfiltered and uncalibrated observing mode, the low SNR regime, and the techniques we use to extract positional measurements.
    At the same time, we will highlight how astrometrically-focused datasets can still contain valuable physical information which, with some care and under some caveats, can be used to extract valuable characterization information.
    Finally, we will briefly discuss how high-precision astrometric observations can indirectly provide physical characterization information, by accurately measuring dynamical properties (e.g. Yarkovsky effect or solar radiation pressure) that can be tied to physical properties of the object.
    
    Speakers: Marco Micheli (ESA PDO NEO Coordination Centre), Francisco Ocaña (ESA PDO / NEOCC)
    
    02_01_Micheli_Ocana.pdf
  - 7
    
    Estimation of the H magnitudes of near-Earth asteroids with time-resolved photometric observations at La Silla and Ondrejov Observatory
    
    We run a long-term project of time-resolved (lightcurve) photometric observations of near-Earth and main-belt asteroids. While our primary scientific interest is to determine other physical parameters of the studied objects (e.g., their spin rates and states, or binary nature), we also, as a by-product, obtain estimates of their absolute magnitudes. We run most of the observations at the 1.54-m Danish telescope on La Silla. The data are calibrated in the Johnson-Cousins VR photometric system using Landolt (1992) standards with the absolute accuracy 0.01 mag. The rotational brightness variations of the observed asteroids are removed by fitting Fouries series with their determined rotational periods, which results in determining their mean V and R magnitudes corresponding to the mean brightness. From the data, the mean (i.e., corresponding to the mean, “rotationally averaged” brightness) H = H_V or H_R of the studied asteroids are estimated. Uncertainties of the estimated absolute magnitudes are predominated by uncertainties of estimated or assumed (in cases where we do not have sufficient observational coverage in solar phase) slope parameters G and they range from ±0.02 to ±0.50 mag with a median of ±0.10 mag. Up to the date (2024-09-11) our sample is 544 H_V and 678 H_R values of NEAs and MBAs, covering a range of H from 8.52 to 30.24, with a median of 19.37. In the recent years, we determine on average 44 new H_V values per year, and we focus our recent efforts on sampling smaller NEAs with a median H around 21. As such, our data set is useful especially for characterizing the NEA population in the size range from 0.1 to 1 km, though the tail of the size distribution of our sample goes well below 100 m and so we obtain a limited characterization also for decameter-sized NEAs.
    
    Speaker: Petr Pravec (Astronomical Institute AS CR, Ondřejov, Czech Republic)
    
    02_02_Pravec.pdf
  - 8
    
    Phase curve fitting in the context of large-scale photometric surveys
    
    Nowadays, we are experiencing a revolution in astronomical surveys. Thanks to ground-based and orbiting telescopes, millions of observations of asteroids in various photometric filters are available. The main objective of our project is to develop tools for reading, processing, and analyzing large volumes of data. We’ve successfully determined phase curves for thousands of asteroids in orange and cyan filters from the latest ATLAS Solar System Catalog (V2) release, with errors less than 15%. Our simplified model, which only considers apparition effects, aligns well with more complex methods and needs far less computational power. Interestingly, our results showed smaller errors in G2 compared to G1, a trend also reported in other studies. This suggests that G2 is less sensitive to lack of data at small phase angles, as indicated by our simulations, which showed more stable values for G2 across different phase angle ranges. This connects to the H, G1, G2 function: G1 primarily captures the opposition effect, while G2 models the linear range. Our catalog-independent algorithms are adaptable to new datasets, including future LSST data, for which we already have preliminary results based on DP0.3 simulations.
    
    Speaker: Milagros Colazo (Institute Astronomical Observatory - Adam Mickiewicz University)
    
    02_03_Colazo.pdf
- Open discussion
Tuesday 12 November
- Photometric observations
  - 9
    
    Absolute magnitudes and photometry from the MPC catalog
    
    The Minor Planet Center (MPC) collects astrometric and photometric data on minor planets, with its catalog currently containing over 450 million data points for nearly 1.4 million objects. Photometric measurements are submitted in various bands and converted to the Johnson-Cousins V-band using band-specific conversion constants. The absolute magnitude H, a proxy for object size, is calculated using a least-squares fit based on the Bowell et al. (1989) equation. This fit incorporates known geocentric and heliocentric distances, phase angle, and converted V-band magnitudes, with a fixed slope parameter of 0.15. We will discuss the benefits of submitting data in ADES format, such as higher photometric precision and uncertainties, as well as future improvements, including the integration of Muinonen's HG1G2 system, fitting the slope parameter (G, G12), and deriving uncertainties for H and possible G. Additionally, we will address potential errors, data rejections due to significant magnitude offsets, objects that have no H derived due to no photometric data and the recent transition to using OrbFit for H calculations.
    
    Speaker: Peter Vereš (Minor Planet Center, Harvard - Smithsonian Center for Astrophysics)
    
    01_01_Veres.pdf
  - 10
    
    Systematic issues in the photometry of MPC observations leading to erroneous size estimates
    
    Most Potentially Hazardous and Near-Earth Objects are not observed by specific
    photometric follow-up telescopes. The only estimate on their physical parameters is
    therefore made based on the discovery observations and their follow-up submitted to the
    MPC. Those observations usually only include low-accuracy photometry.
    Recent studies show that these observations also contain systematic offsets. A debiasing
    of the photometry in MPC observations is able to reduce the photometric errors by more
    than 30%, resulting in better size estimate for asteroids. However, the underlying error
    sources should be analyzed by the individual observers. The methods to do a photometric
    calibration should be more prominent as well.
    The analysis opens new questions on the photometric observations of asteroids:
    - How should observer provide their photometry? Calibrated or raw data?
    - Should MPC/JPL/NEOCC implement corrections for (past) measurments?
    - Is the V-band a suitable reference band for optical observations (anymore)?
    - How can the confusion between ‘band’ and ‘filter’ be minimized?
    - Should there be campaigns focusing on the photometry of NEOs (comparable to
    the timing-accuracy ones)?
    Besides presenting the methods on the debiasing method and its results on photometric
    offsets, we will start a discussion concerning the posed questions in order to improve the
    current situation.
    
    Speaker: Tobias Hoffmann (European Space Agency)
    
    01_02_Hoffmann.pdf
  - 11
    
    A New Statistical Approach to Refining the H-G Model for Near-Earth Asteroids
    
    The NEOCC Aegis Orbit Determination and Impact Monitoring system currently uses the H-G model by Bowell et al. 1989 to determine the absolute magnitude H from photometry data. Typically, it is not possible to determine both the absolute magnitude H and the slope parameter G, therefore G is often fixed to a nominal value of 0.15. However, several studies have shown that the slope parameter may vary widely depending on the composition of the asteroid, and the value 0.15 may not be an accurate guess for the whole NEA population. In this talk, we present a new statistical approach to determine H, based on a population model of the slope parameter G. This also permits an extrapolation of uncertainties in the determination of H. Some preliminary results are shown, and a discussion on possible improvements will be raised.
    
    Speaker: Marco Fenucci (ESA NEOCC/PDO)
    
    01_03_Fenucci.pdf
- Open discussion
- 10:45
  
  Coffee break
- Thermophysical observations
  - 12
    
    NEOs in the thermal infrared
    
    Asteroids reflect only a small fraction of solar radiation, with the majority being emitted at thermal infrared (IR) wavelengths. This high infrared-to-visible flux ratio is especially advantageous for observing near-Earth objects (NEOs), which are often viewed at large phase angles and in close proximity to the Sun. Visible observations are hindered by straylight and rotational variations of the small, illuminated surface areas of irregularly shaped NEOs. In contrast, thermal IR observations present a different scenario:
    small, fast-rotating NEOs exhibit nearly isothermal surfaces with temperatures ranging from 300 to 400 K. Consequently, the likelihood of early detection is enhanced at IR wavelengths, even at large phase angles. Additionally, IR measurements provide valuable constraints on an object's size, albedo, and thermal characteristics indicative for the strengths of the surface material.
    
    This abstract highlights key aspects of NEO thermal IR emission measurements and their interpretation through thermophysical modeling techniques. Part of this work is based on studies conducted in the context of ESA's planned NEOMIR mission.
    
    Speaker: Thomas Müller (MPE)
    
    02_01_Mueller.pdf
  - 13
    
    Determination of Sizes and Other Physical Properties of Near-Earth Objects through Thermophysical Modeling: A Review
    
    The radiometric method has proven highly effective in determining the sizes and albedo of asteroids and near-Earth objects (NEOs). This technique involves measuring the heat flux from these objects in the thermal infrared using telescopes, and modeling these observations as a function of size and other physical parameters. The values of these parameters are constrained by achieving the best fit between the model and the observed data.
    While tremendous progress in our understanding of the NEO population has been achieved through the successful use of simple thermal models, these models are still based on the assumption that NEOs are spherical, and they treat surface temperature distribution using geometric approximations. However, knowledge of NEO shapes from independent techniques, such as optical lightcurve inversion and radar observations, enables the use of more sophisticated thermophysical models (TPMs).
    I will review the various flavors of TPMs currently available in the literature, as well as the latest results based on these TPMs, focusing on their accuracy in determining the sizes of NEOs compared to other techniques and also presenting ancillary physical properties TPM can be used to constrain.
    As technology and observational techniques advance, more data is becoming available. I will present a vision for the imminent era of large infrared and optical surveys, such as NASA’s NEO Surveyor and ESA’s NEO-MIR on the infrared side, and the LSST telescope at the Vera Rubin Observatory, ATLAS, and ESA’s Fly-Eye on the visible side. The clear path is towards combining these multi-epochs and multiwavelength data by means of holistic asteroid physical models (HPM).
    
    Speaker: Marco Delbo (Observatoire de la côte d'Azur)
    
    02_02_Delbo.pdf
- Polarimetric observations
  - 14
    
    The Calern Asteroid Polarimetric Survey version 2.0
    
    Polarimetric observations of NEOs are important for a number of reasons:
    • Determination of the geometric albedo and hence possible derivation of diameter
    • Determination of some surface regolith properties
    • For taxonomic classification purposes
    • Because it is useful to identify special classes of objects having anomalous compositions
    • Because it is useful to identify objects exhibiting cometary properties
    • Because it can be useful for the physical characterization of newly discovered near-Earth objects • Because it provides data to constrain the theories of light scattering from asteroid surfaces
    The above item list shows that polarimetry nicely complements the results coming from other observing techniques, in particular spectroscopy, and allows observers to infer important physical properties of NEOs.
    The Calern Asteroid Polarimetric Survey (CAPS) is a project started in 2018 in collaboration with Observatory of Torino (Italy). It has provided in five years a major database of asteroid polarimetric data. The limited magnitude was V= 14 and the observations were essentially made in V band.
    Since 2023 a new setup has been installed on a 1m in diameter telescope of Observatoire de la Cˆote d’Azur on Plateau de Calern. CAPS version 2.0 relies on a Finnish-UK-Italian-German-French collaboration and that allows us to equip the 1m Omicron telescope of the Calern observing station with DIPol-UF (Double Image Polarimeter-Ultra Fast), capable of high precision (10−5 % ) polarimetric observations simultaneously in three passbands (B,V,R).
    The instrument utilizes electron-multiplied EM CCD cameras for high efficiency and fast image readout. The key features of DIPol-UF are: (i) optical design with high throughput and inherent stability; (ii) great versatility which makes the instrument optimally suitable for observations of bright and faint targets; (iii) control system which allows using the polarimeter remotely.
    One full month over three of telescope time is dedicated to CAPS.
    First observations allows to reach V=16 magnitude asteroid in 60s exposure time with a SNR of about several tens opening new perspectives for NEO polarimetric studies
    
    Speaker: Philippe Bendjoya (Université de la côte d'Azur, Observatoire de la Côte d’Azur, CNRS UMR 7293, Laboratoire Lagrange, France)
    
    03_01_Bendjoya.pdf
- 12:15
  
  Lunch
- Thermophysical observations
  - 15
    
    From NEOWISE to NEO Surveyor: Recent Results and Future Plans
    
    The WISE/NEOWISE mission detected ~3000 near-Earth objects (NEOs) over the course its survey from late 2009 to July 2024. From these data, it is possible to obtain measurements of their physical properties, including effective spherical diameter and visible geometric albedo. Roughly 2000 objects in the sample were detected by the automated moving object pipeline, enabling a robust estimate of the size-frequency and orbital element distributions of the population. Here, we present the latest results from these data now that the NEOWISE mission has concluded its survey. NEOWISE has demonstrated the ability to discover and characterize NEOs in near-real time using a space-based telescope.
    
    These results inform the design of the next generation mission, the Near-Earth Object Surveyor. Unlike WISE/NEOWISE, NEO Surveyor is explicitly designed with the goal of discovering, cataloging, and characterizing the majority of potentially hazardous near-Earth asteroids large enough to cause severe regional damage if they were to impact the Earth. The mission consists of a 50-cm telescope operating at two infrared wavelengths, 4-5.2 and 6-10 um, and will survey from its vantage point at the Sun-Earth L1 Lagrange point. Launching in late 2027, NEO Surveyor should significantly improve our understanding of the small body populations in the inner solar system. We present the NEO Surveyor mission design and describe its expected performance for discovering and characterizing near-Earth asteroids.
    
    Speaker: Yoonyoung Kim (UCLA)
    
    04_01_Kim.pdf
  - 16
    
    NEO Surveyor: Survey cadence and expected initial knowledge of newly discovered NEOs
    
    The Near Earth Object Surveyor mission will survey the sky at infrared wavelengths in order to detect and discover ~100,000 NEOs, with sizes down to 25 m. The mission survey cadence is designed to provide sufficient self-followup to constrain the orbits and sizes of all detected NEOs. However, additional characterization of physical properties such as albedo and spectral taxonomy will require observations at reflected-light wavelengths. In this talk, we will discuss the expected results from the planned survey cadence, the orbital arcs and quality for NEOs that will be detected, and the expected visible-band brightnesses of these objects as determined by the mission's Survey Simulator. We also will discuss the ground- and space-based followup resources that would be most useful for these additional characterization observations.
    
    Speaker: Joseph Masiero (Caltech/IPAC)
    
    04_02_Masiero.pdf
  - 17
    
    NEOMIR: ESA’s space-based infrared mission for NEO detection and early warning
    
    Most current and planned NEO surveys are ground-based and carried out in the visible wavelength range. However, this approach has some limitations, such as (1) weather dependency, (2) that only a portion of the night sky is visible from any given location on Earth, (3) NEOs are difficult to detect at low galactic latitudes and (4) that visible-light surveys can only determine the motion and apparent magnitude of an object, but its physical properties (such as size) can only be inferred indirectly and therefore require additional observations for characterisation.
    
    A space-based mission working in the thermal infrared and placed at the first Sun-Earth Lagrange point would overcome most of these issues: by regularly scanning an area not easily accessible from ground or other space-based NEO surveys, it will be capable of detecting and characterising new NEOs and - in the worst case of an imminent impactor - serve as an early warning system.
    
    ESA is studying a NEO Mission in the Infra-Red (NEOMIR), designed with the aim of discovering the smaller NEO population, which can only be observed when the asteroids get closer to Earth. This is achieved by (1) pointing closer to the Sun and at all Ecliptic latitudes and (2) shortening exposure times and increasing cadence of revisit, ensuring that faster and therefore closer NEOs crossing the field of regard are not missed.
    
    We will present the mission and spacecraft design, the status of the project as well as initial results on expected detection capabilities.
    
    Speaker: Luca Conversi (European Space Agency)
    
    04_03_Conversi.pdf
- 14:30
  
  Coffee break
- Thermophysical observations
  - 18
    
    Thermophysical Modeling of (1566) Icarus from Ground-Based Infrared Observations
    
    The thermal emission from an asteroid is a consequence of its surface temperatures, and the object’s size can be directly estimated from observations in the infrared. Accurately estimating the surface temperature distribution, which depends on several factors, can improve the precision of these size measurements. Key factors include the asteroid’s shape, spin, and thermophysical properties such as thermal inertia and surface roughness. Thermophysical models (TPMs) calculate surface temperatures based on shape and spin parameters, incorporating subsurface heat conduction and small-scale topographic effects (i.e., roughness) that cause shadowing and self-heating effects that influence the surface temperatures. When the asteroid’s brightness is measured or estimated, its albedo can also be derived alongside its size from infrared observations. Additionally, depending on the data quality and observing conditions, the thermal inertia and surface roughness can be constrained to some degree.
    
    We present new thermal infrared observations of the potentially hazardous near-Earth asteroid (1566) Icarus, obtained using the MIRSI instrument at NASA's IRTF during a close approach to Earth in June 2024. The thermal emission of Icarus was too weak to be detected in single, background-subtracted MIRSI frames, but optically bright enough at visible wavelengths to be tracked with the MIRSI Optical Camera (MOC). By employing blind stacking of frames acquired over several hours on three separate nights, we were able to detect and measure its thermal emission at 10-microns. Using a TPM along with pre-existing shape and spin parameters, we estimate the asteroid’s size and surface thermophysical properties. We also obtained simultaneous absolute optical photometry, enabling us to estimate Icarus's albedo and update its shape model.
    
    Speaker: Eric M. MacLennan (University of Helsinki)
    
    05_01_MacLennan.pdf
  - 19
    
    Characterizing Small Recently Discovered NEOs: Diameter, Albedo, and Color Measurements (REMOTE)
    
    Rapidly estimating recently discovered hazardous asteroids' physical properties, such as rotation rate, surface composition, size, and albedo, provides a suitable practice for improving planetary defense techniques by acquiring these measurements hours or days after their discoveries. In this talk, we will present simultaneous optical and mid-infrared observations of recently discovered near-Earth Objects (NEOs) using the Mid-Infrared Spectrograph and Imager (MIRSI) and the MIRSI Optical Camera (MOC) integrated at the NASA Infrared Telescope Facility (IRTF). We will discuss the observability selection, diameter, and albedo estimations of recently discovered NEOs such as 2023 GM, 2023 SP1, 2023 DZ2, and 2024 CG2. We characterized these NEOs days or hours after their discovery confirmation. We will also discuss some of the limitations of performing a rapid-response characterization of these objects with ground-based facilities such as the NASA IRTF.
    
    Speaker: Andy Lopez-Oquendo (NASA Goddard)
    
    05_02_Lopez-Oquendo.pdf
  - 20
    
    Thermal infrared observations of near-Earth asteroids with the TAO 6.5-m telescope
    
    The Institute of Astronomy, School of Science, University of Tokyo, is promoting the TAO (The University of Tokyo Atacama Observatory) project to construct a 6.5-m telescope at an altitude of 5,640 m in Atacama, Chile. This will be the highest astronomical observatory on Earth, providing exceptionally clear skies for infrared observations at wavelengths up to 38 μm. Scientific observations are scheduled to begin in 2025. We plan to perform thermal infrared observations of near-Earth asteroids (NEAs) using the TAO 6.5-meter telescope. The primary objective of these observations is to determine the size and albedo of NEAs, which are essential parameters for understanding their physical properties. The high altitude of the observatory significantly reduces atmospheric interference, allowing for more accurate infrared measurements of NEAs, which will contribute to better assessments of their physical properties and potential hazards to Earth.
    
    Speaker: Jin Beniyama (Observatoire de la Côte d’Azur)
    
    05_03_Beniyama.pdf
- Open discussion
- 16:35
  
  Coffee break
- Polarimetric observations
  - 21
    
    Study of near-Earth asteroids by polarimetric technique (REMOTE)
    
    Polarimetry is an effective tool for remote sensing of asteroid surfaces, mainly for assessing their albedo and surface texture, and searching for surface peculiarities. The main advantage of the polarimetric method of albedo determination is that albedo can be derived directly from polarimetric measurements using simple empirical relationships between polarimetric parameters and albedos. Accurate albedo estimates require polarimetric observations at phase angles above 20-30 deg, which is easily achievable for near-Earth asteroids. Available polarimetric observations of near-Earth asteroids have shown that even a single measurement of the polarization degree at a large phase angle can be sufficient to obtain a reliable albedo and distinguish between low, moderate and high albedo asteroid types. This is of exceptional value for studying potentially hazardous asteroids. We will review the achievements and problems of using polarimetry for studying asteroid surfaces.
    
    Speaker: Irina Belskaya (Institute of Astronomy, V. N. Karazin Kharkiv National University, Sumska Str. 35, Kharkiv 61022, Ukraine)
    
    06_01_Belskaya.pdf
  - 22
    
    Capability of the FoReRo2 instrument and the 2m RCC Telescope at the Bulgarian National Astronomical Observatory Rozhen for polarimetric observations of small solar system bodies
    
    The Two-Channel-Focal-Reducer Rozhen (FoReRo2) was delivered to the Bulgarian National Astronomical Observatory (BNAO) Rozhen based on a contract between the Max-Planck Institute for Solar System Research and the Institute of Astronomy and National Astronomical Observatory (IA and NAO) in 2004. Since then, the FoReRo2 has been used at the f/8 Ritchey-Chrétien focus of the 2m Ritchey-Chrétien-Coudé telescope, where it reduces the original focal ratio to f/2.8, corresponding to an effective focal length of 5.6 m. The FoReRo2 is a multimode instrument offering a wide variety of observing capabilities: broadband and narrowband filter imaging; long slit low dispersion spectroscopy; Fabry-Pérot interferometry; imaging polarimetry; low dispersion spectropolarimetry. The two lenses of the blue and red channels are both colour-corrected in the range of 587-1014 nm and 365-436 nm, respectively.
    One important improvement of the polarimetric mode is a new Wollaston prism. It is placed before the colour divider, and it feeds both channels simultaneously. The split angle of 0.71 degrees guarantees separation without overlapping the ordinary and extraordinary stripes projected on the blue and red CCDs. Combining the Wollaston prism and the available Grism for spectroscopic observations opens the way for carrying out low-dispersion spectropolarimetry.
    In 2018, both channels of the FoReRo2 were equipped with new Andor CCD cameras iKon-L 9363, with the new dual antireflection deep depletion ‘BEX2-DD’ chip.
    In February 2014, in the framework of the agreement for collaborative research actions between Armagh Observatory (AO) and IA and NAO, AO delivered to BNAO a half-waveplate (HWP) for joint polarimetric observations. This allows us to easily apply the beam-swapping technique (BST) described by Bagnulo et al. 2009 which is used to minimise instrumental polarisation introduced by the optical components located between the HWP and the CCDs.
    We will present some results of small solar system bodies’ polarimetric observations to show the polarimetric capabilities of the FoReRo2 instrument, which we believe is invaluable for obtaining polarimetric observations of NEOs not only as one of a few instruments in the part of the world but also in general.
    
    Speaker: Galin Biserov Borisov (Institute of Astronomy and National Astronomical Observatory)
    
    06_02_Borisov.pdf
- Open discussion
Wednesday 13 November
- Polarimetric observations
  - 23
    
    Double Trouble: Polarimetric Insights into the Didymos- Dimorphos post-DART Evolution
    
    On September 26, 2022, the NASA DART (Double Asteroid Redirection Test) spacecraft
    struck Dimorphos, the moonlet of Didymos, to test near-Earth object deflection via impact for planetary defence [1]. As well as causing a change in Dimorphos’ orbital period [2], the impact caused a massive dust cloud to be ejected from the surface, e.g. [3,4]. By studying the characteristics and behaviour of the ejecta cloud, the DART mission offered a rare opportunity to peer under the surface of an asteroid and further our understanding of their global properties.
    Didymos-Dimorphos was monitored in polarimetric mode before and after the impact
    [5,6]. Post-impact measurements revealed a significant drop in polarisation, suggesting
    differences between the ejecta material and that on the original regolith surface.
    Remarkably, even months after the impact, the polarisation remained persistently lower than pre-impact, despite photometric measurements showing that the system’s brightness had returned to “normal” ~23 days post-impact [7]. This suggests the presence of residual ejecta material still within the system months after the impact, either in orbit or deposited on the asteroid surface(s). This highlights the sensitivity of polarimetric measurements, revealing
    details often unattainable from traditional observational techniques.
    In this presentation, we will be discussing these previous results, as well as presenting new polarimetric measurements obtained with VLT this year. In 2024, Didymos-Dimorphos is making another close approach to Earth, allowing us to study the system around two years after the DART impact. Our goal with these observations is to establish whether the polarisation of the system has returned to the pre-impact level or remains at the lower post-impact level and, thus, clarify if or how much ejecta material remains in the system. This may ultimately benefit the Hera team [8] for planning the spacecraft trajectory to avoid damage by impacts of dust particles.
    
    [1] Daly, R., et al. (2023), Natur, 616, 443; [2] Thomas, C. A., et al. (2023), Natur, 616, 448; [3] Li, J.-Y., et al. (2023), Natur, 616, 452; [4] Opitom, C., et al. (2023), A&A, 671, L11; [5] Bagnulo, S., et al. (2023), ApJL, 945, L38; [6] Gray, Z., et al. (2024), PSJ, 5, 18; [7] Graykowski, A., et al. (2024), Natur, 616, 452; Michel, P., et al. (2018), AdSpR, 62, 2261.
    
    Speaker: Zuri Gray
    
    01_01_Gray.pdf
  - 24
    
    Polarimetry as a tool for physical characterization of potentially hazardous NEOs
    
    The degree of linear polarization of sunlight scattered by an asteroid contains valuable information for rapid characterization of the surface properties of Near-Earth objects (NEOs). In the case of atmosphereless bodies the state of linear polarization varies as a function of the phase angle (α) and is described using the so-called Pr parameter.
    
    The properties of the phase-polarization curve of an asteroid are mostly defined by its albedo (pV). Numerous calibrations between polarization and pV have been proposed for main-belt asteroids [1, 2]. However, main-belt asteroids rarely exceed phase angle > 30° while near Earth object can be observed at phase angle as large as 100°. These observations at higher phase angles allow for deeper characterization of the observed object, but there is currently a lack of observations of NEOs in polarimetry to accurately calibrate the albedo-polarization relationship at high phase angles.
    
    In this presentation, I will discuss the current state of NEO observations in polarimetry and how polarimetry could be used to obtain reliable information on the geometric albedo of NEOs. With a proper calibration of the polarization-albedo relation, one could reduce the uncertainty on a newly discovered object by a factor of 10 with one single polarimetric observation obtained at a phase angle > 40°.
    
    [1] A. Cellino, R. Gil-Hutton, A. Dell’Oro, P. Bendjoya, M. Canada-Assandri, M. Di Martino, A new calibration of the albedo– ˜ polarization relation for the asteroids, Journal of Quantitative Spectroscopy and Radiative Transfer 113 (2012) 2552–2560.
    [2] A. Cellino, S. Bagnulo, R. Gil-Hutton, P. Tanga, M. Canada-Assandri, E. Tedesco, On the calibration of the relation between ˜ geometric albedo and polarimetric properties for the asteroids, Monthly Notices of the Royal Astronomical Society 451 (2015) 3473–3488.
    
    Speaker: Maxime Devogele (ESA NEOCC)
    
    01_02_Devogele.pdf
  - 25
    
    Cross validation of albedo determination for 1627 Ivar from three different techniques
    
    Near Earth Objects, with their perihelia < 1.3 AU, represent both a potential hazard and a valuable asset for the foreseeable in-space resource utilization, and are hence of great interest to the scientific community. Their physical parameters, such as diameter, albedo and thermal inertia, can be constrained by a number of techniques, with different types of input requirements and levels of uncertainty on the outcome. We are here presenting new results for the well-known Amor-class NEO 1627 Ivar. Thanks to an improved HV absolute magnitude value of 12.43, we compared results obtained from photometric measurements, thermophysical modelling and polarimetry data. Our fits for albedo, consistent with each other, provide a new value of 0.22, which is significantly higher than previous literature values. Our cross-referencing approach validates the result from polarimetry for rapid asteroid property characterization, a technique requiring significantly fewer observations than previously established ones. Future observations will expand this work to a larger sample size, thus increasing further the reliability of this method.
    
    Speaker: Elena Selmi
    
    01_03_Selmi.pdf
- 10:15
  
  Coffee break
- Polarimetric observations
  - 26
    
    Rapid-Defender: A New Polarimeter for Planetary Defense
    
    Polarization observations are a key tool for swiftly determining the size of a NEO, and consequently, their potential threat to Earth. The degree of linear polarization is inversely proportional to the albedo of the scattering surface of an asteroid. This relation is better constrained at high phase angles at which NEOs are usually observed and where polarization is more significant. This translates into low albedo objects consistently exhibiting higher degree of polarization compared to high albedo objects.
    
    Thus, polarimetry allows for direct albedo measurement without relying on additional data, such as the absolute magnitude. Also, polarimetry measurements are independent on the shape of the observed object, so we are not affected by the rotational phase at which the object is observed. Consequently, determining albedo through polarimetry serves as a crucial complementary and independent method to thermal modeling.
    
    In this talk, I will present the design of Rapid-Defender, a polarimeter specifically designed to rapidly characterize NEOs to assess their hazard to Earth. This instrument features a double Wollaston prism, enabling simultaneous measurement of two orthogonal light intensities and yielding the degree of polarization in a single observation. This allows for the estimation of an object's albedo and size within minutes. Additionally, a half-wave plate located before the double Wollaston prism would allow to self-calibrate our observations by swapping the ordinary and extra-ordinary beam by rotating the plane of polarization of the incoming light before the Wollaston prism. This new instrument would allow to reach V ~ 16 mag when located at 1 meter telescopes, leading to the observation of approximately 20 newly discovered NEOs per year.
    
    Speaker: Estela Fernández-Valenzuela (Florida Space Institute, UCF)
  - 27
    
    Asteroid polarization at near-infrared wavelengths
    
    The polarization of light reflected by asteroids can be used to constrain the albedo of the asteroid's surface, and probe mineralogical surface properties such as grain size and index of refraction. Depending on the specific mineral components of the body these surface properties are expected to change with wavelength, and so observations of the polarization beyond the visible provide powerful probes of the surface conditions. We present recent results from our Palomar WIRC+Pol survey of asteroid polarizations at
    near-infrared wavelengths, and discuss some of the unexpected wavelength-induced changes to the polarimetric-phase relationship. We also discuss the capacity for NIR polarimetry to support near-Earth object analysis.
    
    Speaker: Joseph Masiero (Caltech/IPAC)
    
    02_02_Masiero.pdf
- Open discussion
- 12:00
  
  Lunch
- Miscellaneous observations of NEOs
  - 28
    
    Stellar occultations for NEA high-accuracy astrometry and size determination
    
    The powerful method of stellar occultations is an unbeatable technique uniquely approaching, in some aspects, the performances of planetary space missions. It allows km-level accuracies on the determination of shapes and sizes of objects, and down to a couple of hundred meters for sum-km sized NEAs.
    Furthermore, it allows to derive, from ground using small aperture telescopes, asteroid positions at Gaia-level accuracy [1]. Although challenging, occultation by NEAs and in particular by sub-km NEAs are feasible under some conditions. They require an initial astrometric follow-up to predict reliable occultation events. They also require the deployment of mobile stations equipped with fast cameras across the prediction path. For most our targets, the maximum expected duration of the event is <0.5s. We present here the strengths and limitations of such an approach as well as preliminary results obtained, in particular in the case of (65803) Didymos to support both the DART (NASA) and Hera (ESA) planetary defence missions.
    
    ——————
    [1] Ferreira, J.F., Tanga, et al.: 2022, A&A 658, A73.
    
    Speaker: Damya Souami
    
    03_01_Souami.pdf
  - 29
    
    Asteroid size determination from photometric/spectroscopic observations at the IAC (REMOTE)
    
    Determining the size of an asteroid is scientifcally interesting, but also particularly relevant in the case of those objects that are considered as potential impactors: together with the distance to Earth, is one of the two criteria used to classify an asteroid as Potentially Hazardous. There are several ways to compute this physical property, some of them based on the acquisition of photometric and/or spectroscopic data at visible wavelengths. The most used one is time-series photometry (lightcurve) to compute asteroid’s H magnitude. Once the H magnitude is computed, spectroscopic/color observations provide asteroid’s taxonomy, i.e., its composition and consequently, its albedo. Albedo and H are used to infer diameter. The Observatorios de Canarias (OCAN), managed by the Instituto de Astrofísica de Canarias (IAC), include two observatories, El Roque de Los Muchachos (ORM), in the island of La Palma (MPC code 950) and El Teide (OT), in the island of Tenerife (MPC code 954). These two observatories host a very complete variety of telescopes and instruments (see Table 1), covering the wavelength range from the optical to the near-infrared (0.35 - 2.5 µm), and with sizes that go from the small Atlas-Teide telescope (0.6cm) to the large Gran Telescopio Canarias (GTC, 10.4m), the world’s largest optical to near-infrared telescope. The Solar System Group of the IAC has access to all these facilities through regular competitive calls that are open every semester. In this talk we present the current observational efforts of the group in the caracterization of asteroids, focusing in the determination of their sizes.
    
    Speaker: Julia De León (Instituto de Astrofísica de Canarias, Tenerife, Spain; (2) Departamento de Astrofísica, Universidad de La Laguna, Tenerife, Spain)
    
    03_02_deLeon.pdf
  - 30
    
    ESA’s Flyeye telescopes
    
    The European Space Agency’s Flyeye telescopes will play a crucial role in global efforts to detect and track near-Earth objects (NEOs). Inspired by the compound vision of a fly, the unique design of Flyeye-1 divides its field of view into 16 subfields, enabling the telescope to scan a wide area of the sky every night.
    This talk will explore the design and development of the Flyeye-1, focusing on the current progress of integration and validation tasks underway at the ASI facility in Matera, Italy.
    Additionally, the survey strategy, scheduling and provisions for data processing and photometry will be discussed in detail.
    
    Speakers: Dora Föhring (ESA Near-Earth Coordination Center), Dr Ernesto Doelling (ESA)
    
    03_03_Doelling_Fohring.pdf
  - 31
    
    Observations of NEOs with the TRAPPIST telescopes in support of size determination using polarimetry and radar.
    
    Photometric observations of near-Earth asteroids (NEAs) using the TRAPPIST telescopes are regularly performed notably to support shape modeling using radar data as well as an effort to calibrate the relation between the albedo and the polarization displayed by NEAs. These two methods allow to obtain accurate size determination for these objects.
    The TRAPPIST telescopes are two twin 60-cm robotic telescopes operated by the Liège University (Jehin+2011). TRAPPIST-North (Z53) is located at Oukaïmeden Observatory in Morocco, while TRAPPIST-South (I40) is based at the ESO La Silla Observatory in Chile. About 25% of their observation time is dedicated to asteroid observations.
    Photometry is a necessary complement to radar data, allowing for extended observation times and at a wider range of phase angles than in radar, which is of great importance for spin axis determination and shape modeling purposes. We are supporting shape modeling efforts using archive radar data from the Arecibo Observatory and/or observations from the Goldstone Observatory.
    The degree of linear polarization of the light scattered by an asteroid surface is dependent on its albedo at first order (Belskaya+2015). High albedo results in low polarization and vice versa. To calibrate this relationship for the first time for NEAs at high phase angles, polarimetric observations of NEAs are ongoing which will provide a strong tool to determine the albedo of a given NEA, thus its size, from a single polarimetric observation at phase angle > 40°. To refine or determine the albedo of our targets, we use photometric lightcurves to help determine the spin axis orientation, thus the size of the objects thanks to the radar data. Additionally, observations at low phase angles are used to construct phase curves that allow to refine the absolute magnitude H.
    To date, we have observed over 60 NEAs with available radar data from Arecibo and/or Goldstone and we possess polarimetric data for about 30 of them. Since 2017, we have acquired a dense lightcurve for each of these objects during one or several apparitions, complemented with color data using the B, V, R, and I Cousins filters. Finally, dedicated short observations at low solar phase angles can be acquired. Notable Examples of shape modeling using the TRAPPIST data include NEAs 2005 UD (Kueny+2023) and 1998 OR2 (Devogèle+2024).
    In this presentation, we will highlight the capabilities of the TRAPPIST telescopes, the current dataset of NEA observations, and planned observations of NEAs with existing radar data but remaining to be analyzed.
    Belskaya, I. et al. (2015) Asteroids IV, 151–163. Devogèle, M. et al. (2024) PSJ, 5, 44..
    Jehin, E. et al. (2011) The Messenger, 145, 2–6. Kueny, J. K. et al (2023) PSJ, 4, 56.
    
    Speaker: Elisabeth Petrescu (Space Sciences, Technologies & Astrophysics Research (STAR) Institute University of Liège Allée du 6 Août 19, 4000 Liège, Belgium)
    
    03_04_Petrescu.pdf
- 14:40
  
  Coffee break
- Open discussion
- Final discussions, key takeaway, findings

EU-ESA Workshop on Size Determination of Potentially Hazardous Near-Earth Objects

Rooms H.IV, H.V and H.VI

ESOC