SB1

Colors have been commonly used by researchers to assign taxonomic class to asteroids. Furthermore, the optical design of future space surveys should account for a large number of incidental asteroid observations that they will likely make. It is thus crucial to consider a use of an optimal filter setup for asteroid taxonomic classification. Following the work from Klimczak et al. 2021 which compared different machine learning algorithms (Naive Bayes, Logistic Regression, Support Vector Machine (SVM), Gradient-boosting, Multilayer Perceptrons) on two different parameter sets: principal component directions (PCA) and reflectance values spaced 0.05μm in the 0.45-2.5μm range and spectral slope, we extend this analysis to reflectance colors.  We aim to study the most efficient way to link colors to the Bus-DeMeo taxonomy and obtain a set of narrow band filters that should be used to correctly detect the taxonomic type of an object. 

The set of features used in this work consists of 35 reflectance colors that were selected across the wide range of spectroscopic measurements. All aforementioned machine learning methods are trained to predict taxonomic types on this set, and Feature Selection is performed to assess the importance of individual features and decrease the redundancy of the set. 

We find that Multilayer Perceptron and Support Vector Machines provide the best results on the whole feature set, with 85% balanced accuracy for taxonomic types and 93% for complexes. These results slightly outperform the results from our previous work. Furthermore, we find that satisfactory results can be obtained by reducing the feature set to top 5 features for taxonomic types (retaining 80% balanced accuracy), and top 3 features for complexes (retaining 89% balanced accuracy).

This work has been supported by grant No. 2017/25/B/ST9/00740 from the National Science Centre, Poland.

How to cite: Klimczak, H., Kotłowski, W., Oszkiewicz, D., DeMeo, F., Kryszczyńska, A., Wilawer, E., and Kwiatkowski, T.: Optimization of future multi-filter surveys towards asteroid characterisation, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-796, https://doi.org/10.5194/epsc2022-796, 2022.

Introduction. The NEO Rapid Observation, Characterization and Key Simulations (NEOROCKS) project is funded (2020-2023) through the H2020 European Commission programme to improve our knowledge on near-Earth objects by connecting expertise in performing small body astronomical observations and the related modelling needed to derive their dynamical and physical properties. The Instituto de Astrofísica de Canarias (IAC), and in particular members of the Solar System Group, participate in the NEOROCKS project and currently are devoted to one specific task: to collect observational data, mainly in the visible and near-infrared wavelength regions, of NEAs that have been observed in the past using the Arecibo Planetary Radar. In this work we present preliminary results, focusing on those targets for which the signal-to-noise ratio is satisfactorily high.

Observations. Our observations include spectroscopy, color photometry and lightcurves. They are performed using the facilities located at the Observatorios de Canarias (OOCC), including the El Teide Observatory in the island of Tenerife and the El Roque de los Muchachos Observatory in the island of La Palma. Visible and near-infrared spectra are mainly obtained using the 10.4-m Gran Telescopio de Canarias (GTC) and its visible (OSIRIS) and near-infrared (EMIR) spectrographs. We also use the ALFOSC spectrograph at the 2.5-m Nordic Optical Telescope (NOT). Visible color photometry is obtained using the MuSCAT2 instrument at the 1.5-m Telescopio Carlos Sánchez (TCS). The setup allows us to obtain simultaneous imaging in the g, r, i, and z_s visible bands. Time-series photometry in the visible is obtained using several telescopes, including the 46-cm TAR2, 80-cm IAC-80, and 1-m Jacobus Kapteyn Telescope (JKT).

Results. Spectra in the visible and/or the near-infrared wavelengths, as well as color photometry in the visible, allow us to taxonomically classify the targets and to infer their composition. In the case of having no albedo measurements for any given object, we can also use the taxonomy to have an estimation of the albedo based on the spectral class, and therefore determine the size of the asteroid. Lightcurves allow us to both get the asteroid rotational period and, together with radar data, to obtain the shape and the spin axis orientation of the target. In this way, a full characterization can be obtained for every asteroid observed within this program. So far, we have obtained spectra/colors/lightcurves in the visible for more than 100 NEAs. In this work, we present our most updated results.

Acknowledgements. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870403.

How to cite: Medeiros, H., de León, J., Licandro, J., Popescu, M., Morate, D., and Pinilla-Alonso, N. and the Arecibo Planetary Radar Team*, and the NEOROCKS Team**: NEOROCKS characterization programme of near-Earth asteroids previously observed with radar, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-570, https://doi.org/10.5194/epsc2022-570, 2022.

Primitive asteroids (PAs) are characterized by dark surfaces (albedo < 10%) dominated by carbon compounds. Their reflectance spectra are similar to those of carbonaceous chondrites (CCs), the most pristine meteorites in our records, abundant in hydrated minerals and organics. Studying these life-forming materials in PAs and CCs is important to answer how water and life appeared on Earth.

PAs present rather featureless spectra in visible and near-infrared wavelengths (from 0.5 to 2.5 microns). The most diagnostic and reliable region to study hydrated mineralogies and organics is the 3 microns region. However, observing at those wavelengths is extremely complicated using ground-based telescopes due to Earth's atmosphere, and so, the 3-microns feature can only be appropriately studied using space telescopes (e.g. AKARI). Another feature in visible wavelengths around 0.7 µm, thus accessible from the Earth, is related to the Fe2+ Fe3+ iron transition in hydrated mineralogies (Vilas 1994, Fornasier et al. 2014, Morate et al. 2016). However, this band is shallow. Hiroi et al (1998) have proposed a correlation between the 3 microns band and the UV absorption based on the meteorite spectra. 

In our work, we aim to explore the near UV (NUV hereafter) behavior of PAs and try to relate it with the main characteristics in the visible (0.7 micron band and slope). To accomplish this objective we have observed and explored spectral data for more than a hundred primitive asteroids with different taxonomies using the 3.58-m Telescopio Nazionale Galileo and the 2.54-m Isaac Newton Telescope located at the Roque de Los Muchachos Observatory. All the spectra go down to ~0.35 microns (near-UV or NUV). The ground-based reflectance spectroscopy in NUV needs special cautions such as airmass, and solar analogs (Tatsumi et al. accepted). In addition, we have explored the Hubble Space Telescope archive thanks to the Archival Research Visitor Program from ESA, finding also some primitive asteroids observed at UV. We also selected ~100 PAs from the MoOJA catalog (Morate et al. 2021), that have 5 filters between 0.35 and 0.55 microns and other 7 from 0.55 to 1 microns. This set of filters allows us to obtain information about how strong is the NUV absorption, to characterize the 0.7-micron band, and to compute several spectral slopes.

Results: we have found a correlation of 77% between the difference of spectral slopes between 0.4 - 0.55 microns and 0.55-0.7 microns (associated with the absorption in the UV) and the difference of spectral slopes between 0.55 - 0.7 microns and 0.7-0.9 microns (associated with iron transition at 0.7 microns), see Figure 1. Therefore, this drop in reflectance in the NUV can be used as a proxy for the phyllosilicates to measure the hydration degree of asteroids. Moreover, there are Fe-rich and Mg-rich phases among phyllosilicates, which reflect the amount of water present during their formation. On other hand, we have found a way to describe the beginning of the NUV drop, and among different taxonomies, there is a difference in the wavelength statistically significant, see Figure 2.

Gaia DR3 will provide thousands of low-resolution slit-less spectra of asteroids in the range of 0.35 - 0.90 microns before the meeting. This will constitute the largest dataset of asteroid spectra down to the NUV and we are going to also present how the thousands of asteroid spectra look like in our spectral slope space.

Figure 1. Slope change at 0.55 microns (computed as slope between 0.39 and 0.55 microns minus slope between 0.55 and 0.7) versus slope change at 0.7 microns (computed as slope between 0.55 and 0.7 minus slope between 0.7 and 0.9). The Pearson correlation coeficient between both variables is 0.77.

Figure 2. Histogram for the wavelength where the drop in reflectance downwards UV wavelengths begins for C, B, G and F taxonomies. We can see 2 main groups: one around 0.4 microns composed by F and B types and the other around 0.55 containing mainly C and G types.

REFERENCES

Fornasier, S., Lantz, C., Barucci, M. A., & Lazzarin, M. 2014, Icarus, 233, 163

Hiroi, T. and Zolensky, E.M.: 1998, Antarctic Meteorites XXIII; 23, 30.

Morate, D., de León, J., De Prá, M., et al. 2016, A&A, 586, A129

Tatsumi, E., Tinaut-Ruano, F., de León, J., et al. 2022, A&A, accepted

Vilas, F. 1994, Icarus, 111, 456

How to cite: Tinaut-Ruano, F., Tatsumi, E., de León, J., and Morate, D.: Exploring the near-UV for primitive asteroids using ground-based observations, space telescopes, a survey-like catalog, and following up with Gaia, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1049, https://doi.org/10.5194/epsc2022-1049, 2022.

Using multi-directional measurements of the Transiting Exoplanet Survey Satellite we have been able to perform analysis on small bodies in the Solar System. In order to validate a purely TESS-based shape and spin-axis reconstruction modelling we determined the spin axis of about forty objects, selected from our large light curve sample. We chose those targets that had been measured in at least three TESS sectors and showed an amplitude variation large enough to constraint the direction of the spin axis.  In our simple model the asteroid's shape is approximated with a triaxial ellipsoid, rotating around its shortest axis, with a fixed spin axis direction. Seen from different directions, the light curves and amplitudes would also be different, and we minimize the difference between the predicted and measured amplitudes to obtain the best fitting spin axis direction (lambda_p, beta_p) and shape parameters. Our spin axis solutions are compared to those in the DAMIT database which are based on multi-epoch, mainly ground-based observations. This benchmark study will indicate the capabilities in obtaining shape and spin axis solutions from large surveys. Extracting this information for several such asteroids we can obtain shape and spin axis direction distributions which will be used to infer the formation and dynamics of the asteroid belt using formation and collisional evolution models, for a significantly larger number of targets then before, and using high quality light curve data.  

Figure 1: Light curves of the asteroid (4717) Kaneko observed in six different sectors of TESS

Figure 2: χ² contours obtained from the comparision of observed and predicted light curve amplitudes for (4717) Kaneko. The lowest contours represent our best fitting spin axis solutions in ecliptic coordinates. Circles mark the spin axis solutions provided by DAMIT.

How to cite: Nóra, T., Pál, A., and Kiss, C.: Spin-axis and rotation of small bodies using multi-directional measurements of the Transiting Exoplanet Survey Satellite, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1225, https://doi.org/10.5194/epsc2022-1225, 2022.

Abstract

Until recently, only three large main belt asteroids, Ceres, Vesta and Lutetia, had been imaged with a high level of detail, as they were visited by the space missions Dawn and Rosetta of NASA and the European Space Agency, respectively. The previously small number of detailed observations of asteroids meant that, until now, key characteristics such as their 3D shape or density had remained largely unknown. Between 2017 and 2019, we have been filling this gap by conducting a high-angular-resolution imaging survey of 42 large main-belt asteroids with VLT/SPHERE (ESO large programme; PI: P. Vernazza; ID: 199.C-0074), sampling the main compositional classes. These observations have allowed to cast some light on the following fundamental questions:

– What is the diversity in shape among large asteroids and are the shapes close to equilibrium?
– How do large impacts affect asteroid shape?
– What is the bulk density of large asteroids and is there a relationship with their surface composition? Is there any evidence of differentiation among those bodies?
– Is the density of those bodies that are predicted to be implanted bodies from the outer Solar System (P/D-types) compatible with that of small (D ≤ 300 km) trans-Neptunian objects?
– What physical properties drive the formation of companions around large asteroids?

Importantly, our survey along with previous observations provides evidence in support of the possibility that some C-complex bodies could be intrinsically related to IDP-like P- and D-type asteroids, representing different layers of a same body (C: core; P/D: outer shell). We therefore propose that P/ D-types and some C-types may have the same origin in the primordial trans-Neptunian disk. The main belt would thus host a population of former TNOs much more important than the one previously considered, consisting solely of P and D-type bodies.

Here, we will present an overview of the results obtained from this survey ([1]) with a specific focus on the origin of a large fraction of the asteroid belt (typically C, P and D-type bodies).

References

[1] Vernazza, P., Ferrais, M., Jorda, L., et al. VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. A&A 654, 2021.

How to cite: Vernazza, P., Ferrais, M., Jorda, L., Hanus, J., Carry, B., Marsset, M., Brož, M., and Fetick, R. and the HARISSA team: VLT/SPHERE imaging survey of D>100 km asteroids: Final results and synthesis, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-103, https://doi.org/10.5194/epsc2022-103, 2022.

Introduction

Asteroid families generated by the collisional fragmentation of a common parent body have been identified using clustering methods of asteroids in their proper orbital element space (Broz & Morbidelli 2013; Tsirvoulis et al. 2018; Dermott et al. 2021). However, there is growing evidence that some of the real families are larger than the corresponding cluster of objects in orbital elements, as well as there are families that escaped identification from clustering methods (Milani et al. 2014). An alternative method has been developed by Bolin et al. (2017); Delbo et al. (2017), in order to identify collisional families from the correlation between the asteroid fragment sizes and their proper semimajor axis distance from the family center (V-shape). This method has been shown to be effective in the cases of the very diffused families that have formed Gyrs ago. Based on this method, a 4 Gyr-old (so-called primordial) collisional family of the inner main belt has been identified consisting of low-albedo asteroids (Delbo et al. 2017). The theory of asteroid family evolution predicts that there is an excess on retrograde asteroids in the inward side of the family’s V-shape. For this reason, photometric observations were performed in order to construct their rotational light curves and determine their shape and spin state.

Dataset and Observations

We combined data of asteroid lightcurves that we collected from the databases, sparse photometric data obtained from different surveys and existing shape models. Aiming to enlarge our input dataset used for the shape modelling, which would potentially lead to new and improved shape solutions, we performed additional ground-based photometric observations.

An international observing campaign has been initiated in the framework of our international initiative called Ancient Asteroids[1], aiming to collect dense photometric data for asteroids that belong to the oldest asteroid families (Athanasopoulos et al. 2021).

Method

The photometric datasets include both dense photometric data from ground-based facilities, as well as sparse data from several sky surveys and space missions. Appropriate analysis techniques were used for each type of dataset to extract the asteroid’s rotational light curve and use the convex inversion (CI) method developed by Kaasalainen & Torppa (2021); Kaasalainen et al. (2021). So far, the CI has been used to derive asteroid models for more than 3,460 asteroids that are stored in the DAMIT database.

Results

The spin state for 54 family members was determined by additionally using literature and sparse photometric data from ground and space observatories. Moreover, we measured rotation periods for 8 asteroids for the first time.

Combining new and literature data, we determined shapes and spin states for 54 asteroids that belong to the nominal population of the primordial family. This corresponds to 50% of the population in the sliver between the left-wing border of the Polana and the primordial family (see Fig. 1). Specifically, we calculated 23 new complete asteroid models, 16 revised and 8 new partial models, where 32 asteroids have retrograde rotation and 22 prograde.

Based on our analysis, we indicated 9 interlopers in the sample of 54 studied objects. From these 45 confirmed asteroid family members, 29 asteroids (65%) models have retrograde rotation and 16 prograde, including also the partial solutions.

Fig. 1: Panel A: The primordial family members are presented in proper semi-major axis vs. inverse diameter plane, along with the low albedo asteroids. Yellow diamond markers present members with known spin pole from the literature. Moreover, "plus", cross and square markers show the sources of dense lightcurves for these members. Panel B: The left side of the V-shape of the primordial family, where the red markers show the retrograde and blue markers the retrograde asteroids respectively.

Conclusion

We carried out a campaign of photometric observations of those asteroids that have been claimed to be members of one of the oldest collisional (primordial) families in the Solar System and we extract the lightcurves, spin state and shape for 45 members.

The statistical predominance of the retrograde spin poles is due to a physical process, as it was claimed by Delbo et al. 2017, namely formation as collisional fragments of a common parent body, a subsequent dynamical evolution driven by the Yarkovsky effect. The results of this research constitute corroborating evidence that the asteroids as members of a 4 Gry-old collisional family have a common origin, thus strengthening their family membership.

Acknowledgments

MD and CA acknowledge support from ANR “ORIGINS” (ANR-18-CE31-0014). This work is based on data provided by the Minor Planet Physical Properties Catalogue (MP3C) of the Observatoire de la Côte d’Azur. The research of JH has been supported by the Czech Science Foundation through grant 20-08218S. The work of OP has been supported by INTER-EXCELLENCE grant LTAUSA18093 from the Ministry of Education, Youth, and Sports. Support for T.W.-S.H. was provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51458.001-A awarded by the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555.

We thank the Las Cumbres Observatory and their staff for its continuing support of the ASAS-SN project, supported by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University and funded in part by the Alfred P. Sloan Foundation grant G-2021-14192 and NSF grant AST-1908570. Development of ASAS-SN has been supported by NSF grant AST-0908816, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics at the Ohio State University, the Chinese Academy of Sciences South America Center for Astronomy (CAS-SACA), the Villum Foundation, and George Skestos.

References

Athanasopoulos, D. et al. 2021, in European Planetary Science Congress Vol. 15, EPSC2021–335

Bolin, B. T. et al. 2017, Icarus, 282, 290

Brož, M. & Morbidelli, A. 2013, Icarus, 223, 844

Delbo’, M. et al. 2017, Science, 357, 1026

Dermott, S. F. et al. 2021, MNRAS, 505, 1917

Kaasalainen, M. & Torppa, J. 2001, Icarus, 153, 24

Kaasalainen, M. et al. 2001, Icarus, 153, 37

Milani, A. et al. 2014, Icarus, 239, 46

Tsirvoulis, G. et al. 2018, Icarus, 304, 14

How to cite: Athanasopoulos, D., Hanuš, J., Avdellidou, C., Bonamico, R., Delbo, M., Conjat, M., Ferrero, A., Gazeas, K., Rivet, J.-P., Sioulas, N., and Van Belle, G. and the Ancient Asteroids team: Asteroid spin-states of a 4 Gyr-old collisional family., Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-949, https://doi.org/10.5194/epsc2022-949, 2022.

The Hubble Space Telescope (HST) archives hide many unexpected treasures, such as trails of asteroids, showing a characteristic curvature due to the parallax induced by the orbital motion of the spacecraft. We have explored two decades of HST data for serendipitously observed asteroid trails with a deep learning algorithm on Google Cloud, called AutoML, trained on classifications from the Hubble Asteroid Hunter (www.asteroidhunter.org) citizen science project. 

The project was set up as a collaboration between the ESAC Science Data Centre, Zooniverse, and engineers at Google as a proof of concept to valorize the rich data in the ESA archives. I will present the first results from the project, finding 1,700 asteroid trails in the HST archives (Kruk et al., 2022). Their distribution on the sky is shown in Figure 1.

The majority of the asteroid trails (1,031) we found are faint (typically > 21 mag, see Figure 2) and do not match any entries in the Minor Planet Center database, thus likely  correspond to previously unidentified asteroids (see a few examples in Figure 3). We will argue that a combination of AI and crowdsourcing is an efficient way of exploring increasingly large datasets by taking full advantage of the intuition of the human brain and the processing power of machines. 

The second part of this project aims to analyze in detail these potentially new asteroids and use them to improve our current understanding of the size distribution of small-sized asteroids, and thus help constrain models of the evolution of our Solar System.

Taking into account Hubble’s motion around the Earth, the parallax effect can be computed to obtain the distance to the asteroids by fitting  simulated trajectories to the observed trails and obtaining the best fit (Evans et al. 1998). We show one example of a curve fit to the observed trail in Figure 4. Once we know the distance to the asteroids, we are able to obtain their absolute magnitudes and, combined with an assumed albedo, we can obtain their sizes. This method is also able to estimate an envelope for the asteroid's main orbital parameters. 

Given Hubble’s resolution and capability of reaching faint magnitudes, we expect that many of the new asteroids to be small-sized Main Belt asteroids (diameter <1 km), a population that is too faint and thus difficult to image from ground-based observatories. In addition, with the typical 30 min exposures of Hubble, some of these unknown objects show lightcurves that could be used to infer the rotation and shape of these asteroids, which is helpful for better assessing their type and origin.

This project may serve in the future as a “proof of concept” for an automated detection and analysis pipeline in large astronomical archives or surveys.

Figure 1: Distribution on the sky of the Solar System Objects (SSOs) identified in the HST images in Mollweide projection. The blue stars show the identified, known asteroids. The orange circles show the location of objects for which we did not find any associations with SSOs. The ecliptic is shown with red. The two gaps in this plot correspond to the Galactic plane, which was not observed by HST.

Figure 2: Distribution of apparent magnitudes for the SSOs identified in the HST images. The measured magnitudes for the identified objects (blue bars) and for the objects for which we did not find any associations with known SSOs (orange bars).