Search

Some active asteroids have been proposed to be the result of impact events. Because active asteroids are generally discovered serendipitously only after their tail formation, the process of the impact ejecta evolving into a tail has never been directly observed. NASA's Double Asteroid Redirection Test (DART) mission, apart from having successfully changed the orbital period of Dimorphos, demonstrated the activation process of an asteroid from an impact under precisely known impact conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope (HST) from impact time T+15 minutes to T+18.5 days at spatial resolutions of ~2.1 km per pixel. Our observations reveal a complex evolution of ejecta, which is first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and later by solar radiation pressure. The lowest-speed ejecta dispersed via a sustained tail that displayed a consistent morphology with previously observed asteroid tails thought to be produced by impact. The ejecta evolution following DART's controlled impact experiment thus provides a framework for understanding the fundamental mechanisms acting on asteroids disrupted by natural impact., accepted by Nature

Main Belt Comets (MBCs) exhibit sublimation-driven activity while occupying asteroid-like orbits in the Main Asteroid Belt. MBCs and candidates show stronger clustering of their longitudes of perihelion around 15{\deg} than other objects from the Outer Main Belt (OMB). This potential property of MBCs could facilitate the discovery of new candidates by observing objects in similar orbits. We acquired deep r-band images of 534 targeted asteroids using the INT/WFC between 2018 and 2020. Our sample is comprised of OMB objects observed near perihelion, with longitudes of perihelion between 0{\deg} and 30{\deg} and orbital parameters similar to knowns MBCs. Our pipeline applied activity detection methods to 319 of these objects to look for tails or comae, and we visually inspected the remaining asteroids. Our activity detection pipeline highlighted a faint anti-solar tail-like feature around 2001 NL19 (279870) observed on 2018 November 07, six months after perihelion. This is consistent with cometary activity but additional observations of this object will be needed during its next perihelion to investigate its potential MBC status. If it is active our survey yields a detection rate of $\sim$1:300, which is higher than previous similar surveys, supporting the idea of dynamical clustering of MBCs. If not, it is consistent with previously estimated abundance rates of MBCs in the OMB (, Comment: 13 pages, 9 figures, accepted for publication in MNRAS

The discovery of long period comet C/2014 UN271 (Bernardinelli-Bernstein) announced in June 2021 quickly suggested an inactive nucleus with an absolute magnitude of HV = 7.8 mag [1], which implied a diameter between 130 and 260 km assuming geometric albedos between 2% and 8%. Immediate follow-up observations with our Las Cumbres Observatory (LCO) Outbursting Objects Key project (LOOK) [2] as well as with SkyGems Namibia [3] revealed that the comet was active at 20.18 au. Evidence was quickly found that C/2014 UN271 had been active since 2018 and possibly even active at the time it was first observed in 2014 (at 29 au) [4,5]. After the discovery announcement, follow-up observations with ALMA and HST determined that Bernardinelli-Bernstein has a cometary albedo (0.033 ± 0.009) and an effective diameter of 137 ± 17 km, distinguishing it as the largest observed Oort-cloud comet [6,7]. Prior to the observations of C/2014 UN271, the most distant comet discoveries were C/2010 U3 and C/2017 K2, which were made between 15 and 20 au, but for which pre-discovery images indicate activity beyond 20 au [8,9]. C/2014 UN271 was significantly brighter than those comets at the same distance, which provided an exceptional opportunity to characterize its very distant comet activity close to 20 au. In this presentation we will report the results of our observing program with FORS2 on ESO’s 8-meter VLT in July and August 2021. The VLT/FORS data are interpreted in combination with targeted observations with the 4.1-m SOAR and the long-term monitoring campaign with 1-m facilities within the LOOK Project [10,11]. The multi-facility long-term photometric monitoring of C/2014 UN271 enabled our team to identify three outbursts between June and September 2021, indicating that the comet's optical brightness was dominated by cometary outbursts during the VLT observing run. Our VLT/FORS2 multi-band imaging and spectroscopic observations allowed us therefore to characterize the comet’s outburst in terms of spectral slope and coma morphology, including arc-like features. We will also present our efforts to characterize the comet’s short-term variability and rotation period. References: [1] https://minorplanetcenter.net/mpec/K21/K21M53.html [2] Kokotanekova, R., et al. (2021), ATel, 14733 [3] https://minorplanetcenter.net/mpec/K21/K21M83.html [4] https://minorplanetcenter.net/mpec/K21/K21M83.html [5] Farnham, T. (2021), ATel, 14759 [6] Lellouch, E. et al. (2022) A&,659, L1, 8 [7] Hui, M.-T., et al. (2022) ApJL, 929, 1, L12, 7 [8] Hui, M.-T., et al. (2019) AJ 157 [9] Jewitt, D. et al. (2021) AJ 161, 188 [10] Lister et al., submitted [11] Kelley et al., submitted.

Introduction The Asteroid Terrestrial Last-impact Alert System (ATLAS; Tonry et al., 2018) is a network of robotic telescopes which scan the sky searching for transient phenomena from potentially hazardous Near Earth Objects to distant supernovae. The units at Haleakala and Mauna Loa have been operating since 2015 and 2017 respectively and together they can scan the visible night sky every two nights down to a limiting magnitude of ~19.5 mag in wideband orange and cyan filters. We have matched the ephemerides of ~445,000 asteroids to ATLAS pointings from these two units up to January 2022, providing us with a photometric database of 9.6×107 detections. We can investigate the surface scattering properties of asteroids by observing the change in brightness as a function of the observer-asteroid-Sun phase angle. A well determined phase curve is required to obtain an accurate absolute magnitude for an asteroid, and hence its size if albedo is known. The form of the phase curve depends on the scattering properties of the surface, primarily its composition and roughness, and is described by one or more slope parameters. These slope parameters can be used as a proxy for taxonomic information when more detailed measurements such as spectra are unavailable (e.g. Mahlke et al., 2021). Methods We have analysed the phase curves of all asteroids in our dataset of selected ATLAS photometry, considering o and c filter data separately. We fit a variety of phase curve models to the data using Levenberg-Marquadt least squares fitting but we focused on the results for the HG (Bowell et al., 1989) and HG12* (Penttilä et al., 2016) phase curve models specifically. We found that outlying photometric detections were present in the dataset due to brightness contamination from nearby stars/galaxies or incorrect matching of ephemerides. Such data points were dealt with by initial cuts and an iterative sigma clip during fitting. For simplicity we did not account for rotational brightness variation in the phase curve model as for most asteroids this is not accurately constrained. Rotation effects manifest as an additional variation in brightness which leads to greater uncertainty in determining phase curve parameters. However, we show in our results that the fitted phase curve parameters of an ensemble of objects with sufficient phase angle coverage are still useful for science on a population statistics level. Furthermore, we automatically identify asteroids in the dataset that show a significant apparition effect. The phase curves of these objects display shifts in absolute brightness between different apparitions, caused by changes in the asteroid aspect angle as viewed from Earth. The resulting phase curve parameters were then filtered down to a subset of nearly 100,000 objects with high quality phase curves suitable for comparing the surface properties of different asteroid populations. Objects were selected that have sufficient phase angle coverage, high number of observations, low formal uncertainties from the fitting process, and no strong apparition effects. Results We have derived phase curve parameters for a large number of asteroids obtained in a consistent manner from a single survey over a long baseline and we find this dataset to be useful for a range of science cases. We shall present results comparing the distribution of phase parameters and surface colours measured in the ATLAS c - o filters of different asteroid populations. Considering the Jupiter Trojans, we can compare the properties of the leading and trailing dynamical groups and also assess trends in the “Red” and “Less-Red” compositional classes (Wong & Brown, 2016). Alongside this phase curve database we have generated a list of ~5000 Main Belt and Jupiter Trojan asteroids that exhibit strong apparition effects. Such objects must have high spin axis obliquity and would make interesting targets for studies of asteroid spin axis evolution and shape modelling via lightcurve inversion. Only 25% of the objects in our list are currently present in the Asteroid Lightcurve Database (LCDB; Warner et al., 2021). In general our work highlights the possibilities and potential limitations of sparsely sampled photometric surveys for investigating the properties of large numbers of asteroids. The upcoming Legacy Survey of Space and Time (LSST) will discover a huge number of new asteroids, however, characterisation of these objects will depend on survey design choices such as cadence and filter selection as well as follow up campaigns from other surveys/observers. Figure 1: ATLAS photometry for 766 Moguntia, an asteroid that shows clear apparition effects. Left: Phase curves for each apparition, fixing G = 0.15 and fitting only for H. Observations are colour-coded by apparition. Right: Absolute magnitude of Moguntia over time, where each observation has been corrected for distance and phase effects. Error bars indicate the fitted H and standard deviation of the residuals for each apparition, highlighting the shifts in brightness between apparitions and rotational brightness variations. Acknowledgements This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory (SAAO), and the Millennium Institute of Astrophysics (MAS), Chile. References Bowell, E., Hapke, B., Domingue, D., et al. 1989, Application of Photometric Models to Asteroids., 524–556 Mahlke, M., Carry, B., & Denneau, L. 2021, Icarus, 354, 114094 Penttilä, A., Shevchenko, V., Wilkman, O., & Muinonen, K. 2016, Planetary and Space Science, 123, 117 Tonry, J. L., Denneau, L., Heinze, A. N., et al. 2018, Publications of the Astronomical Society of the Pacific, 130, 064505 Warner, B. D., Harris, A. W., & Pravec, P. 2021, Asteroid Lightcurve Database (LCDB) Bundle V4.0 Wong, I. & Brown, M. E. 2016, The Astronomical Journal, 152, 90

1. Introduction On 26 September 2022 NASA's Double Asteroid Redirection Test (DART) spacecraft will impact Dimorphos, the satellite of asteroid (65803) Didymos. The impact will change Dimorphos' orbital period around Didymos. As Didymos is an eclipsing binary, and on a close flyby of Earth on this date, the period change can be detected by Earth-based observers. Before impact, DART will deploy the Light Italian Cubesat for Imaging of Asteroid (LICIACube) that will provide images of the first instants after impact. ESA’s Hera spacecraft will rendezvous with Didymos four years after the impact. It will perform the measurements necessary to fully understand the effect of the DART impact on Dimorphos, in particular by measuring its mass, and investigating its internal structure, and thus determining the momentum transfer and detailed characterization of the crater left by DART. 2. The Hera Mission Hera launch is planned for October 2024 and arrival at the Didymos system in early 2027. After arrival, it will release two cubesats, Juventas and Milani, and the three spacecraft will investigate the asteroid system for 6 months (see Figure 1). The mission is divided into several phases: The Early Characterisation Phase (6 weeks): Hera will stay around Didymos on hyperbolic arcs at a distance of 20-30 km. The goal is the determination of the global system properties (global shape of asteroids, gravity, dynamic properties). Payload Deposition Phase (4 weeks): Release and commissioning of the cubesats. The trajectories will be of the same type as during Early Characterisation Phase Detailed Characterisation Phase (4 weeks): Hera will orbit Disymos on hyperbolic arcs at distances of 8-20 km. Detailed investigations (mass of Dimorphos, subsurface characterization with radar). Close Observation Phase (6 weeks): Close flybys down to 4 km distance. Detailed characterization of surface properties and shape of DART impact crater. Experimental Phase (6 weeks): Close flybys down to 1 km distance from Dimorphos or less. High resolution observations of DART crater and other surface features. Landing of Cubesats on Dimorphos and of Hera on Didymos. The following payload will study the surface, interior, and environment of the asteroid pair: Two Asteroid Framing Cameras (AFCs, visible imaging and navigation) The Hyperscout-H multispectral imager (spectral imaging 670 - 975 nm) The Thermal InfraRed Imager (TIRI, 8 - 14 μm), contributed by JAXA The Planetary Altimeter (PALT, Laser altimeter) The Radio Science Experiment (RSE) The JUventas RAdar (JURA), a monostatic radar operating at 50-70 MHz The Gravimeter GRASS on Juventas, to measure surface gravity after landing on Dimorphos The Asteroid SPECTrometer (ASPECT), a Fabry-Perot visible and near-IR imaging spectrometer on Milani The VISTA Thermogravimeter (Dust detection and Composition) The Hera payload and cubesats have passed their Critical Design Reviews and are now being manufactured for delivery of the flight models in this year. Figure 1: Overview of the Hera mission at (65803) Didymos. 3. Planetary Defence return Hera will characterize in detail the properties of Dimorphos that are relevant for planetary defense. Its objectives related to the deflection demonstration are the following: Measuring the mass of Dimorphos to determine the momentum transfer efficiency from DART impact. Investigating in detail the crater produced by DART to improve our understanding of the cratering process and the mechanisms by which the crater formation drives the momentum transfer efficiency. Observing subtle dynamical effects (e.g. libration imposed by the impact, orbital and spinexcitation of Dimorphos) that are difficult to detect for remote observers. Characterising the surface and interior of Dimorphos to allow scaling of the momentum transfer efficiency to different asteroids. 4. Science return Even though its requirements are driven by planetary defence, Hera will also provide unique information on many current issues in asteroid science. The reason is that our knowledge of these fascinating objects is still poor, especially for the smallest ones. The recent data obtained by the JAXA Hayabusa2 and NASA OSIRIS-REx missions have revolutionized our understanding of carbonaceous Near-Earth Objects. Hera has the potential to perform similar as it will rendezvous for the first time with a binary asteroid. Dimorphos has a diameter of only 160 m. So far, no mission has visited such a small asteroid. Moreover, for the first time, internal and subsurface properties will be directly measured. From small asteroid internal and surface structures, through rubble-pile evolution, impact cratering physics, to the long-term effects of space weathering in the inner Solar System, Hera will have a major impact on many fields, providing answers to questions such as: How do binaries form? What does a 160 m-size rock in space look like? What is the surface composition? What are its internal properties? What are the surface structure and regolith mobility on both Didymos and Dimorphos? And what will be the size and the morphology of the crater left by DART? These questions will be addressed by Hera as a natural outcome of its planetary defence investigations5. ConclusionThe DART impact on Dimorphos and the accompanying observations by LICIACube and Earth-based observers, together with the detailed investigation of the Didymos system by Hera will be the first full scale asteroid deflection test of mankind.

The phase curve of a small Solar System object shows the change in the object’s reflectance at different Sun-object-observer (phase) angles, and the shape of the phase curve is determined by the physical properties of the object’s surface. Analysing phase curves of small Solar System objects such as Kuiper belt objects (KBOs), Centaurs, and inert Jupiter-family comets (JFCs) can also probe the composition and regolith structure of their surfaces, and by comparing these across populations of objects, can reveal possible evolutionary connections between them. The phase curve of an object can be significantly altered from its nominal shape if the object has undergone epochs of cometary activity during the time across which observations were taken. Furthermore, Centaur activity takes the form of outgassing in between long periods of quiescence, differing significantly from the activity exhibited by JFCs, which is more continuous and strongly coupled to heliocentric distance. As a result, Centaur activity may be difficult to identify from their phase curves if there are large time-intervals between observations. However, if observations are taken at sufficiently high cadence to isolate these outbursts, phase curves can be used to detect and analyse cometary activity out to distances where any coma may not be visible to Earth-based observations. This technique can help shed light on the mechanism responsible for the activity exhibited by Centaurs, which is presently not fully understood, and which is also an important step in understanding cometary evolution in the Solar System. Obtaining such high cadence observations over multiple apparitions for phase curve analysis is difficult to achieve due to limited availability of telescope observation time, the large baseline of observations required, and the challenges of ground-based observing (poor weather and the limited observability of targets throughout the year). To circumvent these difficulties, many previous studies of phase curves augment their datasets by collating photometry obtained from different sources (Alvarez-Candal et al., 2016; Ayala-Loera et al., 2018; Alvarez-Candal et al., 2019). These additional data are often obtained with heterogeneous methods of image processing and data analysis, which might introduce systematic errors in the resulting phase curve. The Asteroid Terrestrial-impact Last Alert System (ATLAS) survey (Tonry et al., 2018a,b) has accumulated serendipitous observations of several bright KBOs, Centaurs, and JFCs, having observed the visible sky over a 2-day cadence, to a limiting magnitude of ~19.5 in two wide-band filters since it first began observations in 2015. ATLAS offers an opportunity to study the phase curves of these objects populated with large datasets and thus use them to search for epochs of cometary activity. Using data from the Haleakalā and Mauna Loa telescopes of the ATLAS 4-telescope survey, we analyse the phase curves of the bright KBOs, Centaurs, and JFCs visible to ATLAS with datasets large enough that we do not need to include additional data from potentially heterogeneous sources, thereby reducing potential systematic errors. Our sample of 18 bright KBOs, JFCs and Centaurs visible to ATLAS span semimajor axes of 5.7 au ≤ a ≤ 67.9 au, absolute magnitudes of −1.1 < HV < 13.3. Each object has on average 181 (c-filter) and 463 (o-filter) data points, exceeding the values of previous studies (Rabinowitz et al., 2007; Schaefer et al., 2009; Alvarez-Candal et al., 2016; Ayala-Loera et al., 2018; Alvarez-Candal et al., 2019), and which sample most of the phase angle range observable from Earth. Figure 1: From Dobson et al. (2021), ATLAS Phase Curves of (2060) Chiron in c-filter (upper plot) and o-filter (lower plot), with data from most recent apparition indicated as black triangles. Black lines represent linear best fits to data. We present our findings on using ATLAS phase curves with their advantage of unprecedentedly large datasets to search for instances of cometary activity exhibited by our sample of KBOs, JFCs and Centaurs since 2015. Notably, we discover a recent brightening of the Centaur (2060) Chiron (Dobson et al. 2021), as seen in Figure 1, which despite the lack of detected coma, is indicative of either an outburst or enhanced activity. Acknowledgements We made use of the ATLAS Forced Photometry Server to obtain our Centaur and KBO photometry. https://fallingstar-data.com/forcedphot/ This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (AT- LAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory (SAAO), and the Millennium Institute of Astrophysics (MAS), Chile. References Alvarez-Candal, A., Ayala-Loera, C., Gil-Hutton, R., et al. 2019, MNRAS, 488, 3035, doi: http://doi.org/10.1093/mnras/stz188010.1093/mnras/stz1880 Alvarez-Candal, A., Pinilla-Alonso, N., Ortiz, J. L., et al. 2016, A&A, 586, A155, doi: http://doi.org/10.1051/0004-6361/20152716110.1051/0004-6361/201527161 Ayala-Loera, C., Alvarez-Candal, A., Ortiz, J. L., et al. 2018, MNRAS, 481, 1848, doi: http://doi.org/10.1093/mnras/sty236310.1093/mnras/sty2363 Dobson, M. M., Schwamb, M. E., Fitzsimmons, A., et al. 2021, Research Notes of the AAS, 5, 9, doi: http://doi.org/10.3847/2515-5172/ac26c9 Rabinowitz, D. L., Schaefer, B. E., & Tourtellotte, S. W. 2007, AJ, 133, 26, doi: http://doi.org/10.1086/50893110.1086/508931 Schaefer, B. E., Rabinowitz, D. L., & Tourtellotte, S. W. 2009, AJ, 137, 129, doi: http://doi.org/10.1088/0004-6256/137/1/12910.1088/0004-6256/137/1/129 Tonry, J. L., Denneau, L., Heinze, A. N., et al. 2018a, , 130, 064505, doi: http://doi.org/10.1088/1538-3873/aabadf10.1088/1538-3873/aabadf Tonry, J. L., Denneau, L., Flewelling, H., et al. 2018b, ApJ, 867, 105, doi: http://doi.org/10.3847/1538-4357/aae38610.3847/1538-4357/aae386

Funding Information: To achieve these objectives, Milani is carrying two scientific payloads, the ASPECT visual and near-infrared (Vis-NIR) imaging spectrometer and the VISTA thermogravimeter aimed at collecting and characterizing volatiles and dust particles below 10 μm. Additionally, navigation payloads include a visible navigation camera and lidar. The Milani consortium is composed of entities and institutions from Italy, the Czech Republic, and Finland. The consortium Prime is Tyvak International, responsible for the whole program management and platform design, development, integration, testing, and final delivery to the customer. Politecnico di Torino is tasked with defining requirements and performing thermal, radiation, and debris analysis. Politecnico di Milano is responsible for mission analysis and GNC. Altec will support the Ground Segment architecture and interface definition. Centro Italiano per la Ricerca Aerospaziale (CIRA) is responsible for the execution of the vehicle environmental campaign. HULD contributes to developing the mission-specific software. VTT is the main payload (ASPECT hyperspectral imager) provider and is supported by the following entities dealing with ASPECT-related development: University of Helsinki (ASPECT calibration); Reaktor Space Lab (ASPECT Data Processing Unit development), Institute of Geology of the Czech Academy of Sciences (ASPECT scientific algorithms requirements and testing); and Brno University of Technology (ASPECT scientific algorithms development). INAF-IAPS is the secondary Payload (VISTA, dust detector) provider. Funding Information: The Mission PI is appointed by ESA and is the primary interface to ESA. The Hera SMB consists of the ESA Hera Project Scientist (ESA PS), the Mission PI, and the Hera Advisory Board, consisting of four mission advisors. The Mission PI chairs the HIT and is supported by the Hera Advisory Board. The tasks of the Hera SMB are 1. advising the Hera mission project team on all aspects related to the Hera mission objectives; 2. ensuring that the WGs’ activities cover the needs of the Hera mission; 3. providing recommendations to ESA concerning the membership in the HIT; and 4. implementing the Publication Policy. Funding Information: Hera is the ESA contribution to the AIDA collaboration. Hera, Juventas, Milani, and their instruments are developed under ESA contract supported by national agencies. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 870377 (project NEO-MAPP), the CNRS through the MITI interdisciplinary programs, ASI, CNES, JAXA, the Academy of Finland project no. 335595, and was conducted with institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. M.L., E.P., P.T .and E.D. are grateful to the Italian Space Agency (ASI) for financial support through Agreement No. 2022-8-HH.0 in the context of ESA’s Hera mission. We are grateful to the whole Hera team, including Working Group core members and other contributors for their continuous efforts and support. Their names can be found on the following website: https:// www.heramission.space/team. Publisher Copyright: © 2022. The Author(s). Published by the American Astronomical Society. Hera is a planetary defense mission under development in the Space Safety and Security Program of the European Space Agency for launch in 2024 October. It will rendezvous in late 2026 December with the binary asteroid (65803) Didymos and in particular its moon, Dimorphos, which will be impacted by NASA’s DART spacecraft on 2022 September 26 as the first asteroid deflection test. The main goals of Hera are the detailed characterization of the physical properties of Didymos and Dimorphos and of the crater made by the DART mission, as well as measurement of the momentum transfer efficiency resulting from DART’s impact. The data from the Hera spacecraft and its two CubeSats will also provide significant insights into asteroid science and the evolutionary history of our solar system. Hera will perform the first rendezvous with a binary asteroid and provide new measurements, such as radar sounding of an asteroid interior, which will allow models in planetary science to be tested. Hera will thus provide a crucial element in the global effort to avert future asteroid impacts at the same time as providing world-leading science.

Between 2010 and 2017 we have collected new optical and radar observations of the potentially hazardous asteroid (2102)~Tantalus from the ESO NTT and Danish telescopes at the La Silla Observatory and from the Arecibo planetary radar. The object appears to be nearly spherical, showing a low amplitude light-curve variation and limited large-scale features in the radar images. The spin-state is difficult to constrain with the available data; including a certain light-curve subset significantly changes the spin-state estimates, and the uncertainties on period determination are significant. Constraining any change in rotation rate was not possible, despite decades of observations. The convex lightcurve-inversion model, with rotational pole at ${\lambda}=210{\pm}41${\deg} and ${\beta}=-30{\pm}35${\deg}, is more flattened than the two models reconstructed by including radar observations: with prograde (${\lambda}=36{\pm}23${\deg}, ${\beta}=30{\pm}15${\deg}), and with retrograde rotation mode (${\lambda}=180{\pm}24${\deg}, ${\beta}=-30{\pm}16${\deg}). Using data from WISE we were able to determine that the prograde model produces the best agreement in size determination between radar and thermophysical modelling. Radar measurements indicate possible variation in surface properties, suggesting one side might have lower radar albedo and be rougher at centimetre-to-decimetre scale than the other. However, further observations are needed to confirm this. Thermophysical analysis indicates a surface covered in fine-grained regolith, consistent with radar albedo and polarisation ratio measurements. Finally, geophysical investigation of the spin-stability of Tantalus shows that it could be exceeding its critical spin-rate via cohesive forces., Comment: Main paper: 14 pages and 12 figure, Appendix: 13 pages and 12 figues

The proposed CUBES spectrograph for ESO's Very Large Telescope will be an exceptionally powerful instrument for the study of comets. The gas coma of a comet contains a large number of emission features in the near-UV range covered by CUBES (305-400 nm), which are diagnostic of the composition of the ices in its nucleus and the chemistry in the coma. Production rates and relative ratios between different species reveal how much ice is present and inform models of the conditions in the early solar system. In particular, CUBES will lead to advances in detection of water from very faint comets, revealing how much ice may be hidden in the main asteroid belt, and in measuring isotopic and molecular composition ratios in a much wider range of comets than currently possible, provide constraints on their formation temperatures. CUBES will also be sensitive to emissions from gaseous metals (e.g., FeI and NiI), which have recently been identified in comets and offer an entirely new area of investigation to understand these enigmatic objects., Accepted for publication in Experimental Astronomy

In the era of Extremely Large Telescopes, the current generation of 8-10m facilities are likely to remain competitive at ground-UV wavelengths for the foreseeable future. The Cassegrain U-Band Efficient Spectrograph (CUBES) has been designed to provide high-efficiency (>40%) observations in the near UV (305-400 nm requirement, 300-420 nm goal) at a spectral resolving power of R>20,000 (with a lower-resolution, sky-limited mode of R ~ 7,000). With the design focusing on maximizing the instrument throughput (ensuring a Signal to Noise Ratio (SNR) ~20 per high-resolution element at 313 nm for U ~18.5 mag objects in 1h of observations), it will offer new possibilities in many fields of astrophysics, providing access to key lines of stellar spectra: a tremendous diversity of iron-peak and heavy elements, lighter elements (in particular Beryllium) and light-element molecules (CO, CN, OH), as well as Balmer lines and the Balmer jump (particularly important for young stellar objects). The UV range is also critical in extragalactic studies: the circumgalactic medium of distant galaxies, the contribution of different types of sources to the cosmic UV background, the measurement of H2 and primordial Deuterium in a regime of relatively transparent intergalactic medium, and follow-up of explosive transients. The CUBES project completed a Phase A conceptual design in June 2021 and has now entered the detailed design and construction phase. First science operations are planned for 2028., SPIE proceedings, SPIE Astronomical Telescopes + Instrumentation 2022, Montr\'eal, Canada; 20 pages, 13 figures, 2 tables

The YORP effect is a small thermal-radiation torque experienced by small asteroids, and is considered to be crucial in their physical and dynamical evolution. It is important to understand this effect by providing measurements of YORP for a range of asteroid types to facilitate the development of a theoretical framework. We are conducting a long-term observational study on a selection of near-Earth asteroids to support this. We focus here on (68346) 2001 KZ66, for which we obtained both optical and radar observations spanning a decade. This allowed us to perform a comprehensive analysis of the asteroid's rotational evolution. Furthermore, radar observations from the Arecibo Observatory enabled us to generate a detailed shape model. We determined that (68346) is a retrograde rotator with its pole near the southern ecliptic pole, within a $ 15^\circ$ radius of longitude $ 170^\circ$ and latitude $ -85^\circ$. By combining our radar-derived shape model with the optical light curves we developed a refined solution to fit all available data, which required a YORP strength of $ (8.43\pm0.69)\times10^{-8} \rm~rad ~day^{-2} $. (68346) has a distinct bifurcated shape comprising a large ellipsoidal component joined by a sharp neckline to a smaller non-ellipsoidal component. This object likely formed from either the gentle merging of a binary system, or from the deformation of a rubble pile due to YORP spin-up. The shape exists in a stable configuration close to its minimum in topographic variation, where regolith is unlikely to migrate from areas of higher potential., To be published in the Monthly Notices of the Royal Astronomical Society (MNRAS). 19 pages, 14 figures

INTRODUCTION The Asteroid Terrestrial Last-impact Alert System (ATLAS) currently consists of two robotic telescopes in Hawaii. Together they scan the entire visible night sky every two nights in wideband orange and cyan filters down to ~19.5 mag. In addition to rapid discovery and follow up of Near-Earth Objects, ATLAS serendipitously observes a large number of asteroids and other moving objects. Since operations began in 2015 ATLAS has collected a substantial photometric database of ~7x107 observations for ~430,000 asteroids. Asteroids are valuable compositional and dynamical tracers that can be used to constrain the processes of Solar System evolution. The observed size distribution of an asteroid population can inform us of its primordial formation and subsequent collisional evolution. Composition of a population can indicate a common origin; such as a protoplanetary disk formation region or a relation through asteroid collisional families. We aim to utilise the ATLAS database to investigate the physical properties of a large number of asteroids, by fitting phase curve models to the photometric observations. The form of the phase curve depends on the physical properties of the asteroid and is parameterised by a variety of models which are each described by absolute magnitude and slope parameters. Absolute magnitude, H, is related to the size and albedo of the body. The form of the phase curve is determined by the slope parameter in the phase function and depends on the surface composition and structure. Hence these parameters may be used as proxies for the properties of asteroids when more detailed information is unavailable. METHODS An analysis of an earlier dataset has been performed using MCMC techniques by Mahlke et al. (2021), providing phase curves in the H, G1, G2 and H, G12* systems (Muinonen et al. 2010, Penttilä et al. 2016). In this work we fit both these systems plus the original H, G system (Bowell et al., 1989) to all asteroids in the ATLAS dataset, using Levenberg-Marquardt least squares fitting. We perform a separate fit in both the o and c ATLAS filters. We have investigated various methods of outlier rejection, and use techniques such as initial data cuts, iterative sigma clipping, and avoidance of low galactic latitudes. These have considerably improved the quality of the dataset. However, in some cases the photometric observations may not be well described by the phase curve model due to physical phenomena such as large amplitude rotational variation, or changing aspect angle for asteroids with high spin obliquities. We have created a dataset of phase curve parameters for ~370,000 asteroids, and identify a subset of high quality phase curves where the phase curve model describes the data well and absolute magnitude and slope have low formal uncertainties. We analyse in detail the distribution of these phase curve parameters and verify them with comparison to previous work and catalogue values. RESULTS In general we find good agreement between our phase curve results and previously published studies, especially for well observed bright asteroids. We identify metrics that indicate the ability to obtain a good fit to the phase function, such as number of observations and phase angle coverage. Furthermore, we demonstrate the scientific value of our database by comparing the properties of the leading and trailing Jupiter Trojans observed with ATLAS. The ability to determine phase curves for large numbers of asteroids, and studying the statistics of their properties, highlights the potential value of sparse photometric survey data for asteroid science. This is especially relevant to the upcoming Legacy Survey of Space and Time (LSST) which will perform photometry of asteroids at a similar cadence to ATLAS. FIGURES Figure 1. The semimajor axis distribution of asteroids in the ATLAS database between 1.5 and 5.5 AU. ATLAS samples a wide range of populations; including the main belt, Hildas and Jupiter Trojans. Figure 2. The distribution of detections for asteroids observed by ATLAS in the o and c filters. Figure 3. Proper orbital elements (semimajor axis versus inclination) of selected asteroids from our database of phase curve fits. Asteroid families appear as distinct clusters with similar surface scattering properties as measured by the phase curve parameter G12*. ACKNOWLEDGEMENTS This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory (SAAO), and the Millennium Institute of Astrophysics (MAS), Chile. JR acknowledges the support of STFC Consolidated Grant ST/P000304/1. REFERENCES Bowell, E., Hapke, B., Domingue, D., et al. 1989, in Asteroids II, ed. R. P. Binzel, T. Gehrels, & M. S. Matthews, 524–556 Mahlke, M., Carry, B., & Denneau, L. 2021, Icarus, 354, 114094, doi: 10.1016/j.icarus.2020.114094 Muinonen, K., Belskaya, I. N., Cellino, A., et al. 2010, Icarus, 209, 542, doi: 10.1016/j.icarus.2010.04.003 Penttilä, A., Shevchenko, V. G., Wilkman, O., & Muinonen, K. 2016, Planet. Space Sci., 123, 117, doi: 10.1016/j.pss.2015.08.010

The Hera mission is in development for launch in 2024 within the ESA Space Safety Program. Hera will contribute to the first deflection test of an asteroid, in the framework of the international NASA- and ESA-supported Asteroid Impact and Deflection Assessment (AIDA) collaboration. Hera will also offer a great science return. 1. IntroductionThe impact of the NASA DART spacecraft on the 160 m-diameter natural satellite called Dimorphos of the binary asteroid 65803 Didymos in late September 2022 will change its orbital period around Didymos. As Didymos is an eclipsing binary, and close to the Earth on this date, the change can be detected by Earth-based observers. Before impact, DART will deploy the Italian LICIACube that will provide images of the first instants after impact. ESA’s Hera spacecraft will rendezvous Didymos four years after the impact. It will perform the measurements necessary to understand the effect of the DART impact on Dimorphos, in particular its mass, its internal structure, the direct determination of the momentum transfer and the detailed characterization of the crater left by DART. 2. Planetary Defense returnHera will characterize in details the properties of a Near-Earth Asteroid that are fully relevant to planetary defense. Its objectives related to the deflection demonstration are the following:•Measuring the mass of Dimorphos to determine the momentum transfer efficiency from DART impact.•Investigating in detail the crater produced by DART to improve our understanding of the cratering process and the mechanisms by which the crater formation drives the momentum transfer efficiency. •Observing subtle dynamical effects (e.g. libration imposed by the impact, orbital and spin excitation of Dimorphos) that are difficult to detect for remote observers. •Characterising the surface and interior of Dimorphos to allow scaling of the momentum transfer efficiency to different asteroids. 3. Science returnEven if its requirements are driven by planetary defense, Hera will also provide unique information on many current issues in asteroid science. The reason is that our knowledge of these fascinating objects is still poor, especially for the smallest ones. The recent data obtained by the JAXA Hayabusa2 and NASA OSIRIS-REx missions have revolutionized our understanding of carbonaceous-type Near-Earth Objects. Hera has the the potential to do similar as it will rendezvous for the first time with a binary asteroid. Its secondary has a diameter of only 160 m in diameter. So far, no mission has visited such a small asteroid. Moreover, for the first time, internal and subsurface properties will be directly measured. From small asteroid internal and surface structures, through rubble-pile evolution, impact cratering physics, to the long-term effects of space weathering in the inner Solar System, Hera will have a major impact on many fields. How do binaries form? What does a 160 m-size rock in space look like? What is the surface composition? What are its internal properties? What are the surface structure and regolith mobility on both Didymos and Dimorphos? And what will be the size and the morphology of the crater left by DART, which will provide the first impact experiment at full asteroid scale using an impact speed close to the average speed between asteroids? These questions and many others will be addressed by Hera as a natural outcome of its investigations focused on planetary defense. 4. InstrumentsHera is equipped with the following payload:- The Asteroid Framing Cameras are both science and navigation cameras. They will provide the target global properties as well as local geomorphology and will investigate the crater. They will also measure the mass of Dimorphos through the “wobble” motion of Didymos.- The Planetary ALTimeter (PALT) will measure the distance to the target and, from close distance, derive shape and topography information complementary to the shape information in framing camera images.- A thermal infrared imager (TIRI) will provide information about thermal properties and spectral information in the mid-infrared.- The Hyperscout-H hyperspectral imager will provide mineralogical information in the spectral range between 450 and 950 nm.- Milani is a 6 unit cubesat that will carry the ASPECT Fabry-Perot imager to derive mineralogical information, and a thermogravimeter for measuring the abundance and constraining the composition of ambient dust particles.- Juventas is a 6 unit cubesat that will carry a monostatic low-frequency radar, and a gravimeter to derive interior and surface properties of the asteroids. Its landing on Dimorphos will also allow an estimate of the surface response to a very slow impact.- The radioscience experiment will measure the gravity field of the Didymos system. It will work in two ways: measurements of the acceleration of the Hera spacecraft by the asteroid pair through the radio link between earth and Hera will be used as well as the intersatellite link between Hera and the two cubesat, which will measure the gravitational parameters from the relative position and velocity of the three spacecraft. 5. NEO-MAPPNEO-MAPP (Near Earth Object Modelling and Payload for Protection) is a project funded by the H2020 program of the European Commission. Hera is its reference mission, and most of the NEO-MAPP activities are aimed at supporting the preparation of Hera. The main goal of NEO-MAPP is to provide significant advances in our modeling of impact physics, binary dynamics and internal properties, as well as in instrumentations and associated measurements by a spacecraft (including those necessary for the physical and dynamical characterization in general). In particular, innovative and synergetic measurement and data-analysis strategies are developed that combine multiple payloads, to ensure optimal data exploitation for Hera and other NEO missions. 6. ConclusionThe measurements performed by Hera will thus provide unique information on many current issues in asteroid science and therefore, the scientific legacy of the Hera mission will extend far beyond the core aims of planetary defense. Hera is thus an amazing European contribution to the international planetary defense and asteroid exploration era. AcknowledgementsWe thank ESA and CNES for support. We also acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870377 (project NEO-MAPP).

2I/Borisov - hereafter 2I - is the first visibly active interstellar comet observed in the solar system. It was discovered on August 20, 2019 and remained visible for several months, permitting observations with various telescopes around the world and in space. We observed 2I with UVES, the high-resolution optical spectrograph mounted on the UT2 telescope at Paranal observatory in Chile, with the objective to characterize its composition. We collected spectra at 12 different epochs between November 15, 2019, and March 16, 2020, covering a heliocentric distance range from 2.1 au pre-perihelion to 2.6 au post-perihelion. Our observations allowed us to make several key measurements, some being made for the first time in the coma of an interstellar comet. We detected emissions from the radicals OH, NH, CN, CH, C2, and NH2, the [OI] forbidden oxygen lines at 557.73, 630.03, and 636.37 nm, as well as several FeI and NiI lines. We derived the abundance of FeI and NiI in the coma using the model described in [1] and find log Q(NiI)=21.88±0.07 molecules/s and Q(FeI)=21.67±0.16 molecules/s, corresponding to log(NiI/FeI)=0.21±0.18. This ratio agrees with the value recently found in solar system comets [1], as shown in Fig. 1. NH2 ortho and para lines from the ro-vibronic (0,8,0) and (0,9,0) bands were measured and allowed us to derive a NH2 ortho-to-para ratio (OPR) of 3.21±0.15, corresponding to an OPR and spin temperature of ammonia of 1.11±0.08 and 31+10/-5 K, respectively. These values are consistent with measurements in solar system comets and likely reflect the physico-chemical conditions in the comet inner coma. We measured the three forbidden oxygen lines and determined the ratio between the intensity of the green [OI] line and the red doublet. We obtained a rather high value G/R=0.31±0.05 close to perihelion, with possibly an increasing trend while the comet moved away from the Sun. This is consistent with the high CO abundance in the coma of 2I and the increase of the CO/H2O ratio with the heliocentric distance [2,3]. Finally, from the OH (0-0) emission band around 309 nm we derived a water production rate of (2.2±0.2) x 1026 molecules/s close to perihelion on December 24 and 26, 2019. In conclusion, high spectral resolution observations of 2I/Borisov and the associated measurements of the NH2OPR and the Ni/Fe abundance ratio are remarkably similar to solar system comets. The measured G/R ratio is unusually high but consistent with the high abundance ratio of CO/H2O found by other investigators [2,3] and potentially suggesting formation in a colder environment. Figure 1: Comparison between the log(Ni/Fe) ratio of 2I and the values measured by [1] for a sample of solar system comets (Jupiter Family Comets (JFC), Oort Cloud Comets (OC), and Halley Type Comets (HTC)). The horizontal line represents the average value for solar system comets from [1]. References: [1] Manfroid, J., Hutsemékers, D., & Jehin, E. 2021, Nature, 593, 372 [2] Cordiner, M. A., Milam, S. N., Biver, N., et al. 2020, Nature Astronomy, 4, 861 [3] Bodewits, D., Noonan, J. W., Feldman, P. D., et al. 2020, Nature Astronomy, 4,86

2I/Borisov - hereafter 2I - is the first visibly active interstellar comet observed in the solar system, allowing us for the first time to sample the composition of a building block from another system. We report on the monitoring of 2I with UVES, the high resolution optical spectrograph of the ESO Very Large Telescope at Paranal, during four months from November 15, 2019 to March 16, 2020. Our goal is to characterize the activity and composition of 2I with respect to solar system comets. We collected high resolution spectra at 12 different epochs from 2.1 au pre-perihelion to 2.6 au post perihelion. On December 24 and 26, 2019, close to perihelion, we detected several OH lines of the 309 nm (0-0) band and derived a water production rate of $2.2\pm0.2 \times 10^{26}$ molecules/s. The three [OI] forbidden oxygen lines were detected at different epochs and we derive a green-to-red doublet intensity ratio (G/R) of $0.31\pm0.05$ close to perihelion. NH$_2$ ortho and para lines from various bands were measured and allowed us to derive an ortho-to-para ratio (OPR) of $3.21\pm0.15$, corresponding to an OPR and spin temperature of ammonia of $1.11\pm0.08$ and $31^{+10}_{-5}$ K, respectively. These values are consistent with the values usually measured for solar system comets. Emission lines of the radicals NH (336 nm), CN (388 nm), CH (431 nm), and C$_2$ (517 nm) were also detected. Several FeI and NiI lines were identified and their intensities were measured to provide a ratio of log (NiI/FeI) = $0.21\pm0.18$ in agreement with the value recently found in solar system comets. Our high spectral resolution observations of 2I/Borisov and the associated measurements of the NH$_2$ OPR and the Ni/Fe abundance ratio are remarkably similar to solar system comets. Only the G/R ratio is unusually high but consistent with the high abundance ratio of CO/H$_2$O found by other investigators., Comment: Accepted for publication in A&A

Context. A large fraction of transneptunian objects are found in binary pairs, ~30% in the cold classical population between $a_\text{hel}$~39 and ~48 AU. Observationally, these binaries generally have components of similar size and colour. Previous work has shown that gravitational collapse of a pebble cloud is an efficient mechanism for producing such systems. Since the discovery of the bi-lobate nature of Arrokoth there is also interest in gravitational collapse as a way to form contact binaries. Aims. Our aim was to investigate formation of binary systems via gravitational collapse, considering a wider range of binary masses than previous studies. We analysed in detail the properties of the bound systems that are formed and compared them to observations. Methods. We performed N-body simulations of gravitational collapse of a pebble cloud using the REBOUND package, with an integrator designed for rotating reference frames and robust collision detection. We conducted a deep search for gravitationally bound particles at the end of the gravitational collapse phase and tested their stability. For all systems produced, not just the most massive binaries, we investigated the population characteristics of their mass and orbital parameters. Gravitational collapse can create binary systems analogous to Arrokoth and collisions in a collapsing cloud should be gentle enough to preserve a bi-lobed structure. Results. Gravitational collapse is an efficient producer of bound planetesimal systems. On average ~1.5 bound systems were produced per cloud in the mass range studied here. As well as the large equal-sized binaries, we found that gravitational collapse produces massive bodies with small satellites and low mass binaries with a high mass ratio. Our results disfavour the collapse of high mass clouds, in line with reported upper mass limits of clouds formed by the streaming instability., 17 pages, 12 figures. Published in Section 10. Planets and planetary systems of Astronomy and Astrophysics. Manuscript has been updated to match the published version, reproduced with permission from Astronomy & Astrophysics. For supplementary animation, see doi.org/10.17034/6f4b3d90-c3ba-4510-add5-69e504480a74

Binary Transneptunian Objects (TNOs) have remained virtually unaltered since the formation of the solar system. They can therefore provide valuable insights into the history and properties of objects from the outer solar system, such as object compositions and dynamical history, including the effects of planetary migration on primordial planetesimal populations. Benecchi et al. 2009 measured the colours of 23 TNO binaries using the Hubble Space Telescope (HST), reporting a strong correlation between primary-secondary F606W-F814W colours. Marsset et al. 2020 extended this work into the NIR, adding a further three TNO binary objects with accurate colour measurements made using the Gemini-North telescope which indicated a similar colour correlation in the infrared. We aim to increase the number of binary TNOs with accurate NIR colour measurements by reprocessing data available in the HST archive using a consistent MCMC-based point spread function (PSF)-fitting algorithm. We explore both the position and brightness parameter space for the binary components. Tiny Tim (Krist et al., 2011) PSFs are generated for each component and planted in a model image that is compared with the HST archive image to identify best-fit PSF parameter values. These values are then used to produce and subtract a final model image, providing accurate likelihood estimates for the in-image position and photometric brightness of each component. We will present the results of applying the algorithm to archival data of 24 known binaries, including both optical and NIR colour measurements of both binary components. We will also provide a measure of our sensitivity to binary component separations and brightness ratios. Our results will be compared to the correlated colours observed by Benecchi et al. (2009) and Marsset et al. (2020). References:S. D. Benecchi, K. S. Noll, W. M. Grundy, M. W. Buie, D. C. Stephens, andH. F. Levison. The correlated colors of transneptunian binaries. Icarus, 200(1):292–303, Mar 2009. doi: 10.1016/j.icarus.2008.10.025.J Krist, R Hook, and F Stoehr. 20 years of hubble space telescope opticalmodeling using tiny tim, 2011. URLhttps://doi.org/10.1117/12.892762.Micha ̈el Marsset, Wesley C. Fraser, Michele T. Bannister, Megan E. Schwamb, Rosemary E. Pike, Susan Benecchi, J. J. Kavelaars, Mike Alexandersen, Ying-Tung Chen, Brett J. Gladman, Stephen D. J. Gwyn, Jean-Marc Petit, and Kathryn Volk. Col-OSSOS: Compositional Homogeneity of Three KuiperBelt Binaries.The Planetary Science Journal, 1(1):16, June 2020. doi:10.3847/PSJ/ab8cc0.

INTRODUCTION In the trans-Neptunian region a relatively large fraction of minor bodies are found to exist as mutually bound binary pairs. The binary fraction varies for different dynamical classes and is highest for the Cold Classicals, which are dynamically unexcited planetesimals generally found at 39 < a < 48 AU, with estimates of up to 30% of the Cold Classicals being binaries (Noll et al. 2008). A distinct feature of the observed trans-Neptunian binaries is that they often have components that are of a similar size. These components also have similar optical colours which likely indicates similar surface compositions. Furthermore, the binary orbits span a range of separations but it is not uncommon for them to be found on wide (abin > 0.05 RHill) dynamically fragile orbits. The binary orbit inclinations range from prograde to retrograde, with ~80% of the known trans-Neptunian binaries having prograde orbits. The discovery of the bi-lobate nature of the Cold Classical contact binary 2014 MU69 Arrokoth, with its similar size and colour lobes, indicates that this object is likely the end state of a previously separated trans-Neptunian binary (Stern et al. 2019). Previous work (Nesvorný et al. 2010) has shown that the formation of planetesimals via gravitational collapse is a promising mechanism that can reproduce the unique properties of the trans-Neptunian binaries. We assume that a slowly rotating cloud of pebbles grows in the protoplanetary disk, for example via the streaming instability. Eventually this cloud collapses under its own self gravity and merging collisions lead to the rapid growth of ~100 km planetesimals. Conservation of angular momentum in the rotating cloud means that these planetesimals are frequently formed as a binary pair. For a given binary orbit the angular momentum is maximised by forming a system with similar mass components. Additional work (Nesvorný et al. 2019) has shown that the inclination distribution of binary planetesimals formed via streaming instability and gravitational collapse is a good match for the observed inclination distribution. METHODS We performed N-body simulations of the gravitational collapse of a pebble cloud using the Symplectic Epicyclic Integrator within the REBOUND package (Rein & Liu 2012). The pebble cloud was approximated by a rotating uniform spherical distribution of 105 computational particles. The radii of the computational particles was inflated in order to replicate the collisional behaviour of a high number density pebble cloud. Collisions were detected when particles were found to be overlapping and we considered the timestep required to ensure that collision detection was robust. We performed a deep search for systems of gravitationally bound particles and tested their stability. For all systems produced we investigated the population characteristics of their mass and orbital parameters. RESULTS Our results demonstrate that gravitational collapse is an efficient producer of bound planetesimal systems, and frequently forms multiple bound systems per cloud. On average there were ~1.5 bound systems produced per cloud in the mass range we have studied. As well as the large equal-sized binaries which were the focus of previous work, we found that gravitational collapse can produce a range of systems such as massive bodies with small satellites and low mass binaries with a high mass ratio. The binaries with equal-size components have bound orbits with low inclinations and low to moderate eccentricity, similar to the observed trans-Neptunian binaries. The additional binaries with dissimilar-sized components or low system mass span a large range of inclinations and eccentricities. Our results disfavour the collapse of the high mass clouds in our dataset (Mc = 1.8e21 kg), as they form large equal-sized binaries that are not observed (figure 1). This finding is in line with reported upper mass limits of clouds formed by the streaming instability. Gravitational collapse of low mass clouds (Mc ≤ 4.2e18 kg) can create binary systems with similar total mass and mass ratio as the contact binary Arrokoth (figure 1). Furthermore, we find that collisions between planetesimals growing in such a collapsing cloud should be gentle enough to preserve a bi-lobed structure, which further supports the gravitational collapse origin of Arrokoth. Figure 1. Primary V band magnitude against magnitude difference between primary and secondary components for binaries produced by gravitational collapse simulations. Marker colour and shape indicates the initial cloud mass. The vertical line indicates the mass ratio cut off of m2/m1 ≥ 10−3 in the orbit search algorithm. The observed trans-Neptunian binaries from Grundy (2019) are shown as black 'x's. We represent 'special cases' and dwarf planets as red '+'s. The red circle with error-bars represents how Arrokoth would appear if its components could be separately resolved. An approximate empirical detection limit (Noll et al. 2008) and a lower magnitude limit of 25 are shown as red dotted lines. REFERENCES Grundy, W. (2019). Mutual Orbits of Binary Transneptunian Objects. Retrieved December 2nd, 2019, from online webpage website: http://www2.lowell.edu/users/grundy/tnbs/ Nesvorný, D., Youdin, A. N., & Richardson, D. C. (2010). Formation of Kuiper Belt Binaries by Gravitational Collapse. The Astronomical Journal, 140, 785–793. https://doi.org/10.1088/0004-6256/140/3/785 Nesvorný, D., Li, R., Youdin, A. N., Simon, J. B., & Grundy, W. M. (2019). Trans-Neptunian binaries as evidence for planetesimal formation by the streaming instability. Nature Astronomy, 3, 808–812. https://doi.org/10.1038/s41550-019-0806-z Noll, K. S., Grundy, W. M., Chiang, E. I., Margot, J.-L., & Kern, S. D. (2008). Binaries in the Kuiper Belt. In The Solar System Beyond Neptune (pp. 345–363). Tucson: The University of Arizona Press. Rein, H., & Liu, S. F. (2012). REBOUND: An open-source multi-purpose N-body code for collisional dynamics. Astronomy and Astrophysics, 537, A128. https://doi.org/10.1051/0004-6361/201118085 Stern, S. A., Weaver, H. A., Spencer, J. R., Olkin, C. B., Gladstone, G. R., Grundy, W. M., … Zurbuchen, T. H. (2019). Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object. Science, 364(6441). https://doi.org/10.1126/science.aaw9771

2I/Borisov is the first active interstellar comet observed in the Solar Sytem, allowing for the first time to sample the composition of a planetary building block from an extrasolar system. We report on the monitoring of 2I with the FORS low resolution spectrograph of the ESO VLT at Paranal during four months, from November 19, 2019 to March 20, 2020. We collected a dozen spectra at 8 different epochs allowing to follow the evolution of the comet activity and composition around perihelion. We also observed with the same instrumental setup an Oort Cloud comet, C/2019 U6 (Lemmon), at about the same heliocentric and geocentric distance than 2I/Borisov at perihelion (rh=Delta=2 au) and with similar AfRho value and Q(CN) in order to use it as a reference for the Solar System. The usual species are detected in the optical spectrum of 2I (CN, C3, C2, and NH2) and their production rates and abundance ratios are computed. The dust production rate and colors are also derived, compared to C/2019 U6 and other comets of the Solar System, and their evolutions are followed with the heliocentric distance. Fig1. Comparison of the optical spectra of the interstellar comet 2I/Borisov at perihelion (rh=Delta=2,0 au) and the Oort Could comet C/2019 U6 (Lemmon) observed with FORS at the ESO VLT (Paranal Observatory). C/2019/U6 was chosen for comparison as a Solar System comet with a typical composition. It was also at the about the same distances to the Sun and Earth, and it had about the same dust and CN production rates as 2I/Borisov as measured by the TRAPPIST-South telescope.

The interstellar comet 2I/Borisov was discovered on August 20, 2019. It is only the second interstellar object to be observed crossing our Solar System, and the first one for which outgassing was detected directly [1]. Early observations indicated that 2I/Borisov is depleted in C2, similarly to about 30% of Solar System comets [2,3]. Preliminary observations with the MUSE IFU performed in November 2019 confirmed that 2I is depleted in C2 but also showed it is rich in NH2 [4]. We present here results from the full observing campaign performed with the MUSE instrument. MUSE is a multi-unit integral field spectrograph mounted on the UT4 telescope of the VLT [5]. The instrument covers the wavelength range from 480 to 930 nm with a resolving power of about 3000. It has a large field of view of 1’x1’ and a spatial resolution of 0.2”, which makes it an ideal instrument to study extended sources. We observed 2I with MUSE on 16 different dates between November 14, 2019 and March 19, 2020. The observations started about one month before the perihelion passage and continued until the comet reached 3 au post-perihelion. This data sets constitutes a great opportunity to study the activity and coma composition of 2I over several months. Our observations allow us to detect emission bands from C2, NH2, and CN. Using a Haser model [6] we derive production rates for those 3 species and follow their evolution. We also study the evolution of the ratio between those production rates, to monitor how the composition of 2I coma changes as a function of time and distance from the Sun. References: [1] Fitzsimmons et al., 2019, The Astrophysical Journal Letters, Volume 885, Issue 1, article id. L9, 6 pp.[2] Opitom et al., 2019, Astronomy & Astrophysics, Volume 631, id.L8, 5 pp.; [3] Lin et al., 2019, The Astrophysical Journal Letters, Volume 889, Issue 2, id.L30;[4] Bannister et al, 2020, submitted to ApJ Letters; [5] Bacon et al, 2010, Proceedings of the SPIE, Volume 7735, id. 773508; [6] Haser, 1957,Bulletin de la Classe des Sciences de l'Académie Royale de Belgique, vol. 43, p. 740-750

The Hera mission is the planetary defence mission approved under the ESA Space Safety Programme. Together with the NASA DART mission, it is the first Asteroid Deflection Demonstration. DART will impact Dimorphos, the natural satellite of Near Earth Asteroid Didymos, in 2022. Using the kinetic impactor technology, it will demonstrate the ability to change the trajectory of a small asteroid. Hera will launch in 2024 and arrive in the Didymos system at the end of 2026. Its primary goals are to accurately measure the mass of Dimorphos and to characterise the crater and effect of the DART impact on this small asteroid. Hera will spend over 2 years matching orbits with Didymos, travelling through near-Earth space and the inner main-belt. This potentially allows Hera to perform an additional asteroid flyby enroute to Didymos. For such a flyby, a number of factors need to be considered for target selection. Primary factors are the accuracy of the orbit of the asteroid, and distance from the nominal trajectory. Resolution of geomorphology requires a minimum number of spatial resolution elements at the desired flyby distance. Previous observations on the form of lightcurves, colours or spectra are important for flyby science planning. Finally, a dynamical class of asteroid may be targeted for specific science priorities. Analysis of the nominal trajectory shows approximately 100 asteroids passing the nominal position of Hera within 0.02 au. Ten of these potential targets are Near-Earth Asteroids, the rest being Mars-crossers and main-belt asteroids. We will present our current knowledge on these targets, and highlight the highest priority targets for further observation in the next 2-3 years. Although ground-based reconnaissance has started on some targets, further observations are required to support the flyby planning process. Figure 1. Osculating orbital elements of asteroids passing close to Hera on its nominal trajectory to the Didymos system, without any additional trajectory adjustment. Red dots indicate a current flyby distance d < 0.01 au, blue circles indicate a current flyby distance 0.01 au < d < 0.02 au. The solid line indicates a perihelion distance of 1.3 au, asteroids to the left of this line are Near-Earth Asteroids.

Exocomets are small bodies releasing gas and dust which orbit stars other than the Sun. Their existence was first inferred from the detection of variable absorption features in stellar spectra in the late 1980s using spectroscopy. More recently, they have been detected through photometric transits from space, and through far-IR/mm gas emission within debris disks. As (exo)comets are considered to contain the most pristine material accessible in stellar systems, they hold the potential to give us information about early stage formation and evolution conditions of extra Solar Systems. In the Solar System, comets carry the physical and chemical memory of the protoplanetary disk environment where they formed, providing relevant information on processes in the primordial solar nebula. The aim of this paper is to compare essential compositional properties between Solar System comets and exocomets. The paper aims to highlight commonalities and to discuss differences which may aid the communication between the involved research communities and perhaps also avoid misconceptions. Exocomets likely vary in their composition depending on their formation environment like Solar System comets do, and since exocomets are not resolved spatially, they pose a challenge when comparing them to high fidelity observations of Solar System comets. Observations of gas around main sequence stars, spectroscopic observations of "polluted" white dwarf atmospheres and spectroscopic observations of transiting exocomets suggest that exocomets may show compositional similarities with Solar System comets. The recent interstellar visitor 2I/Borisov showed gas, dust and nuclear properties similar to that of Solar System comets. This raises the tantalising prospect that observations of interstellar comets may help bridge the fields of exocomet and Solar System comets., Comment: 25 pages, 3 figures. To be published in PASP. This paper is the product of a workshop at the Lorentz Centre in Leiden, the Netherlands

We report the discovery of a relatively bright eclipsing binary system, which consists of a white dwarf and a main sequence K7 star with clear signs of chromospheric and spot activity. The light curve of this system shows $\sim0.2$mag ellipsoidal variability with a period of 0.297549d and a short total eclipse of the white dwarf. Based on our analysis of the spectral and photometric data, we estimated the parameters of the system. The K7V star is tidally deformed but does not fill its Roche lobe (the filling factor is about 0.86). The orbital inclination is $i=73^\circ.1\pm 0^\circ.2$, the mass ratio is $q=M_2/M_1\approx 0.88$. The parameters of the K7V star are $M_2\approx 0.64$M$_{\odot}$, $R_2=0.645\pm 0.012$R$_{\odot}$, $T_2\approx 4070$K. The parameters of the white dwarf are $M_1\approx 0.72$M$_{\odot}$, $R_1=0.013\pm 0.003$R$_{\odot}$, $T_1=8700\pm 1100$K. Photometric observations in different bands revealed that the maximum depth of the eclipse is in the \textit{SDSS r} filter, which is unusual for a system of a white dwarf and a late main sequence star. We suspect that this system is a product of the evolution of a common envelope binary star, and that the white dwarf accretes the stellar wind from the secondary star (the so-called low-accretion rate polar, LARP)., 11 pages 6 figures, accepted for publication in MNRAS

Comet P/2019 LD2 has orbital elements currently resembling those of a Jupiter Trojan, and therefore superficially appears to represent a unique opportunity to study the volatile content and active behavior of a member of this population for the first time. However, numerical integrations show that it was previously a Centaur before reaching its current Jupiter Trojan-like orbit in 2018 July, and is expected to return to being a Centaur in 2028 February, before eventually becoming a Jupiter-family comet in 2063 February. The case of P/2019 LD2 highlights the need for mechanisms to quickly and reliably dynamically classify small solar system bodies discovered in current and upcoming wide-field surveys., Comment: 7 pages, 3 figures. Accepted for publication in Icarus

We report results from new and archival observations of the newly discovered active asteroid (248370) 2005 QN_137, which has been determined to be a likely main-belt comet based on a subsequent discovery that it is recurrently active near perihelion. From archival data analysis, we estimate g'-, r'-, i'-, and z'-band absolute magnitudes for the nucleus of H_g=16.62+/-0.13, H_r=16.12+/-0.10, H_i=16.05+/-0.11, and H_z=15.93+/-0.08, corresponding to nucleus colors of g'-r'=0.50+/-0.16, r'-i'=0.07+/-0.15, and i'-z'=0.12+/-0.14, an equivalent V-band absolute magnitude of H_V=16.32+/-0.08, and a nucleus radius of r_n=1.6+/-0.2 km (using a V-band albedo of p_V=0.054+/-0.012). Meanwhile, we find mean near-nucleus coma colors when 248370 was active of g'-r'=0.47+/-0.03, r'-i'=0.10+/-0.04, and i'-z'=0.05+/-0.05, and similar mean dust tail colors, suggesting that no significant gas coma is present. We find approximate ratios between the scattering cross-sections of near-nucleus dust (within 5000 km of the nucleus) and the nucleus of A_d/A_n=0.7+/-0.3 on 2016 July 22, and 1.8, Comment: 12 pages, 2 figures, accepted for publication in ApJ Letters (submitted version posted here; please contact first author for final accepted version)

Context.The potentially hazardous asteroid (85990) 1999 JV6 has been a target of previously published thermal-infrared observations and optical photometry. It has been identified as a promising candidate for possible Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect detection.Aims.The YORP effect is a small thermal-radiation torque considered to be a key factor in spin-state evolution of small Solar System bodies. In order to detect YORP on 1999 JV6 we developed a detailed shape model and analysed the spin-state using both optical and radar observations.Methods.For 1999 JV6, we collected optical photometry between 2007 and 2016. Additionally, we obtained radar echo-power spectra and imaging observations with Arecibo and Goldstone planetary radar facilities in 2015, 2016, and 2017. We combined our data with published optical photometry to develop a robust physical model.Results.We determine that the rotation pole resides at negative latitudes in an area with a 5° radius close to the south ecliptic pole. The refined sidereal rotation period is 6.536787 ± 0.000007 h. The radar images are best reproduced with a bilobed shape model. Both lobes of 1999 JV6 can be represented as oblate ellipsoids with a smaller, more spherical component resting at the end of a larger, more elongated component. While contact binaries appear to be abundant in the near-Earth population, there are only a few published shape models for asteroids in this particular configuration. By combining the radar-derived shape model with optical light curves we determine a constant-period solution that fits all available data well. Using light-curve data alone we determine an upper limit for YORP of 8.5 × 10−8rad day−2.Conclusions.The bifurcated shape of 1999 JV6 might be a result of two ellipsoidal components gently merging with each other, or a deformation of a rubble pile with a weak-tensile-strength core due to spin-up. The physical model of 1999 JV6 presented here will enable future studies of contact binary asteroid formation and evolution.

The discovery of the first interstellar object passing through the Solar System, 1I/2017 U1 ('Oumuamua), provoked intense and continuing interest from the scientific community and the general public. The faintness of 'Oumuamua, together with the limited time window within which observations were possible, constrained the information available on its dynamics and physical state. Here we review our knowledge and find that in all cases the observations are consistent with a purely natural origin for 'Oumuamua. We discuss how the observed characteristics of 'Oumuamua are explained by our extensive knowledge of natural minor bodies in our Solar System and our current knowledge of the evolution of planetary systems. We highlight several areas requiring further investigation., Comment: Published in Nature Astronomy, 01 July 2019. 16 pages, 4 figures, 1 table

We observed comet 96P/Machholz 1 on a total of 9 nights before and after perihelion during its 2017/2018 apparition. Both its unusually small perihelion distance and the observed fragmentation during multiple apparitions make 96P an object of great interest. Our observations show no evidence of a detectable dust coma, implying that we are observing a bare nucleus at distances ranging from 2.3 AU to 3.8 AU. Based on this assumption we calculated its color, and found average values of g'-r' = 0.50 +/- 0.04, r'-i' = 0.17 +/- 0.03, and i'-z' = 0.06 +/- 0.04. These are notably more blue than those of the nuclei of other Jupiter family and long period comets. Furthermore, assuming a bare nucleus, we found an equivalent nuclear radius of 3.4 +/- 0.2 km with an axial ratio of at least 1.6 +/- 0.1. The lightcurve clearly displays one large peak, one broad flat peak, and two distinct troughs, with a clear asymmetry that suggests that the shape of the nucleus deviates from that of a simple triaxial ellipsoid. This asymmetry in the lightcurve allowed us to constrain the nuclear rotation period to 4.10 +/- 0.03 hours and 4.096 +/- 0.002 hours before and after perihelion, respectively. Within the uncertainties, 96P's rotation period does not appear to have changed throughout the apparition, and we conclude a maximum possible change in rotation period of 130 seconds. The observed properties were compared to those of comet 322P and interstellar object 1I/'Oumuamua in an attempt to study the effects of close perihelion passages on cometary surfaces and their internal structure, and the potential interstellar origin of 96P., Revised to fix typos; 13 pages text, 3 tables, 8 figures; Accepted by ApJL

We present an analysis of long-slit cometary spectroscopy using the dual-arm ISIS spectrograph on the 4.2 m WHT. Eleven comets were observed over two nights in 2016 March and we detected the OH (0-0) emission band at 3085 {\AA} in the spectra of five of these comets. Emission bands of the species NH, CN, C$_{3}$, C$_{2}$, NH$_{2}$ and [OI] were also detected. We used Haser modelling to determine molecular production rates and abundance ratios for the observed species. We found that our average abundances relative to OH and CN were generally consistent with those measured in previous studies., Comment: 13 pages, 7 figures, 9 tables, accepted for publication in MNRAS

Context.The near-Earth asteroid (1917) Cuyo was subject to radar and light curve observations during a close approach in 1989, and observed up until 2008. It was selected as one of our ESO Large Programme targets, aimed at observational detections of the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect through long-term light curve monitoring and physical modelling of near-Earth asteroids.Aims.We aim to constrain the physical properties of Cuyo: shape, spin-state, and spectroscopic and thermo-physical properties of the surface.Methods.We acquired photometric light curves of Cuyo spanning the period between 2010 and 2013, which we combined with published light curves from 1989 to 2008. Our thermal-infrared observations were obtained in 2011. Rotationally resolved optical spectroscopy data were acquired in 2011 and combined with all available published spectra to investigate any surface material variegation.Results.We developed a convex light curve-inversion shape of Cuyo that suggests the presence of an equatorial ridge, typical for an evolved system close to shedding mass due to fast rotation. We determine limits of YORP strength through light curve-based spin-state modelling, including both negative and positive acceleration values, between − 0.7 × 10−8and 1.7 × 10−8rad day−2. Thermo-physical modelling with the ATPM provides constraints on the geometric albedo,pV= 0.24 ± 0.07, the effective diameter,Deff= 3.15 ± 0.08km, the thermal inertia, Γ = 44 ± 9 J m−2s−1∕2K−1, and a roughness fraction of 0.52 ± 0.26. This enabled a YORP strength prediction ofν= (−6.39 ± 0.96) × 10−10rad day−2. We also see evidence of surface compositional variation.Conclusions.The low value of YORP predicted by means of thermo-physical analysis, consistent with the results of the light curve study, might be due to the self-limiting properties of rotational YORP, possibly involving movement of sub-surface and surface material. This may also be consistent with the surface compositional variation that we see. The physical model of Cuyo can be used to investigate cohesive forces as a way to explain why some targets survive rotation rates faster than the fission limit.

The detection of Interstellar Objects passing through the Solar System offers the promise of constraining the physical and chemical processes involved in planetary formation in other extrasolar systems. While the effect of outgassing by 1I/2017 U1 ('Oumuamua) was dynamically observed, no direct detection of the ejected material was made. The discovery of the active interstellar comet 2I/Borisov means spectroscopic investigations of the sublimated ices is possible for this object. We report the first detection of gas emitted by an interstellar comet via the near-UV emission of CN from 2I/Borisov at a heliocentric distance of $r$ = 2.7 au on 2019 September 20. The production rate was found to be Q(CN) = $(3.7\pm0.4)\times10^{24}$ s$^{-1}$, using a simple Haser model with an outflow velocity of 0.5 km s$^{-1}$. No other emission was detected, with an upper limit to the production rate of C$_2$ of $4\times10^{24}$ s$^{-1}$. The spectral reflectance slope of the dust coma over $3900$ \AA\ $< \lambda< 6000$ \AA \ is steeper than at longer wavelengths, as found for other comets. Broad band $R_c$ photometry on 2019 September 19 gave a dust production rate of $Af\rho=143\pm10$ cm. Modelling of the observed gas and dust production rates constrains the nuclear radius to $0.7-3.3$ km assuming reasonable nuclear properties. Overall, we find the gas, dust and nuclear properties for the first active Interstellar Object are similar to normal Solar System comets., Comment: 9 pages, 4 figures, accepted for publication in ApJL

The discovery of the first active interstellar object 2I/Borisov provides an unprecedented opportunity to study planetary formation processes in another planetary system. In particular, spectroscopic observations of 2I allow us to constrain the composition of its nuclear ices. We obtained optical spectra of 2I with the 4.2 m William Herschel and 2.5 m Isaac Newton telescopes between 2019 September 30 and October 13, when the comet was between 2.5 au and 2.4 au from the Sun. We also imaged the comet with broadband filters on 15 nights from September 11 to October 17, as well as with a CN narrow-band filter on October 18 and 20, with the TRAPPIST-North telescope. Broadband imaging confirms that the dust coma colours (B-V=0.82$\pm$0.02, V-R=0.46$\pm$0.03, R-I=0.44$\pm$0.03, B-R=1.28$\pm$0.03) are the same as for Solar System comets. We detect CN emission in all spectra and in the TRAPPIST narrow-band images with production rates between 1.6$\times10^{24}$ and 2.1$\times10^{24}$ molec/s. No other species are detected. We determine three-sigma upper limits for C$_2$, C$_3$, and OH production rates of 6$\times10^{23}$ molec/s, 3$\times10^{23}$ molec/s and 2$\times10^{27}$ molec/s, respectively, on October 02. There is no significant increase of the CN production rate or A(0)f$\rho$ during our observing period. Finally, we place a three-sigma upper limit on the Q(C$_2$)/Q(CN) ratio of 0.3 (on October 13). From this, we conclude that 2I is highly depleted in C$_2$, and may have a composition similar to Solar System carbon-chain depleted comets., Comment: Revised version, Accepted for publication in A&A Letters

The discovery of 1I/2017 U1 ('Oumuamua) has provided the first glimpse of a planetesimal born in another planetary system. This interloper exhibits a variable colour within a range that is broadly consistent with local small bodies such as the P/D type asteroids, Jupiter Trojans, and dynamically excited Kuiper Belt Objects. 1I/'Oumuamua appears unusually elongated in shape, with an axial ratio exceeding 5:1. Rotation period estimates are inconsistent and varied, with reported values between 6.9 and 8.3 hours. Here we analyse all available optical photometry reported to date. No single rotation period can explain the exhibited brightness variations. Rather, 1I/'Oumuamua appears to be in an excited rotational state undergoing Non-Principal Axis (NPA) rotation, or tumbling. A satisfactory solution has apparent lightcurve frequencies of 0.135 and 0.126 hr-1 and implies a longest-to-shortest axis ratio of 5:1, though the available data are insufficient to uniquely constrain the true frequencies and shape. Assuming a body that responds to NPA rotation in a similar manner to Solar System asteroids and comets, the timescale to damp 1I/'Oumuamua's tumbling is at least a billion years. 1I/'Oumuamua was likely set tumbling within its parent planetary system, and will remain tumbling well after it has left ours., 13 pages, 2 figures, to appear in Nature Astronomy

Using the first 18 months of the Pan-STARRS 1 survey we have identified 33 candidate high-amplitude objects for follow-up observations and carried out observations of 22 asteroids. 4 of the observed objects were found to have observed amplitude $A_{obs}\geq 1.0$ mag. We find that these high amplitude objects are most simply explained by single rubble pile objects with some density-dependent internal strength, allowing them to resist mass shedding even at their highly elongated shapes. 3 further objects although below the cut-off for 'high-amplitude' had a combination of elongation and rotation period which also may require internal cohesive strength, depending on the density of the body. We find that none of the 'high-amplitude asteroids' identified here require any unusual cohesive strengths to resist rotational fission. 3 asteroids were sufficiently observed to allow for shape and spin pole models to be determined through light curve inversion. 45864 was determined to have retrograde rotation with spin pole axes $\lambda=218\pm 10^{\circ}, \beta=-82\pm 5^{\circ}$ and asteroid 206167 was found to have best fit spin pole axes $\lambda= 57 \pm 5^{\circ}$, $\beta=-67 \pm 5^{\circ}$. An additional object not initially measured with $A_{obs}>1.0$ mag, 49257, was determined to have a shape model which does suggest a high-amplitude object. Its spin pole axes were best fit for values $\lambda=112\pm 6^{\circ}, \beta=6\pm 5^{\circ}$. In the course of this project to date no large super-fast rotators ($P_{rot} < 2.2$ h) have been identified., Comment: 31 pages; accepted by AJ

Source at https://doi.org/10.1007/s11214-017-0446-5. This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics.