Active Asteroids

As a recently recognized class of objects exhibiting apparently cometary (sublimation-driven) activity yet orbiting completely within the main asteroid belt, main-belt comets (MBCs) have revealed the existence of present-day ice in small bodies in the inner solar system and offer an opportunity to better understand the thermal and compositional history of our solar system, and by extension, those of other planetary systems as well. Achieving these overall goals, however, will require meeting various intermediate research objectives, including discovering many more MBCs than the currently known seven objects in order to ascertain the population's true abundance and distribution, confirming that water ice sublimation is in fact the driver of activity in these objects, and improving our understanding of the physical, dynamical, and thermal evolutionary processes that have acted on this population over the age of the solar system.

Comets and asteroids have traditionally been separated categories, but main belt comets skew this view, portraying a possible intermediate stage between these two endmembers. Investigating the relationship between these bodies can improve our understanding of the formation and evolution of the Solar System and help to identify potentially interesting parent bodies from within our solar system, for future sample return missions. Furthermore, elucidating the ice-organic-silicate ratios of potential meteorite parent bodies can help to explain the observed isotopic ratios and petrography of meteorite samples. While the ice-organic-silicate ratios of particular bodies have been estimated, there has been no study undertaken which compares different types of bodies in terms of their ice-organic-silicate ratios. Therefore, this study presents a geophysical-chemical mass balance model, to estimate the ice-organic-silicate ratios of comets, main belt comets and asteroids. The results drawn from the model form a diagonal trend upon an ice-organic-silicate ternary diagram, in which comets and main belt comets plot together at generally higher ice contents, with asteroids typically plotting at lower ice contents. However, an overlap between all three body types is observed and supports the scenario in which comets, main belt comets and asteroids are genetically linked.

Classically, comets from the outer solar system (beyond the orbit of Neptune), are expected to be icy, and thus active near the Sun, while asteroids in the inner solar system (interior to the orbit of Jupiter) are expected to be relatively ice-deficient, and thus inert. Studies of anomalous objects, most recently 133P/Elst-Pizarro, challenge this classical picture, however, and suggest that either (1) subsurface ice can in fact be preserved over billions of years in small bodies in the inner solar system but still be close enough to the surface to be excavated by an impact by another body, or (2) non-gravitational dynamical evolution (primarily driven by asymmetrical outgassing) of icy bodies from the outer solar system can drive these cometary bodies onto thoroughly asteroid-like orbits, erasing all dynamical signs of their trans-Neptunian origins in the process. The question thus boils down to whether occasionally sublimating icy bodies on stable asteroid-like orbits in the inner solar system, particularly in the main asteroid belt, may in fact be native to the region or whether they must necessarily be recent arrivals.

Main-belt comets are small Solar System bodies located in the asteroid belt that repeatedly exhibit comet-like activity (that is, dust comae or tails) during their perihelion passages, strongly indicating ice sublimation1,2. Although the existence of main-belt comets implies the presence of extant water ice in the asteroid belt, no gas has been detected around these objects despite intense scrutiny with the world's largest telescopes3. Here we present James Webb Space Telescope observations that clearly show that main-belt comet 238P/Read has a coma of water vapour, but lacks a significant CO2 gas coma. Our findings demonstrate that the activity of comet Read is driven by water-ice sublimation, and implies that main-belt comets are fundamentally different from the general cometary population. Whether or not comet Read experienced different formation circumstances or evolutionary history, it is unlikely to be a recent asteroid belt interloper from the outer Solar System. On the basis of these results, main-belt comets appear to represent a sample of volatile material that is currently unrepresented in observations of classical comets and the meteoritic record, making them important for understanding the early Solar System's volatile inventory and its subsequent evolution.

We are searching for new active asteroids using data from the Subaru Hyper Suprime-Cam (HSC) Public Data Release 3 (PDR3). The active asteroids are a poorly understood class of asteroids, which have orbits similar to other asteroids but exhibit tails or comae like comets. Of the over 1.4 million catalogued minor planets, most of which are main belt asteroids, fewer than 100 of the asteroids have exhibited activity, and roughly half of those are thought to be driven by water ice. The true fraction and solar system distribution of active asteroids is poorly known as most have been discovered serendipitously by a variety of methods. In this work, we search for activity among the known asteroids using one of the deepest wide-area public datasets available, the HSC PDR3. The goal of this work is to discover new active asteroid candidates and recurring activity in known active asteroids in archival data, collect contemporary images of those asteroids using ground based telescopes, and assess the true rate of active asteroid activity in a consistent manner using a large dataset. This will lead to insights into the present day distribution of water ice in our solar system.The HSC PDR3 has been collected using one of the largest ground-based optical telescopes, the 8.2 m Subaru telescope atop Maunakea in Hawaii which regularly achieves image quality better than 0.7 arcseconds yielding single-image depths of 25 to 25.5 magnitudes in the typical exposure times used for the PDR3. In addition, HSC itself has an extremely wide field of view, about 1.8 square degrees, for such a large telescope. Over 13,000 fields were imaged in good conditions with HSC as part of the PDR3, representing over 23,000 square degrees. The PDR3 dataset was collected for scientific purposes unrelated to the asteroids but many fields contain serendipitous images of asteroids. We have cross-correlated the PDR3 fields and the Minor Planet Database using the SkyBoT project and find that over 230,000 minor planets (over 15% of all minor planets and the vast majority asteroids) were serendipitously imaged in the dataset for a total of over 1.6 million minor planet images. We have begun the process of constructing thumbnail images of these minor planets from the full HSC PDR3 data release and expect to finish this year.Initially, we are examining images by eye ourselves to determine activity candidates. However, we will launch a Citizen Science campaign akin to activeasteroids.net, which has to date only used data from the DECam public archive. This will allow us to mobilize volunteers to examine more images than we can reliably categorize ourselves. We present first results from this work including an assessment of activity levels for thousands of asteroids most likely to exhibit activity including: previously known active asteroids, distant C-type main belt asteroids, and Centaurs on unstable orbits that could have recently moved inward from the trans-Neptunian region.Figure 1: One of 4 images from the HSC PDR3 archive of known active outer main belt asteroid 331P/Gibbs, exhibiting a faint linear tail.Figure 2: One of 17 images from the HSC PDR3 archive of known Jupiter family comet 242P/Spahr, exhibiting a tail. While not a primary target of this work, many comets are present in the HSC PDR3.

The object P/2013 P5 PANSTARRS was discovered in August 2013, displaying a cometary tail, but its orbital elements indicated that it was a typical member of the inner asteroid main belt. We monitored the object from 2013 August 30 until 2013 October 05 using the CFHT 3.6 m telescope (Mauna Kea, HI), the NTT (ESO, La Silla), the CA 1.23 m telescope (Calar Alto), the Perkins 1.8m (Lowell) and the 0.6 m TRAPPIST telescope (La Silla). We measured its nuclear radius to be r < 0:25 0:29 km, and its colours g 0 r 0 = 0:58 0:05 and r 0 i 0 = 0:23 0:06, typical for an S-class asteroid, as expected for an object in the inner asteroid belt and in the vicinity of the Flora collisional family. We failed to detect any rotational light curve with an amplitude <0.05 mag and a double-peaked rotation period <20 h. The evolution of the tail during the observations was as expected from a dust tail. A detailed Finson-Probstein analysis of deep images acquired with the NTT in early September and with the CFHT in late September indicated that the object was active since at least late January 2013 until the time of the latest observations in 2013 September, with at least two peaks of activity around 2013 June 14 10 d and 2013 July 22 3 d. The changes of activity level and the activity peaks were extremely sharp and short, shorter than the temporal resolution of our observations ( 1 d). The dust distribution was similar during these two events, with dust grains covering at least the 1‐1000 m range. The total mass ejected in grains <1 mm was estimated to be 3:0 10 6 kg and 2:6 10 7 kg around the two activity peaks. Rotational disruption cannot be ruled out as the cause of the dust ejection. We also propose that the components of a contact binary might gently rub and produce the observed emission. Volatile sublimation might also explain what appears as cometary activity over a period of 8 months. However, while main belt comets best explained by ice sublimation are found in the outskirts of the main belt, where water ice is believed to be able to survive buried in moderately large objects for the age of the solar system deeply, the presence of volatiles in an object smaller than 300 m in radius would be very surprising in the inner asteroid belt.

The Kuiper Belt objects (KBOs), the Centaurs, and the Jupiter-family comets (JFCs) form an evolutionary continuum of small outer solar system objects, and their study allows us to gain insight into the history and evolution of the solar system. Broadband photometry can be used to measure their phase curves, allowing a first-order probe into the surface properties of these objects, though limited telescope time makes measuring accurate phase curves difficult. We make use of serendipitous broadband photometry from the long-baseline, high-cadence Asteroid Terrestrial-impact Last Alert System survey to measure the phase curves for a sample of 18 KBOs, Centaurs, and JFCs with unprecedentedly large data sets. We find phase curves with previously reported negative slopes become positive with increased data and are thus due to insufficient sampling of the phase-curve profile, and not a real physical effect. We search for correlations between phase-curve parameters, finding no strong correlations between any parameter pair, consistent with the findings of previous studies. We search for instances of cometary activity in our sample, finding a previously reported outburst by Echeclus and a new epoch of increased activity by Chiron. Applying the main belt asteroid HG 1 G 2 phase-curve model to three JFCs in our sample with large phase angle spans, we find their slope parameters imply surfaces more consistent with those of carbonaceous main belt asteroids than silicaceous ones.

<p indent="0mm">Deep space exploration is one of the frontiers of scientific research and driven by human instincts to explore the unknown. Since the Moon Race in the 1960s and 1970s, deep-space probes have explored many celestial bodies in our solar system, including eight planets. These explorations have yielded tremendous scientific discoveries, but they also revealed many more unsolved mysteries. The 21st century has witnessed a remarkable increase in deep-space exploration activities, with several new agencies launching their spacecraft in the past few decades. As a result of technological advancement, exploration success rates have improved significantly. A systematic review of frontier scientific questions can provide invaluable references for mission planning strategies and boost scientific discoveries and breakthroughs in the future. In general, these questions are related to three primary scientific subjects: (1) One of the primary scientific objectives of deep-space exploration is to study the origin and evolution of the solar system. From the collapse of the solar nebula to the formation of the planetary system, the solar system has undergone a complex evolutionary process. The components of the current solar system, particularly the residual presolar material, can provide critical information for understanding the initial conditions and evolution of the solar system. The Moon, being the nearest celestial body to Earth, serves as an outpost for exploring other planetary bodies and is especially important for testing deep-space exploration equipment and studying solar system evolution. (2) Another objective of deep space exploration is to better understand the evolution of planetary habitability. The Sun is an important factor affecting habitability, and eruptions of solar activities can have a substantial impact on the space environment of the solar system. The environment of the Earth and other planets has changed dramatically over billions of years of evolution. Although life has only been confirmed on Earth, other celestial bodies might also have had habitable conditions along the evolutionary process. Decoding the evolution of the habitable environment of solar system planetary bodies is of great significance for understanding the formation and evolution of habitable Earth and the origin of life on Earth. (3) Extraterrestrial life detection is a critical scientific topic in deep space exploration, with far-reaching implications for understanding the origin, evolution, and distribution of life in solar and extrasolar systems. The current scientific challenges in the detection of extraterrestrial life primarily involve four aspects: the origin and evolution of life, extraterrestrial habitable environments, life signal identification, and analogy of terrestrial extreme environments. In endeavors to answer these scientific questions, more well-planned deep-space exploration missions are required to collect evidence. In order to outpace opponents in the continuing space exploration endeavor, the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and other rising roles have made their ambitious roadmaps. NASA will encourage commercial companies to join future missions through the Artemis program, and has planned missions to explore Venus, asteroids, and icy bodies. The ESA proposed tens of missions in its white paper <italic>Voyage 2050</italic>. In the coming decades, China will lay the groundwork for the International Lunar Research Station through three Chang’e and four Russian Luna missions. Furthermore, Tianwen-2 and Tianwen-3 will return samples from asteroids and Mars, respectively; Tianwen-4 is intended to explore the Jupiter system. The success of these missions will yield unprecedented information for deciphering the mysteries of the solar system and life, as well as considerably expanding our knowledge of the universe we live in.

Deciphering the composition of interstellar, cometary, and asteroidal dust offers an unparalleled opportunity to explore the origin and evolution of our solar system. The goal of the Fragments from the Origins of the Solar System and our Interstellar Locale (FOSSIL) mission is to confirm or disprove expectations that comets deliver fragments of pristine material, that is the most carbon rich and least aqueously altered matter from the early stages of solar system formation, as opposed to asteroidal matter that is heat-treated, carbon-poor, and aqueously altered. The FOSSIL mission concept is designed to unambiguously identify, and comprehensively explore the makeup of interstellar dust particles (ISD) flowing through our solar system today, delivering matter closest to the original solid building blocks of our solar system. The FOSSIL mission concept would comprehensively survey the compositional diversity of a broad range of bodies in our solar system and beyond, returning a powerful set of measurements to test the genetic relationships between the main reservoirs of primitive materials left over from planet formation. FOSSIL would be placed in an Earth-trailing orbit, carrying four state-of-the-art Dust Telescopes (DT) pointed anti-sunward to measure impacting dust particles' mass, composition, charge, and velocity vector. This approach connects decades of ground-based radar measurements of the speeds and directions of meteors from various sources, lacking compositional information, with decades of laboratory work on meteorites' compositions, similarly lacking dynamical information that would reveal their sources. Each DT consists of a Dust Trajectory Sensor (DTS) and an impact ionization reflectron-type time-of-flight (TOF) Composition Analyzer (CA). The DT has strong heritage from instruments already flown and in development: the Cosmic Dust Analyzer (CDA) onboard Cassini, the Lunar Dust Experiment (LDEX) onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, and the Surface Dust Analyzer (SUDA) instrument for the Europa Clipper mission. FOSSIL is a low-risk mission concept that would survey the composition of the solar system's primitive material at a cost far lower than sending probes to a representative sample of small bodies. FOSSIL would go beyond the in situ measurements from Giotto, Ulysses, Stardust, Rosetta, and Cassini, to enable a breakthrough in understanding how the fine dust from comets, asteroids, and interstellar space evolves in our solar system's largest visible structure, the Zodiacal Cloud.

Introduction:The main-belt comets (MBCs) are objects that exhibit cometary activity but occupy stable orbits in the main asteroid belt. MBCs are considered as a hidden reservoir of water ice in the Solar System, and have the potential in terms of scientific research and space resource utilization in the future (Hsieh, 2020). The main-belt comet 133P/Elst-Pizarro (133P) is the first discovered &amp;#160;and best characterized member of the MBCs. Dynamically, 133P fits well in Themis asteroid family (Hsieh et al., 2004). The spectra of 133P resembles best of B-type asteroid and is very similar to those of Themis family members (Hsieh et al., 2008; Licandro et al., 2011). Meteorites that can be compared to the Themis family are CI, CM, CR, CI-unusual, and CM-unusual (Clark et al., 2010).This abstract described a series of methods to manufacture ice-bearing regolith simulant samples for 133P in the laboratory. The characteristics of the analogue are also obtained. The methods in this work can also be applied to the preparation and application of surface regolith simulants of other icy celestial bodies in the Solar System, which can provide scientific basis and ground truth for remote sensing detection, landing sampling and in-situ resource utilization in future probes.&amp;#160;Method:The study involved creating a surface material analogue based on the observed and modeled composition of 133P. The surface regolith of the cometary is mainly composed of water ice, refractory organic matter, and minerals. A basic recipe for the prototype analogue is based on the observation and compositional modeling spectrum of 133P (Rousselot et al., 2011) and the modal mineralogy of the related meteorites. The analogue was developed using a mix of water ice, silicates (serpentine, olivine, pyroxene), amorphous carbon, and organics. Two methods were employed for preparing the ice-dust mixtures:Mixing pre-prepared micron-sized ice particles with comet surface soil mimic powder in a cryogenic freezer.Adding the cometary dust simulant to a dewar with generated ice particles, then separating the mixture after liquid nitrogen volatilization.Fig. 1 Cometary surface regolith ice-dust analogue according to method 1Fig. 2 Cometary surface regolith ice-dust analogue according to method 2Characterization of the analogue included optical microscopy for morphology and particle size, laser diffraction for particle size distribution, X-ray fluorescence (XRF) spectrometry for bulk chemistry, and spectrophotometric analysis for optical properties.Results:The developed analogue successfully mimics the surface regolith of 133P. Optical microscopy revealed that the water ice particles were spherical, ranging from 2-12 &amp;#956;m. Laser diffraction indicated a broad particle size distribution, with the majority of particles falling within the expected size range for cometary dust. XRF spectrometry confirmed that the major elemental composition of the analogue closely matches that of carbonaceous chondrites, specifically CO3 chondrites, which are considered analogues for cometary material. Spectrophotometric analysis showed that the analogue's spectrum aligned well with observed data for 133P, validating the compositional accuracy of the analogue.Fig.3 Spectrum of the cometary material analogue (red line); visible spectrum of 133P reported by Licandro et al. (2011, yellow spots); near-infrared spectrum of 133P reported by Rousselot et al. (2011, blue spots).Conclusion:&amp;#160;Our cometary analogue provides a robust foundation for simulating cometary processes such as sublimation, deposition, and irradiation, crucial for understanding the early Solar System. Additionally, it supports the development of technologies for future space missions, including sample return missions and in-situ resource utilization.

Asteroids and other Small Solar System Bodies (SSSBs) are currently of great scientific and even industrial interest. Asteroids exist as the permanent record of the formation of the Solar System and therefore hold many clues to its understanding as a whole, as well as insights into the formation of planetary bodies. Additionally, SSSBs are being investigated in the context of impact risks for the Earth, space situational awareness and their possible industrial exploitation (asteroid mining). In all these aspects, the knowledge of the geophysical characteristics of SSSB surface and internal structure are of great importance. Given their size, constitution, and the evidence that many SSSBs are not simple monoliths, these bodies should be studied and modelled as self-gravitating granular systems in general, or as granular systems in micro-gravity environments in particular contexts. As such, the study of the geophysical characteristics of SSSBs is a multi-disciplinary effort that lies at the crossroads between Granular Mechanics, Celestial Mechanics, Soil Mechanics, Aerospace Engineering and Computer Sciences.

Advances in the discovery and characterization of asteroids over the past decade have revealed an unanticipated underlying structure that points to a dramatic early history of the inner Solar System. The asteroids in the main asteroid belt have been discovered to be more compositionally diverse with size and distance from the Sun than had previously been known. This implies substantial mixing through processes such as planetary migration and the subsequent dynamical processes.

NASA’s Deep Space 1 mission demonstrated that a spacecraft can be navigated autonomously during deep-space cruise operations using only optical navigation measurements. A methodology is developed to evaluate the feasibility and accuracy of the Deep Space 1 orbit determination approach throughout the solar system as a function of a spacecraft’s imaging capabilities. Feasibility can be addressed by comparing the apparent magnitudes of the known population of asteroids against the imaging system capabilities. An upper limit on the accuracy of the spacecraft position estimate at a given location can be formulated by assuming observations of at least two asteroids simultaneously. Example results are presented for three camera implementations that span the range of capabilities flown in deep space to date using orbit and absolute magnitude data for 50,129 of the brightest known asteroids. Broadly speaking, achievable accuracies range from approximately 200 to 12,000 km () interior to the main asteroid belt and from 50 to 2000 km within the main belt, depending strongly on the chosen camera implementation. Between the main belt and Jupiter, only a current state-of-the-art imaging system is consistently capable of kinematic positioning using only asteroids. Beyond Jupiter, there are insufficient known asteroids to support this approach without including images of planets and moons. Although these levels of accuracy are far inferior to those achievable with radiometric navigation, they may yet be sufficient to satisfy cruise-phase requirements for many deep-space missions.

We present archival observations of main-belt asteroid (248370) 2005 QN173 (also designated 433P) that demonstrate this recently discovered active asteroid (a body with a dynamically asteroidal orbit displaying a tail or coma) has had at least one additional apparition of activity near perihelion during a prior orbit. We discovered evidence of this second activity epoch in an image captured 2016 July 22 with the DECam on the 4 m Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile. As of this writing, (248370) 2005 QN173 is just the eighth active asteroid demonstrated to undergo recurrent activity near perihelion. Our analyses demonstrate (248370) 2005 QN173 is likely a member of the active asteroid subset known as main-belt comets, a group of objects that orbit in the main asteroid belt that exhibit activity that is specifically driven by sublimation. We implement an activity detection technique, wedge photometry, that has the potential to detect tails in images of solar system objects and quantify their agreement with computed antisolar and antimotion vectors normally associated with observed tail directions. We present a catalog and an image gallery of archival observations. The object will soon become unobservable as it passes behind the Sun as seen from Earth, and when it again becomes visible (late 2022) it will be farther than 3 au from the Sun. Our findings suggest (248370) 2005 QN173 is most active interior to 2.7 au (0.3 au from perihelion), so we encourage the community to observe and study this special object before 2021 December.

Active asteroid (6478) Gault sheds mass independent of location along its orbit. Rotational instability is considered to induce the observed activities. If this is the case, because Gault’s breakup event has not been detected, surface failure is likely, implying that its surface materials are constantly ejected while its major body remains intact. Given this scenario, we first constrain Gault’s bulk cohesive strength. We then characterize heliocentric trajectories of ejected particles over thousands of years. The results show that Gault may be sensitive to structural failure at the current spin period (∼2.5 hr). Gault’s bulk density needs to be below 1.75 g cm−3 in order for particles on the equatorial surface to be shed owing to centrifugal forces. In this case, Gault requires cohesive strength of at least ∼200 Pa to maintain the structure at the center, whereas the surface strength needs to be less than ∼100 Pa to induce mass shedding. This suggests that Gault’s structure may consist of a weak surface layer atop a strong core. The trajectories of dust ejected from Gault depend on how efficiently they are accelerated by solar radiation pressure. Escaped particle clouds with sizes of &lt; ∼100 μm could collide with Gault after ∼700–5300 yr with speeds of ∼0.2 km s−1. This implies a temporal increase in the impact flux and complex interactions between the ejected particles and their host body.