Mass and Density of Asteroid (16) Psyche

We apply our novel Markov Chain Monte Carlo (MCMC)-based algorithm for asteroid mass estimation to asteroid (16) Psyche, the target of NASA's eponymous Psyche mission, based on close encounters with 10 different asteroids, and obtain a mass of (1.117 0.039) x 10(-11) M. We ensure that our method works as expected by applying it to asteroids (1) Ceres and (4) Vesta, and find that the results are in agreement with the very accurate mass estimates for these bodies obtained by the Dawn mission. We then combine our mass estimate for Psyche with the most recent volume estimate to compute the corresponding bulk density as (3.88 0.25) g cm(-3). The estimated bulk density rules out the possibility of Psyche being an exposed, solid iron core of a protoplanet, but is fully consistent with the recent hypothesis that ferrovolcanism would have occurred on Psyche.Peer reviewe

Thermal decomposition as an activity driver of near-Earth asteroid (3200) Phaethon

Near-Earth asteroid (3200) Phaethon is an active asteroid with a dust tail repeatedly observed over the past decade for 3 days during each perihelion passage down to a heliocentric distance of 0.14 au. The mechanism causing the activity is still debated, and the suggested mechanisms lack clear supporting evidence. Phaethon has been identified as the likely parent body of the annual Geminid meteor shower, making it one of the few active asteroids associated with a meteoroid stream. Its low albedo and B-type reflectance spectrum indicates that Phaethon's composition is similar to carbonaceous chondrite meteorites, but a connection to a specific meteorite group is ambiguous due to the lack of diagnostic absorption features. In this study, we analyze the mid-infrared emissivity spectrum of Phaethon and find that it is closely associated with the Yamato-group (CY) of carbonaceous chondrites. The CY chondrites represent primitive carbonaceous material that experienced early aqueous alteration and subsequent late-stage thermal metamorphism. Minerals in these meteorites, some of which we identify in Phaethon's spectrum, show evidence of thermal decomposition; notably, the dehydroxylation and transformation of phyllosilicates into poorly crystalline olivine. Additionally, sulfides and carbonates in CYs are known release S2and CO2 gas upon heating to ~700oC. We show that Phaethon's surface temperature during its observed window of activity is consistent with the thermal decomposition temperatures of several components in CY meteorites. All of these lines of evidence are strong indicators that gas release from thermal decomposition reactions is responsible for Phaethon's activity. The results of this study have implications for the formation of the Geminid meteoroid stream, the origins of thermally-altered primitive meteorites, and the destruction of low-perihelion asteroids.Comment: in revie

Asteroidien tunnistaminen tilastollisilla ratainversiomenetelmillä

An efficient and statistically robust solution for the identification of asteroids among numerous sets of astrometry is presented. In particular, numerical methods have been developed for the short-term identification of asteroids at discovery, and for the long-term identification of scarcely observed asteroids over apparitions, a task which has been lacking a robust method until now. The methods are based on the solid foundation of statistical orbital inversion properly taking into account the observational uncertainties, which allows for the detection of practically all correct identifications. Through the use of dimensionality-reduction techniques and efficient data structures, the exact methods have a loglinear, that is, O(nlog(n)), computational complexity, where n is the number of included observation sets. The methods developed are thus suitable for future large-scale surveys which anticipate a substantial increase in the astrometric data rate. Due to the discontinuous nature of asteroid astrometry, separate sets of astrometry must be linked to a common asteroid from the very first discovery detections onwards. The reason for the discontinuity in the observed positions is the rotation of the observer with the Earth as well as the motion of the asteroid and the observer about the Sun. Therefore, the aim of identification is to find a set of orbital elements that reproduce the observed positions with residuals similar to the inevitable observational uncertainty. Unless the astrometric observation sets are linked, the corresponding asteroid is eventually lost as the uncertainty of the predicted positions grows too large to allow successful follow-up. Whereas the presented identification theory and the numerical comparison algorithm are generally applicable, that is, also in fields other than astronomy (e.g., in the identification of space debris), the numerical methods developed for asteroid identification can immediately be applied to all objects on heliocentric orbits with negligible effects due to non-gravitational forces in the time frame of the analysis. The methods developed have been successfully applied to various identification problems. Simulations have shown that the methods developed are able to find virtually all correct linkages despite challenges such as numerous scarce observation sets, astrometric uncertainty, numerous objects confined to a limited region on the celestial sphere, long linking intervals, and substantial parallaxes. Tens of previously unknown main-belt asteroids have been identified with the short-term method in a preliminary study to locate asteroids among numerous unidentified sets of single-night astrometry of moving objects, and scarce astrometry obtained nearly simultaneously with Earth-based and space-based telescopes has been successfully linked despite a substantial parallax. Using the long-term method, thousands of realistic 3-linkages typically spanning several apparitions have so far been found among designated observation sets each spanning less than 48 hours.Lähiasteroidien ihmiskunnalle aiheuttama mahdollinen uhka on lisännyt voimakkaasti kiinnostusta asteroidien etsintään sekä niiden ratojen määrittämiseen. Useiden havaintoprojektien tuloksena on tunnettujen asteroidien lukumäärä kymmenkertaistunut kymmenen viime vuoden aikana vajaaseen neljäänsataantuhanteen, mutta samalla on kertynyt jopa satojatuhansia niukkoja havaintojoukkoja, jotka eivät yksinään riitä radan määrittämiseen niin tarkasti, että kohde voitaisiin tunnistaa muista havainnoista. Kertyneiden havaintojen joukossa voi siis olla useita samasta kohteesta tehtyjä erillisiä, niukkoja havaintojoukkoja. Sen selvittäminen, mitkä havaintojoukot liittyvät samaan kohteeseen, on laskennallisesti raskas tehtävä. Jos esimerkiksi verrattaisiin jokaista n:ää havaintojoukkoa kaikkiin muihin, laskentaan kuluva aika olisi verrannollinen lukuun n^2. Koska seuraavan sukupolven teleskoopeilla toteutettavat lähiasteroidien etsinnät tulevat kasvattamaan havaintomäärää huomattavasti, tarvitaan niin kutsuttuja loglineaarisia tunnistusmenetelmiä, eli menetelmiä joissa laskentaan kuluva aika olisi verrannollinen lukuun nlog(n). Loglineaaristen menetelmien keskeinen etu on siis se, että havaintojen määrän kasvu ei lisää tunnistusongelman ratkaisuun tarvittavaa aikaa suhteettoman paljon. Tässä työssä on kehitetty kaksi tilastolliseen ratainversioon perustuvaa loglineaarista tunnistusmenetelmää erityyppisten asteroidien tunnistusongelmien ratkaisemiseen: yksi lyhyen aikavälin kytkentään päivien tai viikkojen aikaskaalassa ja toinen pitkän aikavälin kytkentään vuosien tai vuosikymmenien aikaskaalassa. Jälkimmäinen ongelma on erityisen vaativa, eikä sille ole ennen tätä työtä ollut tarjolla yleistä ratkaisumenetelmää. Käytännössä asteroidien tunnistus lukuisten niukkojen havaintojoukkojen seasta tapahtuu etsimällä rataratkaisu, joka samanaikaisesti selittää kaikki tietystä kohteesta tehdyt havainnot. Toisin sanoen havainnot siis kytketään yhteen rataratkaisun avulla. Tässä työssä kehitetyissä menetelmissä samasta kappaleesta tehtyjä havaintojoukkoja etsitään vertaamalla tilastollisen inversion avulla kullekin havaintojoukolle laskettujen rataelementtien tai niiden johdannaisten todennäköisyystiheysfunktioita. Tilastollisen inversion avulla havaintojen satunnaisvirheet otetaan huomioon siten, että ne eivät estä asteroidien tunnistamista. Ehdokaskytkentöjen etsintä suoritetaan tehokkaasti yksiulotteistamalla moniulotteinen vertailuparametriavaruus ja käyttämällä hyväksi puumaisia tietorakenteita. Lopuksi kullekin ehdokaskytkennälle pyritään löytämään erilliset havaintojoukot kytkevä rataratkaisu pääasiassa tilastollisen inversion avulla. Kehitettyjä menetelmiä on sovellettu menestyksekkäästi sekä useisiin simuloituihin testiaineistoihin että esimerkiksi maanpäällisten teleskooppien ja Spitzer-avaruusteleskoopin tekemien asteroidihavaintojen tunnistusongelman ratkaisemiseen. Alle kahden vuorokauden pituisten havaintosarjojen joukosta on lisäksi hiljattain löydetty tuhansia realistisia kytkentöjä, jotka viittaavat merkittävään määrään aikaisemmin tunnistamattomiksi jääneitä asteroideja

Evidence of surface heterogeneity on active asteroid (3200) Phaethon

Thermal infrared emission and thermophysical modeling techniques are powerful tools in deciphering the surface properties of asteroids. The near-Earth asteroid (3200) Phaethon is an active asteroid with a very small perihelion distance and is likely the source of the Geminid meteor shower. Using a thermophysical model with a non-convex shape of Phaethon we interpret thermal infrared observations that span ten distinct sightings. The results yield an effective diameter of 5.4 +/- 0.1 km and independent thermal inertia estimates for each sighting. We find that the thermal inertia varies across each of these sightings in a way that is stronger than the theoretical temperature-dependent expectation from radiative heat transfer within the regolith. Thus, we test whether the variation in thermal inertia can be explained by the presence of a regolith layer over bedrock, or by a spatially heterogeneous scenario. We find that a model in which Phaethon's hemispheres have distinctly different thermophysical properties can sufficiently explain the thermal inertias determined herein. In particular, we find that a boundary is located between latitudes -30(o) and +10(o) that separates two regions: a fine-grained southern latitudes and a northern hemisphere that is dominated by coarse-grained regolith and/or a high coverage of porous boulders. We discuss the implications related to Phaethon's activity, potential association with 2005 UD, and the upcoming DESTINY+ mission.Peer reviewe

SHINeS : Space and High-Irradiance Near-Sun Simulator

We present SHINeS, a space simulator which can be used to replicate the thermal environment in the immediate neighborhood of the Sun down to a heliocentric distance r ~ 0.06 au. The system consists of three main parts: the solar simulator which was designed and constructed in-house, a vacuum chamber, and the probing and recording equipment needed to monitor the experimental procedures. Our motivation for building this experimental system was to study the effect of intense solar radiation on the surfaces of asteroids when their perihelion distances become smaller than the semi-major axis of the orbit of Mercury. Comparisons between observational data and recent orbit and size-frequency models of the population of near-Earth asteroids suggest that asteroids are super-catastrophically destroyed when they approach the Sun. Whereas the current models are agnostic about the disruption mechanism, SHINeS was developed to study the mechanism or mechanisms responsible. The system can, however, be used for other applications that need to study the effects of high solar radiation on other natural or artificial objects.Peer reviewe

The population of natural Earth satellites

Peer reviewe

Surveys, Astrometric Follow-up & Population Statistics

Asteroid surveys are the backbone of asteroid science, and with this in mind we begin with a broad review of the impact of asteroid surveys on our field. We then provide a brief history of asteroid discoveries so as to place contemporary and future surveys in perspective. Surveys in the United States have discovered the vast majority of the asteroids and this dominance has been consolidated since the publication of Asteroids III. Our descriptions of the asteroid surveys that have been operational since that time are focussed upon those that have contributed the vast majority of asteroid observations and discoveries. We also provide some insight into upcoming next-generation surveys that are sure to alter our understanding of the small bodies in the inner solar system and provide evidence to untangle their complicated dynamical and physical histories. The Minor Planet Center, the nerve center of the asteroid discovery effort, has improved its operations significantly in the past decade so that it can manage the increasing discovery rate, and ensure that it is well-placed to handle the data rates expected in the next decade. We also consider the difficulties associated with astrometric follow-up of newly identified objects. It seems clear that both of these efforts must operate in new modes in order to keep pace with expected discovery rates of next-generation ground- and space-based surveys.Comment: Chapter to appear in the book ASTEROIDS IV, (University of Arizona Press) Space Science Series, edited by P. Michel, F. DeMeo and W. Bottk

The Saricicek howardite fall in Turkey : Source crater of HED meteorites on Vesta and impact risk of Vestoids

The Saricicek howardite meteorite shower consisting of 343 documented stones occurred on September 2, 2015 in Turkey and is the first documented howardite fall. Cosmogenic isotopes show that Saricicek experienced a complex cosmic-ray exposure history, exposed during 12-14Ma in a regolith near the surface of a parent asteroid, and that an 1m sized meteoroid was launched by an impact 22 +/- 2Ma ago to Earth (as did one-third of all HED meteorites). SIMS dating of zircon and baddeleyite yielded 4550.4 +/- 2.5Ma and 4553 +/- 8.8Ma crystallization ages for the basaltic magma clasts. The apatite U-Pb age of 4525 +/- 17Ma, K-Ar age of 3.9Ga, and the U,Th-He ages of 1.8 +/- 0.7 and 2.6 +/- 0.3Ga are interpreted to represent thermal metamorphic and impact-related resetting ages, respectively. Petrographic; geochemical; and O-, Cr-, and Ti-isotopic studies confirm that Saricicek belongs to the normal clan of HED meteorites. Petrographic observations and analysis of organic material indicate a small portion of carbonaceous chondrite material in the Saricicek regolith and organic contamination of the meteorite after a few days on soil. Video observations of the fall show an atmospheric entry at 17.3 +/- 0.8kms(-1) from NW; fragmentations at 37, 33, 31, and 27km altitude; and provide a pre-atmospheric orbit that is the first dynamical link between the normal HED meteorite clan and the inner Main Belt. Spectral data indicate the similarity of Saricicek with the Vesta asteroid family (V-class) spectra, a group of asteroids stretching to delivery resonances, which includes (4) Vesta. Dynamical modeling of meteoroid delivery to Earth shows that the complete disruption of a 1km sized Vesta family asteroid or a 10km sized impact crater on Vesta is required to provide sufficient meteoroids 4m in size to account for the influx of meteorites from this HED clan. The 16.7km diameter Antionia impact crater on Vesta was formed on terrain of the same age as given by the He-4 retention age of Saricicek. Lunar scaling for crater production to crater counts of its ejecta blanket show it was formed 22Ma ago.Peer reviewe