Publications (16)18.13 Total impact
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Conference Proceeding: Spitzer Observations Of The Ultrafast-Rotator (54509) 2000 PH5
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ABSTRACT: We report preliminary results of Spitzer observations of the ultra-fast rotating (P~12 min) small (D~150 m) near-Earth asteroid (54509) 2000 PH5. We sampled its lightcurve at three thermal wavelengths with a temporal resolution of some ten seconds per data point. We employed a preliminary model of our target’s shape and spin state (courtesy of P.A. Taylor and J.-L Margot) and a detailed thermophysical model to determine its size, albedo, and thermal inertia. From knowledge of the thermal inertia, conclusions can be drawn on the presence or absence of loose material (regolith) on the asteroid surface. We expect our target to be regolith-free since its gravity cannot match the centrifugal force. Thermal inertia is also a key parameter in the determination of the Yarkovsky and YORP effects, two non-gravitational effects that affect the orbits and spin states of small asteroids. Vokrouhlický et al. (2005) have proposed that the Yarkovsky effect could be observable in the case of 2000 PH5.IAU Symposium S236 (Near-Earth objects) at the XXVI IAU General Assembly; -
Conference Proceeding: Physical Properties of Asteroid (10302) 1989 ML, a Potential Spacecraft Target, from Spitzer Observations
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ABSTRACT: We report on results from recent Spitzer observations of near-Earth asteroid (10302) 1989 ML, which is among the lowest-ranking objects in terms of the specific momentum Δv required to reach it from Earth. It was originally considered as a target for Hayabusa and is now under consideration as a target of the planned ESA mission Don Quijote. Unfortunately, little is known about the physical properties of 1989 ML, in particular its size and albedo are unknown. Its exhibits an X type reflection spectrum, so depending on its albedo, 1989 ML may be an E, M, or P type asteroid. Provisional results from thermal-infrared observations carried out with Spitzer indicate that the albedo of 1989 ML is compatible with an M- or E-type classification. We will discuss our results and their implications for the physical properties and the rotation period of 1989 ML, and its importance as a potential spacecraft target. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.38th DPS-Meeting of the American Astronomical Society; -
Conference Proceeding: Spitzer observations of mutual events in the binary system (617) Patroclus-Menoetius
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ABSTRACT: We report Spitzer observations of the binary Trojan system (617) Patroclus-Menoetius during two mutual events, when respectively one component shadowed and occulted the other. Observing the thermal response to mutual shadowing with spectral ( 8--33 µm) and temporal resolution allowed us to determine the system's thermal inertia in a uniquely direct way. Furthermore, our analysis provided an accurate determination of the system's size which is methodologically independent of the estimate by Berthier et al. (this session). Our results allow a more reliable estimate of the system's bulk density (the total mass was determined from the system's mutual orbit; Marchis et al., 2006; Berthier et al., this session). This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. This material is partly based upon work supported by the national Aeronautics and Space Administration issue through the Science Mission Directorate Research and Analysis Programs number NNG05GF09G.American Astronomical Society, DPS meeting 39; -
Conference Proceeding: Peculiar Betulia Revisited: A Near-Earth Asteroid with a Bare-Rock Surface?
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ABSTRACT: The small C-type asteroid (1580) Betulia is an unusual near-Earth object (NEO) with a lightcurve that changes dramatically with changing solar phase angle, presumably due to a highly irregular shape and/or unusual topographic features. Earlier thermal-infrared observations indicated a surface of high thermal inertia, which is consistent with a lack of thermally insulating regolith. Absence of regolith might be expected in the case of small NEOs with weak gravities, which may be unable to retain collisional debris. However, recent infrared observations of other asteroids of comparable size indicate that regolith is normally present. Knowledge of the thermal properties of NEOs is crucial for meaningful calculations of the Yarkovsky effect, which is invoked to explain the delivery of collisional fragments from the main belt into near-Earth orbits, and apparently has a significant influence on the orbital evolution of potentially hazardous NEOs. We observed Betulia in June 2002 with the 3-m NASA Infrared Telescope Facility. Our database is sufficiently broad to allow the use of more sophisticated thermal models than were available for earlier radiometric observations. It is important to bear the unusual nature of Betulia in mind when interpreting observational data. Fits to our data obtained with a new thermophysical model imply an effective diameter of 4.57±0.46 km and an albedo of 0.077±0.015, and indicate a moderate surface thermal inertia of around 180 Jm<sup>-2</sup>s<sup>-0.5</sup>K<sup>-1</sup>. It is difficult to reconcile our results with those of earlier work, which indicate a larger diameter for Betulia and a high-thermal-inertia surface of bare rock. While the thermal inertias of NEOs appear to be significantly higher than those of large main-belt asteroids, to our knowledge no convincing evidence has yet been found for very high-thermal-inertia “bare-rock” surfaces amongst NEOs: further observations are required to probe smaller objects.37th DPS-Meeting of the American Astronomical Society; -
Article: Thermal inertia of near-Earth asteroids and implications for the magnitude of the Yarkovsky effect
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ABSTRACT: Thermal inertia determines the temperature distribution over the surface of an asteroid and therefore governs the magnitude the Yarkovsky effect. The latter causes gradual drifting of the orbits of km-sized asteroids and plays an important role in the delivery of near-Earth asteroids (NEAs) from the main belt and in the dynamical spreading of asteroid families. At present, very little is known about the thermal inertia of asteroids in the km size range. Here we show that the average thermal inertia of a sample of NEAs in the km-size range is 200 ± 40 Jm<sup>−2</sup> s<sup>−0.5</sup> K<sup>−1</sup>. Furthermore, we identify a trend of increasing thermal inertia with decreasing asteroid diameter, D. This indicates that the dependence of the drift rate of the orbital semimajor axis on the size of asteroids due to the Yarkovsky effect is a more complex function than the generally adopted D<sup>−1</sup> dependence, and that the size distribution of objects injected by Yarkovsky-driven orbital mobility into the NEA source regions is less skewed to smaller sizes than generally assumed. We discuss how this fact may help to explain the small difference in the slope of the size distribution of km-sized NEAs and main-belt asteroids.Icarus 190(2007-03-30-1):236-249. · 3.38 Impact Factor -
Article: Physical characterization of the potentially hazardous high-albedo Asteroid (33342) 1998 WT24 from thermal-infrared observations
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ABSTRACT: The potentially hazardous Asteroid (33342) 1998 WT24 approached the Earth within 0.0125 AU on 2001 December 16 and was the target of a number of optical, infrared, and radar observing campaigns. Interest in 1998 WT24 stems from its having an orbit with an unusually low perihelion distance, which causes it to cross the orbits of the Earth, Venus, and Mercury, and its possibly being a member of the E spectral class, which is rare amongst near-Earth asteroids (NEAs). We present the results of extensive thermal-infrared observations of 1998 WT24 obtained in December 2001 with the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii and the ESO 3.6-m telescope in Chile. A number of thermal models have been applied to the data, including thermophysical models that give best-fit values of 0.35±0.04 km for the effective diameter, 0.56±0.2 for the geometric albedo, pv, and 100–300 J m−2 s−0.5 K−1 for the thermal inertia. Our values for the diameter and albedo are consistent with results derived from radar and polarimetric observations. The albedo is one of the highest values obtained for any asteroid and, since no other taxonomic type is associated with albedos above 0.5, supports the suggested rare E-type classification for 1998 WT24. The thermal inertia is an order of magnitude higher than values derived for large main-belt asteroids but consistent with the relatively high values found for other near-Earth asteroids. A crude pole solution inferred from a combination of our observations and published radar results is β=−52°, λ=355° (J2000), but we caution that this is uncertain by several tens of degrees.Icarus. -
Article: The surface properties of small asteroids: Peculiar Betulia - a case study
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ABSTRACT: We present the results of extensive thermal–infrared observations of the C-type near-Earth Asteroid (1580) Betulia obtained in June 2002 with the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Betulia is a highly unusual object for which earlier radiometric observations, interpreted on the basis of simple thermal models, indicated a surface of high thermal inertia. A high thermal inertia implies a lack of thermally insulating regolith. Radiometric observations of other asteroids of comparable size indicate that regolith is present in nearly all cases. Knowledge of the surface thermal properties of small near-Earth asteroids is crucial for meaningful calculations of the Yarkovsky effect, which is invoked to explain the delivery of collisional fragments from the main belt into near-Earth orbits, and apparently has a significant influence on the orbital evolution of potentially hazardous near-Earth asteroids. Furthermore, apart from being an indicator of the presence of thermally insulating regolith on the surface of an asteroid, the thermal inertia determines the magnitude of the diurnal temperature variation and is therefore of great importance in the design of instrumentation for lander missions to small asteroids. In the case of Betulia our database is sufficiently broad to allow the use of more sophisticated thermal models than were available for earlier radiometric observations. The measured fluxes have been fitted with thermal-model emission continua to determine the asteroid's size and geometric albedo, pv. Fits obtained with a new thermophysical model imply an effective diameter of 4.57±0.46 km and an albedo of 0.077±0.015 and indicate a moderate surface thermal inertia of around 180 J m<sup>−2</sup> s<sup>−0.5</sup> K<sup>−1</sup>. It is difficult to reconcile our results with earlier work, which indicate a larger diameter for Betulia and a high-thermal-inertia surface of bare rock.Icarus 179(1):95-108. · 3.38 Impact Factor -
Conference Proceeding: Thermische Modellierung erdnaher Asteroiden
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ABSTRACT: Die ca. 3000 zur Zeit bekannten erdnahen Asteroiden sind nicht nur als kleinste beobachtbare Himmelskörper interessant, sondern auch als potentielle Erd-Impaktoren. Für die Erforschung ihrer Verbindung mit Hauptgürtelasteroiden und Meteoriten, für die Planung von Raumfahrtmissionen und zur Einschätzung der Gefahr die vom Einschlagsrisiko ausgeht, müssen ihre physikalischen Eigenschaften, allen voran die Größe, bekannt sein, die aus optischen Beobachtungen allein nicht ableitbar sind. Daher beobachten und modellieren wir die von erdnahen Asteroiden emittierte Wärmestrahlung im mittleren Infrarot (5–20µm, M-, N- und Q-Band), die Rückschlüsse über Größe, optische Reflektivität, mesoskopische Oberflächenrauhigkeit und thermische Trägheit der Objekte zulässt. An der thermischen Trägheit lässt sich erkennen, inwieweit das Oberflächenmaterial pulverisiert ist (Regolith) oder aus anstehendem Gestein besteht, wie man es für kleine Körper mit geringer Schwerkraft erwartet. Besonders detaillierte Ergebnisse liegen vor für den ca. 500m kleinen Asteroiden (25143) Itokawa, das Ziel der japanischen ’sample-return’-Mission Hayabusa. Entgegen früherer Erwartungen sind unsere Daten unverträglich mit einer felsigen Oberflächengestalt von Itokawa; sie deuten vielmehr auf eine von Regolith dominierte Oberfläche hin. Diese Behauptung ist ab Sommer 2005, der Ankunft von Hayabusa, überprüfbar und von großer Bedeutung für die Planung der Probenentnahme.69. Jahrestagung der Deutschen Physikalischen Gesellschaft; -
Article: Eclipsing binary Trojan asteroid Patroclus: Thermal inertia from Spitzer observations
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ABSTRACT: We present mid-infrared observations of the binary L5-Trojan system (617) Patroclus–Menoetius before, during, and after two shadowing events, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. For the first time, we effectively observe changes in asteroid surface temperature in real time, allowing the thermal inertia to be determined very directly. A new detailed binary thermophysical model is presented which accounts for the system’s known mutual orbit, arbitrary component shapes, and thermal conduction in the presence of eclipses.We obtain two local thermal-inertia values, representative of the respective shadowed areas: and . The average thermal inertia is estimated to be , potentially with significant surface heterogeneity. This first thermal-inertia measurement for a Trojan asteroid indicates a surface covered in fine regolith. Independently, we establish the presence of fine-grained (<a few μm) silicates on the surface, based on emissivity features near 10 and similar to those previously found on other Trojans.We also report V-band observations and report a lightcurve with complete rotational coverage. The lightcurve has a low amplitude of peak-to-peak, implying a roughly spherical shape for both components, and is single-periodic with a period equal to the period of the mutual orbit, indicating that the system is fully synchronized.The diameters of Patroclus and Menoetius are 106±11 and , respectively, in agreement with previous findings. Taken together with the system’s known total mass, this implies a bulk mass density of , significantly below the mass density of L4-Trojan asteroid (624) Hektor and suggesting a bulk composition dominated by water ice.All known physical properties of Patroclus, arguably the best studied Trojan asteroid, are consistent with those expected in icy objects with devolatilized surface (extinct comets), consistent with what might be implied by recent dynamical modeling in the framework of the Nice Model.Icarus. -
Article: The size and albedo of Rosetta fly-by target 21 Lutetia from new IRTF measurements and thermal modeling
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ABSTRACT: Recent spectroscopic observations indicate that the M-type asteroid 21 Lutetia has a primitive, carbonaceous-chondrite-like (C-type) surface composition for which a low geometric albedo would be expected; this is incompatible with the IRAS albedo of 0.221+/- 0.020. From new thermal-infrared spectrophotometric measurements and detailed thermophysical modeling we infer that Lutetia has a diameter of 98.3 +/- 5.9 km and a geometric albedo of 0.208 +/- 0.025, in excellent agreement with the IRAS value. We can thus rule out a low albedo typical of a C-type taxonomic classification. Furthermore, we find that Lutetia's thermal properties are well within the range expected for large asteroids; we find no evidence for unusually high thermal inertia.Astronomy and Astrophysics 447(2006-03):1153-1158. · 4.59 Impact Factor -
Conference Proceeding: Indications for Regolith on Itokawa from Thermal-Infrared Observations
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ABSTRACT: We report preliminary results of thermal-infrared observations of the near-Earth asteroid (25143) Itokawa made with the ESO 3.6m telescope in 2001 and the NASA IRTF in 2004. Using these and data from the literature we investigated the size and thermal properties of Itokawa. We were not able to obtain an un-ambiguous result for Itokawa’s size but we could reliably constrain the thermal properties of the surface. We use a detailed thermophysical model accounting explicitly for Itokawa’s shape and spin state, heat conduction, and surface roughness. Our results indicate that Itokawa’s surface is dominated by regolith rather than by bare rock; a surface of bare rock only is ruled out, although boulders covering a moderate percentage of the surface are not excluded.The first Hayabusa Symposium; -
Article: Size, Albedo, and Taxonomic Type of Potential Spacecraft Target Asteroid (10302) 1989 ML
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ABSTRACT: The Amor-type near-Earth Asteroid (10302) 1989 ML has an “Earth-like” orbit (period 1.44 yr, eccentricity 0.14, inclination 4.4°), therefore the energy required to reach it from the Earth is relatively small making it a very attractive target for rendezvous missions. We have observed 1989 ML in the thermal–infrared using the Spitzer Space Telescope, and compared these data with optical and near-infrared observations. The Spitzer results imply a diameter of 0.28±0.05 km and a geometric albedo of 0.37±0.15; together with the reflectance spectrum they are consistent with the relatively rare E classification.Icarus 187(2):611-615. · 3.38 Impact Factor -
Conference Proceeding: Thermal Inertia of near-Earth Asteroids and Strength of the Yarkovsky Effect
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ABSTRACT: Thermal inertia is the physical parameter that controls the temperature distribution over the surface of an asteroid. It affects the strength of the Yarkovsky effect, which causes orbital drift of km-sized asteroids and is invoked to explain the delivery of near-Earth asteroids (NEAs) from the main belt. Moreover, measurements of thermal inertia provide information on the presence or absence of loose surface material, such as thermally insulating regolith or dust. At present, very little is known about the thermal inertia of asteroids in the km size range. Using an extensive dataset of thermal infrared observations obtained at the Keck 1, the ESO 3.6m and the IRTF telescopes, we find that the mean thermal inertia of near-Earth asteroids in the km-size range is 200 ± 50 J m<sup>-2</sup> s<sup>-0.5</sup> K<sup>-1</sup> corresponding to a surface thermal conductivity of 0.03 ± 0.01 W m<sup>-1</sup>K<sup>-1</sup>. Combining this result with published values of asteroid thermal inertias, we also identify a trend of increasing thermal inertia with decreasing asteroid size. As a consequence, the dependence of the Yarkovsky-induced semimajor axis drift rate on object diameter, D, departs from the 1/D dependence commonly assumed in models of the dynamical evolution of asteroids.38th DPS-Meeting of the American Astronomical Society; -
Conference Proceeding: Thermal Inertia of Near-Earth Asteroids and Magnitude of the Yarkovsky Effect
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ABSTRACT: Thermal inertia is the physical parameter that controls the temperature distribution over the surface of an asteroid. It affects the strength of the Yarkovsky effect, which causes orbital drift of km-sized asteroids and is invoked to explain the delivery of near-Earth asteroids (NEAs) from the main belt. Measurements of thermal inertia provide information on the presence or absence of loose surface material, such as thermally insulating regolith or dust. Such information is not only important for scientific studies of asteroid surface properties, but it is also vital for the design of lander missions and in the development of technology for the deflection of hazardous asteroids. At present, very little is known about the thermal inertia of asteroids in the km size range. Here we report on a method that has allowed us to derive a mean value for the thermal inertia of near-Earth asteroids on the basis of multi-wavelength thermal-infrared observations. We obtain a mean value of 200 ± 50 J m<sup>-2</sup>s<sup>-0.5</sup>K<sup>-1</sup> corresponding to a surface thermal conductivity of 0.03 ± 0.01 W m<sup>-1</sup>K<sup>-1</sup>. We also identify a trend of increasing thermal inertia with decreasing asteroid size. As a consequence, the dependence of the Yarkovsky-induced semimajor axis drift rate on object diameter, D, departs from the 1/D dependence commonly assumed in models of the dynamical evolution of asteroids.European Planetary Science Congress 2006; -
Article: Physical characterization of the potentially-hazardous high-albedo Asteroid (33342) 1998 WT<sub>24</sub> from thermal-infrared observations
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ABSTRACT: The potentially-hazardous asteroid (33342) 1998 WT<sub>24</sub> approached the Earth within 0.0125 AU on 2001 Dec. 16 and was the target of a number of optical, infrared, and radar observing campaigns. Interest in 1998 WT<sub>24</sub> stems from its having an orbit with an unusually low perihelion distance, which causes it to cross the orbits of the Earth, Venus, and Mercury, and its possibly being a member of the E spectral class, which is rare amongst near-Earth asteroids (NEAs). We present the results of extensive thermal-infrared observations of 1998 WT<sub>24</sub> obtained in December 2001 with the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii and the ESO 3.6-m telescope in Chile. A number of thermal models have been applied to the data, including thermophysical models that give best-fit values of 0.35 ± 0.04 km for the effective diameter, 0.56 ± 0.2 for the geometric albedo, pv, and 100 – 300 J m<sup>-2</sup> s<sup>-0.5</sup> K<sup>-1</sup> for the thermal inertia. Our values for the diameter and albedo are consistent with results derived from radar and polarimetric observations. The albedo is one of the highest values obtained for any asteroid and, since no other taxonomic type is associated with albedos above 0.5, supports the suggested rare E-type classification for 1998 WT<sub>24</sub>. The thermal inertia is an order of magnitude higher than values derived for large main-belt asteroids but consistent with the relatively high values found for other near-Earth asteroids. A crude pole solution inferred from a combination of our observations and published radar results is β = -52°, λ = 355° (J2000), but we caution that this is uncertain by several tens of degrees.Icarus 188:414-424. · 3.38 Impact Factor -
Article: A survey of Karin cluster asteroids with the Spitzer Space Telescope
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ABSTRACT: The Karin cluster is one of the youngest known families of main-belt asteroids, dating back to a collisional event only 5.8±0.2 Myr ago. Using the Spitzer Space Telescope we have photometrically sampled the thermal continua (3.5–22 μm) of 17 Karin cluster asteroids of different sizes, down to the smallest members discovered so far, in order to make the first direct measurements of their sizes and albedos and study the physical properties of their surfaces. Our targets are also amongst the smallest main-belt asteroids observed to date in the mid-infrared. The derived diameters range from 17.3 km for 832 Karin to 1.5 km for 75176, with typical uncertainties of 10%. The mean albedo is pv=0.215±0.015, compared to 0.20±0.07 for 832 Karin itself (for H=11.2±0.3), consistent with the view that the Karin asteroids are closely related physically as well as dynamically. The albedo distribution (0.12⩽pv⩽0.32) is consistent with the range associated with S-type asteroids but the variation from one object to another appears to be significant. Contrary to the case for near-Earth asteroids, our data show no evidence of an albedo dependence on size. However, the mean albedo is lower than expected for young, fresh “S-type” surfaces, suggesting that space weathering can darken main-belt asteroid surfaces on very short timescales. Our data are also suggestive of a connection between surface roughness and albedo, which may reflect rejuvenation of weathered surfaces by impact gardening. While the available data allow only estimates of lower limits for thermal inertia, we find no evidence for the relatively high values of thermal inertia reported for some similarly sized near-Earth asteroids. Our results constitute the first observational confirmation of the legitimacy of assumptions made in recent modeling of the formation of the Karin cluster via a single catastrophic collision 5.8±0.2 Myr ago.Icarus.
