arXiv subjects

Simon F. Green

Publications and source records attributed to Simon F. Green.

22 records · Page 2Linked to original sources

arXiv2012

The Influence of Rough Surface Thermal-Infrared Beaming on the Yarkovsky and YORP Effects

It is now becoming widely accepted that photon recoil forces from the asymmetric reflection and thermal re-radiation of absorbed sunlight are, together with collisions and gravitational forces, primary mechanisms governing the dynamical and physical evolution of asteroids. The Yarkovsky effect causes orbital semi-major axis drift and the YORP effect causes changes in the rotation rate and pole orientation. We present an adaptation of the Advanced Thermophysical Model (ATPM) to simultaneously predict the Yarkovsky and YORP effects in the presence of thermal-infrared beaming caused by surface roughness, which has been neglected or dismissed in all previous models. Tests on Gaussian random sphere shaped asteroids, and on the real shapes of asteroids (1620) Geographos and (6489) Golevka, show that rough surface thermal-infrared beaming enhances the Yarkovsky orbital drift by typically tens of percent but it can be as much as a factor of two. The YORP rotational acceleration is on average dampened by up to a third typically but can be as much as one half. We find that the Yarkovsky orbital drift is only sensitive to the average degree, and not to the spatial distribution, of roughness across an asteroid surface. However, the YORP rotational acceleration is sensitive to the surface roughness spatial distribution, and can add significant uncertainties to the predictions for asteroids with relatively weak YORP effects. To accurately predict either effect the degree and spatial distribution of roughness across an asteroid surface must be known.

astro-ph.EP↗

arXiv2011

Physical characterisation of low delta-V asteroid (175706) 1996 FG3

Asteroid (175706) 1996 FG3 is a binary asteroid and the baseline target for the proposed MarcoPolo-R sample-return mission. We present thermal IR photometry obtained with the ESO VLT+VISIR together with optical photometry obtained with the ESO NTT+EFOSC2 . An absolute visual magnitude H_V = 17.833 \pm 0.024 and phase parameter G = -0.041 \pm 0.005 is derived. The Near-Earth Asteroid Thermal Model (NEATM) has been fitted to the measured fluxes to derive a geometric visual albedo p_v = 0.046 \pm 0.014, effective diameter at the observed aspect D_eff = 1.68 \pm 0.25 km, and beaming parameter η = 1.15 for phase angle α = 11.7°. The Advanced Thermophysical Model (ATPM) has been fitted to the measured fluxes to derive a more accurate effective diameter D_eff = 1.71 \pm 0.07 km and albedo p_v = 0.044 \pm 0.004. Based on the ATPM results, assuming the same albedo for primary and secondary, we derive a primary mean spherical diameter D_p = (1.69 +0.18/-0.12) km, secondary diameter D_s = 0.51 \pm 0.03 km, and a secondary orbital semi-major axis a = (2.8 +1.7/-0.7) km. A low surface thermal inertia Γ = 120 \pm 50 J m^-2 s^-1/2 K^-1 was also derived, suggesting a dusty surface and raising questions as to the binary formation mechanism of this asteroid. These physical properties are used to predict a Yarkovsky drift in semi-major axis of (-60 +31/-45) m yr-1.

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arXiv2009

Investigation of Systematic Bias in Radiometric Diameter Determination of Near-Earth Asteroids: the Night Emission Simulated Thermal Model (NESTM)

The Near-Earth Asteroid Thermal Model (NEATM, Harris, 1998) has proven to be a reliable simple thermal model for radiometric diameter determination. However NEATM assumes zero thermal emission on the night side of an asteroid. We investigate how this assumption affects the best-fit beaming parameter, overestimates the effective diameter and underestimates the albedo at large phase angles, by testing NEATM on thermal IR fluxes generated from simulated asteroid surfaces with different thermal inertia. We compare NEATM to radar diameters and find that NEATM overestimates the diameter when the beaming parameter is fitted to multi-wavelength observations and underestimates the diameter when the default beaming parameter is used. The Night Emission Simulated Thermal Model (NESTM) is introduced. NESTM models the night side temperature as an iso-latitudinal fraction (f) of the maximum day side temperature (Maximum temperature calculated for NEATM with beaming parameter = 1). A range of f is found for different thermal parameters, which depend on the thermal inertia. NESTM diameters are compared with NEATM and radar diameters, and it is shown that NESTM may reduce the systematic bias in overestimating diameters. It is suggested that a version of the NESTM which assumes the thermal inertia = 200 S.I. units is adopted as a default model when the solar phase angle is greater than 45 degrees.

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arXiv2006

GEO debris and interplanetary dust: fluxes and charging behavior

In September 1996, a dust/debris detector: GORID was launched into the geostationary (GEO) region as a piggyback instrument on the Russian Express-2 telecommunications spacecraft. The instrument began its normal operation in April 1997 and ended its mission in July 2002. The goal of this work was to use GORID's particle data to identify and separate the space debris to interplanetary dust particles (IDPs) in GEO, to more finely determine the instrument's measurement characteristics and to derive impact fluxes. While the physical characteristics of the GORID impacts alone are insufficient for a reliable distinction between debris and interplanetary dust, the temporal behavior of the impacts are strong enough indicators to separate the populations based on clustering. Non-cluster events are predominantly interplanetary, while cluster events are debris. The GORID mean flux distributions (at mass thresholds which are impact speed dependent) for IDPs, corrected for dead time, are 1.35x10^{-4} m^{-2} s^{-1} using a mean detection rate: 0.54 d^{-1}, and for space debris are 6.1x10^{-4} m^{-2} s^{-1} using a mean detection rate: 2.5 d^{-1}. Beta-meteoroids were not detected. Clusters could be a closely-packed debris cloud or a particle breaking up due to electrostatic fragmentation after high charging.

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