141 research outputs found
Recommended from our members
Thermal infrared and optical observations of near-Earth asteroids.
Increased physical characterisation of Near-Earth Asteroids (NEAs) is important for understanding their origin and evolution, the links between meteorites and their parent bodies, and for assessing the impact hazard. NEAs are also representative of small main belt asteroids. Optical observations of 13 NEAs taken at the Jacobus Kapteyn Telescope in 2001 and 2002 are presented, from which composite lightcurves, mean magnitudes, absolute visual magnitudes, rotation periods and lightcurve amplitudes are derived. Thermal infrared photometry and spectrophotometry of 10 NEAs taken at the United Kingdom Infrared Telescope (UKIRT) in March and September 2002 are presented. The Standard Thermal Model (STM), Fast Rotating Model (FRM) and Near-Earth Asteroid Thermal Model (NEATM) have been fitted to the measured fluxes to derive geometric albedos (pv), effective diameters (Deff) and beaming parameters (rj). The NEATM assumes zero thermal emission on the night side of an asteroid, affecting the best-fit rj, overestimating Deff and underestimating pv at large phase angles. The Night Emission Simulated Thermal Model (NESTM) is introduced. NESTM models the night side temperature (Tnight) as an iso-latitudinal fraction if) of the maximum day side 1/4temperature (Tmax calculated for NEATM with rj = 1): Tnight = fTmax cos (f), where (j) is the latitude. A range of / is found for different thermal parameters, which depend on the surface thermal inertia (T). NESTM is tested on thermal IR fluxes generated from simulated asteroid surfaces with different T. NESTM, NEATM and radar diameters are compared and it is found that NESTM removes a systematic bias of NEATM that overestimates asteroid diameters. From these tests, it is suggested that a version of the NESTM which assumes T = 200 J m-2 s-1/2 K-1 (f~ 0.6) is adopted as a default model when the solar phase angle is greater than 45°
Recommended from our members
Mid-infrared reflectance spectroscopy of calcium-aluminium-rich inclusions to identify primitive near Earth asteroids
The recent availability of mid-IR spectroscopy from space telescopes has led to studies attempting to characterise the composition of near Earth asteroids (NEAs) by comparison to spectroscopic observations of meteorites in the laboratory (e.g. [1, 2]). Mid-IR of carbonaceous chondrite meteorites has also recently been in focus to identify primitive asteroids and the aqueous alteration processes occurring on them [3]. Of special interest among the components of carbonaceous chondrites are the refractory calcium-aluminium-rich inclusions (CAIs) because they are formed very early on in the solar system. We have gathered reflectance spectra in the 2.5-16.0 μm range of the phases (spinel, olivine, melilite, pyroxene, etc.) in CAI sections of meteorites Allende (CV3.3), Vigarano (CV3.3) and Ornans (CO3.3) with a Perkin Elmer AutoIMAGE FTIR microscope, and compared them to mid-IR spectra of several NEA targets (253 Mathilde, 243 Ida, 1917 Cuyo, and the target of OSIRIS-Rex mission 101955 Bennu) of the Spitzer Space Telescope's Infrared Spectrometer (IRS [4]). Spectra from Spitzer were extracted with an optimal extraction method [5] and corrected with a near earth thermal model [6]. Reflectance spectra obtained from the CAIs were compared to spectra in the Keck/NASA RELAB database [7]. Overall, we see some features on the surfaces of the observed asteroids that may correspond to the pristine CAIs from the chondrites, though identification of these features is not easy.
References: [1] Emery, J. P. et al. (2006) Icarus, 182, 496-512. [2] Lim, L. F. et al. (2011) Icarus, 213, 510-523. [3] Trigo-Rodriguez, J. M. et al. (2014) Mon. Not. R. Astron. Soc. 437, 227-240. [4] Houck, J. R. et al. (2004) Astron. Phy. J. Suppl. 154, 18-24. [5] Lebouteiller, V. et al. (2010) Publ. Astron. Soc. Pac., 122.888, 231-240. [6] Harris, A. W. (1998) Icarus 131, 291-301. [7] Pieters, C. M., and Hiroi, T. (2004). Lunar Planet. Sci. XXXV, # 1720
Recommended from our members
Optical and thermal infrared observations of six near-Earth asteroids in 2002
We present thermal infrared photometry and spectrophotometry of six Near-Earth Asteroids (NEAs) using the 3.8 in United Kingdom Infrared Telescope (UKIRT) together with quasi-simultaneous optical observations of five NEAs taken at the 1.0 m Jacobus Kapteyn Telescope (JKT). For Asteroid (6455) 1992 HE we derive a rotational period P = 2.736 +/- 0.002 h, and an absolute visual magnitude H = 14.32 +/- 0.24. For Asteroid 2002 HK12 we derive H = 18.22 (+0.37 -0.30). The Standard Thermal Model (STM), the Fast Rotating Model (FIRM) and the Near-Earth Asteroid Thermal Model (NEATM) have been fitted to the measured fluxes to derive albedos and effective diameters. The derived geometric albedos and effective diameters are (6455) 1992 HE: p(v) = 0.26 +/- 0.08, D-eff = 3.55 +/- 0.5 km; 1999 HF1: p(v) = 0.18 +/- 0.07, D-eff = 3.73 (+1.0. -0.5) km; 2000 ED104: p(v) = 0.18 +/- 0.05, D-eff = 1.21 +/- 0.2 km: 2002 HK12: p(v) = 0.24 (+0.25. -0.11), D-eff 0.62 +/- 0.2 km: 2002 NX18: p(v) = 0.031 +/- 0.009 D-eff = 2.24 +/- 0.3 km; 2002 QE(15): p(v) = 0.15 (+0.08, -0.06), D-eff = 1.94 +/- 0.4 km. The limitations of using the NEATM to observe NEAs at high phase angles are discussed. (c) 2004 Elsevier Inc. All rights reserved
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.Comment: 48 pages, 10 Figures, 5 Table
Physical model of near-Earth asteroid (1917) Cuyo from ground-based optical and thermal-IR observations
Context: The near-Earth asteroid (1917) Cuyo was subject to radar and lightcurve 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 YORP effect through long-term lightcurve monitoring and physical modelling of near-Earth asteroids.
Aims: We aimed to constrain physical properties of Cuyo: shape, spin-state, and spectroscopic & thermophysical properties of the surface.
Methods: We acquired photometric lightcurves of Cuyo spanning the period between 2010 and 2013, which we combined with published lightcurves from 1989-2008. 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 lightcurve-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 lightcurve-based spin-state modelling, including both negative and positive acceleration values, between -0.7x10-8 rad day-2 and 1.7x10-8 rad 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 ± 0.08 km, the thermal inertia, 44 ±- 9 J m-2s-1/2K-1, and a roughness fraction of 0.52 ± 0.26. This enabled a YORP strength prediction of (-6.39 ± 0.96)x10-10 rad day-2. We also see evidence of surface compositional variation.
Conclusions: The low value of YORP predicted by means of thermophysical analysis, consistent with the results of the lightcurve 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
Effect of multivitamin and multimineral supplementation on cognitive function in men and women aged 65 years and over : a randomised controlled trial
Background: Observational studies have frequently reported an association between cognitive function and nutrition in later life but randomised trials of B vitamins and antioxidant supplements have mostly found no beneficial effect. We examined the effect of daily supplementation with 11 vitamins and 5 minerals on cognitive function in older adults to assess the possibility that this could help to prevent cognitive decline. Methods: The study was carried out as part of a randomised double blind placebo controlled trial of micronutrient supplementation based in six primary care health centres in North East Scotland. 910 men and women aged 65 years and over living in the community were recruited and randomised: 456 to active treatment and 454 to placebo. The active treatment consisted of a single tablet containing eleven vitamins and five minerals in amounts ranging from 50–210 % of the UK Reference Nutrient Intake or matching placebo tablet taken daily for 12 months. Digit span forward and verbal fluency tests, which assess immediate memory and executive functioning respectively, were conducted at the start and end of the intervention period. Risk of micronutrient deficiency at baseline was assessed by a simple risk questionnaire. Results: For digit span forward there was no evidence of an effect of supplements in all participants or in sub-groups defined by age or risk of deficiency. For verbal fluency there was no evidence of a beneficial effect in the whole study population but there was weak evidence for a beneficial effect of supplementation in the two pre-specified subgroups: in those aged 75 years and over (n 290; mean difference between supplemented and placebo groups 2.8 (95% CI -0.6, 6.2) units) and in those at increased risk of micronutrient deficiency assessed by the risk questionnaire (n 260; mean difference between supplemented and placebo groups 2.5 (95% CI -1.0, 6.1) units). Conclusion: The results provide no evidence for a beneficial effect of daily multivitamin and multimineral supplements on these domains of cognitive function in community-living people over 65 years. However, the possibility of beneficial effects in older people and those at greater risk of nutritional deficiency deserves further attention.Peer reviewedPublisher PD
Measurement requirements for a near-Earth asteroid impact mitigation demonstration mission
A concept for an Impact Mitigation Preparation Mission, called Don Quijote,
is to send two spacecraft to a Near-Earth Asteroid (NEA): an Orbiter and an
Impactor. The Impactor collides with the asteroid while the Orbiter measures
the resulting change in the asteroid's orbit, by means of a Radio Science
Experiment (RSE) carried out before and after impact. Three parallel Phase A
studies on Don Quijote were carried out for the European Space Agency: the
research presented here reflects outcomes of the study by QinetiQ. We discuss
the mission objectives with regards to the prioritisation of payload
instruments, with emphasis on the interpretation of the impact. The Radio
Science Experiment is described and it is examined how solar radiation pressure
may increase the uncertainty in measuring the orbit of the target asteroid. It
is determined that to measure the change in orbit accurately a thermal IR
spectrometer is mandatory, to measure the Yarkovsky effect. The advantages of
having a laser altimeter are discussed. The advantages of a dedicated
wide-angle impact camera are discussed and the field-of-view is initially sized
through a simple model of the impact.Comment: 28 page
- …