Diameters, Volumes and bulk densities

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Diameters, Volumes and bulk densities

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Diameters,Volumes and bulk densities of 40 asteroids

Dave Herald


An overview of recent results


Why are diameters important?Diameters are a primary characteristic of asteroidsSeveral Infra-red satellite missions have measured diameters for 1000’s of asteroidsInfra-Red Astronomy Satellite - IRAS (1992)Asteroid catalogue using AKARI - AKARI (AcuA) (2011)Wide-field Infrared Survey Explorer - WISE (2013)[Other measures from 2MASS, SMBAS, SDSS MOC, SKAD, MSX & ISO]Important for planning occultation observations


Why is density important?The density of an asteroid tells us much about its composition and originBecause some asteroids are analogs to the building blocks of the solar system, the density and internal structure could tell us much about the formation conditions and evolution processes of planets and the solar systemTo determine density, we need to know the mass and volume of the asteroid


The challengeDensity is Mass/VolumeNeed to use:Reliable mass measurements, andReliable volume measurementsMass measurements are only available for a relatively small number of asteroidsSatellites like IRAS, WISE & AKARI (AcuA) derive mean diameters from IR measurements for a huge number of asteroids. But the linkage to ‘real’ diameters is largely untested.


The Mass of asteroidsMost mass determinations are derived from orbit deflections/perturbationsGaia will greatly increase the number of asteroids with reliable (<20%) mass determinationFor binary asteroids, masses can be determined from the system dynamics


Volume of an asteroidLiterature provides measures of the ‘mean diameter’ of an asteroidA ‘mean diameter’ is simply the diameter of a sphere that is in some manner equal to the asteroid.For density measurements, we want the VOLUME-equivalent sphereLight reflection measurements inherently provide the SURFACE-equivalent sphere


Examples For a sphere, surface-equivalent dia is the same as the volume-equivalent diaFor a cube, the surface-equivalent dia is 12% larger than volume-equivalent dia.For a slightly-elongated cube, difference is 14%For an asteroid like 128 Kleopatra (217km x 94km x 94km) difference is 16%


Mean Diameter - summaryPhotometric measurements give a mean diameter based on equal surface areaDensity determinations require knowledge of the volumeThe mean diameter based on equal volume is almost always smaller than that based on equal surface areaFor irregular asteroids, the difference can be greater than 15%We need a method to derive volume equivalent diameters


Shape of an asteroidShape can be determined by:Space craft imaging (exceedingly rare)Direct imaging with large telescopes (very rare)Radar imaging (mainly NEO’s)Multiple occultations – excellent single-event profiles, but exceedingly hard to combine results from different eventsLight curve inversion - common


Light curve inversionAll asteroids exhibit light variation as they rotateLight curve inversion is only possible when there is significant variation in the inclination of the asteroid’s axis of rotation as viewed from EarthThe technique models the shape of the asteroid that would reproduce the light curvesVolume-equivalent and Surface-equivalent diameters are given by the model


Light curve inversion limitationsAssumes the surface of the asteroid is of uniform albedoFor most models, the surface is assumed to be entirely convexGenerally there is an ambiguity in the determination of the rotational axis  two shape models (with one being wrong!)The size of the asteroid is completely indeterminate


Light-inversion shape model(135) Hertha


Some illustrative fits to light curvesof (135) Hertha


Asteroidal occultations -gives profile & dimensions to ~1km(Asteroid dia = 77km; apprnt dia = 0.071”; resolution ~1mas)


Can fit shape model to occultation


Can vary shape model rotation


Mean diameters determinedVolume equivalent 79kmSurface equivalent 84kmIR SatellitesIRAS 79kmAcuA 73kmWISE 77km


Fit model to multiple occultations(9) Metis(216) Kleopatra51 Nemausa


Comparison – diameters from satellites Compared ‘measured’ diameters against those from IRAS, AcuA and WISEMean differences are: IRAS +1% AcuA -2% WISE +5%Standard deviations IRAS 8% AcuA 8% WISE






is best

HOWEVER for individual asteroids




are equally reliable – at



Bulk density of asteroids:Combine Mass with VolumeTypical bulk densities of meteoritesChondrites 3.3; Carbonaceous chondrites 2.1-3.5; stony irons 4.5; Irons 7-8


Any questions?!227 September 2017

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