Interferometry of ASTEROIDS Marco Delbo CNRS - Observatoire de la Cote d’Azur, Nice, France Collaborators: A. Matter (Bonn, Germany), B. Carry (ESA), S. Ligori (Torino, Italy), P. Tanga (Nice, France), and G. Van Belle (Flagstaff, USA) ESAC - Madrid - Aug 25, 2011 Outline • Introduction: why to study asteroids and what do their physical properties tell us? • Main Belt Asteroids • Size, shape, density and internal structure • Interferometry of asteroids • Data analysis models, potential targets. • First results of VLTI-MIDI observations. • Future projects/perspectives. Asteroids of the main belt Plotted here are the positions of the first 5000 asteroids every 5 days. Asteroids of the main belt Plotted here are the positions of the first 5000 asteroids every 5 days. Near-Earth Asteroids (NEAs) Some are from the main belt Some are dead comets dynamical lifetime: 107y ∼ Size distribution of main belt asteroids Bottke et al. 2005 Assumption of spherical shapes Size distribution of main belt asteroids Bottke et al. 2005 Assumption of spherical shapes D(km) θ(mas)= 0.72 ∆(AU) 1.5 D ∼ × ∆ θ(mas) D(km) at the center∼ of the Belt Size distribution of main belt asteroids Bottke et al. 2005 Assumption of at the center of the MB spherical shapes for D=100km F (12µm) 20(Jy) ∼ D(km) F D2 θ(mas)= ∝ 0.72 ∆(AU) 1.5 D ∼ × ∆ θ(mas) D(km) at the center∼ of the Belt NEAs: size distribution Images of asteroids from spacecrafts DAWN in orbit around (4) Vesta Images of asteroids from spacecrafts Rosetta flyby of 21 Lutetia Covered with a regolith, estimated to be 600 m thick, The regolith softens the outlines of many of the larger craters. size: 132 × 101 × 76 km mass: 1.7 x 1018 kg [pre Rosetta estimate: (2.2-2.6) x 1018 kg] Images of asteroids from spacecrafts 66×48×46 km 18×10×9 km 54×24×15 km Mission NEAR, 1998 (NASA) Mission Galileo, 1993 (NASA) images from the NEAR shoemaker mission (NASA) (433) Eros: size = 23 km 2nd largest near-Earth asteroids Discovered in Nice in 1898 First detailed images of the surface of an asteroid (433 Eros) 25143 Itokawa (viewed by Hayabusa) size: 535 × 294 × 209 m mass: (3.58±0.18)×1010 kg density:1.9±0.13 g/cm³ Asteroids and the origin of our solar system • Debris of the planet formation process • Small → little alteration → conserve pristine material Asteroids and the origin of our solar system • Debris of the planet formation process • Small → little alteration → conserve pristine material • Asteroids suffered collisional evolution. • Sizes, shapes, and bulk densities tell us about their collisional histories Simulation of disruption and reaccumulation by P. Michel Britt et al.: Asteroid Density, Porosity, and Structure 487 tural weakness that break apart during impacts to form what oid 4 Vesta has a bulk density consistent with basaltic mete- become meteorites. Macroporosoity defines the internal struc- orites overlying an olivine mantle and metal-rich core. The ture of an asteroid. Those with low macroporosity are solid, primitive C-type asteroid 1 Ceres has a bulk density similar coherent objects, while high macroporosities values indicate to primitive CI meteorites (for definitions of meteorite types loosely consolidated objects that may be collections of rub- see McSween, 1999). However, the smallest of these three ble held together by gravity (see Richardson et al., 2002). asteroids is an order of magnitude more massive than the next well-characterized asteroids and these less-massive 1.2. Current Measurements of asteroids exhibit some intriguing trends. In general, S-type Asteroid Bulk Density asteroids appear to have higher bulk densities than C-type asteroids, but the range in both groups is large. The M-type Spacecraft missions and advances in asteroid optical and asteroid 16 Psyche, which is interpreted to have a mineral- radar observations have revolutionized our knowledge of ogy analogous to Fe-Ni meteorites, shows a bulk density in asteroid bulk density. Shown in Table 1 and Fig. 1 is a sum- the range of hydrated clays. This indicates either very high mary of published mass and volume measurements. The porosity or a misidentification of the mineralogy. In the case methods of mass and volume determination are discussed of 16 Psyche, in addition to spectra and albedo consistent in section 2.0, but a glance at Table 1 shows that before the with metal, radar-albedo data strongly indicate a largely 1990s bulk-density measurements were limited to a handful metallic surface. of the largest asteroids. In the past 10 years, the accuracy and breadth of these measurements has exploded and pro- 2. THE DETERMINATION OF ASTEROID duced our first picture of the density structure of the aster- MASSES, VOLUMES, AND oid belt. The largest three asteroids, Ceres, Pallas, and Vesta, BULK DENSITIES have been studied for decades and have well-constrained values. These objects make up most of the mass of the aster- Though the number of asteroid density measurements oid belt. As shown in Fig. 1 in comparison with meteorite has begun to increase rapidly in the last few years, still only grain densities, these density values seem to make miner- a tiny fraction of the known asteroids have usable density alogical sense. Because common geologic materials can measurements. A short history of the efforts to determine vary by almost a factor of 4 in their grain density, asteroid the masses of asteroids has been provided by Hilton (2002). bulk-density measurements need to be interpreted in terms Asteroid masses have been reliably determined from asteroid- of the object’s mineralogy. The differentiated V-type aster- asteroid or asteroid-spacecraft perturbations. That is, the mass Densities of asteroids & meteorite analogs 1E + 22 1 Ceres (G) 2 Pallas (B) 4 Vesta (V) 1E + 20 87 Sylvia (P) 16 Psyche (M) 22 Kalliope (M) 45 Eugenia (C) 20 Massalia (S) 762 Pulcova (F) 11 Parthenope (S) 21 Hermione (C) 1E + 18 90 Antiope (C) Average S Average C 243 Ida (S) 253 Mathilde (C) Phobos 1E + 16 Mass in Kilograms (log scale) Mass in Kilograms Ordinary Chondrite Densities Grain CI Grain Density CI Grain 433 Eros (S) Deimos Density CV Grain CM Grain Density CM Grain 1E + 14 0.5 1 1.5 2 2.5 3 3.5 4 Density (g/cm3) Britt et al. 2002 Fig. 1. Bulk densities of measured asteroids with the grain densities of common meteorites for comparison. Also included in the plot are the asteroidlike moons of Mars, Phobos and Deimos, as well as estimates for the average C- and S-type asteroids (Standish, 2001). Several asteroids in Table 1 with large error bars have been left off the plot for clarity. Densities of asteroids & meteorite analogs 1E + 22 1 Ceres (G) 2 Pallas (B) 4 Vesta (V) 1E + 20 M 87 Sylvia (P) 16 Psyche (M) ρ = 22 Kalliope (M) 45 Eugenia (C) 20 Massalia (S) 762 Pulcova (F) V 11 Parthenope (S) 21 Hermione (C) 1E + 18 90 Antiope (C) Average S Average C 2243 Ida (S) 2 253σ Mathildeρ (C) σM σD Phobos 1E + 16 = +9 Mass in Kilograms (log scale) Mass in Kilograms Ordinary Chondrite Densities Grain ρ M Density CI Grain D 433 Eros (S) Deimos Density CV Grain CM Grain Density CM Grain 1E + 14 0.5 1 1.5 2 2.5 3 3.5 4 Density (g/cm3) Britt et al. 2002 Methods of physical characterization Size Volume Shape Mass Density Methods of physical characterization 2 Size Radiometry in the thermal infrared (FIR∝D ) Stellar occultation timing Volume Visible photometry (lightcurves) Shape Mass Density Methods of physical characterization 2 Size Radiometry in the thermal infrared (FIR∝D ) Stellar occultation timing Volume Visible photometry (lightcurves) Shape Asteroid-asteroid perturbation Asteroid-planet (mars) perturbation Mass Asteroid-spacecraft perturbation Density Methods of physical characterization 2 Size Radiometry in the thermal infrared (FIR∝D ) Stellar occultation timing Volume Visible photometry (lightcurves) Shape Asteroid-asteroid perturbation Asteroid-planet (mars) perturbation Mass Asteroid-spacecraft perturbation Period and semimajor axis P 2 4π2 = of asteroid satellites a3 GM Density Adaptive optics, visible photometry Optical interferometers Keck LBT ESO-VLT Optical interferometers Keck LBT ESO-VLT Interferometry and physical characterization of asteroids Size from 1 visibility Volume Shape Mass Density Interferometry and physical characterization of asteroids Size from 1 visibility Volume visibility as a function of time as the asteroid rotates Shape Mass Density PROGRAMME JEUNES Projet BACCI CHERCHEUSES ET JEUNES CHERCHEURS DOCUMENT SCIENTIFIQUE EDITION 2010 3.3. DESCRIPTION DES TRAVAUX PAR TÂCHE / DETAILED DESCRIPTION OF THE WORK ORGANISED BY TASKS Task Sub Tasks (Work Packages) Manager Participants Time 0. Organization MDB MDB, 6 PT 1 1. Astronomical 1.A Interferometric data MDB MDB 18 observations and 1.B Thermal infrared data PT 6 data treatment. 1.C Visible lightcurves MM 2.4 1.D Spectroscopic data HC 3 DH 4 SL 10 AC 3 PD 16 CDDC 8 2. Data modeling 2.A Lightcurve inversion PT MDB 12 and determination 2.B global modeling PT 8 of physical 2.C physical modeling PM 7 Interferometryquantities and physical MM 1.6 HC 2 DH 8 characterization of asteroids SL 2 AC 6 PD 8 CDDC 16 Team members: MDB=Marco Delbo; PT= Paolo Tanga; PM=Patrick Michel; MM=Michael Mueller from(post-doc@UMR6202 1 visibility Cassiopée until 08/2011); HC=Humberto Campins; Size DH=Daniel Hestroffer; SL=Sebastiano Ligori; AC=Alberto Cellino; PD=post-doc recruited with ANR funds; CDDC=Non-permanent research position (CDD chercheur) financed through ANR; The time is given in terms of person/work/months. Volume 3.3.1 TÂCHEvisibility 1 / TASK 1as ASTRONOMICAL a function OBSERVATIONS AND DATA TREATMENT Task 1 includesof time all activities as the devoted to the acquisition of the astronomical observations needed for the project: for each of our targets, we will obtain interferometric observations; photometric lightcurves in visible light; thermal infrared fluxes; spectroscopic observations in the visibleasteroid and near infrared.