Accurate Insights on Multiple Asteroid Systems: Shape, Bulk Density and Formation

Grant #: NNX11AD62G
Senior Scientist: Franck Marchis

It was only when the first images of the asteroid (243) Ida captured by the Galileo spacecraft revealed the presence of a small satellite named Dactyl, that the existence of binary asteroids was unambiguously confirmed. The advent of high angular resolution imaging, radar observations, and robust lightcurve studies permitted the discovery of a large number on multiple asteroids. Over the past 6 years, we gathered in the Virtual Observatory Binary Asteroids Database (VOBAD) the characteristics of known multiple asteroid systems. At the time of writing, 192 companions of asteroids are known in each population of small solar system bodies: 90 MBA, 34 NEA, 4 Jupiter-Trojans, 44 TNOs and 20 unclassified asteroids. 

Our VOBAD database revealed that this interesting population of multiple asteroids is poorly known. In fact for less than half of them we have an estimate of their mutual orbit, and for only 20% a size estimate by radiometric data (IRAS) is available. Very few of them have a recorded visible (24%) or a NIR spectra (14%) making their taxonomic class, a first order estimate of their composition unknown. Finally, only for 1/5 of them can we estimate their bulk density and porosity, a key parameter to determine the internal structure and composition of these asteroids. 

We propose to conduct a robust and consistent observing program to gather detailed information of a large sample of known multiple asteroid systems. Our strategy for the definition of target sample is based on the level of difficulty of the observations. On the first year we will focus our observing program on ~40 bright MBA and NEA (V<16.5, D>20 km for MBA). The second year will be dedicated to collect the characteristics of ~20 faint MBA and NEA (D<20 km). Finally, in the last year, we will observe 20 multiple systems members of the TNOs and the Jupiter-Trojan populations. Combining high angular resolution observations from ground-based AO systems and HST, lightcurve observations from medium size telescope, low resolution spectroscopy in visible and near-infrared with medium size telescope and mid-IR/Far-IR spectroscopy from observations taken with ground-based telescopes and SOFIA and extracted from Herschel archive, we will be able to build a consistent model of these asteroids providing a good estimate of their size, shape, bulk density and surface composition. 

Based on this survey, we will be able to perform statistical studies and i) Determine the relationship between the bulk density of multiple asteroids and their taxonomic classes; ii) Investigate the distribution of the density of asteroids with respect to their size and their distance to the sun; iii) Estimate the multiplicity rate of asteroids per population, size, taxonomic class and distance to the sun; iv) Determine the internal structure and composition of several large components of multiple systems. Ultimately this study will be useful to select the most interesting multiple asteroid systems for a space mission exploration. 

This work specifically addresses NASA Strategic Goal 2 ("Expand scientific understanding of the Earth and the universe in which we live") and the following sub-goals: 

- "Advance scientific knowledge of the origin and history of the solar system, and the potential for life elsewhere." 

- "Perform basic research to understand the hazards and resources available as humans explore space".