Size, Mass and Density of Asteroids (SiMDA) - A Web Based Archive and Data Service

Context

In the past about 10-15 years ground- and space-based observations of asteroids, results from space missions, and new efforts in laboratory and modeling work has advanced significantly, providing further and new data in many domains. On basis of these data, including much more volume and mass estimates, and from that density averages per taxonomic class, new statistical approaches revealed further structures in the asteroid belt and population, with implication on the description of the current and former state of the Solar System (DeMeo & Carry 2013). They suggest a more dynamic evolution of it (supporting the Grand Tack and Nice models) than thought before, but also rises new questions (see,e.g., DeMeo & Carry 2014, Michel et al. 2015).

Motivation and Aim

Size (volume), mass, density, and macro porosity are important physical properties for our understanding of the internal structure and morpholgy of asteroids (Scheeres et al. 2015). Though, we yet have density estimates for only some hundred objects (Carry 2012, this work), and only about 1/3 of them have a relative accuracy better than 20%. With very few exceptions, bulk densities are calculated from individual size and mass estimates, which themselves are derived using different methods. The number of density estimates is limited by the number of available mass estimates. Moreover, the relative accuracy of mass estimates is in many cases lower compared to that of size estimates, and, sometimes even incompatible between different methods within the formal uncertainties. The situation for sizes is generally better in terms of quantity, but also accuracy. Nevertheless even these comparable small relative errors of volume estimates can result in significant uncertainties of the derived density because of the relation ρ ~ D^-3, were D is the volume-equivalent diameter. Comparing results derived by different methods (for both properties mass and volume), constraint by realistic density ranges, can reveal limitations and biases of the individual methods.

Up to now, no machine-readable, easy to use and open-access data collection was available. Purpose of SiMDA (Size, Mass and Density of Asteroids) is not only to serve as an up-to-date and complete collection of size and mass estimates, providing researchers with ready to use data, but also to support some visualization and interactive handling of the data addressing the issues mentioned before.

Data Sets

Starting point for this work was the data compilation by Carry (2012), with ~1000 mass and ~1500 volume estimates. These estimates were evaluated for each object (in most cases averaged), in order to derive a single value for the size, mass and bulk density for finally 287 small bodies (a dozen comets included).

In the work presented here, all data and references given in Carry (2012) were verified by examination of the original work (where possible) and completed if necessary, and then added to the data base. For the time after that date (2011/2012), the literature (and the web) was and is regularly searched for publications of mass and / or size estimates or any direct density measurements, which are then added to the SiMDA data base. This is of course an ongoing process. To name some examples: further volume data will become available due to continued light-curve and stellar occultation observations (both domains with large contributions by amateur astronomers), and by using combined-data asteroid shape and size modeling (see, e.g. Hanuš et al. 2017). The observation of asteroids by Gaia will allow to determine the mass of about 100 asteroids with a relative accuracy better than 50% (Mouret et al. 2008), and planetary ephemerides (INPOP, JPL DE) will provide new mass estimates on a regular basis as well. Currently (May 2020) ~1750 mass and ~4000 diameter estimates for ~425 objects are archived.

Implementation and Usage

SiMDA is implemented as web application, written in Python (https://www.python.org), and using Django (https://www.djangoproject.com) as framework. Data are stored in a SQLite (https://www.sqlite.org) data base. Plotly (https://www.plotly.com) provides visualization. Tabula (https://tabula.technology/) and Excalibur (https://excalibur-py.readthedocs.io/en/master/) is used to extract tabular data from PDFs, where necessary.

SiMDA (https://kretlow.de/simda) provides size and mass estimates and the references of the original works. These data are completed by taxonomic information, if available. The web user interface allows to search for an object or to select it from a list. All available size and mass data for the selected object are displayed in tabular form and visualized as scatter plot. Average values and a derived bulk density are calculated on the fly, including their standard errors. It is possible to interactively change this calculations, i.e. to select / deselect individual size and / or mass estimates and to update the numerical results and plots.

The second main feature is to provide a data catalog for all objects. A single ‘best’ value for size, mass and the bulk density is calculated and the resulting catalog can be downloaded in different formats for own research and further processing.

References

B. Carry, 2012. Density of asteroids. Planetary and Space Science, Volume 73, p. 98-118.

F.E. DeMeo, B. Carry, 2013. The taxonomic distribution of asteroids from multi-filter all-sky photometric surveys. Icarus, Volume 226, Issue 1, p. 723-741.

F.E. DeMeo, B. Carry, 2014. Solar System evolution from compositional mapping of the asteroid belt. Nature, Volume 505, Issue 7485, p. 629-634.

J. Hanuš, M. Viikinkoski, F. Marchis, J. Ďurech, M. Kaasalainen, M. Delbo', D. Herald, E. Frappa, T. Hayamizu, S. Kerr, S. Preston, B. Timerson, D. Dunham, and J. Talbot, 2017. Volumes and bulk densities of forty asteroids from ADAM shape modeling. Astronomy & Astrophysics, Volume 601, A114, 41 pp.

P. Michel, F.E. DeMeo, and W.F. Bottke, 2015. Asteroids: Recent Advances and New Perspectives. In "Asteroids IV" (P. Michel et al., Eds.). pp. 3-10. Univ. of Arizone, Tucson.

S. Mouret, D. Hestroffer, and F. Mignard, 2008. Asteroid mass determination with the Gaia mission. A simulation of the expected precisions. Planetary and Space Science, Volume 56, Issue 14, p. 1819-1822.

D. J. Scheeres, D. Britt, B. Carry, and K.A. Holsapple, 2015. Asteroid Interiors and Morphology. In "Asteroids IV" (P. Michel et al., Eds.). pp. 3-10. Univ. of Arizone, Tucson.