The Minor Planet Physical Properties Catalogue: Connection with the Virtual European Solar and Planetary Access of EUROPLANET and the big data challenge for planetary science

Introduction

The small bodies (also known as minor planets) in our solar system are usually discovered by telescopic surveys. The results of these observations are organised by the Minor Planet Center (MPC), which determines orbits from the astrometric observations of the surveys. As of May 15, 2022, the MPC has an orbit database of 1,193,635 minor planets. However, the MPC data do not tell us what a minor body actually is, as they not contain physical properties such as albedo, diameter, volume, shape, composition, mass, bulk density, period of rotation and direction in the sky of the axis of rotation. Asteroids have very diverse values of these physical properties, whose knowledge is important for scientific studies of our solar system and its origin, planetary formation and evolution, space exploration, and planetary defence.

The physical properties of small bodies are spread across many publications, several websites (e.g. the NASA planetary data service) and archives. Many compositionally diagnostic spectra are also presented in a series of publications, but their source files are not necessarily public. This poses a fundamental problem to our ability to massively exploit the physical properties of minor bodies. Moreover, the values ​​of these physical parameters are extremely heterogeneous, obtained by several different groups, project teams and telescopic surveys, and even individual researchers, using different techniques. For example, thermal models are used to interpret observations in the mid-infrared (between 3.5 and 100 μm) from space and from the ground for the measurement of diameters and albedos. However, other techniques such as radar or the measurement of stellar occultation times provide measurements of diameters.

 

A centre of minor planets’ physical properties 

The Minor Planet Physical Properties Catalogue, or MP3C for short, collects and organises in a single place and makes available to the community values of physical properties of asteroids and other small bodies of our solar system. The MP3C is designed for very heterogeneous and big amount data: as of March 2022 it contains 3,710,587 measured properties, all of which are fully referenced to their published sources. The MP3C offers two main interfaces to explore the data (Figs. 1 and 2), namely (i) a web portal (mp3c.oca.eu) that allows to perform extraction of physical properties values on the basis of a single or list of minor bodies identifiers (names, numbers), but also on ranges of physical and dynamical properties; (ii) a data server offering a Table Access Protocol (TAP), which is defined and compatible with the standards of the Virtual Observatory (VO). The data server is accessible at https://dachs.oca.eu. MP3C is registered as service of the Virtual Observatory and therefore searchable and accessible using Virtual Observatory tools such as TOPCAT (Fig. 2). Using TOPCAT and/or the TAP, a human or robotic user can perform queries in a standardised database language (ADQL). MP3C contains sizes, albedos, absolute magnitudes, masses and rotational period, along with osculating and proper orbital elements of 1,169,632 minor bodies. At the time of writing we are also implementing in the MP3C table of spectral classes, colours, rotation vector orientation, and later reflectance spectra of minor planets.

 

Interface with the Virtual European Solar and Planetary Access of EUROPLANET

In addition to the general TAP interface, the best current determinations of all parameters and objects are accessible via the EPN-TAP protocol. EPN-TAP, designed in Europlanet/VESPA, uses a specific metadata vocabulary to uniformly describe Solar System data, which facilitates cross-searches in various data services - for instance, several services providing spectra of small bodies are also accessible in this format. All EPN-TAP services can be queried from the VESPA portal (http://vespa.obspm.fr) or from scripts in various languages, e.g. for mass processing.

 

The future big data challenge

In the future, the problem of the spreading of small-body physical properties will become very important: Soon, the Near-Earth Object Surveyor Telescope, decided to be implemented by NASA's Planetary Defense Coordination Office in the fall of 2019, with a launch in 2026 will determine the sizes and albedos of nearly 8 million asteroids from observations of their thermal emission. At the end of 2022 LSST will be commissioned and will begin operations. About two years later, LSST will start publishing the data. In a single visit, LSST detects up to 5,000 solar system objects. Over its 10 year lifespan, LSST could catalog over 5 million Main Belt asteroids, almost 300,000 Jupiter Trojans, over 100,000 near-Earth objects, and over 40,000 trans-neptunian objects. Many of these objects will receive 100s of observations in multiple bandpasses. This amounts to an increase of at least 10× the known population, with similar increases in the number of objects with enough data to generate light-curves and colours. We therefore imagine a very significant increase in the physical properties of asteroids and comets. Clearly the latter is a major big-data problem, which will necessitate scientific and technical implementation skills. 

 

Acknowledgments

M. Delbo and C. Avdellidou acknowledge support from the ANR Origins (ANR-18-CE31-13-0014). The work of C. Avdellidou was also partially funded by he French National Research Agency under the project “Investissements d’Avenir” UCAJEDI with the reference number ANR-15-IDEX-01 (2018-2020). The work of S. Erard for linking MP3C to EUROPLANET VESPA was supported by the the Europlanet-2024 Research Infrastructure project that received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 871149.

Figure 1: (Left) HTML output of all properties listed by MP3C for Pallas. (Right) Search form by list of object identifiers and/or range of their physical and orbital properties.

Figure 2: The MP3C service is accessible via the TAP protocol by the applications of the Virtual Observatory (here, TOPCAT). (Left) List of tables with their description, including the EPNCore table available in particular for the VESPA service. (Right) Graphs and tables obtained by submitting ADQL queries.