Physical Properties of Near-Earth Asteroids

The recent large number of new optical measurements from various surveys allowed to significantly increase the number of asteroids with known physical properties to more than 3,000 (https://astro.troja.mff.cuni.cz/projects/damit/). In particular, these properties include rotation states, i.e., sidereal rotation period and the orientation of the spin axis, and convex 3D shape models. Those are routinely derived from disk-integrated optical photometry by the convex inversion method. Photometric datasets usually consist of (i) dense-in-time lightcurves - temporal changes of asteroids' brightness within one observing night, thus during several hours, and (ii) sparse-in-time measurements that cover a large time interval of several years, but the typical difference between two consecutive measurements ranges from days to even months. These sparse data are internally calibrated, which makes them a valuable target for the inversion of asteroids' physical properties. Only photometric datasets that sufficiently sample various observing geometries lead to the unique determination of physical properties. It turned out that main-belt asteroids often have such datasets, so the majority of successful shape solutions belong to this category of asteroids. Obtaining good photometry for near-Earth asteroids (NEA) is challenging due to their smaller apparent brightness, larger proper motion on the sky, or less convenient observing opportunities (e.g., closer to the Sun, short observing windows, long intervals between possible observations). Therefore, shape and rotation state properties have been so far derived for only a few tens of NEAs. In our work, we analyze photometric datasets that we gathered from various sources (such as Gaia DR2, ATLAS, ASAS-SN) in an attempt to at least double the number of shape solutions for NEAs. Shapes and rotation state properties of NEAs are important for understanding their origin and their collisional and dynamical evolution. We will discuss our results in this context. Moreover, we will also discuss the role of stellar occultations - an asteroid is passing in front of a bright star, which is occulted, the duration of this event is directly related to the asteroids' dimension. Various observers distributed on the ground can sample different parts of the asteroid, which can lead to an accurate size estimate, and possibly even to a shape model refinement. Within the Unistellar's eVscope network of citizen astronomers, we are targeting stellar occultations by NEAs in order to estimate their dimensions with an accuracy non-achievable by commonly used techniques such as thermal modeling. This task is quite challenging, however, we will report the first successful observations of these events by Unistellar's citizen astronomers.