Project Asteroids@home

Asteroids@home is a volunteer distributed computing project developed at the Astronomical Institute,Charles University in Prague, in cooperation with Radim Vančo from CzechNationalTeam. The project is directed by Josef Durech. It runs on the Berkeley Open Infrastructure for Network Computing (BOINC) software platform and uses power of volunteers’ computers to solve the lightcurve inversion problem for many asteroids.

Why distributed computing?

With huge amount of photometric data coming from big all-sky surveys as well as from backyard astronomers, the lightcurve inversion becomes a computationaly demanding process. In the future, we can expect even more data from surveys that are either already operating (PanSTARRS) or under construction (Gaia, LSST). Moreover, data from surveys are often sparse in time, which means that the rotation period – the basic physical parameter – cannot be estimated from the data easily. Contrary to classical lightcurves where the period is “visible” in the data, a wide interval of all possible periods has to be scanned densely when analysing sparse data. This fact enormously enlarges the computational time and the only practical way to efficiently handle photometry of hundreds of thousands of asteroids is to use distributed computing. Moreover, the problem is ideal for parallelization – the period interval can be divided into smaller parts that are searched separately and then the results are joined together.

Why to study asteroids?

• The large discrepancy between the huge number of all known asteroids and the small number of those with known basic physical parameters (shape, spin, period) is a strong motivation for further research.
• Knowing the physical properties of a significant part of the asteroid population is necessary for understanding the origin and evolution of the whole solar system.
• Thermal emission of small asteroids can significantly change their orbit (Yarkovsky efect), which can be crucial for predicting the probability of their collision with the Earth. To be able to compute how the thermal emission affects the orbit, we have to know the spin (and also the shape, to a certain extent) of the object.

Scientific objectives

The aim of the project is to derive shapes and spin for a significant part of the asteroid population. As input data, we use any asteroid photometry that is available. The results are asteroid convex shape models with the direction of the spin axis and the rotation period. The models will be published in peer-reviewed journals and then made public in the DAMIT database.

Basics about asteroids

Asteroids are small bodies of the solar system. Most of them orbit between Mars and Jupiter in the so-called main belt. Some of them, however, have orbits that come close to the Earth’s orbit or even cross it. These are called near-Earth asteroids. Asteroids can be described as irregular solid bodies without any atmosphere or coma. Their size ranges from hundreds of kilometers for the largest ones to meters for the smalles ever detected.

So far, there are almost half a million known asteroids – we know their orbit in the solar system (by measuring their position at different times) and their approximate size (by measuring their brightness and knowing their distance). To learn more about their physical properties, other observing techniques have to be used. One of them is photometry – we measure brightness variations caused by rotation. By this technique, rotation periods were derived for several thousands of asteroids

Asteroid lightcurves

Similarly to planets, asteroids shine by the reflected sunglight. Because the distance of an asteroid to the Sun and the Earth changes as the asteroid and the Earth orbit the Sun, the brightness of the asteroid also changes with time. Apart from this easily predictable change of brightness, asteroids also exhibit brightness variations that are caused by their irregular shape and their rotation.

Asteroids rotate, the cross-section of the visible and illuminated part of their surface varies with time and so varies their brightness. This brightness variation is called a lightcurve. By measuring lightcurves, we can measure asteroid rotation periods. The shape of a lightcurve depends on the mutual geometry of the Sun, the Earth, and the asteroid (which is known because we know the orbit of the asteroid in the solar system), and on asteroid spin axis orientation and shape (which we do not know).

Lightcurve inversion

If there are enough lightcurves from different geometries available, the shape model, the spin axis direction, and the rotation period of an asteroid can be derived. For example, an almost spherical asteroid would be constantly bright, whereas an elongated asteroid would exhibit large brightness variations when viewed edge-on and small variations when viewed pole-on. The process of the shape and spin reconstruction from lightcurves is called lightcurve inversion. From a mathematical point of view, lightcurve inversion is a nice and interesting example of an inverse problem. It can be shown that a uniqe convex shape model of an asteroid can be derived from its lightcurves. From an astronomical point of view, the lightcurve inversion method enables us to reveal basics physical characteristics of individual asteroids by inverting their lightcurves. So far, models for more than 200 asteroids have been derived this way. They are stored in the Database of Asteroid Models from Inversion Techniques (DAMIT).