New method identifies families of asteroids with greater precision

Developed by an international group led by Brazilian researchers, the innovation combines orbital position, color and emitted light to identify members of a family of asteroids.

By Elton Alisson

Agência FAPESP – An international group of astronomists, led by researchers of the Orbital Dynamics and Planetology Group in the Engineering School of the Universidade Estadual Paulista (Guaratinguetá campus) have developed a new method to identify families of asteroids.

The method, the result of a FAPESP-funded research project, was presented during a meeting of the Dynamic Astronomy Division of the American Astronomy Society (AAS) held May 5–9, 2013, in Paraty, Rio de Janeiro and was described in an article published in the July edition of the Monthly Notices of the Royal Astronomical Society.

“The new method allows for identification of members of a family of asteroids with greater precision than before,” says Valério Carruba, a professor at Unesp-Guaratinguetá and the author of the study.

The families of asteroids are formed by parts of larger asteroids that collide, breaking into fragments of different sizes. After being ejected, these fragments tend to travel in similar trajectories around the Sun and slowly distance themselves from each other over time.

Some pieces end up in unstable orbits, which deviate toward dangerous incursions in the solar system, whereas other pieces become integrated into the populations of asteroids near Earth.

In addition to identifying families of asteroids, scientists are also currently using models based on surveys of rocky objects that are close to large-scale asteroids, given a certain distance between them, according to Carruba.

The problem with these methods, according to the researcher, is that they only take into account the so-called “proper elements” or the orbital position of the rocky objects. They do not take into account other important features that could be used to identify and classify them as members of a family of asteroids. Among these features are colors and the amount of light they reflect – the geometric albedo – because asteroids of the same family have the same color in visible light and reflect similar amounts of light.

As a result, many objects currently identified as members of families of existing asteroids are in reality interlopers that entered or previously occupied the same orbital region.

In collaboration with colleagues from the National Institute of Space Research in Rio de Janeiro, the Southwest Research Institute in the United States, and University Pierre & Marie Curie and the Paris Observatory (SYRTE), both in France, Carruba developed a method that combines orbital position, colors and geometric albedo to identify families of asteroids.

With the new method, according to Carruba, it is possible to significantly reduce the probability of classifying intrusive objects as members of certain families.

“The new method has high efficiency in identifying objects that have a high probability of being parts of real families of asteroids because they can delimit the number of interlopers. This improves the identification of the orbital limits occupied for each group,” he explains.

Data from space missions

According to Carruba, the development of a method to identify families of asteroids was made possible by data obtained by the Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Explorer (WISE) space missions conducted by the U. S. National Aeronautics and Space Agency (NASA).

The most ambitious space survey underway, the SDSS mission is comprised of several telescopes that observe the sky in several bandwidths. Since it was initiated in 2000, the mission has collected data on more than 200,000 objects, including asteroids. The objects are detailed in Mobile Object Catalogues (MOCs), which NASA makes accessible to the public via the internet.

Through these catalogues, the researcher has access to data on the photometry of asteroids, as well as the color and light they emit in several bandwidths. The problem is that there are several asteroid spectra that cannot be identified solely based on photometry because, although they present the same type of visible spectrum, they have distinct values of geometric albedo.

“Information on the light reflected by asteroids can be used in a complementary manner to identify an object as belonging to a certain family of asteroids,” stresses Carruba. “Until 2002, however, we only had these data for approximately 2,000 objects.”

Beginning in 2011, however, the WISE mission began to make geometric albedo data for more than 100,000 asteroids available. The telescope utilized in the mission uses infrared waves to make observations.

The larger an object is, the more heat it emits, and when the size of an asteroid can be measured, it is possible to determine its reflexive properties. Therefore, asteroids circling the Sun in similar orbits that were once thought to belong to a single family could in fact be members of different families.

“Information on the orbital position, colors and geometric albedos of all asteroids is still not available,” commented Carruba. “However, even with a reduced number of objects, it is possible to determine the families of asteroids with greater precision,” he affirms.

Ages of families

The new method was tested and applied to all known families of asteroids found in the main asteroid belt between the orbits of Mars and Jupiter at the beginning of 2013.

The method will allow scientists to more accurately calculate the ages of families of asteroids.

According to Carruba, to have a notion of the age of a family of asteroids, one must have an estimate of the orbital dispersion of its members, which the new method considers.

“If there is not a good estimate of the dispersion of a family of asteroids, of the objects that are really part of it and the spectral type of its members, the age calculation could be wrong. This has very important implications for our comprehension of the evolutionary process that shape the main asteroid belt,” says Carruba.

The article A multidomain approach to asteroid families’ identification (doi:10.1093/mnras/stt884), by Carruba et al., can be read by subscribers at mnras.oxfordjournals.org/content/433/3/2075.abstract?sid=9deb1321-2e70-4bef-a30e-a1a87dd94704.