FM01: A Century of Asteroid Families

Artist’s impression of an asteroid breaking up after colliding with another object. (NASA / JPL-CalTech)

In addition to celebrating the centennial of the IAU at this year’s General Assembly, we also celebrate the 100th anniversary of the discovery of asteroid families. In a 1918 paper in the Astronomical Journal, “Groups of Asteroids Probably of Common Origin,” Kiyotsugu Hirayama identified three groups of asteroid orbits that were clustered in parameter space in a way that did not appear random. He noted that the orbital parameters could all result from secular perturbations by Jupiter of the orbits of a group of bodies with small initial relative velocities. He showed that the clustering was seen in orbital mean motion, inclination, longitude of the node, eccentricity, and longitude of perihelion. Further, he suggested that these three groups of asteroids each originated with the breakup of a single parent body and coined the term “asteroid families” to describe them. All of this in a four-page paper written when only 790 asteroids were known!

A century later we have increased the size our asteroid catalog by a factor of 1000; advanced our understanding of gravitational and non-gravitational orbital perturbations; photometrically surveyed asteroids from the ultraviolet to the infrared; collected thousands of colors, sizes, albedos, light curves, spectra, and shapes; numerically simulated impact formation of families and their evolution over millions or billions of years; and even sent spacecraft to visit more than a dozen asteroids, most recently the near-Earth asteroid Ryugu, which Japan’s spacecraft Hayabusa2 is now exploring. Throughout this incredible growth in our knowledge, all of the evidence we have found supports the prediction made in the title of Hirayama’s paper, that families are “of common origin”.

Today, we know of more than 100 asteroid families. Each formed when two objects collided, destroying the smaller one and either excavating a crater in the larger one or completely shattering it. The velocity of the ejected fragments changed their orbits from that of the original body. In the Main Belt the velocity change is small compared to the orbital velocity, so the pieces remain tightly clustered, while for more distant populations the change in orbital energy from the impact has larger effects on the orbits, sending fragments on totally different courses around the Sun and requiring different techniques for uncovering them.

Our goal with Focus Meeting 1 is both to look backward, reviewing all we have learned in the 100 years since the discovery of asteroid families, and to look forward at how upcoming surveys, missions, and techniques will lead us into the second century of family studies. In the near term, the next generation of surveys will expand our catalogs by another order of magnitude, pushing the detection of family members down to objects much smaller than a kilometer. Cataloging the population at these sizes is critical to finding the youngest, and thus newest, families. Small objects are also the most strongly affected by the Yarkovsky effect and thus will have undergone the most significant orbital evolution.

The Large Synoptic Survey Telescope (LSST), currently under construction, and the proposed NEOCam mission, currently in Extended Phase A study, have the potential to detect millions of Main Belt asteroids and provide colors, diameters, albedos, and coarse light curves for the majority of them. With these surveys combined, we will have an unprecedented dataset with which to identify families, measure their compositions and compositional gradients, and investigate size- and time-dependent processes that alter asteroid surfaces.

As observational surveys have expanded, so too has our ability to simulate asteroids’ evolution under gravitational and non-gravitational effects. The growth of, and increasingly easy access to, high performance computing centers has expanded our ability to study family evolution on gigayear timescales while still preserving the fine temporal resolution needed for objects entering the inner Solar System. Direct backward numerical integration has allowed us to set precise ages for young families, and identification of new young families will expand this list. As parallel CPU processing and GPU capabilities continue to improve, so too will our ability to constrain the scale and effects of the impacts that form families, the resonances that are important to shaping them, and the connection between families and near-Earth asteroids.

Asteroid families have provided unique insights into the forces shaping both our Solar System and other planetary systems. Families provide us observable evidence of nearby, large-scale catastrophic impacts, giving us the tools to test impact physics on planetary scales. Families also allow us to probe the heterogeneity of the protoplanetary disk in the terrestrial planet region and the homogeneity of the parent bodies prior to the family-forming collision. In the next century asteroid families will continue to be a touchstone for planetary science, providing insights and test populations for planetary formation models not available in any other way.

JOE MASIERO is a scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, and Deputy Principal Investigator of the NEOWISE mission. His research focuses on discovery and characterization of asteroids and asteroid families.