Asteroid Family Physical Properties Joseph R. Masiero NASA Jet Propulsion Laboratory/Caltech Francesca DeMeo Harvard/Smithsonian Center for Astrophysics Toshihiro Kasuga Planetary Exploration Research Center, Chiba Institute of Technology Alex H. Parker Southwest Research Institute An asteroid family is typically formed when a larger parent body undergoes a catastrophic collisional disruption, and as such family members are expected to show physical properties that closely trace the composition and mineralogical evolution of the parent. Recently a number of new datasets have been released that probe the physical properties of a large number of asteroids, many of which are members of identified families. We review these data sets and the composite properties of asteroid families derived from this plethora of new data. We also discuss the limitations of the current data, and the open questions in the field. 1. INTRODUCTION techniques that rely on simulating the non-gravitational forces that depend on an asteroid’s albedo, diameter, and Asteroid families provide waypoints along the path of density. dynamical evolution of the solar system, as well as labo- In Asteroids III, Zappala` et al. (2002) and Cellino et ratories for studying the massive impacts that were com- al. (2002) reviewed the physical and spectral properties mon during terrestrial planet formation. Catastrophic dis- (respectively) of asteroid families known at that time. Zap- ruptions shattered these asteroids, leaving swarms of bod- pala` et al. (2002) primarily dealt with asteroid size distri- ies behind that evolved dynamically under gravitational per- butions inferred from a combination of observed absolute turbations and the Yarkovsky effect to their present-day lo- H magnitudes and albedo assumptions based on the subset cations, both in the Main Belt and beyond. The forces of of the family members with well-measured values. Surveys the family-forming impact and the gravitational reaccumu- in the subsequent years have expanded the number of mea- lation of the collisional products also left imprints on the sured diameters and albedos by nearly two orders of magni- shapes, sizes, spins, and densities of the resultant family tude, allowing for more accurate analysis of these families. members (see chapter by Michel et al., in this volume). By Cellino et al. (2002) discussed the spectroscopic proper- studying the physical properties of the collisional remnants, ties of the major asteroid families known at that time. The we can probe the composition of the parent asteroids, im- principal leap forward since Asteroids III in the realm of portant source regions of transient populationslike the near- spectroscopy has been the expansion of spectroscopic char- arXiv:1502.00961v1 [astro-ph.EP] 3 Feb 2015 Earth objects, and the physical processes that asteroids are acterization to near-infrared (NIR) wavelengths. The de- subjected to on million- and billion-year timescales. velopment of more sensitive instrumentation covering the In the thirteen years since the publication of Asteroids 1 µm and 2 µm silicate absorption features and new observ- III, research programs and sky surveys have produced phys- ing campaigns to acquire NIR spectra for a large number of ical observations for nearly two orders of magnitude more objects have revolutionized studies of asteroid composition asteroids than were previously available. The majority of and space weathering. these characterized asteroids are members of the Main Belt, By greatly expanding the number of family members and approximately one third of all known Main Belt aster- with measured physical properties, new investigations of as- oids (MBAs) with sizes larger than a few kilometers can be teroid families can be undertaken. Measurements of colors associated with asteroid families. As such, these data sets and albedo allow us to identify outliers in our population represent a windfall of family physical property informa- lists and search for variations in surface properties of fam- tion, enabling new studies of asteroid family formation and ily members that might indicate heterogeneity of the par- evolution. These data also provide a feedback mechanism ent body or weathering processes. Diameter measurements for dynamical analyses of families, particularly age-dating 1 let us build a size frequency distribution and estimate the color information obtained during the course of the SDSS original parent body size, both of which are critical to prob- survey can be used to infer the spectroscopic properties of ing the physics of giant impacts. Spectra provide detailed tens of thousands of asteroids at optical wavelengths. mineralogical constraints of family members, allowing for The Sloan Digital Sky Survey, primarily designed to more sensitive tests of space weathering and parent hetero- measure the redshifts of a very large sample of galaxies, geneity, albeit for a smaller sample size, while also probing serendipitously observed many asteroids over the course of the formation environment and allowing for comparisons to its several survey iterations. Under standard survey oper- meteorite samples. ations with 53.9 second exposures, its five-color camera In this chapter, we highlight the datasets that have been was sensitive to stationary sources as faint as r ∼ 22.2, obtained since Asteroids III which have greatly expanded with similar performance in u and g and somewhat brighter our ability to understand asteroid families. We discuss their limits in i and z (21.3 and 20.5, respectively). To enable implications for specific families, and tabulate average pho- the accurate determination of photometric redshifts, high- tometric, albedo, and spectroscopic properties for 109 fam- precision internal and absolute calibrations were essential. ilies identified in the chapter by Nesvorny´ et al. in this vol- The care and effort expended on these calibrations carried ume. We also discuss the key questions that have been an- over into the dataset of moving object photometry, resulting swered since Asteroids III, the ones that remain open, and in the largest well-calibrated dataset of multi-band asteroid the new puzzles that have appeared over the last decade. photometry to date. As of the latest data release, the Moving Object Catalog 4 (MOC4, Parker et al. 2008) contains asteroid observa- 2. NEW DATA SETS tions from 519 survey observing runs. The automatic flag- ging and analysis of moving objects required that they be The field of asteroid research has benefited in the last 13 brighter than magnitude r = 21.5. The brightest object in years from a huge influx of data. Many of these large sets the sample is r ∼ 12.91, giving the survey a dynamic range of asteroid characterization data (including photometric and of over 8.5 magnitudes. The large sample size and dynamic thermophysical data) have been ancillary results of surveys range of this survey make it a powerful tool for studying primarily designed to investigate astrophysical sources be- luminosity functions of dynamically- or photometrically- yond the solar system. Simultaneously, observing programs selected sub-populations of the moving objects, of which designed to acquire more time-intensive data products such asteroid families are a natural example. as asteroid spectra, photometric light curves, or polarimet- Because of the much smaller sample size of aster- ric phase curves have also blossomed. We review below the oids with u-band observations having photometric errors main datasets that have advanced family characterization in < 0.1 mag (44, 737, compared with 442, 743 with a sim- recent years. ilar precision in r-band), most large-scale asteroid stud- ies using the SDSS photometry have considered only the 2.1 Optical colors from SDSS four longer-wavelength bands. These are often further collapsed into two principle components, the a⋆ color The Sloan Digital Sky Survey (SDSS, York et al. (a⋆ = 0.89 (g − r)+0.45 (r − i) − 0.57) and the (i − z) 2000) produced one of the first extremely large, homo- color (see Ivezic´ et al. 2001). geneous data sets that contained information about as- Using the SDSS data set, Ivezic´ et al. (2002) showed teroid surface properties at optical wavelengths. These that asteroid families are easily identified from their opti- data are archived in the SDSS Moving Object Catalog cal colors. They found four primary color classes of fami- (http://www.astro.washington.edu/users/ivezic/sdssmoc/sdssmoc.htmllies, which) describe one of four characteristic compositions: which currently contains 471, 569 entries of moving objects Vesta, Koronis, Eos, and Themis. Jedicke et al. (2004) and from survey scans conducted up to March 2007. The cata- Nesvorny´ et al. (2005) expanded upon this to investigate log entries can be associated with 104, 449 unique known space weathering and find that older asteroid families have moving objects, however ∼ 250, 000 entries do not have steeper spectral slopes from 0.55 to 1 micron than younger corresponding associations in the orbital element catalogs asteroid families. implying the potential for a significant benefit from future Szabo´ & Kiss (2008) used SDSS photometry to con- data mining efforts. strain the shape distributions of eight large asteroid fami- While the SDSS 5-color photometric system (u,g,r,i,z, lies. By assuming the observed magnitude differences be- with central wavelengths of 0.3543 µm, 0.4770 µm, tween different epochs of observation were uncorrelated, 0.6231 µm, 0.7625 µm, 0.9134 µm, respectively) was not they showed that both older families and families closer designed with asteroid taxonomy in mind (unlike previous to the Sun tend to have more spherical shape distributions. surveys such as the Eight Color Asteroid Survey, Zellner et This effect is what would be expected for a system where al. 1985a), the sheer size of the dataset coupled with its ex- small-scale cratering collisions redistributed material into tremely well-characterized performance has made SDSS an gravitational lows, resulting in a more spherical shape. immensely valuable asset for defining asteroid families and Parker et al. (2008) used the family color relationships exploring their properties.