Asteroids IV

1. INTRODUCTION A variety of remote-sensing techniques have given rise to much of our understanding of the small-body populations in our solar system. Groundbased surveys operating at visible wavelengths have discovered most of the minor planets that are known today. At present, roughly 700,000 asteroids have been discovered in the main belt between Mars and Jupiter, ~13,000 near-Earth objects (NEOs) are known at all sizes, and some 5000 jovian Trojans have been found. While these numbers are thought to represent only a tiny fraction of the small bodies believed to exist in our solar system, much can be learned about these populations by studying the physical and dynamical properties of representative samples (e.g., Bottke et al., 2005; Mainzer et al., 2011b; Grav et al., 2011a; see also the chapter by Jedicke et al. in this volume). Moreover, information about the probable composition and mineralogy of asteroids can be learned from visible and near-infrared (VNIR) spectroscopy and spectrophotometry. Such studies have been carried out for ~2500 asteroids [e.g., the Massachusetts Institute of Technology (MIT)-University of Hawaii (UH)-Infrared Telescope Facility (IRTF) Joint Campaign for NEO Spectral Reconnaisance (Tholen and Barucci, 1989; Xu et al., 1995; Bus and Binzel, 2002; DeMeo et al., 2009; Reddy, 2010; Kuroda et al., 2014; and many others)]. Compared to broadband imaging, spectrally resolved observations require brighter sources for the same signal-tonoise ratio, which limits the number of targets observable. The fourth release of the Sloan Digital Sky Survey (SDSS) Moving Object Catalog (Stoughton et al., 2002; Abazajian et al., 2003) provided ~100,000 observations in u, g, r, i, and z filters, leading to taxonomic classifications for ~64,000 asteroids (Carvano et al., 2010). Asteroid lightcurves, sometimes combined with stellar occultation data, have been inverted to obtain models of shapes and rotational states for hundreds of objects (Kaasalainen et al., 2001; Durech et al., 2010; see also the chapter by Durech et al. in this volume). Radar observations can produce the most detailed information about shape, size, orbit, and spin state short of visiting a body with spacecraft (e.g., Ostro et al., 2002; see the chapter by Benner et al. in this volume). Since radar echoes must be sent to and received from a body, sensitivity drops as distance to the fourth power, so objects must make fairly close approaches in order to be detected. Approximately 600 asteroids have been observed with radar to date (http://echo.jpl.nasa.gov/ asteroids/index.html). Polarimetry offers a means of studying asteroid surface properties, but since only a small fraction of the total luminosity is polarized, this technique has been applied to ~300 asteroids thus far (Lupishko and Vasilyev, 2012; Gil-Hutton and Canada-Assandri, 2012). While powerful, these techniques have only been employed on a small fraction of asteroids. Until recently, for the vast majority of asteroids, nothing was known except 89 Mainzer A., Usui F., and Trilling D. E. (2015) Space-based thermal infrared studies of asteroids. In Asteroids IV (P. Michel et al., eds.), pp. 89–106. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816532131-ch005. Space-Based Thermal Infrared Studies of Asteroids Amy Mainzer Jet Propulsion Laboratory, California Institute of Technology Fumihiko Usui The University of Tokyo David E. Trilling Northern Arizona University Large-area surveys operating at mid-infrared wavelengths have proven to be a valuable means of discovering and characterizing minor planets. Through the use of radiometric models, it is possible to derive physical properties such as diameters, albedos, and thermal inertia for large numbers of objects. Modern detector array technology has resulted in a significant improvement in spatial resolution and sensitivity compared with previous generations of spacebased infrared telescopes, giving rise to a commensurate increase in the number of objects that have been observed at these wavelengths. Spacebased infrared surveys of asteroids therefore offer an effective method of rapidly gathering information about the orbital and physical properties of small-body populations. The AKARI, Wide-field Infrared Survey Explorer (WISE)/ NearEarth Object Wide-field Infrared Survey Explorer (NEOWISE), Spitzer Space Telescope, and Herschel Space Observatory missions have significantly increased the number of minor planets with well-determined diameters and albedos. 90   Asteroids IV for absolute visible magnitudes (denoted H) and orbital parameters. Observing small bodies at thermal infrared (IR) wavelengths with space telescopes complements other techniques such as visible-light groundbased surveys, VNIR spectroscopy and spectrophotometry, radar studies, and in situ spacecraft visits. If a telescope can be cryogenically cooled such...