Physical properties of asteroid 308635 (2005 YU₅₅) derived from multi-instrument infrared observations during a very close Earth approach

Abstract

The near-Earth asteroid 308635 (2005 YU 55 ) is a potentially hazardous asteroid which was discovered in 2005 and passed Earth on Nov. 8, 2011 at 0.85 lunar distances. This was the closest known approach by an asteroid of several hundred metres in diameter since 1976 when an object of similar size passed at 0.5 lunar distances. We observed 2005 YU 55 from the ground with a recently developed mid-IR camera (miniTAO/MAX38) in N and Q bands and with the Submillimeter Array (SMA) at 1.3 mm. In addition, we obtained space observations with Herschel/PACS at 70, 100, and 160 μm. Our thermal measurements cover a wide range of wavelengths from 8.9 μm to 1.3 mm and were taken after opposition at phase angles between -97 • and -18 • . We performed a radiometric analysis via a thermophysical model and combined our derived properties with results from radar, adaptive optics, lightcurve observations, speckle, and auxiliary thermal data. We find that 308635 (2005 YU 55 ) has an almost spherical shape with an effective diameter of 300 to 312 m and a geometric albedo p V of 0.055 to 0.075. Its spin axis is oriented towards celestial directions (λ ecl , β ecl ) = (60 • ± 30 • , -60 • ± 15 • ), which means it has a retrograde sense of rotation. The analysis of all available data combined revealed a discrepancy with the radar-derived size. Our radiometric analysis of the thermal data together with the problem to find a unique rotation period might be connected to a non-principal axis rotation. A low to intermediate level of surface roughness (rms mean slope in the range 0.1-0.3) is required to explain the available thermal measurements. We found a thermal inertia in the range 350−800 Jm −2 s −0.5 K −1 , very similar to the rubble-pile asteroid 25 143 Itokawa and indicating a surface with a mixture of low conductivity fine regolith with larger rocks and boulders of high thermal inertia.

Asteroid Thermophysical Modeling

“…In general, utilized shape models are derived from radar observations, inversion of optical light-curves, in-situ spacecraft images, or stellar occultation timing (see Durech et al, 2015, for a review of asteroid shape modeling). If no shape model is available, one typically falls back to a sphere or an ellipsoid (e.g., Müller et al, 2013Müller et al, , 2014a.…”

Space-Based Thermal Infrared Studies of Asteroids

“…However, Main Belt asteroids were used as calibration sources (Müller et al 2014a), and measurements of individual targets of interest were made for ∼10 asteroids (e.g. Leyrat et al 2012;O'Rourke et al 2012;Müller et al 2013Müller et al , 2014b.…”

“…Some mid-IR instruments are available on groundbased telescopes, especially at high-altitude observatories in Hawaii and Chile, leading to observations of several hundred objects to date (e.g. Hansen 1976;Cruikshank 1977;Lebofsky et al 1978;Morrison & Lebofsky 1979;Delbó et al 2003Delbó et al , 2011Matter et al 2011;Müller et al 2012Müller et al , 2013Wolters et al 2005Wolters et al , 2008.…”

NEOWISEREACTIVATION MISSION YEAR ONE: PRELIMINARY ASTEROID DIAMETERS AND ALBEDOS

“…Observations in other wavelengths, such as the infrared (e.g., Hansen 1976;Cruikshank 1977;Lebofsky et al 1978;Morrison & Lebofsky 1979;Delbó et al 2003Delbó et al , 2011Wolters et al 2005Wolters et al , 2008Matter et al 2011;Müller et al 2012Müller et al , 2013 or radio (e.g., Ostro et al 2002;Benner et al 2015), are needed to determine these quantities precisely. At present, well-determined diameters and albedos have been measured for less than a quarter of known asteroids.…”