Albedos of Main-Belt Comets 133p/Elst-Pizarro and 176p/Linear

We present the determination of the geometric R -band albedos of two main-belt comet (MBC) nuclei based on data from the Spitzer Space Telescope and a number of ground-based optical facilities. For 133P/Elst-Pizarro, we find an albedo of p R = 0.05 ± 0.02 and an effective radius of r e = 1.9 ± 0.3 km (estimated semiaxes of a ∼ 2.3 km and b ∼ 1.6 km). For 176P/LINEAR, we find an albedo of p R = 0.06 ± 0.02 and an effective radius of r e = 2.0 ± 0.2 km (estimated semiaxes of a ∼ 2.6 km and b ∼ 1.5 km). In terms of albedo, 133P and 176P are similar to each other and are typical of other Themis family asteroids, C-class asteroids, and other comet nuclei. We find no indication that 133P and 176P are compositionally unique among other dynamically similar (but inactive) members of the Themis family, in agreement with previous assertions that the two objects most likely formed in situ. We also note that low albedo ( p R < 0.075) remains a consistent feature of all cometary (i.e., icy) bodies, whether they originate in the inner solar system (the MBCs) or in the outer solar system (all other comets).