Physical Observations of Icy Minor Bodies in the Outer Solar System

Icy minor bodies in the outer solar system like TNOs (Trans-Neptunian Objects), Centaurs, cometary nuclei and unusual asteroids are thought to be candidates of planetesimal aggregate bodies in the formation theory of the solar system. Therefore, studies of the physical properties of these objects provide clues to the initial conditions for the solar system. However, their physical properties are still poorly known even the most basic parameters like size and albedo because of their extreme faintness. Typical brightness of TNOs in V-band is fainter than 21 magnitude which makes the use of the largest telescopes an indispensable requirement. Observations were carried out to reveal the characteristics of these faint objects using the largest telescopes. Photometric and low dispersion spectroscopic observations of 16 TNOs and 1 Centaurs in optical and near-IR wavelength were performed for the purpose of colour taxonomical study as the first approach. On the contrary to former studies, absolute magnitude of TNOs and Centaurs shows no significant correlation with the (V-I) colour. Our new measurements of their colours showed no clear bi-modality and continuous diversity from solar to reddened colour of their surface. The surface of TNOs and Centaurs can be modified by resurfacing process of the intrinsic cometary activity. Bi-colour lightcurve observations in high spatial resolution mode, together with deep imasing were performed in order to confirm a cometary activity of TNO 1996 TO66. The inhomogeneous (V - R) colour variation of TNO over one rotation was obtained. It supports the existence of TNO\u27s patchy surface assumption like Pluto\u27s spotted surface. However, cometary coma was not detected down to 29 mag/sq.arcsec. As a next step toward the physical studies of these objects, the determination method of the most basic parameters, size and albedo, and the observing feasibility for the outer minor bodies are discussed. As a trial case, the MUSES-C spacecraft mission target, NEA 1998 SF36 was analyzed from thermal observations in mid-IR (N-band). The derived radius and albedo of were 178 (+13/-14) m and 0.29 (+0.05/-0.04), respectively. Finally, the capability of adoption to the faint objects such like TNOs and Centaurs are presented with the possible future facilities