Spectroscopy of Kuiper Belt Objects and Centaurs

Due to their faintness, only a small fraction of the Kuiper Belt Objects (KBOs) and Centaurs discovered so far have been studied by spectroscopy. The spectra acquired have revealed a large variety of spectral shapes. Very few of them present absorptions. Only water ice (for a few Centaurs and KBOs) and methane (for the KBO 50000 Quaoar) have been identified unambiguously at the surface of these primitive bodies. Some weak features are found in the near infrared (for the KBO 26375 (1999DE9)) as well as in the visible (for the KBOs 47932 (2000 GN171), 38628 Huya, and 2003 AZ84) that are tentatively assigned to the presence of hydrated silicates. Other features (a narrow one due to methanol ice - or ice of a similar compound -, and a broad one that could be due to olivine) appear only in spectra of the Centaur 5145 Pholus. A few objects have heterogeneous surfaces revealed by differences in spectra recorded for different rotational phases. We will review the spectroscopic investigations and modelling attempts made so far, and see what can be deduced from them. We will then evaluate the needs for additional spectral data and laboratory data to optimize the scientific return of the observations that have been carried out. Some concluding remarks on object selections for future spectroscopic investigations and target selections for space exploration will be made.