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Asteroids as far-infrared photometric standards for ISOPHOT *
T.G. Müller 1, 3 and
J.S.V. Lagerros 2
Received 11 February 1998 / Accepted 22 June 1998
Asteroids are used as far-IR calibrators for the imaging photopolarimeter ISOPHOT on board the Infrared Space Observatory ISO. For the 10 selected objects we compiled a large uniform database 1 of 678 individual observations, ranging from 7-.
Applying a new thermophysical model to the observations we derived thermal properties of the selected asteroids, based on spin-vector solutions, direct size measurements and the HG-magnitude system.
Our investigations indicate very rough surfaces, reflected in the beaming effect, and very low levels of heat conduction, expressed in thermal inertias between 5 and . Due to scattering processes in the porous regolith, the emissivity varies significantly with wavelength. In case of Vesta we find emissivities as low as 0.6 in the far-IR / submillimetre region.
By entering the combined results into the thermophysical model we defined new photometric standards for the far-IR. The absolute accuracy for thermal flux or lightcurve predictions is 5- for the first category objects and 10- for the secondaries. The methods and procedures discussed here are included in the first update of the ISOPHOT calibration in 1998.
Key words: minor planets, asteroids radiation mechanisms: thermal infrared: solar system
* The ISOPHOT data are based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.
Send offprint requests to: T.G. Müller, (email@example.com)
Online publication: September 8, 1998