Issue |
A&A
Volume 381, Number 1, JanuaryI 2002
|
|
---|---|---|
Page(s) | 324 - 339 | |
Section | Celestial mechanics and astrometry | |
DOI | https://doi.org/10.1051/0004-6361:20011491 | |
Published online | 15 January 2002 |
Asteroids as calibration standards in the thermal infrared for space observatories *,**
1
ISO Data Centre, Astrophysics Division, Space Science Department of ESA, Villafranca, PO Box 50727, 28080 Madrid, Spain
2
Astronomiska observatoriet, Box 515, 752 37 Uppsala, Sweden e-mail: Johan.Lagerros@astro.uu.se
Corresponding author: T. G. Müller, Thomas.Mueller@esa.int
Received:
5
June
2001
Accepted:
19
October
2001
Asteroids have been used extensively as calibration sources for the
Infrared Space Observatory (ISO) and are planned to be used by
future groundbased, airborne and space-based projects in the
thermal infrared (IR) and in the sub-millimetre.
We summarize the general IR observational parameters with a
focus on space observatories and discuss brightness variations, apparent
velocities and background influences.
During the ISO mission ten well-studied asteroids were used for
the photometric calibration of ISOPHOT, but additionally the
bright asteroids turned out to be of great interest for many technical
tests and calibration aspects. We evaluated the different applications,
like testing the photometry of the spectrometers,
validation of relative spectral response functions, determination
of beam profiles or colour correction tests.
The description of the asteroids' thermal emission has been obtained by
a recent thermophysical model (TPM). The important model aspects
are size, albedo, shape together with the spin vector, a beaming model,
thermal inertia and a wavelength-dependent emissivity.
With a large sample of observational data provided by three different
ISO instruments we had for the first time the
possibility to study the thermal emission of several asteroids in
detail. The intercomparison between results from different instruments
allowed us to distinguish between observational errors and model
shortcomings. It turned out that the accuracy of TPM predictions
is in many cases strongly related to the limited knowledge of
the asteroid shapes. The concepts of beaming, thermal inertia
and wavelength dependent emissivities were nicely confirmed for
a wide range of observing and illumination geometries under many
aspect angles for different asteroids. The TPM predictions for
Ceres, Pallas and Vesta are accurate
within 5% over the full wavelength range from 5 to 200 ,
for Hygiea and a few other asteroids the predictions and
observations agree within 10 to 15%. We found similar emissivity
behaviour for the four large asteroids over the full ISO wavelength
range. Up to now, no clear spectral features have been seen in the
asteroid far-IR spectra.
Key words: minor planets, asteroids / radiation mechanisms: thermal / infrared: solar system
© ESO, 2002
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