Standard Thermal Model
The Standard Thermal Model (STM) of asteroids was developed and
refined by a number of investigators, primarily during the 1970's and
1980's. Its history, background, and application is discussed by
Lebofsky
and Spencer (1989). The most important aspects can be
summarised as follows:
- The asteroid is assumed to be smooth, spherical and slowly
rotating. Each point on the surface is in instantaneous
equilibrium with the solar radiation, no heat is conducted
into the surface or to any neighbouring parts of the surface.
In older texts, this was often referred to as the ``dusty''
model.
- To correct for any beaming, shape or
conductivity effects, the so-called beaming parameter is
introduced. Furthermore, the flux at non-zero solar phase
angles is obtained by applying an empirical phase correction
to the flux calculated at opposition.
There are other variants of the STM. The most important is perhaps the
Fast Rotating Model (FRM). In FRM, the asteroid is assumed
to rotate so fast that the surface is in equilibrium with the mean
diurnal insolation. Furthermore, the spin vector is usually taken to be
perpendicular to the orbital plane. It follows that the
temperature is a function of the latitude only.
STM and FRM represent two extreme cases in terms of heat conduction
versus spin period. In STM, the heat conduction is very slow as
compared to the spin period, and vice versa for FRM. FRM is a poor
model for main-belt asteroids in the mid-infrared, and is primarily
used in other applications.
Johan Lagerros