  Infrared measurements of asteroids are crucial for the determination of
physical and thermal properties of individual objects, and for the
understanding of the small-body populations in the solar system as a whole. But
standard radiometric methods can only be applied if the orbit of an object is
known, hence its position at the time of the observation. We present MIRI
observations of the outer-belt asteroid 10920 and an unknown object, detected
in all 9 MIRI bands in close proximity to 10920. We developed a new method
"STM-ORBIT" to interpret the multi-band measurements without knowing the
object's true location. The method leads to a confirmation of radiometric
size-albedo solution for 10920 and puts constraints on the asteroid's location
and orbit in agreement with its true orbit. Groundbased lightcurve observations
of 10920, combined with Gaia data, indicate a very elongated object (a/b >=
1.5), with a spin-pole at (l, b) = (178{\deg}, 81{\deg}), and a rotation period
of 4.861191 h. A thermophysical study leads to a size of 14.5 - 16.5 km, a
geometric albedo between 0.05 and 0.10, and a thermal inertia in the range 9 to
35 Jm-2s-0.5K-1. For the newly discovered MIRI object, the STM-ORBIT method
revealed a size of 100-230 m. The new asteroid must be on a very
low-inclination orbit and it was located in the inner main-belt region during
JWST observations. A beaming parameter {\eta} larger than 1.0 would push the
size even below 100 meter, a main-belt regime which escaped IR detections so
far. These kind of MIRI observations can therefore contribute to formation and
evolution studies via classical size-frequency studies which are currently
limited to objects larger than about one kilometer in size. We estimate that
MIRI frames with pointings close to the ecliptic and only short integration
times of a few seconds will always include a few asteroids, most of them will
be unknown objects.
