Currently, there is no way to determine the object class (roughly spectral type) of an asteroid short of visiting it.

However, for comets the CometVessel module can override the knowledge base entry, displaying information about the comet's orbit instead of its size, and it seems like the purpose of VesselModule is to be accessible from unloaded contexts like the tracking station. Create a VesselModule that displays the asteroid's class, and add it to asteroids (but not comets?).