>>12599240because you take the light spectra from the object over several days and or weeks and look at the frequency of brightness and dimming, work out the mostly likely rotation period. Work out the albedo, look at what it's made of from the light spectra.
imagine it's shaped like an orange, the brightness and dimming is going to be a very shallow frequency dip, being an astronomer you know what objects in the asteroid belt and comets look like from previous data. You look at this thing, wtf am I looking at. etc etc
