Hydrous asteroids formed from dust including ice of volatile species such as H₂O and CO₂ in outer cold regions of the early Solar System. Combined X-ray computed tomography and electron microscope analysis of samples from hydrous asteroids unveiled multiple events involved in evolutions of hydrous asteroids. Small and ultraporous lithology discovered in Acfer 094 carbonaceous chondrite was proven to be a fossil asteroidal ice originally formed as compact icy dust aggregates around the H₂O snow line in the solar nebula. The compact dust aggregates formed by sintering of fluffy ice-bearing dust. This finding further suggested the Acfer 094 parent body formation by agglomeration of fluffy dust with and without ice through its radial migration from the outer to the inner Solar System across the H₂O snow line. The combined analysis method was also applied for asteroid Ryugu samples collected by Hayabusa 2 sample return mission. The analysis revealed the presence of carbonated water preserved as a small fluid inclusion in a pyrrhotite grain. This provided evidence of Ryugu’s parent body formation in the outer Solar System beyond the CO₂ snow line. Analysis on space weathered Ryugu samples revealed modification processes of the asteroid Ryugu surface via cometary dust impacts. These findings provided important pieces to understand the long history of asteroid Ryugu.