In 1918, while the Japanese astronomer Kiyotsugu Hirayama was studying at Yale University, he began to examine asteroid motions.
By plotting the mean motion, eccentricity and inclination of the asteroid orbits, he discovered that some of the objects formed groupings.
That is, prior to the breakup, the parent body was partly segregated with denser materials moving toward the core.
[5] Objects that share similar orbits with the Eos family but do not have this spectrum are assumed to be random interlopers.
Evolutionary models of this spread in the rotation rate of the Eos family implies that this group may be comparable to the age of the Solar System.
[6] Numerical simulations of the collision that created the Eos family suggest that the smaller body was about a tenth the mass of the parent and struck from a direction out of the ecliptic plane.
[7] Not all fragments of the original parent body have remained in the orbital zone occupied by the Eos family.