section_path: Classification/Orbital classification
section_name: Orbital classification
paragraph_text: Orbital classification
  
  Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past. Families are more common and easier to identify within the main asteroid belt, but several small families have been reported among the Jupiter trojans. Main belt families were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.
  
  About 30–35% of the bodies in the asteroid belt belong to dynamical families, each thought to have a common origin in a past collision between asteroids. A family has also been associated with the plutoid dwarf planet Haumea.
  
  Some asteroids have unusual horseshoe orbits that are co-orbital with Earth or another planet. Examples are 3753 Cruithne and 2002 AA 29. The first instance of this type of orbital arrangement was discovered between Saturn 's moons Epimetheus and Janus. Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their earlier status. Both Earth and Venus are known to have quasi-satellites.
  
  Such objects, if associated with Earth or Venus or even hypothetically Mercury, are a special class of Aten asteroids. However, such objects could be associated with the outer planets as well.