The Flora Asteroid Clan


Flora asteroid family:  Number of members  ~ 819,  Largest member: 8 Flora, 136km, 

Average distance from Sun: 2.169 -2.211 AU's         

Age: 500 million - 1 billion years                                                                                                                                                                        

In 1918, the Japanese astronomer K. Hirayama first recognized some non random concentrations of asteroid elements.  He noticed that certain “groups” of asteroids had similar orbital elements, and hence he first introduced the concept of “asteroid families”, and identified three of them: Koronos, Eos, and Themis.  One year later he included the Flora group, its name derived from its primary member, the asteroid 8 Flora. The names of these groups were chosen by the parent (brightest) asteroid that the smaller group asteroids follow. These asteroid families provide a unique opportunity to investigate the mineralogical composition of the interior of solar system bodies, since they have components that are considered as remnants of energetic inter-asteroidal collisions.  These are believed to represent the leftover fragments of a much larger parent body. 

Some of the more common asteroid families include the Trojans, which are actually not an asteroid family, but a group of asteroids caught in the Sun-Jupiter gravitational equilibrium points known as L3 and L4. These are the well known Lagrangian points which come from the solution of the restricted 3-body problem in celestial mechanics.

The asteroid family concept was first confirmed by well known celestial mechanician Dirk Brouwer (who was head of the group to solve the equations of motion for NASA in the 1950’s to initiate satellite launches and tracking). He computed the proper elements of 1537 asteroids and added a few other families. The Flora family was then split into four subfamilies. Later astronomers disputed the four subfamily theory, claiming that this idea was a spurious consequence of the method used. 

Investigators still recognized the difficulty of identifying sub-groups of asteroid families such as in the Flora group. Problems in determining the proper elements, and the high density background plus the probable multi-collisional origin of this family contributed to the complexity of this region.  An animation of the asteroid family collision theory is presented in Figure 1 below. J. G. Williams in 1992 even suggested that the Flora family could be broken up into 10 separate families suggesting that many collisional events could have caused this splitting.


                                   Figure 1. Animation of Collision Theory of Creation of Asteroid Family.

With the many problems in identifying and defining asteroid families such as the Flora family, P. Farinella and others (in a LPI publication) proposed the new term “clans”.  This term represents groupings in which unequivocal membership definition and/or separation from other background groups is impossible. Thus in clans, the expected number of interlopers (non asteroid members passing through the orbital space) can be a significant fraction of the total number of asteroid members of a particular family.  This problem is similar to the extremely difficult task of identification of members of open (galactic) star clusters. In open clusters, the determination of membership requires accurate proper motions, and B-V magnitudes to filter out background stars.

Some interesting questions come about the study of such asteroid families:

1.     What exactly are asteroid families?

     2.     How do we identify a family?

     3.      How did they originate?  How do they evolve?  Do they gain/lose members?

     4.      How old is the family?

     5.      What causes interlopers to appear within a asteroid family?

     6.      How many asteroid families are there? (Some 40% of the numbered asteroids are members of over 100 families.)

     7.      And why are there so many asteroids between Mars and Jupiter?

     8.      Why are most of these asteroids so small?

     9.      Is their a mass history of the asteroid belt?

 Asteroid families are groups of asteroids that share the same or similar proper orbital elements (semimajor axis, eccentricity, inclination).  The inner zone of the asteroid belt is made up of three main clans: Flora, Nysa and Vesta. The largest of them is those associated with the asteroid 8 Flora, details of the Flora clan appear below:

               Flora Clan  Statistics


Largest Member:             8 Flora, size: 136 km

Next largest member:       36 Ariadne, size: 66 km

Estimated No. members:  819, all under 31 km in size

Location:                        Semi-major axis: 2.169 – 2.211 AU

No. of Interlopers:           170

Estimated age of Clan:     500 million – 1 billion years

Other prominent member: 951 Gaspra, studied by Galileo spacecraft

 By studying the orbital characteristics of over 12,487 main belt (within orbits of  Mars and Jupiter) asteroids, and using two independent techniques, in 1995 Zappala found the total number of significant clustering of asteroids to be 63.  Yet there are many more groupings of asteroids if one adds spectroscopic techniques, pushing the number to over 100.    

The identification of family members can be done not only by orbital elements but by examining the spectra of its members.  In a spectroscopic study of 47 Flora members, in 1998 Brazilian astronomers found 42 members with nearly identical spectral characteristics (asteroid DNA). The remaining 5 differed significantly, including the asteroid 4278 Harvey, whose spectra is very similar to 4 Vesta (a V-type classification).  4278 Harvey is thus assumed to be an interloper, that is an escaped member of another asteroid family, namely 4 Vesta.

How did 4278 Harvey (and other interlopers) end up in a similar orbit as the Flora family?   Theories of the origin of asteroid families suggest in the early history of the solar system a single large parent body suffered a collision. Fragments from this collision stayed in the orbit at the time and like debris from a comet’s tail, began to disperse around the Sun. Thus the asteroids in a particular family should and do have similar composition. Over time, additional collisions occurred and the same happened for other large bodies in the orbits between Mars and Jupiter, thus creating over time thousands of smaller asteroids, and thus more families.  In addition to the collisions, individual asteroids were influenced by the gravitational effect of Jupiter (and Mars), changing their orbital characteristics.  Thus some would escape their parent body orbit and stray into other orbits. This is one way that interlopers arrive in other families. The spectra/composition of these interlopers is good indicators of which parent asteroid was their origin.



Florczak, M., Barucci, M.A., Doressoundiram, A., Lazzaro, D., Angeli, A., Dotto, E., 1998, A Visible Spectroscopic Survey of the Flora Clan, Icarus, 133, p. 233-246.

Lodders, K., Fegley, B., 1998, The Planetary Scientist's Companion, Oxford University Press, NewYork, p. 241-246.

Marzari, F., Farinella, P., Davis, D.R., 1999, Origin, Aging, and Death of Asteroid Families, Icarus, 142, p. 63-77. 

Nesvorny, D., Morbidelli, A., Vokrouhlicky, D., Bottke, W.F., Broz, M., 2002, The Flora Family: A Case of the Dynamically Dispersed Collisional Swarm?, Icarus, 157, p. 155-172.