Flora family

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The Flora or Florian family of asteroids is a large grouping of S-type asteroids in the inner main belt, whose origin and properties are relatively poorly understood at present. Roughly 4-5% of all main belt asteroids belong to this family.

Because of its poorly defined boundaries, and the location of Flora itself near the edge, this asteroid group has also sometimes been called the Ariadne(an) family, when Flora did not make it into the group during an analysis (e.g. the WAM analysis by Zappala 1995).

The Flora family of asteroids may be the source of the Chicxulub (Cretaceous–Paleogene) impactor, the likely culprit in the extinction of the dinosaurs.[1]


Location and structure of the Flora family

The largest member is 8 Flora, which measures 140 km in diameter, and comprises about 80% of the total family mass. Nevertheless, the parent body was almost certainly disrupted by the impact/s that formed the family, and Flora is probably a gravitational aggregate of most of the pieces.[citation needed] 43 Ariadne makes up much of the remaining mass (about a further 9%), with the remaining family members being fairly small, below 30 km in diameter.

A noticeable fraction of the parent body has been lost from the family since the original impact, presumably due to later processes such as e.g. secondary collisions. For example, it has been estimated that Flora contains only about 57% of the parent body's mass (Tanga 1999), but about 80% of the mass in the present family.

The Flora family is very broad and gradually fades into the background population (which is particularly dense in this part of space) in such a way that its boundaries are very poorly defined. There are also several non-uniformities or lobes within the family, one cause of which may have been later secondary collisions between family members. Hence, it is a classical example of a so-called 'asteroid clan' (see asteroid family). Curiously, the largest members, 8 Flora and 43 Ariadne, are located near the edge of the family. The reason for this unusual mass distribution within the family is unknown at present.

951 Gaspra, a medium-sized core family member was visited by the Galileo spacecraft on its way to Jupiter, and is one of the most extensively studied asteroids. Studies of Gaspra suggests that the family's age is of the order of 200 million years (indicated by the crater density), and that the parent body was at least partially differentiated (indicated by the high abundance of olivine) (Veverka 1994).

The Flora family members are considered good candidates for being the parent bodies of the L chondrite meteorites (Nesvorny 2002), which contribute about 38% of all meteorites impacting the Earth. This theory is supported by the family's location close to the unstable zone of the \nu_6\,\! secular resonance, and because the spectral properties of family members are consistent with being the parent bodies of this meteorite type.

The Flora family was one of the five original Hirayama families that were first identified. It has a high number of early discovered members both because S-type asteroids tend to have high albedo, and because it is the closest major asteroid grouping to Earth.

Location and size[edit]

A HCM numerical analysis (Zappala 1995) determined a large group of 'core' family members, whose proper orbital elements lie in the approximate ranges

ap ep ip
min 2.17 AU 0.109 2.4°
max 2.33 AU 0.168 6.9°

The boundaries of the family are, however, very indistinct. At the present epoch, the range of osculating orbital elements of these core members is

a e i
min 2.17 AU 0.053 1.6°
max 2.33 AU 0.224 7.7°

Zappala's 1995 analysis found 604 core members, and 1027 in a wider group. A search of a recent proper element database (AstDys)for 96944 minor planets in 2005 yielded 7438 objects lying within the rectangular-shaped region defined by the first table above. However, this also includes parts of the Vesta and Nysa families in the corners so that a more likely membership estimate is 4000-5000 objects (by eye). This means that the Flora family represents 4-5% of all main belt asteroids.


Because of the high background density of asteroids in this part of space, one might expect that a great number of interlopers (asteroids unrelated to the collision that formed the family) would be present. However, few have been identified. This is because interlopers are hard to distinguish from family members because the family is of the same spectral type (S) that dominates the inner main belt overall. The few interlopers that have been identified are all small (Florczak et al. 1998, and also by inspection of the PDS asteroid taxonomy data set for non S-type members.) They include 298 Baptistina, 422 Berolina, 2093 Genichesk, 2259 Sofievka (the largest, with a 21 km diameter), 2952 Lilliputia, 453 Tea, 3533 Toyota, 3850 Peltier, 3875 Staehle, 4278 Harvey, 4396 Gressmann, and 4750 Mukai.


  1. ^ J.D. Harrington (2010-02-02). "Suspected Asteroid Collision Leaves Trailing Debris". NASA Release : 10-029. Retrieved 2010-02-03. 
  • Zappalà, Vincenzo; Bendjoya, Philippe; Cellino, Alberto; Farinella, Paolo; and Froeschlé, Claude; Asteroid Families: Search of a 12,487-Asteroid Sample Using Two Different Clustering Techniques, Icarus, Volume 116, Issue 2 (August 1995), pages 291-314
  • Florczak, M., et al.; A Visible Spectroscopic Survey of the Flora Clan, Icarus Vol. 133, p. 233 (1998).
  • PDS asteroid taxonomy data set
  • Bus, Schelte J.; and Binzel, Richard P.; Phase II of the Small Main-Belt Asteroid Spectroscopic Survey, Icarus Vol. 158, p. 106 (2002). Data set online here.
  • Nesvorný, D., et al.; The Flora Family: A Case of the Dynamically Dispersed Collisional Swarm?, Icarus, Vol. 157, p. 155 (2002).
  • AstDys site. Proper elements for 96944 numbered minor planets.
  • Tanga, P., et al.; On the Size Distribution of Asteroid Families: The Role of Geometry, Icarus, Vol. 141, p. 65 (1999).
  • Veverka, J., et al.; Galileo's Encounter with 951 Gaspra: Overview, Icarus, Vol. 107, p. 2 (1994).